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µ-Law Compressed Sound Format

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

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 Explanation of format description terms

LC experience or existing holdings None
LC preference None

Sustainability factors Explanation of format description terms

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, with later revisions. (Extract from the Blue Book). G.711 is freely available at http://www.itu.int/rec/T-REC-G.711-198811-I/en. The Blue Book is ITU's Telecommunication Policies for the Americas (Blue Book).
Adoption Not investigated for this format description.
    Licensing and patents Not investigated for this format description.
Transparency Depends upon algorithms and tools to read; will require sophistication to build tools.
Self-documentation Not investigated for this format description.
External dependencies Not investigated for this format description.
Technical protection considerations Not investigated for this format description.

Quality and functionality factors Explanation of format description terms

Sound
Normal rendering Compressed encoding intended for low bandwidth transmission.
Fidelity (high audio resolution) Low; this is a format to carry voices to telephones using reduced bandwidth.
Multiple channels Stereo appears to be possible, a least in a WAVE file; see http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/Samples.html.
Support for user-defined sounds, samples, and patches Not applicable
Functionality beyond normal rendering None.

File type signifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable.  Depends on wrapper.
Internet Media Type Not applicable.  Depends on wrapper.
Magic numbers Not found.  Comments welcome.   

Notes Explanation of format description terms

General

From Archived Voice Codecs FAQ from Nuvoton:"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 as 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."

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Format specifications Explanation of format description terms


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Last Updated: Wednesday, 08-Jan-2014 12:16:55 EST