ITU-T G 711-1988 PULSE CODE MODULATION (PCM) OF VOICE FREQUENCIES《语音频率的脉冲编码调制(PCM)》.pdf

1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/ further amended)1 GeneralThe characteristics given below are recommended for encoding voice-frequency signals.2 Sampling rateThe nominal value recommended for the sampling rate is 8000 sa

2、mples per second. The tolerance on that rateshould be 50 parts per million (ppm).3 Encoding law3.1 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 -law. Thedefinition of these laws

3、is given in Tables 1a/G.711 and 1b/G.711 and Tables 2a/G.711 and 2b/G.711 respectively.When using the -law in networks where suppression of the all 0 character signal is required, the charactersignal corresponding to negative input values between decision values numbers 127 and 128 should be 0000001

4、0 andthe 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 inaccordance with the A-law

5、. Where both countries have adopted the same law, that law should be used on digital pathsbetween them. Any necessary conversion will be done by the countries using the -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 PCMEvery “decision val

6、ue“ and “ quantized value“ of the A (resp. ) law should be associated with a “uniformPCM value“. (For a definition of “decision value“ and “quantized value“, see Recommendation G.701 and in particularFigure 2/G.701). This requires the application of a 13 (14) bit uniform PCM code. The mapping from A

7、-law PCM, and -law PCM, respectively, to the uniform code is given in Tables 1/G.711 and 2/G.711. The conversion to A-law or-law values from uniform PCM values corresponding to the decision values, is left to the individual equipmentspecification. One option is described in Recommendation G.721, 4.2

8、.8 subblock COMPRESS.4 Transmission of character signalsWhen character signals are transmitted serially, i.e. consecutively on one physical medium, bit No. 1 (polaritybit) is transmitted first and No. 8 (the least significant bit) last.2 Fascicle III.4 - Rec. G.7115 Relationship between the encoding

9、 laws and the audio levelThe relationship between the encoding laws of Tables 1/G.711 and 2/G.711 and the audio signal level isdefined as follows:A sine-wave signal of 1 kHz at a nominal level of 0 dBm0 should be present at any voice frequency output ofthe PCM multiplex when the periodic sequence of

10、 character signals of Table 5/G.711 for the A-law and ofTable 6/G.711 for the -law is applied to the decoder input.The resulting theoretical load capacity (Tmax) is +3.14 dBm0 for the A-law, and +3.17 dBm0 for the -law.Note - The use of another digital periodic sequence representing a nominal refere

11、nce frequency of 1020 Hz ata nominal level of -10 dBm0 (preferred value, see Recommendation O.6) or 0 dBm0 is acceptable, provided that thetheoretical accuracy of that sequence does not differ by more than 0.03 dB from a level of -10 dBm0 or 0 dBm0respectively. In accordance with Recommendation O.6,

12、 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.Fascicle III.4 - Rec. G.711 3 4 Fascicle III.4 - Rec. G.711 Fascicle III.4 - Rec. G.711 5 6 Fascicle III.4 - Rec. G.711 Fa

13、scicle III.4 - Rec. G.711 7TABLE 3/G.711-A conversion-lawDecoder outputvalue numberA-lawDecoder outputvalue number-lawDecoder outputvalue numberA-lawDecoder outputvalue number01234567891011121314151617181920212223242526272829303132333435363738394041424311223344556677889101112131415161718192021222324

14、2527293133343536373839404445464748495051525354555657585960616263646566676869707172737475767778798081828384858687.1274142434446484950515253545556575859606162646566676869707172737475767778798182838485868788.128Notes relative to Table 3/G.711Note 1 - The input signals to an A-law decoder will normally

15、include even bit inversion as applied in accordance with Note 2 ofTable 1a/G.711. Consequently the output signals from a -A converter should have even bit inversion embodied within the converteroutput.8 Fascicle III.4 - Rec. G.711Note 2 - If a -A conversion is followed by an A- conversion, most of t

16、he octets are restored to their original values. Only thoseoctets which correspond to -law decoder output value numbers 0, 2, 4, 6, 8, 10, 12, 14 are changed (the numbers being increased by1). Moreover, in these octets, only bit No. 8 (least significant bit in PCM) is changed. Accordingly, the doubl

17、e conversion-A- is transparent to bits Nos. 1-7.Similarly, if an A- conversion is followed by a -A conversion, only the octets corresponding to A-law decoder output valuenumbers 26, 28, 30, 32, 45, 47, 63 and 80 are changed. Again, only bit No. 8 is changed, i.e. the double conversion A-A, too, istr

18、ansparent to bits No. 1-7.A consequence of this property is that in most of the analogue voice frequency signal range the additional quantizing distortioncaused by -A- or A-A conversion is considerably lower than that caused by either -A or A- conversion (see RecommendationG.113).The A-A transparenc

19、y for bits 1 to 7 was achieved by modifying the table slightly from the optimum conversion in that -80 isconverted to A-81 instead of A-80, and A-80 is converted to -79 instead of -80. This has an insignificant effect on quantizingdistortion.Fascicle III.4 - Rec. G.711 9TABLE 4/G.711-A conversionA-l

20、awDecoder outputvalue number-lawDecoder outputvalue numberA-lawDecoder outputvalue number-lawDecoder outputvalue number1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950135791113151617181920212223242526272829303132323333343435353637383940414243444546474848494

21、95051515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798.128525354555657585960616263646465666768697071727374757677787979808182838485868788899091929394959697.12710 Fascicle III.4 - Rec. G.711Notes relative to Table 4/G.711Note 1 - The output signals of an

22、A-law decoder will have even bit inversion as applied within the encoder in accordance with Note 2of Table 1a/G.711. Consequently the input signals to an A- converter will already be in this state, so that removal of even bitinversion should be embodied within the converter.Note 2 - If a -A conversi

23、on is followed by an A- conversion, most of the octets are restored to their original values. Only thoseoctets which correspond to -law decoder output value numbers 0, 2, 4, 6, 8, 10, 12, 14 are changed (the numbers being increased by1). Moreover, in these octets, only bit 8 (least significant bit i

24、n PCM) is changed. Accordingly, the double conversion -A- istransparent to bits 1 to 7.Similarly, if an A- conversion is followed by a -A conversion, only the octets corresponding to A-law decoder output valuenumbers 26, 28, 30, 32, 45, 47, 63 and 80 are changed. Again, only bit 8 is changed, i.e. t

25、he double conversion A-A, too, istransparent to bits 1 to 7.A consequence of this property is that in most of the analogue voice frequency signal range the additional quantizing distortioncaused by -A- or A-A conversion is considerably lower than that caused by either -A or A- conversion (see Recomm

26、endationG.113).The A-A transparency for bits 1 to 7 was achieved by modifying the table slightly from the optimum conversion in that -80 isconverted to A-81 instead of A-80, and A-80 is converted to -79 instead of -80. This has an insignificant effect on quantizingdistortion.TABLE 5/G.711 TABLE 6/G.711A-law -law100001111200000000311111111410011001500000000610011001700000000801100110100001111200000000300000000410011001511111111610011001711111111801100110