INTERNATIONAL  TELECOMMUNICATION  UNION

		     ITU-T RECOMMENDATION SUMMARY


Rec. No. : G.727

Title :	5-, 4-, 3- and 2-bits sample embedded adaptive differential 
	pulse code modulation (ADPCM)

Study Group : XV - Transmission Systems and Equipment

Version : New

Date of adoption : 1990

Notes :	


Recommendation G.727 contains the specification of an embedded 
Adaptive Differential Pulse Code Modulation (ADPCM) algorithms with 
5-, 4-, 3- and 2-bits per sample (i.e., at rates of 40, 32, 24 and 16 
kbit/s). The characteristics indicated are recommended for the 
conversion of 64 kbit/s. A-law or µ-law PCM channels to/ from variable 
rate-embedded ADPCM channels.

Recommendation G.727 defines the transcoding law when the source signal 
is a pulse-code modulation signal at a pulse rate of 64 kbit/s 
developed from voice frequency analog signals as fully specified by 
Blue Book Volume, Recommendation G.711.

Applications where the encoder is aware and the decoder is not aware 
of the way in which the ADPCM codeword bits have been altered, or 
when both the encoder and decoder are aware of the ways the codewords 
are altered, or where neither the encoder nor the decoder are aware 
of the ways in which the bits have been altered can benefit from 
other embedded ADPCM algorithms.

The embedded ADPCM algorithms specified in Recommendation G.727 are 
extensions of the ADPCM algorithms defined in Recommendation G.726 and 
are recommended for use in packetized speech systems operating 
according to the Packetized Voice Protocol (PVP) specified in draft 
Recommendation G.764.

PVP is able to relieve congestion by modifying the size of a speech 
packet when the need arises. Utilizing the embedded property of the 
algorithm described here, the least significant bit(s) of each 
codeword can be disregarded at packetization points and/or 
intermediate nodes to relieve congestion. This provides for 
significantly better performance than by dropping packets during 
congestion.

Recommendation G.727 first outlines a description of the ADPCM 
transcoding algorithm. It provides then the principles and functional 
descriptions of the ADPCM encoding and decoding algorithms, 
respectively. It contains also the computational details of the 
algorithm.



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