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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 ╡-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.
To order the complete text of this Recommendation, please use the
Order Form for ITU-T Recommendations. An electronic version of this
form is available on ITUDOC (Winword: UPI=ITU-5265; ASCII: UPI=ITU-2488).