G.711 represents logarithmic pulse-code modulation (PCM) samples for signals of voice frequencies, sampled at the rate of 8000 samples/second.
G.711.0 (G.711 LLC) - Lossless compression of G.711 pulse code modulation was approved by ITU-T in September 2009. It reduces the bandwidth usage by as much as 50 percent.
G.711.1 is an extension to G.711, published as ITU-T Recommendation G.711.1 in March 2008. Its formal name is Wideband embedded extension for G.711 pulse code modulation.
G.711 μ-law tends to give more resolution to higher range signals while G.711 A-law provides more quantization levels at lower signal levels. When using μ-law G.711 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......
Where s is the sign bit, and the ellipsis represents additional low-order bits that are not encoded. So for example, 1000'0000'1010'1111 maps to 1000'1010 (according to the first row of the table), and 0000'0001'1010'1111 maps to 0001'1010 (according to the second).
This can be seen as a floating point number with 4 bits of mantissa and 3 bits of exponent.
In addition, the standard specifies that all resulting even bits are inverted before the octet is transmitted. This is to provide plenty of 0/1 transitions to facilitate the clock recovery process in the PCM receivers. Thus, a silent A-law encoded PCM channel has the 8 bit samples coded 0x55 instead of 0x00 in the octets (or 0xD5 if the sign bit happens to be set).
Note that the ITU define bit 1 to have the value 128 and bit 8 to have the value 1.
The more widely accepted convention has bit 7 = 128 and bit 0 = 1.
Note that when data is sent over E0 (G.703), MSB (signbit) is sent first and LSB is sent last.
Where s is the sign bit, and the ellipsis represents additional low-order bits that are not encoded.
In addition, the standard specifies that all result bits are inverted before the octet is transmitted. Thus, a silent μ-law encoded PCM channel has the 8 bit samples coded 0xFF instead of 0x00 in the octets.
Also the "trick" of adding 32 means μ-law does not encode all 14-bit values; inputs must be within ±8159.
G.711.1 is compatible with G.711 at 64 kbit/s, hence an efficient deployment in existing G.711-based voice over IP (VoIP) infrastructures is foreseen. The G.711.1 coder can encode signals at 16 kHz with a bandwidth of 50–7000 Hz at 80 and 96 kbit/s, and for 8-kHz sampling the output may produce signals with a bandwidth ranging from 50 up to 4000 Hz, operating at 64 and 80 kbit/s.
Category:Audio codecs Category:Speech codecs Category:ITU-T recommendations
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