Communications Toolbox

compand

Source code mu-law or A-law compressor or expander

Syntax

out = compand(in,param,v) out = compand(in,Mu,v,'mu/compressor') out = compand(in,Mu,v,'mu/expander') out = compand(in,A,v,'A/compressor') out = compand(in,A,v,'A/expander')

Description

out = compand(in,param,v) implements a µ-law compressor for the input vector in. Mu specifies µ and v is the input signal's maximum magnitude. out has the same dimensions and maximum magnitude as in.

out = compand(in,Mu,v,'mu/compressor') is the same as the syntax above.

out = compand(in,Mu,v,'mu/expander') implements a µ-law expander for the input vector in. Mu specifies µ and v is the input signal's maximum magnitude. out has the same dimensions and maximum magnitude as in.

out = compand(in,A,v,'A/compressor') implements an A-law compressor for the input vector in. The scalar A is the A-law parameter, and v is the input signal's maximum magnitude. out is a vector of the same length and maximum magnitude as in.

out = compand(in,A,v,'A/expander') implements an A-law expander for the input vector in. The scalar A is the A-law parameter, and v is the input signal's maximum magnitude. out is a vector of the same length and maximum magnitude as in.

Note The prevailing parameters used in practice are µ= 255 and A = 87.6.

Examples

The examples below illustrate the fact that compressors and expanders perform inverse operations.

compressed = compand(1:5,87.6,5,'a/compressor')
expanded = compand(compressed,87.6,5,'a/expander')

The output is

compressed =

    3.5296    4.1629    4.5333    4.7961    5.0000


expanded =

    1.0000    2.0000    3.0000    4.0000    5.0000

Algorithm

For a given signal x, the output of the µ-law compressor is

where V is the maximum value of the signal x, µ is the µ-law parameter of the compander, log is the natural logarithm, and sgn is the signum function (sign in MATLAB).

The output of the A-law compressor is

where A is the A-law parameter of the compander and the other elements are as in the µ-law case.

References

[1] Sklar, Bernard, Digital Communications: Fundamentals and Applications, Englewood Cliffs, N.J., Prentice-Hall, 1988.

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