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(Extract from the "LUE"OOK)
NOTES
1 ITU-T Recommendation G.711 was published in Fascicle III.4 of the Blue Book. This file is an extract from
the Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, the
contents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).
2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both a
telecommunication administration and a recognized operating agency.
ITU 1988, 1993
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.
Fascicle III.4 - Rec. G.711 1
Recommendation G.711
Fascicle III.4 - Rec. G.711
PULSE CODE MODULATION (PCM) OF VOICE FREQUENCIES
(Geneva, 1972; further amended)
1 General
The characteristics given below are recommended for encoding voice-frequency signals.
2 Sampling rate
The nominal value recommended for the sampling rate is 8000 samples per second. The tolerance on that rate
should be ± 50 parts per million (ppm).
3 Encoding law
3.1 Eight binary digits per sample should be used for international circuits.
3.2 Two encoding laws are recommended and these are commonly referred to as the A-law and the µ-law. The
definition of these laws is given in Tables 1a/G.711 and 1b/G.711 and Tables 2a/G.711 and 2b/G.711 respectively.
When using the
µ
-law in networks where suppression of the all 0 character signal is required, the character
signal corresponding to negative input values between decision values numbers 127 and 128 should be 00000010 and
the value at the decoder output is -7519. The corresponding decoder output value number is 125.
3.3 The number of quantized values results from the encoding law.
3.4 Digital paths between countries which have adopted different encoding laws should carry signals encoded in
accordance with the A-law. Where both countries have adopted the same law, that law should be used on digital paths
between them. Any necessary conversion will be done by the countries using the µ-law.
3.5 The rules for conversion are given in Tables 3/G.711 and 4/G.711.
3.6 Conversion to and from uniform PCM
Every "decision value" and " quantized value" of the A (resp. µ) law should be associated with a "uniform
PCM value". (For a definition of "decision value" and "quantized value", see Recommendation G.701 and in particular
Figure 2/G.701). This requires the application of a 13 (14) bit uniform PCM code. The mapping from A-law PCM, and
µ-law PCM, respectively, to the uniform code is given in Tables 1/G.711 and 2/G.711. The conversion to A-law or
µ-law values from uniform PCM values corresponding to the decision values, is left to the individual equipment
specification. One option is described in Recommendation G.721, § 4.2.8 subblock COMPRESS.
4 Transmission of character signals
When character signals are transmitted serially, i.e. consecutively on one physical medium, bit No. 1 (polarity
bit) is transmitted first and No. 8 (the least significant bit) last.
2 Fascicle III.4 - Rec. G.711
5 Relationship between the encoding laws and the audio level
The relationship between the encoding laws of Tables 1/G.711 and 2/G.711 and the audio signal level is
defined as follows:
A sine-wave signal of 1 kHz at a nominal level of 0 dBm0 should be present at any voice frequency output of
the PCM multiplex when the periodic sequence of character signals of Table 5/G.711 for the A-law and of
Table 6/G.711 for the
µ
-law is applied to the decoder input.
The resulting theoretical load capacity (T
max
) is +3.14 dBm0 for the A-law, and +3.17 dBm0 for the
µ
-law.
Note - The use of another digital periodic sequence representing a nominal reference frequency of 1020 Hz at
a nominal level of -10 dBm0 (preferred value, see Recommendation O.6) or 0 dBm0 is acceptable, provided that the
theoretical accuracy of that sequence does not differ by more than ± 0.03 dB from a level of -10 dBm0 or 0 dBm0
respectively. In accordance with Recommendation O.6, the specified frequency tolerance should be 1020 Hz + 2 Hz,
-7 Hz.
If a sequence representing -10 dBm0 is used, the nominal value at the voice frequency outputs should be
-10 dBm0.
