INTERNATIONAL TELECOMMUNICATION UNION
CCITT G.726
THE INTERNATIONAL
TELEGRAPH AND TELEPHONE
CONSULTATIVE COMMITTEE
GENERAL ASPECTS OF DIGITAL
TRANSMISSION SYSTEMS;
TERMINAL EQUIPMENTS
40, 32, 24, 16 kbit/s ADAPTIVE
DIFFERENTIAL PULSE CODE
MODULATION (ADPCM)
Recommendation G.726
Geneva, 1990
FOREWORD
The CCITT (the International Telegraph and Telephone Consultative Committee)
is a permanent organ of the International Telecommunication Union (ITU). CCITT is
responsible for studying technical, operating and tariff questions and issuing
Recommendations on them with a view to standardizing telecommunications on a
worldwide basis.
The Plenary Assembly of CCITT which meets every four years, establishes the
topics for study and approves Recommendations prepared by its Study Groups. The
approval of Recommendations by the members of CCITT between Plenary Assemblies is
covered by the procedure laid down in CCITT Resolution No. 2 (Melbourne, 1988).
Recommendation G.726 was prepared by Study Group XV and was approved under
the Resolution No. 2 procedure on the 14 of December 1990.
___________________
CCITT NOTE
In this Recommendation, the expression "Administration" is used for
conciseness to indicate both a telecommunication Administration and a recognized
private operating agency.
ITU 1990
All rights reserved. No part of this publication may be reproduced or utilized in
any form or by any means, electronic or mechanical, including photocopying and
microfilm, without permission in writing from the ITU.
Recommendation G.726 1
Recommendation G.726
Recommendation G.726
40, 32, 24, 16 kbit/s ADAPTIVE DIFFERENTIAL PULSE
CODE MODULATION (ADPCM)
1)
1 General
The characteristics below are recommended for the conversion of a 64 kbit/s
A-law or µ-law pulse code modulation (PCM) channel to and from a 40, 32, 24 or
16 kbit/s channel. The conversion is applied to the PCM bit stream using an ADPCM
transcoding technique. The relationship between the voice frequency signals and the
PCM encoding/decoding laws is fully specified in Recommendation G.711.
The principal application of 24 and 16 kbit/s channels is for overload
channels carrying voice in Digital Circuit Multiplication Equipment (DCME).
The principal application of 40 kbit/s channels is to carry data modem
signals in DCME, especially for modems operating at greater than 4800 kbit/s.
Sections 1.1 and 1.2 of this Recommendation provide an outline description of
the ADPCM transcoding algorithm, §§ 2 and 3 provide the principles and functional
descriptions of the ADPCM encoding and decoding algorithms respectively, whilst § 4
is the precise specification for the algorithm computations. Networking aspects and
digital test sequences are addressed in Appendices I and II, respectively, to this
Recommendation.
Simplified block diagrams of both the ADPCM encoder and decoder are shown in
Figure 1/G.726.
In § 4, each sub-block in the encoder and decoder is precisely defined using
one particular logical sequence. If other methods of computation are used, extreme
care should be taken to ensure that they yield exactly the same value for the output
processing variables. Any further departures from the processes detailed in § 4 will
incur performance penalties which may be severe.
Note 1 – Prior to the definition of this Recommendation, other ADPCM
algorithms of performance similar to the 40 kbit/s algorithm specified here have
been incorporated in DCME designs and used in telecommunications networks. These
algorithms may be considered by bilateral agreement for limited DCME applications
under certain circumstances. Technical descriptions providing information on two
such algorithm approaches can be found in COM XVIII No. 101 and COM XVIII No. 102 of
the 1984-1988 Study Period.
Note 2 – The assignment of 16, 24, 32 and 40 kbit/s DCME channels and the
associated selection of coding rates are beyond the scope of this Recommendation;
see, for example, Recommendation G.763 (revised, 1990).
Note 3 – Signalling and multiplexing considerations are beyond the scope of
this Recommendation; see, for example, Recommendations G.761 and G.763 (revised,
1990).
______________________________
1)
This Recommendation completely replaces the text of Recommendations G.721 and G.723
published in Volume III.4 of the Blue Book. It should be noted that systems designed in
accordance with the present Recommendation will be compatible with systems designed in
accordance with the Blue Book version.
2 Recommendation G.726
+
–
+
+
+
+
T1508190-92
Convert to
uniform PCM
Adaptative
quantizer
Adaptive
predictor
Inverse
adaptive
quantizer
Synchronous
coding
adjustment
Conve rt to PCM
Adaptive
predictor
Inverse
adaptive
quantizer
ADPCM
Input
Quantized
difference
signal
Reconstruc-
ted signal
Signal
estimate
64 kbit/s
output
ADPCM
output
Difference signal
Input
signal
64 kbit/s
PCM
input
Signal estimate
Quantized
difference
signal
a) Encoder
b) Decoder
FIGURE 1/G.726
Simplified block diagrams
Reconstruc-
ted signal
1.1 ADPCM encoder
Subsequent to the conversion of the A-law or µ-law PCM input signal to
uniform PCM, a difference signal is obtained, by subtracting an estimate of the
input signal from the input signal itself. An adaptive 31-, 15-, 7-, or 4-level
quantizer is used to assign five, four, three or two binary digits, respectively, to
the value of the difference signal for transmission to the decoder. An inverse
quantizer produces a quantized difference signal from these same five, four, three
or two binary digits, respectively. The signal estimate is added to this quantized
difference signal to produce the reconstructed version of the input signal. Both the
reconstructed signal and the quantized difference signal are operated upon by an
adaptive predictor which produces the estimate of the input signal, thereby
completing the feedback loop.
1.2 ADPCM decoder
The decoder includes a structure identical to the feedback portion of the
encoder, together with a uniform PCM to A-law or µ-law conversion and a synchronous
coding adjustment.
The synchronous coding adjustment prevents cumulative distortion occurring on
synchronous tandem codings (ADPCM-PCM-ADPCM, etc., digital connections) under
certain conditions (see § 3.7). The synchronous coding adjustment is achieved by
adjusting the PCM output codes in a manner which attempts to eliminate quantizing
distortion in the next ADPCM encoding stage.
Recommendation G.726 3
2 ADPCM encoder principles
Figure 2/G.726 is a block schematic of the encoder. For each variable to be
described, k is the sampling index and samples are taken at 125 µ s intervals. A
fundamental description of each block is given below in §§ 2.1 to 2.8.
s(k)
s (k)
l
d(k)
y (k)
l
l(k)
y(k)
a (k)
1
t (k)
d
t (k)
r
d (k)
q
a (k)
2
s (k)
r
k)
e
T1508200-92
Input PCM
format
conversion
Difference
signal
computation
Adaptive
quantizer
Inverse
adaptive
quantizer
Adaptive
predictor
Tone and
transition
detector
Adaptation
speed
control
Quantizer
scale factor
adaptation
Reconstructed
signal
calculator
ADPCM
output
FIGURE 2/G.726
Encoder block schematic
(
S
2.1 Input PCM format conversion
This block converts the input signal s (k) from A-law or µ-law PCM to a
uniform PCM signal s
l
(k).
2.2 Difference signal computation
This block calculates the difference signal d (k) from the uniform PCM signal
s
l
(k) and the signal estimate s
e
(k):
d (k) = s
l
(k) − s
e
(k) (2-1)
2.3 Adaptive quantizer
A 31-, 15, 7- or 4-level non-uniform adaptive quantizer is used to quantize
the difference signal d (k) for operating at 40, 32, 24 or 16 kbit/s, respectively.
Prior to quantization, d (k) is converted to a base 2 logarithmic representation and
scaled by y (k) which is computed by the scale factor adaptation block. The
normalized input/output characteristic (infinite precision values) of the quantizer
is given in Tables 1/G.726 through 4/G.726.
