G.711 is a narrowband audio codec originally designed for use in telephony that provides toll-quality audio at 64 kbit/s. It is an ITU-T standard (Recommendation) for audio encoding, titled Pulse code modulation (PCM) of voice frequencies released for use in 1972. G.711 passes audio signals in the frequency band of 300–3400 Hz and samples them at the rate of 8000 Hz, with the tolerance on that rate of 50 parts per million (ppm). It uses one of two different logarithmic companding algorithms: μ-law, which is used primarily in North America and Japan, and A-law, which is in use in most other countries outside North America. Each companded sample is quantized as 8 bits, resulting in a 64 kbit/s bit rate. G.711 is a required standard in many technologies, such as in the H.320 and H.323 standards. It can also be used for fax communication over IP networks (as defined in T.38 specification). Two enhancements to G.711 have been published: G.711.0 utilizes lossless data compression to reduce the bandwidth usage and G.711.1 increases audio quality by increasing bandwidth.
Features 8 kHz sampling frequency 64 kbit/s bitrate (8 kHz sampling frequency × 8 bits per sample) Typical algorithmic delay is 0.125 ms, with no look-ahead delay G.711 is a waveform speech coder G.711 Appendix I defines a packet loss concealment (PLC) algorithm to help hide transmission losses in a packetized network G.711 Appendix II defines a discontinuous transmission (DTX) algorithm which uses voice activity detection (VAD) and comfort noise generation (CNG) to reduce bandwidth usage during silence periods PSQM testing under ideal conditions yields mean opinion scores of 4.45 for G.711 μ-law, 4.45 for G.711 A-law PSQM testing under network stress yields mean opinion scores of 4.13 for G.711 μ-law, 4.11 for G.711 A-law
Types G.711 defines two main companding algorithms, the μ-law algorithm and A-law algorithm. Both are logarithmic, but A-law was specifically designed to be simpler for a computer to process. The standard also defines a sequence of repeating code values which defines the power level of 0 dB. The μ-law and A-law algorithms encode 14-bit and 13-bit signed linear PCM samples (respectively) to logarithmic 8-bit samples. Thus, the G.711 encoder will create a 64 kbit/s bitstream for a signal sampled at 8 kHz. 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. The terms PCMU, G711u and G711MU are also used for G.711 μ-law, and PCMA and G711A for G.711 A-law.
A-law
A-law encoding thus takes a 13-bit signed linear audio sample as input and converts it to an 8 bit value as follows:
Where s is the sign bit, s is its inverse (i.e. positive values are encoded with MSB = s = 1), and bits marked x are discarded. Note that the first column of the table uses different representation of negative values than the third column. So for example, input decimal value −21 is represented in binary after bit inversion as 1000000010100, which maps to 00001010 (according to the first row of the table). When decoding, this maps back to 1000000010101, which is interpreted as output value −21 in decimal. Input value +52 (0000000110100 in binary) maps to 10011010 (according to the second row), which maps back to 0000000110101 (+53 in decimal). This can be seen as a floating-point number with 4 bits of mantissa m (equivalent to a 5-bit precision), 3 bits of exponent e and 1 sign bit s, formatted as seeemmmm with the decoded linear value y given by formula
y = ( − 1 ) s ⋅ ( 16 ⋅ min { e , 1 } + m + 0.5 ) ⋅ 2 max { e , 1 } , {\displaystyle y=(-1)^{s}\cdot (16\cdot \min\{e,1\}+m+0.5)\cdot 2^{\max\{e,1\}},}
which is a 13-bit signed integer in the range ±1 to ±(212 − 26). Note that no compressed code decodes to zero due to the addition of 0.5 (half of a quantization step). In addition, the standard specifies that all resulting even bits (LSB is even) 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 0xD5 instead of 0x80 in the octets. When data is sent over E0 (G.703), MSB (sign) is sent first and LSB is sent last. ITU-T STL defines the algorithm for decoding as follows (it puts the decoded values in the 13 most significant bits of the 16-bit output data type).
See also "ITU-T Software Tool Library 2009 User's manual" that can be found at.
μ-law
The μ-law (sometimes referred to as ulaw, G.711Mu, or G.711μ) encoding takes a 14-bit signed linear audio sample in two's complement representation as input, inverts all bits after the sign bit if the value is negative, adds 33 (binary 100001) and converts it to an 8 bit value as follows:
Where s is the sign bit, and bits marked x are discarded. In addition, the standard specifies that the encoded bits are inverted before the octet is transmitted. Thus, a silent μ-law encoded PCM channel has the 8 bit samples transmitted 0xFF instead of 0x00 in the octets. Adding 33 is necessary so that all values fall into a compression group and it is subtracted back when decoding. Breaking the encoded value formatted as seeemmmm into 4 bits of mantissa m, 3 bits of exponent e and 1 sign bit s, the decoded linear value y is given by formula
y = ( − 1 ) s ⋅ [ ( 33 + 2 m ) ⋅ 2 e − 33 ] , {\displaystyle y=(-1)^{s}\cdot [(33+2m)\cdot 2^{e}-33],}
which is a 14-bit signed integer in the range ±0 to ±8031. Note that 0 is transmitted as 0xFF, and −1 is transmitted as 0x7F, but when received the result is 0 in both cases.
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