Documentation

compand

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

Syntax

out = compand(in,Mu,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,Mu,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

collapse all

Generate a data sequence.

data = 2:2:12;

Compress the input sequence using an μ-law compander. The typical value for μ is 255. The data ranges between 8.1 and 12 instead of between 2 and 12.

compressed = compand(data,255,max(data),'mu/compressor')
compressed = 1×6

8.1644    9.6394   10.5084   11.1268   11.6071   12.0000

Expand the compressed signal. The expanded sequence is nearly identical to the original.

expanded = compand(compressed,255,max(data),'mu/expander')
expanded = 1×6

2.0000    4.0000    6.0000    8.0000   10.0000   12.0000

Generate a data sequence.

data = 1:5;

Compress the input sequence using an A-law compander. The typical value for A is 87.5. The data ranges between 3.5 and 5 instead of between 1 and 5.

compressed = compand(data,87.6,max(data),'a/compressor')
compressed = 1×5

3.5296    4.1629    4.5333    4.7961    5.0000

Expand the compressed signal. The expanded sequence is nearly identical to the original.

expanded = compand(compressed,87.6,max(data),'a/expander')
expanded = 1×5

1.0000    2.0000    3.0000    4.0000    5.0000

Algorithms

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

$y=\frac{V\mathrm{log}\left(1+\mu |x|/V\right)}{\mathrm{log}\left(1+\mu \right)}\mathrm{sgn}\left(x\right)$

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

 Sklar, Bernard, Digital Communications: Fundamentals and Applications, Englewood Cliffs, NJ, Prentice-Hall, 1988.