Revised Recommendation M.590
SETTING UP AND LINING UP A CIRCUIT FITTED WITH A COMPANDOR
1. The compandor should first be tested in accordance with the appropriate
design information which should be made available in a suitable form to repeater
station staff. In particular, because the unaffected level of a compandor is
defined with reference to an 800 Hz signal, it should be verified for each type of
compandor that use of a reference test frequency of 1 020 Hz produces the same
results as using a reference test frequency of 800 Hz.
2. Circuits with compandors fitted should be lined up to achieve the same
limits as circuits without compandors. The compandor should be fitted to the
circuit only after the circuit without its compandor is considered satisfactory in
respect of loss and loss/frequency response. It should be noted that to achieve the
limits for loss/frequency response on the companded circuit without equalization,
it will be necessary for the loss/frequency response of the uncompanded circuit
to be within one half of the circuit limits.
3. Measurements of total distortion and idle channel noise
After compandors have been fitted, total distortion and idle channel
noise measurements should be made. The test signal used for the total distortion
measurement should be applied at the unaffected level of the compandor.
In the case of a circuit which is fitted with a compandor to subjectively
reduce the noise generated within a terrestrial circuit section, the measurements
should be noted.
In the case of a circuit which is fitted with a compandor to subjectively
reduce the noise generated by a satellite circuit section, the procedure is as
follows:
- 1 For analogue routed circuits the noise objectives given in
Table 4/M.580 for the appropriate terrestrial length
of circuit should be combined with the total
distortion objective for the satellite channel2 to produce a
total distortion objective for the whole circuit. See
Annex A to this Recommendation for an example of this
calculation.
1 It should be noted that in the case of mixed analogue/digital circuits,
if the unaffected level is other than -10 dBm0 then this procedure will
produce less accurate results and in this case it should be used as a
general guide only.
2 The total distortion objective for INTELSAT single sideband Standard B
satellite channels is -41 dBm0p (FM companded circuit).
- For mixed analogue/digital routed circuits the total distortion
objectives given in Table 6/M.580 for the appropriate
terrestrial length of circuit should be combined with
the total distortion objective for the satellite
channel2 to produce a total distortion objective for the
whole circuit. See Annex A to this Recommendation for an
example of this calculation.
- If the total distortion which is measured is higher than the
calculated total distortion objective, then a fault
should be suspected and action should be taken to
locate and remedy any fault where possible.
- When the total distortion measurement has been made and is found
to meet the calculated total distortion objectives,
an idle channel noise measurement should be made.
- The idle channel noise measurement should be compared with the
maintenance objective given in Table 4/M.580 for the
appropriate length of circuit, taking into account the
Note associated with this table that the satellite
section of the circuit may be considered to have an
equivalent length of 2,500 km. This is a valid
consideration provided that the total distortion objective of the
satellite channel is not greater than -30 dBm0p.
- If the measured value is higher by 5 dB or more than the noise
objective given in Table 4/M.580 or is higher than -37 dBm0p,
whichever is the more stringent requirement, then a fault should be
suspected and action should be taken to locate and remedy any
fault where possible.
4. A speaking test should be made on the circuit to verify the correct
operation of the compandors.
5. Designations
Companded circuits and groups of circuits which are all companded
should be designated in accordance with Recommendation M.140.
Note - Repeater station staff should be well instructed as to the subjective
effect of errors and the location of faults affecting compandors.
2 The total distortion objective for INTELSAT single sideband Standard B
satellite channels is -41 dBm0p (FM companded circuit).
ANNEX A
(to revised Recommendation M.590)
Total distortion and idle channel noise objectives for circuits
which are fitted with compandors in order to subjectively reduce
the effect of noise generated on satellite channels
A.1 A distinction is made between
a) circuits which are fitted with compandors to subjectively improve
the noise generated by a terrestrial section; and
b) circuits which are fitted with compandors to subjectively improve the noise generated by a satellite section.
This distinction is made because in case a) it is not possible to specify
noise or total distortion objectives. However, in case b), the satellite system operator
can specify noise objectives for the satellite section. Thus these limits can be
combined with those contained in Recommendation M.580 to calculate an overall limit.
A.2 Examples of calculations of total distortion for case b)
Example 1
Consider an analogue circuit which has a terrestrial length of 1,600 km and
which is routed via a satellite which has a specified idle channel noise objective of
-41 dBm0p for analogue channels.
From Table 4/M.580, the noise objective for a length of l,600 km is
-49 dBm0p.
Combining -41 dBm0p and 49 dBm0p gives a total distortion of
-40.36 dBm0p.
Thus the total distortion objective should be -40 dBm0p.
Example 2
Consider a mixed analogue/digital circuit which has a terrestrial length of
l,600 km, two analogue/digital conversions using 8 bit coding and which is routed via
a satellite which has a specified idle channel noise objective of -41 dBm0p for
analogue channels.
From Table 6/M.580, the total distortion objective for a length of 1,600 km
is -42 dBm0p.
Combining -41 dBm0p and -42 dBm0p gives a total distortion of -38.46 dBm0p.
Thus the total distortion objective should be -38 dBm0p.