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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
Convert 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%NCODER
B$EC ODER
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.
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