5i' FASCICLE VII.3 Recommendations T.0-T.63 TERMINAL EQUIPMENT AND PROTOCOLS FOR TELEMATIC SERVICES Blanc MONTAGE: PAGE 2 = PAGE BLANCHE "Telematic services" is used provisionally and includes such services as Videotex, Teletex, facsimile, etc. COLLABORATION WITH OTHER | INTERNATIONAL ORGANIZATIONS ON CCITT-DEFINED TELEMATIC SERVICES Recommendation A.21 published in Volume I is reproduced below for the convenience of the reader. Recommendation A.21 COLLABORATION WITH OTHER INTERNATIONAL ORGANIZATIONS ON CCITT-DEFINED TELEMATIC SERVICES (Geneva, 1980, amended at Malaga-Torremolinos, 1984) The CCITT, considering (a) that, according to Article 1 of the agreement between the United Nations and the International Telecommunication Union, the United Nations recognizes the International Telecommunication Union as the specialized agency responsible for taking such action as may be appropriate under its basic instrument for the accomplishment of the purposes set forth therein; (b) that Article 4 of the International Telecommunication Con- vention | (Nairobi, 1982) states that the purposes of the Union are: "a) to maintain and extend international cooperation between all Members of the Union for the improvement and rational use of telecommunications of all kinds, as well as to promote and to offer technical assistance to developing countries in the field of telecommunications; " b) to promote the development of technical facili- ties and their most efficient operation with a view to improving the efficiency of telecommunication services, increasing their use- fulness and making them, as far as possible, generally available to the public; " c) to harmonize the actions of nations in the attainment of those ends."; (c) that Article 40 of the Convention states that "in further- ance of complete international coordination on matters affecting telecommunication, the Union shall cooperate with international organizations having related interests and activities"; (d) that this cooperation has to recognize the advisory capa- city of organizations participating in the work of CCITT; (e) that, in the study of terminals for new CCITT-defined telematic services (e.g. for Teletex , Telefax , Datafax , Bureaufax , Videotex services), ISO in particular is invited to give advice to CCITT based on their work on data systems and data communications; (f ) that this cooperation has to be organized in a manner that will avoid duplication of work and of decisions that would be contrary to the principles set out above; recognizes the following principles (1) It is the responsibility of the CCITT alone to make the decisions regarding the operational, technical (including factors needed to ensure international interworking) and tariff principles of the CCITT-defined services. (2) While the CCITT will define many of the relevant factors for the CCITT-defined telematic services, other international organizations will be invited to give specialist advice to CCITT on subjects that are of mutual interest, such as: - character sets and coding; - end-to-end control procedures including error protection; - interfaces between terminals and circuit ter- minating equipment; - terminal transmitter distortion and receiver mar- gin; - paper sizes and text formatting. (3) Standardization, if required, of hardware and software implementation of terminals, such as printing systems, paper feed, character type fonts, paper characteristics, etc., are outside the scope of CCITT. Recommendation A.22 COLLABORATION WITH OTHER INTERNATIONAL ORGANIZATIONS ON INFORMATION TECHNOLOGY (Melbourne, 1988) The CCITT, considering (a) that the purposes of the International Telecommunication Union and the recognition of CCITT relations with other organiza- tions were given in 1964 and later, in CCITT Recommendation A.20 which concerns data transmission; (b) that the principles of responsibility in regard to CCITT-defined Telematic services were given in 1980 and later, in Recommendation A.21 which mentions some subjects of mutual interest; (c) that CCITT Resolution No. 7 in 1984 further recognized common interests with ISO and IEC concerning Information Technology and cooperation with them by appropriate means, recognizes the following principles (1) that in accordance with CCITT Recommendations A.20 and A.21 and Resolution No. 7, every effort should be made in estab- lishing respective study programmes to identify overlapping studies with a view to avoiding duplication of work; (2) that where subjects are identified in which coordination seems desirable, text should be drawn up mutually and kept aligned; (3) that in carrying on the respective programmes of Informa- tion Technology studies, collaborative meetings at appropriate lev- els should be scheduled, where necessary. In drafting aligned text, it is necessary to take into account the respective timing for approvals and publication, particularly with the ISO/IEC Joint Technical Committee 1 (JTC1) on Information Technology; (4) that commonality of text with ISO/IEC and cross-references is considered desirable in certain areas of mutual interest, such as: - Message Handling Systems, - Directory Systems, - Open Systems Interconnection (OSI) architecture - service definitions and protocol specifications, - certain areas of Interworking, - certain aspects of Telematic Services, - Document Architecture, - certain aspects of ISDN. Recommendation T.0 CLASSIFICATION OF FACSIMILE APPARATUS FOR DOCUMENT | TRANSMISSION OVER THE PUBLIC NETWORKS (Geneva, 1976; amended at Geneva, 1980; Malaga-Torremolinos, 1984 and Melbourne, 1988) 1 For document facsimile transmission by international commun- ications carried on the public networks there is a need for provid- ing sufficient operating speeds to meet users' requirements. 2 Users' requirements may best be served at the present time by classifying the following four basic categories of document fac- simile apparatus. 2.1 Apparatus for use over the public telephone network Group 1 (see Note 1) Apparatus which uses double sideband modulation without any special measures to compress the bandwidth of the transmitted sig- nal and which is suitable for the transmission of documents of ISO A4 size at nominally 4 lines per mm in about six minutes via a telephone-type circuit. Apparatus in this group may be designed to operate at a lower definition suitable for the transmission of documents of ISO A4 size in a time between three and six minutes. Group 2 (see Note 2) Apparatus which exploits bandwidth compression techniques in order to achieve a transmission time of about three minutes for the transmission of an ISO A4 size document at nominally 4 lines per mm via a telephone-type circuit. Bandwidth compression in this context includes encoding and/or vestigial sideband working but excludes processing of the document signal to reduce redundancy. Group 3 (see Note 3) Apparatus which incorporates means for reducing the redundant information in the document signal prior to the modulation process and which can achieve a transmission time of about 1 minute for a typical typescript document of ISO A4 size via a telephone-type circuit. The apparatus may incorporate bandwidth compression of the line signal. 2.2 Apparatus for use over the public data networks Group 4 (see Note 4) Apparatus which incorporates means for reducing the redundant information in the document signal prior to transmission mainly via public data networks (PDNs). The apparatus will utilize procedures applicable to the PDN and will assure an essentially error-free reception of the document. The apparatus may also be used on the public telephone network where an appropriate modulation process will be utilized. 3 The users will choose among this apparatus, in accordance with their needs and the facilities afforded by the connection and the network. 4 Procedures for Groups 1, 2 and 3 document facsimile transmission in the public switched telephone network should be in accordance with Recommendation T.30. 5 Procedures for Group 4 document facsimile transmission should be in accordance with Recommendations T.62, T.62 | fIbis , and T.70. Note 1 - This apparatus has been standardized in Recommendation T.2. Note 2 - This apparatus has been standardized in Recommendation T.3. Note 3 - This apparatus has been standardized in Recommendation T.4. Note 4 - This apparatus has been standardized in Recommendations T.6, T.503, T.521 and T.563. 6 Annex A contains definitions for terms used in the T-series Recommendations applicable to facsimile apparatus. ANNEX A (to Recommendation T.0) Definitions for terms used in the T-series Recommendations applicable to fascimile apparatus The following definitions apply to Recommendations T.1, T.2, T.3 and T.4: A.1 dead sector (Recommendations T.1, T.2) In drum apparatus, that portion of the drum surface the scan- ning time of which cannot be used for picture signal transmission. A.2 drum factor (Recommendation T.1) In drum apparatus, the ratio of the usable scanning length of the drum to its diameter. A.3 facsimile (Series T) The process of scanning a document (page), converting the image scanned into electrical signals for transmission to a remote receiver and the conversion of the received signals to produce a copy of the image originally scanned. A.4 factor of cooperation (Recommendation T.1) The product of the total scanning line length and the scanning density. A.5 flat-bed transmitter (Recommendation T.1) Apparatus in which the original document is placed flat and scanned line by line. A.6 index of cooperation (Recommendations T.1, T.2, T.3) Quotient of the factor of cooperation divided by the quan- tity ~~. In the case of a drum apparatus, the index of cooperation is also equal to the product of the drum diameter and the scanning density. A.7 judder, longitudinal (Recommendation T.1) Effect due to the irregular rotation of the drum or helix causing, on the reproduced picture, slight waviness or breaks in lines that are regular on the original document. A.8 judder, transverse (Recommendation T.1) Effect due to irregularity of the scanning pitch resulting in concurrent overlapping and underlapping in the reproduced picture. A.9 lost time (Recommendation T.3) The portion of the scanning line period which cannot be used for picture signal transmission. Note - In the case of drum apparatus, this is the same as the dead sector scanning time. A.10 nominal black (white) (Recommendation T.1) Level or frequency of the signal corresponding to a pure black (white). A.11 pel (Series T) A contraction of "picture element". A.12 phasing (Recommendations T.1, T.2, T.3) At the receiver, ensuring the exact coincidence of the mid- point of the scanning field, with the corresponding point at the transmitter so as to ensure the correct positioning of the picture on the recording medium. A.13 phasing signal (Recommendations T.1, T.2, T.3) A signal sent by the transmitter for phasing purposes. Note - Phasing is known as "phase white (black)" if the phas- ing signal is a black (white) signal of which a short interruption corresponding to the white (black) is sent during the lost time. A.14 phototelegraphy (Recommendation T.1) Method of reception of facsimile telegraphy which is chiefly intended for the reproduction of graded tonal densities and in which a photographic process is used at the receiver. A.15 picture element (Recommendations T.3, T.4) a) at transmission: The part of the area of the original document which coincides with the scanning spot at a given instant and which is of one intensity only, with no distinction of the details that may be included. b) at reception: The area of the finest detail that can be effectively reproduced on the recording medium. A.16 reproduction ratio (Recommendation T.1) The ratio of the linear dimensions of the reproduced document to the corresponding dimensions of the original document. A.17 resolution (Series T) A measure of the capability for delineating picture detail. In Group 3 and Group 4 facsimile transmission resolution is expressed as picture elements or pels per mm (horizontal resolution) and lines per mm (vertical resolution). A.18 scanning density (Recommendations T.1, T.2, T.3) Number of scanning pitches per unit length. A.19 scanning line (Recommendations T.1, T.2, T.3) The area explored by the scanning spot in one sweep from one side to the other of the scanning field. A.20 scanning pitch (Recommendation T.1) The distance between the corresponding edges of two consecu- tive scanning lines. A.21 skew (Recommendation T.3) A defect in reproduction in which lines that should be at right-angles to the scanning direction are inclined to it, owing to a difference between the scanning speeds at transmission and recep- tion. A.22 synchronization (Recommendation T.1) The establishment of equal scanning line frequencies at the transmitter and receiver. Recommendation T.1 STANDARDIZATION OF PHOTOTELEGRAPH APPARATUS (former CCIT Recommendation D.1; amended at New Delhi, 1960; Geneva, 1964; Mar del Plata, 1968; Malaga-Torremolinos, 1984 and Melbourne, 1988) The CCITT, considering that the transmission of pictures is possible only if certain characteristics of the transmitting and receiving equipments are identical, unanimously declares the view that phototelegraph apparatus and the associated modulating and demodulating equipment should be constructed and employed according to the following standards: 1 Scanning track At the transmitting apparatus the message area should be scanned in a "negative" direction. The orientation of the document in relation to the scanning plane will depend upon its dimensions and is of no consequence. At the receiving apparatus scanning takes place in a "nega- tive" direction for "positive" reception and in a "positive" direc- tion for "negative" reception. 2 Index of cooperation The normal index is 352 (corresponds to a factor of coopera- tion of 1105). The preferred alternative index, for use when less dense scan- ning is required, or when the characteristics of circuits (and par- ticularly combined radio and metallic circuits) so demand, is 264 (a factor of cooperation of 829). The admissible tolerances on the above-mentioned values are _ | %. 3 Dimensions of apparatus 3.1 Apparatus with drum scanning The most currently used drum diameters are 66, 70 and 88 mm. The drum factor of the sending apparatus shall not be more than 2.4. The drum factor of the receiving apparatus shall not be less than 2.4. The width of the picture-retaining device (dead sector) may not exceed 15 mm. An allowance of 3% of the total length of a scan- ning line is also made for phasing. Thus, since the total cir- cumference of a drum of the diameter of 66 mm is about 207 mm, the usable circumference will be at least 186 mm. 3.2 Apparatus with flat-bed scanning The total lengths of the most current scanning lines are 207, 220 and 276 mm of which 15 mm are not used for effective transmis- sion, because of the possibility that the receiving station may use a drum apparatus. Before transmitting a picture to a receiving station using a drum apparatus, it is necessary to ensure that the value of ratio: otal length of a scanning line ____________________________________ x ~~ is less or at most equal to the drum factor of the receiver used. 3.3 Table 1/T.1 gives corresponding values of index of cooperation M , factor of cooperation C , drum diameter D , total length of scanning line L , scanning pitch P and scanning density F for apparatus in most common use. H.T. [T1.1] TABLE 1/T.1 _________________________ Measured in the direction perpendicular to the scanning line. _________________________________________________________ M C D (mm) L (mm) P (mm) F (lines/mm) _________________________________________________________ 264 829 66 207 1/4.77 4.77 264 829 70 220 1/3.77 3.77 264 829 88 276 1/3.77 3.77 350 1099 70 220 1/5.77 5.77 352 1105 66 207 3/16 7 16/3 352 1105 88 276 1/4.77 4.77 _________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note - The maximum dimensions of the pictures to be transmitted result from the parameters given in the table. Tableau 1/T.1 [T1.1], p. 4 Reproduction ratio In the case where apparatus working with different lengths of scanning line (but with the same index of cooperation) are inter- connected, there will be a slight change in size and the reproduc- tion will bear the same proportion as the original, the ratio being that of the total lengths of the scanning lines. 5 Drum rotation speed - scanning line frequency 5.1 Table 2/T.1 gives the normal and approved alternative com- binations of drum rotation speeds or of scanning line frequencies and indices of cooperation. H.T. [T2.1] TABLE 2/T.1 ______________________________________________________________________________________________ Metallic circuits { { ______________________________________________________________________________________________ Normal conditions 60 90 352 352 264 ______________________________________________________________________________________________ { Alternatives for use when the phototelegraph apparatus and metallic circuits are suitable } 90 120 150 { 264 and 352 264 and 352 264 } ______________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | Note 1 - In the case of transmitters operating on metallic cir- cuits, the index 264 is not intended to be used with an 88-mm drum. In the case of transmitters operating on combined metallic and radio circuits, the index 264 associated with a drum diameter of 88-mm is intended to be used only exceptionally. Note 2 - The provisions given in the table are not intended to require the imposition of such standards on users who use their own equipment for the transmission of pictures over leased circuits. However, the characteristics of the apparatus used should be compa- tible with the characteristics of the circuits used. Tableau 2/T.1 [T2.1], p. 5.2 The speed of transmitters must be maintained as nearly as possible to the nominal speed and in any case within _ | 0 parts in 106 of the nominal speed. The speed of receivers must be adju- stable and the range of adjustment should be at least _ | 0 parts in 106 from the nominal speed. After regulation, the speeds of the transmitting and receiving sets should not differ by more than 10 parts in 106. 6 Judder The stability of the speed during one rotation should be such that the maximum shift of the drum surface from the average posi- tion should not exceed one quarter of the scanning pitch P at nor- mal index 352, which means that the maximum angle of the oscilla- tions should not exceed 0.08 degree measured from the average posi- tion. 7 Synchronization When phototelegraph stations have available a standard of fre- quency which is better than _ | parts in 106, verification of the synchronism between the two stations may be dispensed with. In view of the saving of time, this method should be adopted wherever pos- sible. To compare the speeds of a transmitter and a receiver, an alternating current whose frequency bears an unvarying relationship to the transmitter speed and has a nominal value of 1020 Hz is used. Where there is the possibility that the transmitter and receiver may be connected by a circuit liable to introduce fre- quency drifts, for example, by a carrier telephone circuit, the use of the simple 1020-Hz synchronizing tone is unsatisfactory. The preferred method of overcoming this difficulty is to transmit the phototelegraph carrier (of about 1900 Hz) modulated by the 1020-Hz synchronizing tone. At the receiving end, the 1020-Hz synchronizing frequency is restored by detection and can then be used in the normal manner. 8 Phasing Phasing is performed after the speeds of the transmitter and receiver drums have been equalized. For phasing purposes, the transmitter sends a series of alter- nating white and black signals in such a way that the black lasts 95% and the white 5% of the total scanning line period (admissible tolerance: _ | .5% of the total duration of a scanning line). The apparatus must be so adjusted that the pulses corresponding to the signal for white are transmitted: - during scanning of the " dead sector ", when drum apparatus is used, - during "lost time", when flat-bed apparatus is used, and that they are placed at the middle of the dead sector (or of the interval corresponding to the lost time ). (Tolerance admitted in the position of the "white" pulses: __E = _ 1% of a "total scanning line length".) At the receiving station, phasing signals are used to start the apparatus so that short white pulses occur in the middle of the "lost time" (tolerance admitted: __R = _ 2% of a "total scanning line length"). Note - These tolerances allow for the fact that the restitu- tion of the original may deviate from its nominal position by 3% of a "total scanning line length", when the sending and receiving sta- tions are operating with the maximum authorized drift in the same direction. 9 Contrast The transmitter must transmit the original document without changing the contrast of the tone scales of the picture to be transmitted. 10 Modulation and demodulation equipment 10.1 Amplitude modulation Phototelegraph equipment shall normally provide for transmis- sion and reception of an amplitude modulated audio-frequency car- rier, which is the normal mode of transmission for international metallic circuits. The level of the output signal of the transmitter shall be greatest for white and least for black. It is desirable that the ratio of nominal white signal to nominal black signal should be approximately 30 decibels. To simplify multi-destination operation and AM/FM conversion for radio operation it is desirable that the amplitude of the transmitted signal should vary linearly with the photocell voltage and that no corrections for tone scale should be made at the photo- telegraph transmitting station. For audio-frequency telephone circuits, the frequency of the picture carrier-current is fixed at about 1300 Hz. This frequency gives the least delay distortion on lightly loaded underground cables. In the case of carrier telephone circuits providing a transmission band from 300 to 3400 Hz, a carrier-current frequency of about 1900 Hz is recommended. 10.2 Frequency modulation Preferably phototelegraph apparatus should also provide for transmission and reception of a frequency-modulated audio-frequency carrier for use when necessary: a) on combined metallic and radio circuits; b) on wholly metallic circuits. In such a case, the characteristics of the frequency-modulated output should be: mean frequency 1900 Hz white frequency 1500 Hz black frequency 2300 Hz The deviation of frequency should vary linearly with photocell voltage or, in the case of conversion from amplitude modulation to frequency modulation, with the amplitude of the amplitude-modulated carrier. The stability of the transmission must be such that the frequency corresponding to a given tone does not vary by more than 8 Hz in a period of 1 second and by more than 16 Hz in a period of 15 minutes. The receiving apparatus must be capable of operating correctly when the drift of black and white frequencies received does not exceed their nominal value by more than _ | 2 Hz. Note - It is recognized that there are difficulties operating with these frequency limits on the public switched telephone net- work (PSTN) where certain types of signalling equipment are used. By prior agreement between users on the PSTN, alternative frequen- cies of 1300 Hz for white and 2100 Hz for black may be used. 11 Positive or negative reception Selection of positive or negative reception should be made by adjustment at the receiver. The adaptation of the transmitted sig- nals to the characteristics of the photographic materials must also be effected at the receiving end according to the type of reproduc- tion, negative or positive. 12 Colour transmission (optional) 12.1 Phototelegraphy apparatus constructed in accordance with this Recommendation can be used in colour phototelegraphy by split- ting the spectrum of light reflected from the picture elements into three basic colours and transmitting the three resulting signals sequentially. Then each signal can be treated and transmitted as a phototelegraphy signal as specified in this Recommendation above. 12.2 The splitting of light reflected from picture element into three spectral components should be performed simultaneously. Thus synchro and phase coincidence and electronic colour correction can be achieved. 12.3 The triad RGB (red, green, blue) shall be used as a basis of main colours. The red colour shall be in range of 575-700 nm, green 485-575 nm, blue 400-485 nm. Note - For the high quality reproduction of art images by means of graphic facilities, transmission of fourth components (i.e. black overtone) is desirable. 12.4 The order of signal transmission shall be as follows: red, green, blue. In the case of negative reproduction the order of colour separated signals transmission is reversed. 12.5 The speeds of the transmitting and receiving sets should not differ by more than 1 part in 107. Recommendation T.2 STANDARDIZATION OF GROUP 1 FACSIMILE APPARATUS FOR DOCUMENT TRANSMISSION (Mar del Plata, 1968; amended at Geneva, 1972 and 1976) The CCITT, considering (a) that there is a requirement for Group 1 facsimile apparatus which enables an ISO A4 document to be transmitted over a telephone-type circuit in approximately six minutes; (b) that document facsimile transmission may be requested alternately with telephone conversation or when either or both sta- tions are unattended; in both cases the facsimile operation should conform to Recommendation T.30, unanimously declares the view that Group 1 facsimile apparatus for use on the general switched telephone network and international leased circuits should be designed and operated according to the following standards: 1 Scanning track The message area should be scanned in the same direction in the transmitter and receiver. Viewing the message area in a verti- cal plane, the scanning direction should be from left to right, and subsequent scans should be adjacent and below the previous scan. 2 Index of cooperation The nominal index of cooperation is 264. In cases where a lower vertical resolution is acceptable, and by agreement between the users, an optional index of cooperation of 176 may be used. These values should be observed with a nominal tolerance of _ | % for each equipment. 3 Dimensions of apparatus 3.1 The apparatus should accept documents up to a minimum of ISO A4 size (nominally 210 mm x 297 mm). 3.2 The total scanning line length ( active sector plus dead sector) should be nominally 215 mm. Nominally 200 mm should be available for scanning or recording, the remainder being the dead sector 3.3 For any one document the nominal number of scans should be 1144 for an index of cooperation of 264 (762 scans for an index of 176). The receiver should be capable of recording nominally 1144 scans per document for an index of cooperation of 264 (or 762 scans for an index of 176). 3.4 Apparatus with other dimensions may be used provided that the index of cooperation is respected, that the total scanning line length lies between 210 and 250 mm and the usable recording line length retains the same ratio to the total scanning line length. 4 Scanning density Scanning density is normally 3.85 lines per mm. 5 Scanning line frequency In the subscriber-to-subscriber service via the general switched telephone network, the scanning line frequency should be 180 lines per minute (see Note). For leased circuits operation the best line frequency, which may be higher or lower than 180 lines per minute, may be chosen according to the circuit characteristics. The scanning line frequency during the transmission should be kept within _ | 0 parts in 106 of the nominal value. Note - With manual control at the two ends of connection set up over the general switched telephone network, another scanning line frequency (e.g. 240 per minute) may be chosen by agreement between the two operators. 6 Phasing The duration of the phasing signal for transmitters should be 15 _ 1 seconds. In a preferred method of phasing (see Note 1), the transmitter sends a series of alternating white and black signals in such a way that the white pulse (phasing pulse) is 4 to 6% of the total scan- ning line length and the leading edge is 2 to 3% in advance of the middle of the dead sector. Receiving apparatus should synchronize the middle of its dead sector 0.5 to 4.5% lagging the leading edge of the received phasing pulse (see Note 2). Note 1 - In a permitted method of phasing for present genera- tion machines, the transmitter sends a series of white and black signals in such a way that the white pulse is 2 to 12% of the total scanning line length and the leading edge is 2 to 3% in advance of the middle of the dead sector. Note 2 - Maximum reduction of recorded scanning line length due to synchronizing misalignment should not exceed 3% of total scanning line length. Maximum reduction of recorded scanning line length due to the combined effect of deviations of the transmitter and receiver scanning line frequencies should not exceed 4% of total scanning line length. The effect of these reductions of recorded scanning line length may cause it to be less than the nom- inal 200 mm. 7 Modulation and demodulation equipments 7.1 Amplitude modulation | (for leased circuits only) The facsimile signal level is higher for black and lower for white. The carrier frequency should range between 1300 and 1900 Hz and will depend upon the characteristics of the circuits used. 7.2 Frequency modulation | (for leased circuits and for switched connections) The frequency corresponding to black will normally be f0 + 400 Hz and the frequency corresponding to white will nor- mally be f0 - 400 Hz (see Note). For switched connections f0 = 1700 Hz (provisional). For leased circuits f0should range between 1300 and 1900 Hz: the choice of the centre frequency f0will depend upon the circuit characteris- tics. However, if the user, in some cases, wishes to use the apparatus on switched connections, f0 = 1700 Hz. The stability of the transmitter must be such that the signi- ficant frequencies do not vary by more than 32 Hz from their nomi- nal value in a period of 15 minutes. Note - Attention is drawn to the fact that there are some equipments currently in operation for which black and white ele- ments are represented in the opposite sense, but the preferred standard for new equipment is as indicated above. 7.3 Power at the transmitter output When amplitude modulation is used, the power of black at the transmitter output must be able to be adjusted between -7 dBm and 0 dBm. The white level must be approximately 15 dB below the black level. In frequency-modulated systems, the level at the output of the facsimile apparatus must be able to be adjusted between -15 dBm and 0 dBm. The equipment should be so designed that there is no possibil- ity of this adjustment being tampered with by an operator. 7.4 Power at the receiver input The facsimile receiver must be so designed that it functions correctly when the input power ranges between 0 dBm and -40 dBm, the latter value being considered provisional. In the case of amplitude modulation, this concerns the power of the black signal. No control of receiver sensitivity should be provided for operator use. Recommendation T.3 STANDARDIZATION OF GROUP 2 FACSIMILE APPARATUS | FOR DOCUMENT TRANSMISSION (Geneva, 1976; amended at Geneva, 1980) The CCITT, considering (a) that Recommendation T.2 refers to Group 1 type apparatus for ISO A4 document transmission in approximately six minutes; (b) that there is a demand for Group 2 apparatus which enables an ISO A4 document to be transmitted over a telephone-type circuit in approximately three minutes; (c) that the Group 2 apparatus reproduces document quality similar to Group 1 apparatus; (d) that such a service may be requested either alternatively with telephone conversation, or when either or both stations are not attended; in both cases, the facsimile operation will follow Recommendation T.30; (e) interconnection between two machines of different designs, both conforming to Recommendation T.3 as published in the Orange Book may give a lower guaranteed reproducible area in cer- tain cases; unanimously declares the view that Group 2 facsimile apparatus for use on the general switched telephone network and international leased circuits shall, in future, be designed and operated in accordance with this Recom- mendation. Apparatus conforming to Recommendation T.3 of the Orange Book may continue in service. 1 Scanning track The message area should be scanned in the same direction in the transmitter and receiver. Viewing the message area in a verti- cal plane, the scanning direction should be from left to right and subsequent scans should be adjacent and below the previous scan. In order to avoid loss of information, users should insert documents so that the scanning direction is from the wider to the narrower margin. 2 Dimensions of apparatus The following dimensions are recommended but apparatus with other dimensions may be used provided that the factor of coopera- tion is respected and that the total scanning line length lies between 215 and 222 mm. Factor of cooperation (FOC) 829 _ 1% Total scanning line-length (TLL) 215 mm Usable scanning line-length (ULL) 205 mm minimum The end of ULL shall lie between 0 and 1 mm within the nominal position of the right hand edge of an ISO A4 size docu- ment. Input document size Up to a minimum of ISO A4 size (nominally 210 x 297 mm) The foregoing dimensions give rise to the following approxi- mate secondary dimensions. Index of cooperation (IOC) 264 Scanning density 3.85 line/mm Number of scanning lines in a document 297 mm long 1145 3 Scanning line frequency 3.1 The scanning line frequency should be 360 lines per minute. Note - Another scanning line frequency (e.g. 300 lines per minute) may be chosen by agreement between the two stations. 3.2 The scanning line frequency during the transmission should be kept within _ | parts in 106 of the nominal value. 4 Phasing For phasing prior to transmission of document information the transmitter sends a signal as shown in Figure 1/T.3. The start of the carrier indicates the end of the lost time time to the start of carrier should be _ | .5% of TLL. The receiver should phase its reference point with an accuracy of _ | % of TLL to this start of the carrier. The reference point should be 209 mm before the end of ULL (see Figure 2/T.3). Note - In the case of the multiple page transmission the phasing procedure may need to be repeated between pages. Figure 1/T.3, (M) p. Figure 2/T.3, (M) p. 5 Document transmission 5.1 During transmission of document information the transmitter should transmit full amplitude carrier during the lost time for at least 4% of the transmission time for TLL. The phase of the carrier may be reversed at the end of this signal. Following the lost time the information transmitted to the receiver is the one contained in ULL. The value of ULL is defined in S 2. The receiver must be able to reproduce at least ULL on the reproduction media after the end of the lost time. 5.2 This signal may be used at the receiver as an amplitude reference independent of the document signal and also to indicate to the receiver that document transmission is still in progress. 6 Paper alignment All types of apparatus should be designed to ensure that paper is positioned within _ | .5 mm of nominal in the horizontal direc- tion. In the vertical direction it must be ensured that paper is positioned in such a way that scanning and recording start not more than 4 mm down the document from the top edge. 7 Modulation and demodulation 7.1 Equivalent modulated waveforms with vestigial sideband amplitude modulation - phase modulation characteristics should be used for Group 2 apparatus operated on leased circuits and on the general switched telephone network. The carrier frequency should be 2100 _ 10 Hz (see Note). A white signal should be represented by maximum carrier and a black signal by minimum (at least 26 dB below white) or no carrier. The phase of the carrier representing white may be reversed after each transition through black. Note - It should be noted that there are equipments in ser- vice using, inter alia, a carrier frequency of 2048 Hz. 7.2 The facsimile transmission could contain a limited range of half-tones represented by a carrier amplitude less than that sent during the lost time. 7.3 A vestigial sideband filter symmetrical about the 2100-Hz carrier frequency should be provided at the transmitter. The response of this filter should conform to Figure 3/T.3. The accu- racy of this filter is not critical but the relative transmittance at the carrier frequency should be 0.5 with a tolerance of _ | .05 and the characteristic should be sensibly symmetrical about the carrier frequency. Figure 3/T.3, (M) p. 7.4 The receiving apparatus must be capable of operating correctly when the drift of carrier frequency does not exceed its nominal value by more than _ | 6 Hz. 7.5 A fixed compromise equalizer, an automatic equalizer or an adaptive equalizer may be provided in the receiver but this should be a matter for further study. 8 Power at the transmitter output The power of the white signal should be adjustable from -15 dBm to 0 dBm but the equipment should be so designed that there is no possibility of this adjustment being tampered with by an operator. The black level should be at least 26 dB below the white level. 9 Power at the receiver input The receiving apparatus should be capable of functioning correctly when the received signal level for white is within the range 0 dBm to -40 dBm, the latter value being considered provi- sional. No control of receiver sensitivity should be provided for operator use. APPENDIX I (to Recommendation T.3) Guaranteed reproducible area for Group 2 machines conforming to Recommendation T.3 Figure I-1/T.3, (M), p.6 I.1 Horizontal loss Figure I-2/T.3, (M), p.7 H.T. [T1.3] TABLE I-1/T/3 Results _____________________________________________________________ Left margin (mm) Right margin (mm) Horizontal losses 205 210 205 210 _____________________________________________________________ | | | | | | _________________________________________________________________________________ Transmitter a _0.5% 1.11 1.11 1.11 1.11 Phasing Receiver b _1.0% 2.22 2.22 2.22| | 2.22 _________________________________________________________________________________ Skew c { _5 x 10 | uDlF2616 } 2.46 2.46 2.46 2.46 _________________________________________________________________________________ Enlarging d - - 6.46 6.62 _________________________________________________________________________________ Paper insertion f 1.50 1.50 1.50 1.50 _________________________________________________________________________________ Min. x - - 1.00 1.00 Lost time Max. x 5.00 - - - _________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau I-1/T.3 [T1.3], p.8 I.2 Vertical loss Figure I-3/T.3, (M), p.9 H.T. [T2.3] TABLE I-2/T.3 ___________________________________________________ Top (mm) Bottom (mm) ___________________________________________________ Gripping loss g 4.0 4.0 ___________________________________________________ Paper insertion h ___________________________________________________ Enlarging + FOC i - 15.15 ___________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau I-2/T.3 [T2.3], p.10 Recommendation T.4 STANDARDIZATION OF GROUP 3 FACSIMILE APPARATUS FOR DOCUMENT TRANSMISSION (Geneva, 1980, amended at Malaga-Torremolinos, 1984 and Melbourne, 1988) The CCITT, considering (a) that Recommendation T.2 refers to Group 1 type apparatus for ISO A4 document transmission over a telephone-type circuit in approximately six minutes; (b) that Recommendation T.3 refers to Group 2 type apparatus for ISO A4 document transmission over a telephone-type circuit in approximately three minutes; (c) that there is a demand for Group 3 apparatus which enables an ISO A4 document to be transmitted over a telephone-type circuit in approximately one minute; (d) that for a large number of applications black and white reproduction is sufficient; (e) that such a service may be requested either alternatively with telephone conversation, or when either or both stations are not attended; in both cases, the facsimile operation will follow Recommendation T.30; unanimously declares the view that Group 3 facsimile apparatus for use on the general switched telephone network and international leased circuits should be designed and operated according to the following standards: 1 Scanning track The message area should be scanned in the same direction in the transmitter and receiver. Viewing the message area in a verti- cal plane, the picture elements should be processed as if the scan- ning direction were from left to right with subsequent scans adja- cent and below the previous scan. 2 Dimensions of apparatus Note - The tolerances on the factors of cooperation are subject to further study. 2.1 The following dimensions should be used: a) a standard resolution and an optional higher resolution of 3.85 line/mm _ 1% and 7.7 line/mm _ 1% respectively in vertical direction, b) 1728 black and white picture elements along the standard scan line length of 215 mm _ 1%, c) optionally, 2048 black and white picture ele- ments along a scan line length of 255 mm _ 1%, d) optionally, 2432 black and white picture ele- ments along a scan line length of 303 mm _ 1%, and, for equipment which provides A5 and/or A6 facilities: e) optionally, 864 black and white picture elements along a scan line length of 107 mm _ | %, f ) optionally, 1216 black and white picture ele- ments along a scan line length of 151 mm _ | %, g) Optionally, 1728 black and white picture ele- ments along a scan line length of 107 mm _ | %, h) Optionally, 1728 black and white picture ele- ments along a scan line length of 151 mm _ | %. The normal method of interworking when transmitting from an A5 or A6 machine to an A4 machine not signalling such capabilities, is that the A5 or A6 content will be enlarged to fill the A4 page (see also Note 3). This means that if the document is then retransmit- ted, or if it has been stored for later retransmission, it will be received without additional reduction. Where the full image contents being received from an A4 machine need to be maintained, g) or h) respectively should be used. Interworking between equipments with A5/A6 and A4 facilities and between equipments with combinations of these facilities is shown in Annex C. Note 1 - Cases e) to h) describe equipments which may be implemented singly or in any combination and would not, for A5/A6 facsimile equipments, require implementation of a) or b). These equipments may be implemented with cases different for sending and receiving. Note 2 - In cases e) to h), 1728 pels will always be provided to the coder (see Annex C). In cases e) and f ), the additional pels required are produced by pel processing (i.e., either by picture processing or by adding white pels on each side of the central picture information) prior to coding. Note 3 - It could be possible, by a setting on the A5/A6 transmitting machine, to send the document so that it is received equal size on an A4 machine not signalling such capabilities. In this case the vertical resolution will be 3.85 (or 7.7) line/mm. The user should be made aware that in this particular equal size case if the received copy is transmitted back to the A5/A6 machine the subsequent copy will be reduced. Note 4 - Some Administrations may require that equipments using e) or f ) dimensions, when working with a receiver not sig- nalling such capabilities, insert a message e.g., "ISO A6" or "ISO A5", as the case may be, into the picture at the transmitting side. 2.2 Input documents up to a minimum of ISO A4 size should be accepted. Note - The size of the guaranteed reproducible area is shown in Appendix I. 3 Transmission time per total coded scan line The total coded scan line is defined as the sum of DATA bits plus any required FILL bits plus the end-of-line (EOL) bits. For the optional two-dimensional coding scheme as described in S 4.2, the total coded scan line is defined as the sum of DATA bits plus any required FILL bits plus the EOL bits plus a tag bit. To handle various printing methods, several optional minimum total coded scan line times are possible in addition to the 20 mil- liseconds standard. 3.1 The minimum transmission times of the total coded scan line should conform to the following: 1) Alternative 1, where the minimum transmission time of the total coded scan line is the same both for the standard resolution and for the optional higher resolution: a) 20 milliseconds recommended standard, b) 10 milliseconds recognized option with a manda- tory fall-back to the 20 milliseconds standard, c) 5 milliseconds recognized option with a manda- tory fall-back to the 10 milliseconds option and the 20 mil- liseconds standard, d) 0 millisecond recognized option with a mandatory fall-back to the 5 milliseconds option, the 10 milliseconds option and the 20 milliseconds standard, and an optional fall-back to the 40 milliseconds option, e) 40 milliseconds recognized option. 2) Alternative 2, where the minimum transmission time of the total coded scan line for the optional higher resolu- tion is half of that for the standard resolution (see Note). These figures refer to the standard resolution: a) 10 milliseconds recognized option with a manda- tory fall-back to the 20 milliseconds standard, b) 20 milliseconds recommended standard, c) 40 milliseconds recognized option. The identification and choice of this minimum transmission time is to be made in the pre-message (phase B) portion of Recommendation T.30 control procedure. Note - Alternative 2 applies to equipment with printing mechanisms which achieve the standard vertical resolution by print- ing two consecutive, identical higher resolution lines. In this case, the minimum transmission time of the total coded scan line for the standard resolution is double the minimum transmission time of the total coded scan line for the higher resolution. 3.2 The maximum transmission time of any total coded scan line should be less than 5 seconds. When this transmission time exceeds 5 seconds, the receiver must proceed to disconnect the line. 3.3 Error correction mode For the optional error correction mode, an HDLC frame struc- ture is utilized to transmit the total coded scan line. This error correction mode is defined in Annex A. 4 Coding scheme 4.1 One-dimensional coding scheme The one-dimensional run length coding scheme recommended for Group 3 apparatus is as follows: 4.1.1 Data A line of Data is composed of a series of variable length code words. Each code word represents a run length of either all white or all black. White runs and black runs alternate. A total of 1728 picture elements represent one horizontal scan line of 215 mm length. In order to ensure that the receiver maintains colour syn- chronization, all Data lines will begin with a white run length code word. If the actual scan line begins with a black run, a white run length of zero will be sent. Black or white run lengths, up to a maximum length of one scan line (1728 picture elements or pels ) are defined by the code words in Tables 1/T.4 and 2/T.4. The code words are of two types: Terminating code words and Make-up code words one Make-up code word followed by a Terminating code word. Run lengths in the range of 0 to 63 pels are encoded with their appropriate Terminating code word. Note that there is a dif- ferent list of code words for black and white run lengths. Run lengths in the range of 64 to 1728 pels are encoded first by the Make-up code word representing the run length which is equal to or shorter than that required. This is then followed by the Ter- minating code word representing the difference between the required run length and the run length represented by the Make-up code 4.1.2 End-of-line (EOL) This code word follows each line of Data. It is a unique code word that can never be found within a valid line of Data; there- fore, resynchronization after an error burst is possible. In addition, this signal will occur prior to the first Data line of a page. Format: 000000000001 4.1.3 Fill A pause may be placed in the message flow by transmitting Fill. Fill may be inserted between a line of Data and an EOL, but never within a line of Data. Fill must be added to ensure that the transmission time of Data, Fill and EOL is not less than the minimum transmission time of the total coded scan line established in the pre-message control procedure. Format: variable length string of 0s. H.T. [T1.4] TABLE 1/T.4 Terminating codes ________________________________________________________________ White run length Code word Black run length Code word ________________________________________________________________ 0 00110101 0 0000110111 1 000111 1 010 2 0111 2 11 3 1000 3 10 4 1011 4 011 5 1100 5 0011 6 1110 6 0010 7 1111 7 00011 8 10011 8 000101 9 10100 9 000100 10 00111 10 0000100 11 01000 11 0000101 12 001000 12 0000111 13 000011 13 00000100 14 110100 14 00000111 15 110101 15 000011000 16 101010 16 0000010111 17 101011 17 0000011000 18 0100111 18 0000001000 19 0001100 19 00001100111 20 0001000 20 00001101000 21 0010111 21 00001101100 22 0000011 22 00000110111 23 0000100 23 00000101000 24 0101000 24 00000010111 25 0101011 25 00000011000 26 0010011 26 000011001010 27 0100100 27 000011001011 28 0011000 28 000011001100 29 00000010 29 000011001101 30 00000011 30 000001101000 31 00011010 31 000001101001 32 00011011 32 000001101010 33 00010010 33 000001101011 34 00010011 34 000011010010 35 00010100 35 000011010011 36 00010101 36 000011010100 37 00010110 37 000011010101 38 00010111 38 000011010110 39 00101000 39 000011010111 40 00101001 40 000001101100 41 00101010 41 000001101101 42 00101011 42 000011011010 43 00101100 43 000011011011 44 00101101 44 000001010100 45 00000100 45 000001010101 46 00000101 46 000001010110 47 00001010 47 000001010111 48 00001011 48 000001100100 49 01010010 49 000001100101 50 01010011 50 000001010010 51 01010100 51 000001010011 52 01010101 52 000000100100 53 00100100 53 000000110111 54 00100101 54 000000111000 55 01011000 55 000000100111 56 01011001 56 000000101000 57 01011010 57 000001011000 58 01011011 58 000001011001 59 01001010 59 000000101011 60 01001011 60 000000101100 61 00110010 61 000001011010 62 00110011 62 000001100110 63 00110100 63 000001100111 ________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 1/T.4 [T1.4],p.11 H.T. [T2.4] TABLE 2/T.4 Make-up codes ______________________________________________________________________ White run lengths Code word Black run lengths Code word ______________________________________________________________________ 64 11011 64 0000001111 128 10010 128 000011001000 192 010111 192 000011001001 256 0110111 256 000001011011 320 00110110 320 000000110011 384 00110111 384 000000110100 448 01100100 448 000000110101 512 01100101 512 0000001101100 576 01101000 576 0000001101101 640 01100111 640 0000001001010 704 011001100 704 0000001001011 768 011001101 768 0000001001100 832 011010010 832 0000001001101 896 011010011 896 0000001110010 960 011010100 960 0000001110011 1024 011010101 1024 0000001110100 1088 011010110 1088 0000001110101 1152 011010111 1152 0000001110110 1216 011011000 1216 0000001110111 1280 011011001 1280 0000001010010 1344 011011010 1344 0000001010011 1408 011011011 1408 0000001010100 1472 010011000 1472 0000001010101 1536 010011001 1536 0000001011010 1600 010011010 1600 0000001011011 1664 011000 1664 0000001100100 1728 010011011 1728 0000001100101 EOL 000000000001 EOL 000000000001 ______________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note - It is recognized that machines exist which accommodate larger paper widths maintaining the standard horizontal resolution. This option has been provided for by the addition of the Make-up code set defined as follows: Tableau 2/T.4 [T2.4], p.12 4.1.4 Return to control (RTC) The end of a document transmission is indicated by sending six consecutive EOLs. Following the RTC signal, the transmitter will send the post message commands in the framed format and the data signalling rate of the control signals defined in Recommendation T.30. Format: 000000000001 . | | | | | | | | | 000000000001 (total of 6 times) Figures 1/T.4 and 2/T.4 clarify the relationship of the sig- nals defined herein. Figure 1/T.4 shows several scan lines of data starting at the beginning of a transmitted page. Figure 2/T.4 shows the last coded scan line of a page. The identification and choice of either the standard code table or the extended code table is to be made in the pre-message (phase B) portion of Recommendation T.30 control procedures. Figure 1/T.4, (M), p. Figure 2/T.4, (M), p. 4.2 Two-dimensional coding scheme The two-dimensional coding scheme is an optional extension of the one-dimensional coding scheme specified in S 4.1 and is as fol- lows: 4.2.1 Data 4.2.1.1 Parameter K In order to limit the disturbed area in the event of transmis- sion errors, after each line coded one-dimensionally, at most K -1 successive lines shall be coded two-dimensionally. A one-dimensionally coded line may be transmitted more frequently than every K lines. After a one-dimensional line is transmitted, the next series of K -1 two-dimensional lines is initiated. The maximum value of K shall be set as follows: Standard vertical resolution: K = 2 Optional higher vertical resolution: K = 4. Note 1 - Some Administrations pointed out that for the optional higher vertical resolution K may optionally be set to a lower value. Note 2 - Some Administrations reserve the right to approve only such apparatus for use in the facsimile service in their respective countries which will be able to produce a visible sign on its received facsimile message indicating that two-dimensional coding has been used in the transmission process. 4.2.1.2 One-dimensional coding This conforms with the description of Data in S 4.1.1. 4.2.1.3 Two-dimensional coding This is a line-by-line coding method in which the position of each changing picture element on the current or coding line is coded with respect to the position of a corresponding reference element situated on either the coding line or the reference line which lies immediately above the coding line. After the coding line has been coded it becomes the reference line for the next coding line 4.2.1.3.1 Definition of changing picture elements | (see Figure 3/T.4) A changing element is defined as an element whose "colour" (i.e. black or white) is different from that of the previous ele- ment along the same scan line a0 The reference or starting changing element on the cod- ing line. At the start of the coding line a0is set on an imaginary white changing element situated just before the first element on the line. During the coding of the coding line, the position of a0is defined by the previous coding mode. (See S 4.2.1.3.2.) a1 The next changing element to the right of a0on the coding line. a2 The next changing element to the right of a1on the coding line. b1 The first changing element on the reference line to the right of a0and of opposite colour to a0. b2 The next changing element to the right of b1on the reference line. Figure 3/T.4, (M) p. 4.2.1.3.2 Coding modes One of the three coding modes are chosen according to the cod- ing procedure described in S 4.2.1.3.3 to code the position of each changing element along the coding line. Examples of the three cod- ing modes are given in Figures 4/T.4, 5/T.4 and 6/T.4. a) Pass mode This mode is identified when the position of b2lies to the left of a1. When this mode has been coded, a0is set on the element of the coding line below b2in preparation for the next coding (i.e. on a`0). Figure 4/T.4, (M) p. However, the state where b2occurs just above a1, as shown in Figure 5/T.4 is not considered as a pass mode. Figure 5/T.4, (M) p. b) Vertical mode When this mode is identified, the position of a1is coded relative to the position of b1. The relative distance a1b1can take on one of seven values V(0), VR(1), VR(2), VR(3), VL(1), VL(2) and VL(3), each of which is represented by a separate code word. The subscripts R and L indicate that a1is to the right or left respec- tively of b1, and the number in brackets indicates the value of the distance a1b1. After vertical mode coding has occurred, the posi- tion of a0is set on a1, (see Figure 6/T.4). c) Horizontal mode When this mode is identified, both the run-lengths a0a1and a1a2are coded using the code words H + M(a0a1) + M(a1a2). H is the flag code word 001 taken from the two-dimensional code table (Table 3/T.4). M(a0a1) and M(a1a2) are code words which represent the length and "colour" of the runs a0a1and a1a2respectively and are taken from the appropriate white or black one-dimensional code tables (Tables 1/T.4 and 2/T.4). After a horizontal mode coding, the position of a0is set on a2(see Figure 6/T.4). 4.2.1.3.3 Coding procedure The coding procedure identifies the coding mode that is to be used to code each changing element along the coding line. When one of the three coding modes has been identified according to Step 1 or Step 2 mentioned below, an appropriate code word is selected from the code table given in Table 3/T.4. The coding procedure is as shown in the flow diagram of Figure 7/T.4. Figure 6/T.4, (M) p. Note - It does not affect compatibility to restrict the use of pass mode in the encoder to a single pass mode. Variations of the algorithm which do not affect compatibility should be the sub- ject of further study. Step 1 i) If a pass mode is identified, this is coded using the word 0001 (Table 3/T.4). After this processing, picture element a`0just under b2is regarded as the new starting picture element a0for the next coding. (See Figure 4/T.4.) ii) If a pass mode is not detected then proceed to Step 2. Step 2 i) Determine the absolute value of the relative distance a1b1. ii) If | d1b1 | 3, as shown in Table 3/T.4, a1b1is coded by the vertical mode, after which position a1is regarded as the new starting picture element a0for the next coding. iii) If | d1b1 | > 3, as shown in Table 3/T.4, following horizontal mode code 001, a0a1and a1a2are respectively coded by one-dimensional coding. After this processing position a2is regarded as the new starting picture element a0for the next coding. H.T. [T3.4] TABLE 3/T.4 Two-dimensional code table _________________________________________________________________________________ Mode Elements to be coded Notation Code word _________________________________________________________________________________ Pass b 1, b 2 P 0001 _________________________________________________________________________________ Horizontal a 0a 1, a 1a 2 H { 001 + M(a 0a 1) + M(a 1a 2) (see Note 1) } _________________________________________________________________________________ a 1 just under b 1 a 1b 1 = 0 V(0) 1 a 1b 1 = 1 V R(1) 011 a 1 to the right of b 1 Vertical V L(1) | | | | | | | | | | | | | | | | | | | | 010 | | | | | | | | a 1b 1 = 2 _________________________________________________________________________________ Extension { 2-D (extensions) 1-D (extensions) } { 0000001xxx 000000001xxx (see Note 2) } _________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note 1 - Code M( | of the horizontal mode represents the code words in Tables 1/T.4 and 2/T.4. Note 2 - It is suggested the uncompressed mode is recognized as an optional extension of two-dimensional coding scheme for Group 3 apparatus. The bit assignment for the xxx bits is 111 for the uncompressed mode of operation whose code table is given in Table 4/T.4. Note 3 - Further study is needed to define other unspecified xxx bit assignments and their use for any further extensions. Note 4 - If the suggested uncompressed mode is used on a line designated to be one-dimensinally coded, the coder must not switch into uncompressed mode following any code word ending in the sequence 000. This is because any code word ending in 000 followed by a switching code 000000001 will be mistaken for an end-of-line code. Tableau 3/T.4 [T3.4], p.19 H.T. [T4.4] TABLE 4/T.4 Uncompressed mode code words _________________________________________________________________________________ { Entrance code to uncompressed mode } { On one-dimensionally coded line: 000000001111 On two-dimensionally coded line: 0000001111 } _________________________________________________________________________________ Uncompressed mode code { Image pattern 1 01 001 0001 00001 00000 } { Code word 1 01 001 0001 00001 000001 } _________________________________________________________________________________ { Exit from uncompressed mode code } { 0 00 000 0000 } { 0000001T 00000001T 000000001T 0000000001T 00000000001T T denotes a tag bit which tells the colour of the next run (black = 1, white = 0). } _________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau 4/T.4 [T4.4], p.20 4.2.1.3.4 Processing the first and last picture elements in a line a) Processing the first picture element The first starting picture element a0on each coding line is imaginarily set at a position just before the first picture ele- ment, and is regarded as a white picture element (see S 4.2.1.3.1). The first run length on a line a0a1is replaced by a0a1-1. Therefore, if the first run is black and is deemed to be coded by horizontal mode coding, then the first code word M(a0a1) corresponds to a white run of zero length (see Figure 10/T.4, Exam- ple 5). b) Processing the last picture element The coding of the coding line continues until the position of the imaginary changing element situated just after the last actual element has been coded. This may be coded as a1or 2. Also, if b1and/or b2are not detected at any time during the coding of the line, they are positioned on the imaginary changing element situated just after the last actual picture element on the refer- ence line. 4.2.2 Line synchronization code word To the end of every coded line is added the end-of-line (EOL) code word 000000000001. The EOL code word is followed by a single tag bit which indicates whether one- or two-dimensional coding is used for the next line. In addition, EOL plus the tag bit 1 signal will occur prior to the first Data line of a page. Format: EOL + 1: one-dimensional coding of next line EOL + 0: two-dimensional coding of next line Figure 7/T.4, (MC), p.21 4.2.3 Fill Fill is inserted between a line of Data and the line synchron- ization signal, EOL + tag bit, but is not inserted in Data. Fill must be added to ensure that the transmission time of Data, Fill and EOL plus tag bit is not less than the minimum transmission time of the total coded scan line. Format: variable length string of 0 s. 4.2.4 Return to control (RTC) The format used is six consecutive line synchronization code words, i.e., 6 x (EOL + 1). To further clarify the relationship of the signals defined herein, Figures 8/T.4 and 9/T.4 are offered in the case of K = 2. Figure 8/T.4 shows several scan lines of data starting at the beginning of a transmitted page. Figure 9/T.4 shows the last several lines of a page. Figure 8/T.4, (M) p. Figure 9/T.4, (M) p. 4.2.5 Coding examples Figure 10/T.4 shows coding examples of the first part of scan lines and Figure 11/T.4 coding examples of the last part, while Figure 12/T.4 shows other coding examples. The notations P, H and V in the figures are, as shown in Table 3/T.4, the symbols for pass mode, horizontal mode and vertical mode respectively. The picture elements marked with black spots indicate the changing picture ele- ments to be coded. Figure 10/T.4, (M) p. Figure 11/T.4, (M) p. Figure 12/T.4, (M) p. 4.3 Error limiting mode One-dimensional coding scheme with the division of scan line into parts. The one-dimensional coding scheme with the division of scan line into parts is an optional extension of the one-dimensional coding scheme specified in Annex B. 5 Modulation and demodulation Group 3 apparatus operating on the general switched telephone network shall utilize the modulation, scrambler, equalization and timing signals defined in Recommendation V.27 | fIter , specifi- cally SS 2, 3, 7, 8, 9, 11 and the Appendix. 5.1 The training signal to be used shall be the long training sequence with protection against talker echo. (See Recommendation V.27 ter , S 2.5.1, Table 3/V.27 ter ). 5.2 The data signalling rates to be used are 4800 bit/s and 2400 bit/s as defined in Recommendation V.27 | fIter . Note 1 - Some Administrations pointed out that it would not be possible to guarantee the service at a data signalling rate higher than 2400 bit/s. Note 2 - It should be noted that there are equipments in ser- vice using, inter alia, other modulation methods. Note 3 - Where quality of communication service can success- fully support higher speed operation, such as may be possible on leased circuits or high-quality switched circuits, Group 3 apparatus may optionally utilize the modulation, scrambler, equali- zation and timing signals defined in Recommendation V.29, specifi- cally SS 1, 2, 3, 4, 7, 8, 9, 10 and 11. Under this option the data should be non-multiplexed and limited to the data signalling rates of 9600 bit/s and 7200 bit/s. 6 Power at the transmitter output The average power should be adjustable from -15 dBm to 0 dBm but the equipment should be so designed that there is no possibil- ity of this adjustment being tampered with by an operator. Note - The power levels over the international circuits will conform to Recommendation V.2. 7 Power at the receiver input The receiving apparatus should be capable of functioning correctly when the received signal level is within the range of 0 dBm to -43 dBm. No control of receiver sensitivity should be provided for operator use. 8 Implementation of apparatus Although paper sizes are referred to, this does not always require a physical paper scanner and/or printer to be implemented. Details may be defined by Administrations. If the message is not generated from a physical scanner or displayed on paper, then the signals appearing across the network interface shall be identical to those which would be generated if paper input and/or output had been implemented. ANNEX A (to Recommendation T.4) Optional error correction mode A.1 Introduction This annex specifies the message format required for document transmission incorporating the optional error correction capabil- ity. A.2 Definitions The definitions contained in Recommendations T.4 and T.30 shall be applied unless explicitly amended. A.3 Message format An HDLC frame structure is utilized for all binary coded fac- simile message procedures. The basic HDLC structure consists of a number of frames each of which is subdivided into a number of fields. It provides for frame labelling and error checking. Specific examples are given in Figures A-1/T.4 and A-2/T.4 of formats used for binary coded signalling. These examples show an initial partial page (PP) frame structure and a last PP frame structure. In the following descriptions of the fields, the order in which the bits are transmitted is from the most to the least signi- ficant bit, i.e., from left to right as printed. The exception to this is the frame number (see S A.3.6.1). The equivalent between binary notation symbols and the signi- ficant condition of the signalling code should be in accordance with Recommendation V.1. A.3.1 Synchronization A synchronization sequence shall precede all binary coded information whenever a new transmission begins. The synchronization shall be a training sequence and a series of flag sequences for nominal 200 ms, tolerance +100 ms. Note - Continuous flags have two zeros as shown in the fol- lowing diagram: 0111 1110 0111 1110 0111 1110 A.3.2 Flag sequence (F) The eight bit HDLC flag sequence is used to denote the begin- ning and end of the frame for the facsimile message procedure. The flag sequence is also used to establish bit and frame synchroniza- tion. To facilitate this the synchronization defined in A.3.1 should be used prior to the first frame. Subsequent frames and end of the last frame need one or more than one flag sequence. Format: 0111 1110 Note - The leading flag of a frame may be the trailing flag of the previous frame. Figure A-1/T.4, (N), p. Figure A-2/T.4, (N), p. A.3.3 Address field (A) The eight bit HDLC address field is intended to provide iden- tification of specific station(s) in a multi-point arrangement. In the case of transmission on the general switched telephone network, this field is limited to a single format. Format: 1111 1111 A.3.4 Control field (C) The eight bit HDLC control field provides the capability of encoding the command unique to the facsimile message procedure. Format: 1100 X000 The X bit is set to 0 for the FCD frame (facsimile coded data frame) and the RCP frame (return to control for partial page frame). A.3.5 Facsimile control field (FCF) In order to distinguish between the FCD frame (facsimile coded data frame) and the RCP frame (return to control for partial page frame), the FCF for the in-message procedure is defined as follows: 1) FCF for the FCD frame Format: 0110 0000 2) FCF for the RCP frame Format: 0110 0001 A.3.6 Facsimile information field (FIF) The facsimile information field is a length of 257 or 65 octets (see Note 1) and is divided into two parts, the frame number and the facsimile data field (see Note 2). Note 1 - This does not include bit stuffing to preclude non-valid flag sequences. Note 2 - There is no information field in the RCP frame. A.3.6.1 Frame number This is an eight bit binary number. The frame number is defined to be the first eight bits of the facsimile information field. The least significant bit is transmitted first. The frame number 0-255 (maximum number is 255) is used to identify the facsimile data field (see Recommendation T.30, Annex A). The frame 0 is transmitted first in each block. A.3.6.2 Facsimile data field The coding schemes specified in S 4 are valid with the follow- ing notes. 1) The facsimile data field is a length of 256 or 64 octets. 2) The total coded scan line is defined as the sum of DATA bits plus the EOL bits. For the optional two-dimensional coding scheme as described in S 4.2, the total coded scan line is defined as the sum of DATA bits plus the EOL bits plus a tag bit 3) At the end of facsimile data field, if neces- sary, Pad bits may be used to align on octet boundaries and frame boundaries (see Notes 1 and 2). The format is a variable length string of zeros. Note 1 - The receiver is able to receive both Pad bits and Fill bits. Note 2 - The facsimile data field length of the final frame including RTC signal may be less than 256 or 64 octets. A.3.7 Frame checking sequence (FCS) The FCS shall be a 16 bit sequence (see Recommendation T.30, S 5.3.7). A.3.8 Return to control for partial page (RCP) The end of a partial page transmission is indicated by sending three consecutive RCP frames (see Note). Following these RCP frames, the transmitter will send the post message commands in the framed format and the data signalling rate of the control signals defined in Recommendation T.30, Annex A. Note - The flag sequence following the last RCP frame shall be less than 50 ms. ANNEX B (to Recommendation T.4) Optional error limiting mode Note - The text of Annex B shall be refined and studied dur- ing the next study period. B.1 Data B.1.1 The division of a scan line into parts In order to limit the disturbed area in the event of transmis- sion error, the scan lines are divided into parts before coding. The number of parts shall be used as follows: a) standard, 12 parts in a line composed of 1728 black and white picture elements, b) optionally, 15 parts in a line composed of 2048 black and white picture elements, c) optionally, 17 parts in a line composed of 2432 black and white picture elements. Note - For alternatives b) and c), the last part of a scan line can be shortened and then will contain 32 and 128 pels respec- tively. B.1.2 Scan line coding All parts of a scan line are divided into whites (W) if they are composed of all white picture elements and not-white (NW) if they contain at least one black element. The coding procedure is as shown in the flow diagram of Figure B-1/T.4. B.1.2.1 Shaping the extended description of a scan line For each coded scan line the extended scan line description (ELD) is shaped. ELD represents a sequence, where the bit number is equal to the part number in a scan line, i.e., each part has corresponding bit in the sequence. This bit is equal to "1", if the part is "NW" and it is equal to "0" if the part is "W". B.1.2.2 Scan line part coding W-parts are not encoded. The coding of each NW-part is independent of the coding of other parts in the given scan line. In the NW-part the white and black runs alternate. The coding always begins with a white run. If the actual scan line begins with a black run then a white run length of zero will be sent. Run lengths are encoded using Tables 1/T.4 and 2/T.4 as described in S 4.1.1. The last run of each NW-part is not encoded. Resulted coded run lengths (CRL) are sent directly one after another. B.1.2.3 Code bit number variation (CBNV) It is necessary to code and send the number of coded bits for each NW-part. For this purpose the code bit number of the previous NW-part qi\d\u(em1is subtracted from the code bit number of the given NW-part qi. The resulting difference qi - qi\d\u(em1is coded by using code words listed in Table B-1/T.4. For the first NW-part in a scan line q0is taken to be 40. In the code words given in Table B-1/T.4 the bit X corresponds to the sign of the difference qi - qi\d\u(em1. When the difference is positive, bit X equals "0", but when the difference is negative bit X equals "1". Figure B-1/T.4, (N), p.29 H.T. [1T5.4] TABLE B-1/T.4 Code table for the code bit number variation _____________________________________________________________________________________________________ Absolute value of variation Code Absolute value of variation Code _____________________________________________________________________________________________________ 0 100000 51 X11111 010101 1 X00001 52 X11111 010110 2 X00010 53 X11111 010111 3 X00011 54 X11111 011000 4 X00100 55 X11111 011001 5 X00101 56 X11111 011010 6 X00110 57 X11111 011011 7 X00111 58 X11111 011100 8 X01000 59 X11111 011101 9 X01001 60 X11111 011110 10 X01010 61 X11111 100000 11 X01011 62 X11111 100001 12 X01100 63 X11111 100010 13 X01101 64 X11111 100011 14 X01110 65 X11111 100100 15 X01111 66 X11111 100101 16 X10000 67 X11111 100110 17 X10001 68 X11111 100111 18 X10010 69 X11111 101000 19 X10011 70 X11111 101001 20 X10100 71 X11111 101010 21 X10101 72 X11111 101011 22 X10110 73 X11111 101100 23 X10111 74 X11111 101101 24 X11000 75 X11111 101110 25 X11001 76 X11111 101111 26 X11010 77 X11111 110000 27 X11011 78 X11111 110001 28 X11100 79 X11111 110010 29 X11101 80 X11111 110011 30 X11110 81 82 { X11111 110100 X11111 110101 } 31 X11111 000001 83 X11111 110110 32 X11111 000010 84 X11111 110111 33 X11111 000011 85 X11111 111000 34 X11111 000100 86 X11111 111001 35 X11111 000101 87 X11111 111010 36 X11111 000110 88 X11111 111011 37 X11111 000111 89 X11111 111100 38 X11111 001000 90 X11111 111101 39 X11111 001001 91 X11111 X11110 010000 40 X11111 001010 92 X11111 X11111 000001 41 X11111 001011 93 X11111 X11111 000010 42 X11111 001100 94 X11111 X11111 000011 43 X11111 001101 95 X11111 X11111 000100 44 X11111 001110 96 X11111 X11111 000101 45 X11111 001111 97 X11111 X11111 000110 46 X11111 010000 98 X11111 X11111 000111 47 X11111 010001 99 X11111 X11111 001000 48 X11111 010010 100 X11111 X11111 001001 49 X11111 010011 101 X11111 X11111 001010 50 X11111 010100 102 X11111 X11111 001011 _____________________________________________________________________________________________________ | | | | | Tableau B-1/T.4 [1T5.4], p. H.T. [2T5.4] TABLE B-1/T.4 (cont.) ___________________________________________________________________________________________________________ Absolute value of variation Code Absolute value of variation Code ___________________________________________________________________________________________________________ 103 X11111 X11111 001100 119 X11111 X11111 011100 104 X11111 X11111 001101 120 X11111 X11111 011101 105 X11111 X11111 001110 121 X11111 X11111 011110 106 X11111 X11111 001111 122 X11111 X11111 100000 107 X11111 X11111 010000 123 X11111 X11111 100001 108 X11111 X11111 010001 124 X11111 X11111 100010 109 X11111 X11111 010010 125 X11111 X11111 100011 110 X11111 X11111 010011 126 X11111 X11111 100100 111 X11111 X11111 010100 127 X11111 X11111 100101 112 X11111 X11111 010101 128 X11111 X11111 100110 113 X11111 X11111 010111 129 X11111 X11111 100111 114 X11111 X11111 010111 130 X11111 X11111 101000 115 X11111 X11111 011000 131 X11111 X11111 101001 116 X11111 X11111 011001 132 X11111 X11111 101010 117 X11111 X11111 011010 133 X11111 X11111 101011 118 X11111 X11111 011011 134 X11111 X11111 101100 ___________________________________________________________________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note - Bit X corresponds to the sign of the variation. Tableau B-1/T.4 [2T5.4], p. B.1.3 Data format The data format for the scan line containing several NW-parts is shown in Figure B-2/T.4 and containing only one NW-part is shown in Figure B-3/T.4. The data format for the scan line containing all whites is shown in Figure B-4/T.4. Figure B-2/T.4, (N), p.32 Figure B-3/T.4, (N), p.33 Figure B-4/T.4, (N), p.34 B.2 End of line (EOL) This code word follows each line of data. There is a slight probability of occurrence of the same bit combination for ELD and the code word EOL. This should be taken account in the decoding algorithm. In addition, EOL is sent prior to the format data line of the page. Format: 000000000001 B.3 Fill A pause in the message may be filled as described in S 4.1.3. B.4 Return to control (RTC) The return to control should comply with S 4.1.4. Note - When decoding, the correction of the corrupted parts can be performed by replacing the corrupted part with the corresponding uncorrupted part from the previous line. The exceed- ing of the value 144 by the decoded part length or the absence of code word of the given part in the code table vocabulary can be shown as a sign for replacement. ANNEX C (to Recommendation T.4) Interworking between equipments with A5/A6 and A4 facilities and between equipments with combinations of these facilities Tableau C-1/T.4 [T6.4] A L'ITALIENNE MONTAGE: A COLLER LE TITRE , p.35 APPENDIX I (to Recommendation T.4) Guaranteed reproducible area for Group 3 apparatus conforming to Recommendation T.4 FIGURE I-1/T.4, p. + Remarques Figure I-2/T.4, (M), p.37 H.T. [T7.4] TABLE I-1/T.4 Horizontal losses _________________________________________ Printer/scanner a _ | .5 mm _________________________________________ Enlarging b _ | .1 mm _________________________________________ Skew c _ | .6 mm _________________________________________ Positioning errors d _ | .5 mm _________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Tableau I-1/T.4 [T7.4], p.38 Figure I-3/T.4, (M), p.39 H.T. [T8.4] TABLE I-2/T.4 Vertical losses ____________________________________________________ Paper insertion f { _ | .07 mm } ____________________________________________________ Skew g _ | .87 mm ____________________________________________________ Scanning density tolerance h _ | .97 mm ____________________________________________________ Gripping loss i { _ | .07 mm } ____________________________________________________ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Note - Scanning density tolerance will reduce to 0 mm on roll-fed machines. Tableau I-2/T.4 [T8.4], p.40