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G.711

May 07, 2023

G.711 is a narrowband audio codec originally designed for use in telephony 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.

G.711
Pulse code modulation (PCM) of voice frequencies
StatusIn force
Year started1972
Latest version(02/00)
February 2000
OrganizationITU-T
Related standardsG.191, G.711.0, G.711.1, G.729
Domainaudio compression
Websitehttps://www.itu.int/rec/T-REC-G.711

G.711 is an ITU-T standard (Recommendation) for audio companding, titled Pulse code modulation (PCM) of voice frequencies released for use in 1972. It is a required standard in many technologies, such as in the H.320 and H.323 standards.[1] 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

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.

The terms PCMU, G711u or G711MU for G711 μ-law, and PCMA or G711A for G711 A-law, are used.[2]

A-law

Main article: A-law algorithm

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
[note 1]		 Compressed code
XOR 01010101 		Linear output code
[note 2]
s0000000abcdx s000abcd s0000000abcd1
s0000001abcdx s001abcd s0000001abcd1
s000001abcdxx s010abcd s000001abcd10
s00001abcdxxx s011abcd s00001abcd100
s0001abcdxxxx s100abcd s0001abcd1000
s001abcdxxxxx s101abcd s001abcd10000
s01abcdxxxxxx s110abcd s01abcd100000
s1abcdxxxxxxx s111abcd s1abcd1000000
  1. ^ This value is produced by taking the two's complement representation of the input value, and inverting all bits after the sign bit if the value is negative.
  2. ^ Signed magnitude representation

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

 

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[3] 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). 

void alaw_expand(lseg, logbuf, linbuf)  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.[4]

μ-law

Main article: μ-law algorithm

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:

Linear input value
[note 1]		 Compressed code
XOR 11111111 		Linear output value
[note 2]
s00000001abcdx s000abcd s00000001abcd1
s0000001abcdxx s001abcd s0000001abcd10
s000001abcdxxx s010abcd s000001abcd100
s00001abcdxxxx s011abcd s00001abcd1000
s0001abcdxxxxx s100abcd s0001abcd10000
s001abcdxxxxxx s101abcd s001abcd100000
s01abcdxxxxxxx s110abcd s01abcd1000000
s1abcdxxxxxxxx s111abcd s1abcd10000000
  1. ^ This value is produced by taking the two's complement representation of the input value, inverting all bits after the sign bit if the value is negative, and adding 33.
  2. ^ Signed magnitude representation. Final result is produced by decreasing the magnitude of this value by 33.

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

 

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.

G.711.0

G.711.0, also known as G.711 LLC, utilizes lossless data compression to reduce the bandwidth usage by as much as 50 percent.[5] The Lossless compression of G.711 pulse code modulation standard was approved by ITU-T in September 2009.[6][7]

G.711.1

G.711.1 "Wideband embedded extension for G.711 pulse code modulation" is a higher-fidelity extension to G.711, ratified in 2008 and further extended in 2012.[8]

G.711.1 allows a series of enhancement layers on top of a raw G.711 core stream (Layer 0): Layer 1 codes 16-bit audio in the same 4kHz narrowband, and Layer 2 allows 8kHz wideband using MDCT; each uses a fixed 16kbps in addition to the 64kbps core. They may be used together or singly, and each encodes the differences from the previous layer. Ratified in 2012, Layer 3 extends Layer 2 to 16kHz "superwideband," allowing another 16kbps for the highest frequencies, while retaining layer independence. Peak bitrate becomes 96 kbps in original G.711.1, or 112 kbps with superwideband. No internal method of identifying or separating the layers is defined, leaving it to the implementation to packetize or signal them.

A decoder that doesn't understand any set of fidelity layers may ignore or drop non-core packets without affecting it, enabling graceful degradation across any G.711 (or original G.711.1) telephony system with no changes.

Also ratified in 2012 was G.711.0 lossless extended to the new fidelity layers. Like G.711.0, full G.711 backward compatibility is sacrificed for efficiency, though a G.711.0 aware node may still ignore or drop layer packets it doesn't understand.

Licensing

The patents for G.711, released in 1972, have expired, so it may be used without the need for a licence.[1]

See also

References

  1. ^ a b c "G.711 : Pulse code modulation (PCM) of voice frequencies". www.itu.int. from the original on 2019-06-17. Retrieved 2019-11-11.
  2. ^ "Video/Voice/Speech Codecs". Grandstream=. Retrieved 19 July 2020.
  3. ^ G.191 : Software tools for speech and audio coding standardization. Function alaw_expand in file Software/stl2009/g711/g711.c. Itu.int. Retrieved on 2013-09-18.
  4. ^ G.191 : ITU-T Software Tool Library 2009 User's manual. Itu.int (2010-07-23). Retrieved on 2013-09-18.
  5. ^ ITU-T (2009-07-17). "ITU-T Newslog - Voice codec gets new lossless compression". from the original on 2016-03-03. Retrieved 2010-02-28.
  6. ^ ITU-T. "G.711.0 : Lossless compression of G.711 pulse code modulation". Retrieved 2010-02-28.
  7. ^ Recent Audio/Speech Coding Developments in ITU-T and future trends (PDF), August 2008, retrieved 2010-02-28
  8. ^ G.711.1 : Wideband embedded extension for G.711 pulse code modulation, ITU-T, 2012, retrieved 2022-12-24

External links

  • ITU-T Recommendation G.711
  • ITU-T G.191 software tools for speech and audio coding, including G.711 C code
  • RFC 3551 - RTP Profile for Audio and Video Conferences with Minimal Control - G.711 - PCMA and PCMU definition.
  • RFC 4856 - Registration of Media Type audio/PCMA and audio/PCMU
  • RFC 5391 - RTP Payload Format for ITU-T Recommendation G.711.1 (PCMA-WB and PCMU-WB)

May 07, 2023
narrowband, audio, codec, originally, designed, telephony, that, provides, toll, quality, audio, kbit, passes, audio, signals, range, 3400, samples, them, rate, samples, second, with, tolerance, that, rate, parts, million, uniform, logarithmic, quantization, w. G 711 is a narrowband audio codec originally designed for use in telephony 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 m law which is used primarily in North America and Japan and A law which is in use in most other countries outside North America G 711Pulse code modulation PCM of voice frequenciesStatusIn forceYear started1972Latest version 02 00 February 2000OrganizationITU TRelated standardsG 191 G 711 0 G 711 1 G 729Domainaudio compressionWebsitehttps www itu int rec T REC G 711G 711 is an ITU T standard Recommendation for audio companding titled Pulse code modulation PCM of voice frequencies released for use in 1972 It is a required standard in many technologies such as in the H 320 and H 323 standards 1 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 Contents 1 Features 2 Types 2 1 A law 2 2 m law 3 G 711 0 4 G 711 1 5 Licensing 6 See also 7 References 8 External linksFeatures Edit8 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 m law 4 45 for G 711 A law citation needed PSQM testing under network stress yields mean opinion scores of 4 13 for G 711 m law 4 11 for G 711 A law citation needed Types EditG 711 defines two main companding algorithms the m 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 m 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 m 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 or G711MU for G711 m law and PCMA or G711A for G711 A law are used 2 A law Edit Main article A law algorithm 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 note 1 Compressed codeXOR 01010101 Linear output code note 2 s0000000abcdx span style text decoration overline s span 000abcd s0000000abcd1s0000001abcdx span style text decoration overline s span 001abcd s0000001abcd1s000001abcdxx span style text decoration overline s span 010abcd s000001abcd10s00001abcdxxx span style text decoration overline s span 011abcd s00001abcd100s0001abcdxxxx span style text decoration overline s span 100abcd s0001abcd1000s001abcdxxxxx span style text decoration overline s span 101abcd s001abcd10000s01abcdxxxxxx span style text decoration overline s span 110abcd s01abcd100000s1abcdxxxxxxx span style text decoration overline s span 111abcd s1abcd1000000 This value is produced by taking the two s complement representation of the input value and inverting all bits after the sign bit if the value is negative Signed magnitude representation Where s is the sign bit span style text decoration overline s span 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 span style text decoration overline s span eeemmmm 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 3 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 void alaw expand lseg logbuf linbuf long lseg short linbuf short logbuf short ix mant iexp long n for n 0 n lt lseg n ix logbuf n 0x0055 re toggle toggled bits ix amp 0x007F remove sign bit iexp ix gt gt 4 extract exponent mant ix amp 0x000F now get mantissa if iexp gt 0 mant mant 16 add leading 1 if exponent gt 0 mant mant lt lt 4 0x0008 now mantissa left justified and 1 2 quantization step added if iexp gt 1 now left shift according exponent mant mant lt lt iexp 1 linbuf n logbuf n gt 127 invert if negative sample mant mant See also ITU T Software Tool Library 2009 User s manual that can be found at 4 m law Edit Main article m law algorithm The m law sometimes referred to as ulaw G 711Mu or G 711m 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 Linear input value note 1 Compressed codeXOR 11111111 Linear output value note 2 s00000001abcdx s000abcd s00000001abcd1s0000001abcdxx s001abcd s0000001abcd10s000001abcdxxx s010abcd s000001abcd100s00001abcdxxxx s011abcd s00001abcd1000s0001abcdxxxxx s100abcd s0001abcd10000s001abcdxxxxxx s101abcd s001abcd100000s01abcdxxxxxxx s110abcd s01abcd1000000s1abcdxxxxxxxx s111abcd s1abcd10000000 This value is produced by taking the two s complement representation of the input value inverting all bits after the sign bit if the value is negative and adding 33 Signed magnitude representation Final result is produced by decreasing the magnitude of this value by 33 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 m 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 G 711 0 EditG 711 0 also known as G 711 LLC utilizes lossless data compression to reduce the bandwidth usage by as much as 50 percent 5 The Lossless compression of G 711 pulse code modulation standard was approved by ITU T in September 2009 6 7 G 711 1 EditG 711 1 Wideband embedded extension for G 711 pulse code modulation is a higher fidelity extension to G 711 ratified in 2008 and further extended in 2012 8 G 711 1 allows a series of enhancement layers on top of a raw G 711 core stream Layer 0 Layer 1 codes 16 bit audio in the same 4kHz narrowband and Layer 2 allows 8kHz wideband using MDCT each uses a fixed 16kbps in addition to the 64kbps core They may be used together or singly and each encodes the differences from the previous layer Ratified in 2012 Layer 3 extends Layer 2 to 16kHz superwideband allowing another 16kbps for the highest frequencies while retaining layer independence Peak bitrate becomes 96 kbps in original G 711 1 or 112 kbps with superwideband No internal method of identifying or separating the layers is defined leaving it to the implementation to packetize or signal them A decoder that doesn t understand any set of fidelity layers may ignore or drop non core packets without affecting it enabling graceful degradation across any G 711 or original G 711 1 telephony system with no changes Also ratified in 2012 was G 711 0 lossless extended to the new fidelity layers Like G 711 0 full G 711 backward compatibility is sacrificed for efficiency though a G 711 0 aware node may still ignore or drop layer packets it doesn t understand Licensing EditThe patents for G 711 released in 1972 have expired so it may be used without the need for a licence 1 See also EditList of codecs Comparison of audio coding formats RTP audio video profile Au file formatReferences Edit a b c G 711 Pulse code modulation PCM of voice frequencies www itu int Archived from the original on 2019 06 17 Retrieved 2019 11 11 Video Voice Speech Codecs Grandstream Retrieved 19 July 2020 G 191 Software tools for speech and audio coding standardization Function alaw expand in file Software stl2009 g711 g711 c Itu int Retrieved on 2013 09 18 G 191 ITU T Software Tool Library 2009 User s manual Itu int 2010 07 23 Retrieved on 2013 09 18 ITU T 2009 07 17 ITU T Newslog Voice codec gets new lossless compression Archived from the original on 2016 03 03 Retrieved 2010 02 28 ITU T G 711 0 Lossless compression of G 711 pulse code modulation Retrieved 2010 02 28 Recent Audio Speech Coding Developments in ITU T and future trends PDF August 2008 retrieved 2010 02 28 G 711 1 Wideband embedded extension for G 711 pulse code modulation ITU T 2012 retrieved 2022 12 24External links EditITU T Recommendation G 711 ITU T G 191 software tools for speech and audio coding including G 711 C code Code Project C implementation of G 711 with source code RFC 3551 RTP Profile for Audio and Video Conferences with Minimal Control G 711 PCMA and PCMU definition RFC 4856 Registration of Media Type audio PCMA and audio PCMU RFC 5391 RTP Payload Format for ITU T Recommendation G 711 1 PCMA WB and PCMU WB Retrieved from https en wikipedia org w index php title G 711 amp oldid 1129346997, wikipedia, wiki, book, books, library,

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