G.711
Encyclopedia
G.711 is an ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

 standard for audio companding
Companding
In telecommunication, signal processing, and thermodynamics, companding is a method of mitigating the detrimental effects of a channel with limited dynamic range...

. It is primarily used in telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....

. The standard was released for usage in 1972. Its formal name is Pulse code modulation (PCM
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...

) of voice frequencies
. It is required standard in many technologies, for example in H.320
H.320
H.320 is an umbrella recommendation by the ITU-T for running Multimedia over ISDN based networks. The main protocols in this suite are H.221, H.230, H.242, audio codecs such as G.711, and video codecs such as H.261 and H.263....

 and H.323
H.323
H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

 specifications. It can also be used for fax
Fax
Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...

 communication over IP networks (as defined in T.38
T.38
T.38 is an ITU recommendation for allowing transmission of fax over IP networks in real time.- History :The T.38 fax relay standard was devised in 1998 as a way to permit faxes to be transported across IP networks between existing Group 3 fax terminals. T.4 and related fax standards were published...

 specification). G.711, also known as Pulse Code Modulation (PCM), is a very commonly used waveform codec. G.711 uses a sampling rate of 8,000 samples per second, with the tolerance on that rate 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 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
Lossless data compression
Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange...

 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 x 8 bits per sample)
  • Typical algorithmic delay is 0.125 ms, with no look-ahead delay
  • G.711 is a waveform
    Waveform
    Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...

     speech coder
  • G.711 Appendix I defines a Packet Loss Concealment
    Packet Loss Concealment
    Packet loss concealment is a technique to mask the effects of packet loss in VoIP communications. Because the voice signal is sent as packets on a VoIP network, they may travel different routes to get to destination. At the receiver a packet might arrive very late, corrupted or simply might not...

     (PLC) algorithm to help hide transmission losses in a packetized network
  • G.711 Appendix II defines a Discontinuous Transmission
    Discontinuous Transmission
    Discontinuous transmission is a means by which a mobile telephone is temporarily shut off or muted while the phone lacks a voice input.-Misconception:...

     (DTX) algorithm which uses Voice Activity Detection
    Voice activity detection
    Voice activity detection , also known as speech activity detection or speech detection, is a technique used in speech processing in which the presence or absence of human speech is detected.. The main uses of VAD are in speech coding and speech recognition...

     (VAD) and Comfort Noise Generation
    Comfort noise
    Comfort noise is synthetic background noise used in radio and wireless communications to fill the artificial silence in a transmission resulting from voice activity detection or from the audio clarity of modern digital lines....

     (CNG) to reduce bandwidth usage during silence periods
  • PSQM
    PSQM
    PSQM is a computational and modeling algorithm defined in ITU Recommendation ITU-T P.861 that objectively evaluates and quantifies voice quality of voice-band speech codecs....

     testing under ideal conditions yields Mean Opinion Score
    Mean Opinion Score
    The Mean Opinion Score test has been used for decades in telephony networks to obtain the human user's view of the quality of the network. In multimedia especially when codecs are used to compress the bandwidth requirement , the mean opinion score ...

    s of 4.45 for G.711 u-law, 4.45 for G.711 a-law
  • PSQM
    PSQM
    PSQM is a computational and modeling algorithm defined in ITU Recommendation ITU-T P.861 that objectively evaluates and quantifies voice quality of voice-band speech codecs....

     testing under network stress yields Mean Opinion Score
    Mean Opinion Score
    The Mean Opinion Score test has been used for decades in telephony networks to obtain the human user's view of the quality of the network. In multimedia especially when codecs are used to compress the bandwidth requirement , the mean opinion score ...

    s of 4.13 for G.711 u-law, 4.11 for G.711 a-law

Types

G.711 defines two main compression algorithms, the µ-law algorithm (used in North America & Japan) and A-law algorithm
A-law algorithm
An A-law algorithm is a standard companding algorithm, used in European digital communications systems to optimize, i.e., modify, the dynamic range of an analog signal for digitizing.It is similar to the μ-law algorithm used in North America and Japan....

 (used in Europe and the rest of the world). Both are logarithmic
Logarithmic scale
A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...

, 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
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

.

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
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...

 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.

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:
Linear input code Compressed code
s0000000wxyza... s000wxyz
s0000001wxyza... s001wxyz
s000001wxyzab... s010wxyz
s00001wxyzabc... s011wxyz
s0001wxyzabcd... s100wxyz
s001wxyzabcde... s101wxyz
s01wxyzabcdef... s110wxyz
s1wxyzabcdefg... s111wxyz


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
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...

 number with 4 bits of mantissa
Mantissa
* The mantissa is the significand in a common logarithm or floating-point number.* Metaphorically, it is the part of the self that eludes conscious awareness or self-understanding.* An addition of little importance.Mantissa may also refer to:...

 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
Clock recovery
Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...

 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
G.703
G.703 is an ITU-T standard for transmitting voice or data over digital carriers such as T1 and E1. G.703 provides specifications for pulse code modulation .G.703 also specifies E0...

), MSB (signbit) is sent first and LSB is sent last.

μ-Law

μ-law encoding takes a 14-bit signed linear audio sample as input, increases the magnitude by 32 (binary 100000), and converts it to an 8 bit value as follows:
Linear input code Compressed code
s00000001wxyza... s000wxyz
s0000001wxyzab... s001wxyz
s000001wxyzabc... s010wxyz
s00001wxyzabcd... s011wxyz
s0001wxyzabcde... s100wxyz
s001wxyzabcdef... s101wxyz
s01wxyzabcdefg... s110wxyz
s1wxyzabcdefgh... s111wxyz


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.711.0

G.711.0, also known as G.711 LLC, utilizes lossless data compression
Lossless data compression
Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange...

 to reduce the bandwidth usage by as much as 50 percent.. The Lossless compression of G.711 pulse code modulation standard was approved by ITU-T in September 2009. 

G.711.1

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.711.1, allows the addition of narrowband
Narrowband
In radio, narrowband describes a channel in which the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.The opposite of...

 and/or wideband
Wideband
In communications, wideband is a relative term used to describe a wide range of frequencies in a spectrum. A system is typically described as wideband if the message bandwidth significantly exceeds the channel's coherence bandwidth....

 (16000 samples/s) enhancements, each at 25 % of the bitrate of the (included) base G.711 bitstream, leading to data rates of 64, 80 or 96 kbit/s.

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.

The G.711.1 encoder creates embedded bitstream structured in three layers corresponding to three available bit rates: 64, 80 and 96 kbit/s. The bitstream does not contain any information on which layers are contained, an implementation would require outband signalling on which layers are available. The three G.711.1 layers are: log companded pulse code modulation (PCM) of the lower band including noise feedback, embedded PCM extension with adaptive bit allocation for enhancing the quality of the base layer in the lower band, and weighted vector quantization coding of the higher band based on modified discrete cosine transform
Modified discrete cosine transform
The modified discrete cosine transform is a Fourier-related transform based on the type-IV discrete cosine transform , with the additional property of being lapped: it is designed to be performed on consecutive blocks of a larger dataset,...

ation (MDCT).

Two extensions for G.711.1 are planned in 2010: superwideband extension (bandwidth to 14000 Hz) and lossless bitstream compression.

Licensing

Since G.711 was released in 1972 its patents have long since expired, so it is freely available.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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