An A-law algorithm is a standard

companding In telecommunication 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 expanding, ...

algorithm, used in

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

an 8-bit PCM digital

communications Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...

systems to optimize, i.e. modify, the

dynamic range Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...

of an

analog signal An analog signal or analogue signal (see spelling differences) is any continuous signal representing some other quantity, i.e., ''analogous'' to another quantity. For example, in an analog audio signal, the instantaneous signal voltage varies c ...

for digitizing. It is one of two versions of the

G.711 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 ...

standard from

ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...

, the other version 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 a human vocal communication using language. Each language uses Phonetics, phonetic combinations of vowel and consonant sounds that form the sound of its words (that is, all English words sound different from all French words, even if ...

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 typographic approximation, approximated as u-law) is a companding algorithm, primarily used in 8-bit PCM Digital data, digital telecommunication systems in North America and Jap ...

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

Comparison to μ-law

See also

External links