An A-law algorithm is a standard

companding In telecommunications and signal processing, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited dynamic range. The name is a portmanteau of the words compressing and expandi ...

algorithm, used in

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

an 8-bit PCM

digital communications Data communication, including data transmission and data reception, is the transfer of data, signal transmission, transmitted and received over a Point-to-point (telecommunications), point-to-point or point-to-multipoint communication chann ...

systems to optimize, i.e. modify, the

dynamic range Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and their effect on motion Brands and ent ...

of an

analog signal An analog signal (American English) or analogue signal (British and Commonwealth English) is any continuous-time signal representing some other quantity, i.e., ''analogous'' to another quantity. For example, in an analog audio signal, the ins ...

for digitizing. It is one of the two companding algorithms in the G.711 standard from

ITU-T The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating Standardization, standards fo ...

, the other being the similar μ-law, used in North America and Japan. For a given input

x

, the equation for A-law encoding is as follows:

\dfrac, & \dfrac \leq , x, \leq 1, \end

where

A

is the compression parameter. In Europe,

A = 87.6

. A-law expansion is given by the inverse function:

F^(y) = \sgn(y) \begin
    \dfrac, &  , y,  < \dfrac,
    \\
    \dfrac, & \dfrac \leq , y,  < 1.
\end

The reason for this encoding is that the wide

speech Speech is the use of the human voice as a medium for language. Spoken language combines vowel and consonant sounds to form units of meaning like words, which belong to a language's lexicon. There are many different intentional speech acts, suc ...

does not lend itself well to efficient linear digital encoding. A-law encoding effectively reduces the dynamic range of the signal, thereby increasing the coding efficiency and resulting in a signal-to-

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...

ratio that is superior to that obtained by linear encoding for a given number of bits.

Comparison to μ-law

The

μ-law algorithm The μ-law algorithm (sometimes written Mu (letter), mu-law, often abbreviated as u-law) is a companding algorithm, primarily used in 8-bit PCM Digital data, digital telecommunications systems in North America and Japan. It is one of the two c ...

provides a slightly larger dynamic range than the A-law at the cost of worse proportional distortion for small signals. By convention, A-law is used for an international connection if at least one country uses it.

External links

Waveform Coding Techniques
- Has details of implementation (but note that the A-law equation is incorrect)
A-law implementation in C-language with example code
{{Compression Methods Audio codecs Data compression

Comparison to μ-law

See also

External links