1. Features

2. Types
     A-Law    μ-Law  

3. G.711.0

4. G.711.1

5. Licensing

6. See also

7. References

8. External links

G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation (PCM) of voice frequencies. It is a required standard in many technologies, for example in H.320 and H.323 specifications. It can also be used for fax communication over IP networks (as defined in T.38 specification). G.711, also known as Pulse Code Modulation (PCM), is a very commonly used waveform codec. G.711 is a narrowband audio codec that provides toll-quality audio at 64 kbit/s. G.711 passes audio signals in the range of 300–3400 Hz and samples them at the rate of 8,000 samples per second, with the tolerance on that rate of 50 parts per million (ppm). Non-uniform (logarithmic) quantization with 8 bits is used to represent each sample, resulting in a 64 kbit/s bit rate. There are two slightly different versions: 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.

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

• Sampling frequency 8 kHz
• 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.^[1]

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.

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 {{code|s}} is the sign bit, {{overline|s}} is its inverse (i.e. positive values are encoded with MSB = {{var|{{overline|s}}}} = 1), and bits marked {{code|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 {{var|m}}, 3 bits of exponent {{var|e}} and 1 sign bit {{var|s}} formatted as {{overline|s}}eeemmmm with the decoded linear value {{var|y}} given by formula

which is a 13-bit signed integer in the range ±1 to ±(2{{sup|12}} − 2{{sup|6}}). 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^[4] 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).

  long            lseg;  short          *linbuf;  short          *logbuf;

  short           ix, mant, iexp;  long            n;

  for (n = 0; n < lseg; n++)  {    ix = logbuf[n] ^ (0x0055);	/* re-toggle toggled bits */

    ix &= (0x007F);		/* remove sign bit */    iexp = ix >> 4;		/* extract exponent */    mant = ix & (0x000F);	/* now get mantissa */    if (iexp > 0)      mant = mant + 16;		/* add leading '1', if exponent > 0 */

    mant = (mant << 4) + (0x0008);	/* now mantissa left justified and */    /* 1/2 quantization step added */    if (iexp > 1)		/* now left shift according exponent */      mant = mant << (iexp - 1);

    linbuf[n] = logbuf[n] > 127	/* invert, if negative sample */      ? mant      : -mant;  }

See also "ITU-T Software Tool Library 2009 User's manual" that can be found at.^[5]

μ-Law

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:

• Poindexter, Georgia
• Poindexter Peak
• Poindimié, New Caledonia
• Poinding
• Poine
• Poing (disambiguation)
• Poing, Germany
• Poinguinim
• Poing (video game)
• Poinsenot
• Poinsetta bowl
• Poinsettia: Amairo Winter Memories
• Poinsettia ~Amairo Winter Memories~
• Poinsettia Bowl broadcasters
• Poinsettia (cocktail)
• Poinsettia day
• Poinsettia (disambiguation)
• Poinsettia Heights
• Poinsettia Heights, Fort Lauderdale, Florida
• Poinsettias
• Poinsett State Park
• Poinson-les-Fayl
• Poinson-les-Grancey
• Poinson-les-Nogent
• Poinson-lès-Fayl