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µ-Law Compressed Sound Format
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Format Description Properties
• ID: fdd000039
• Short name: µ-Law
• Content categories: sound
• Format category: bitstream encoding
• Last significant update: 2005-06-22
• Draft status: Partial (low priority for LC)
Identification and description
Full name | [µ-Law (Mu-Law) telephony companding algorithm, from ITU-T G.711] |
Description | Standard companding algorithm used in digital communications systems in North America and Japan (telephones, for the most part) to optimize the dynamic range of an analog signal (generally a voice) for digitizing, i.e., to compress 16 bit LPCM (Linear Pulse Code Modulated) data down to 8 bits of logarithmic data. See also Notes below. µ-Law is similar to the A-Law algorithm used in Europe. |
Production phase | Final-state for end-user delivery. |
Relationship to other formats | |
Used by | WAVE, WAVE Audio File Format |
Used by | Other file formats, not documented at this time |
Local use
LC experience or existing holdings | None |
LC preference | None |
Sustainability factors
Disclosure | Open standard. Developed by ITU-T (International Telecommunication Union, Telecommunication Standardization Sector). |
Documentation | ITU-T Recommendation G.711, General Aspects of Digital Transmission Systems. Terminal Equipments [sic]. Pulse Code Modulation (PCM) of Voice Frequencies. 1972, later revisions. "Extract from the Blue Book." |
Adoption | Not investigated for this page. |
Licensing and patent claims |
Not investigated for this page. |
Transparency | Depends upon algorithms and tools to read; will require sophistication to build tools. |
Self-documentation | Not investigated for this page. |
External dependencies | Not investigated for this page. |
Technical protection considerations | Not investigated for this page. |
Quality and functionality factors
Normal rendering for sound | Not applicable. |
Fidelity (support for high audio resolution) | Low; this is a format to carry voices to telephones using reduced bandwidth. |
Sound field (support for multi-channel audio) | Stereo appears to be possible, a least in a WAVE file; see http://www.tsp.ece.mcgill.ca/MMSP/Documents/AudioFormats/WAVE/Samples.html. |
Functionality beyond normal sound rendering | Not applicable. |
File type signifiers
Tag type | Value | Note |
Filename Extension | n/a | |
Internet Media Type | n/a | |
Magic numbers | n/a | |
Notes
General | From http://www.winbond-usa.com/products/codecs/faqs.html:
"µ-law and A-law are audio compression schemes defined by ITU-T G.711 that compress 16 bit linear data down to 8 bits of logarithmic data. The encoding process (referred to logarithmic companding) breaks the linear data into segments with each progressively higher segment doubling in size. This ensures that the lower amplitude signals (where most of the information in speech takes place) get the highest bit resolution while still allowing enough dynamic range to encode high amplitude signals. Though this method does not provide a very high compression ratio (roughly 2:1), it does not require much processing power to decode.
"Mu-law (also written µ-Law) is the encoding scheme used in North America and Japan for voice traffic. A-Law (or a-Law) is used in Europe and throughout the rest of the world. The two schemes are very similar. Both break the total dynamic range into eight positive and eight negative segments. Bit 1 (MSB) identifies the polarity, bits 2,3,4 identify the segment, and the last four bits quantize the value within the segment. The differences are in the actual coding levels and the bit inversion. Nevertheless, both systems offer 2:1 bit compression, thus doubling the capacity of a digital transmission circuit while maintaining 'toll quality' voice reproduction."
From http://www.theparticle.com/cs/bc/mcs/notes0007.html: "The µ-Law inputs a 14bit sample, and via a non-linear transform outputs an 8bit sample. A-Law is similar, except it starts with a 13bit sample. The telephone signal is sampled at 8kHz, at 14bits each, that's 112,000 bits/s. At a compression factor of 1.75, the encoder outputs 64,000 bits/s." |
History | |
Format specifications
URLs
Print
• ITU-T Recommendation G.711, General Aspects of Digital Transmission Systems. Terminal Equipments [sic]. Pulse Code Modulation (PCM) of Voice Frequencies. 1972, later revisions. "Extract from the Blue Book."
Useful references
URLs
•
http://www.winbond-usa.com/products/codecs/faqs.html
•
Wikipedia article on Mu-Law (http://en.wikipedia.org/wiki/Mu-law_algorithm)
•
Wikipedia article on G.711 (http://en.wikipedia.org/wiki/G.711)
•
Sample files including µ-Law in WAVE wrapper (http://www.tsp.ece.mcgill.ca/MMSP/Documents/AudioFormats/WAVE/Samples.html)
•
Includes segment on µ-Law and A-Law (http://www.theparticle.com/cs/bc/mcs/notes0007.html)
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Last updated Monday, 06-Mar-2006 07:38:53 EST