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Application Report Abstract
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mu-Law and A-Law Companding Using the TMS320C2xx DSP

Companding (compression and expansion) is a method commonly used in telephony applications to increase dynamic range while keeping the number of bits used for quantization constant. The compression is lossy, but provides lower quantization error at small amplitude values than at larger values.

Mu-law is a companding standard commonly used in the United States. It takes 14-bit data values and compresses them to eight-bit values. A-law is the standard used in Europe. It takes 13-bit data values and compresses them to eight-bit values. For more information on these standards, see the references at the end of this application note.

In these C-callable subroutines, expanded input and output are expected to be in signed left-aligned form. For a-law, this means the least-significant three bits are zero after expansion, and are ignored during compression. For mu-law, the two least significant bits are zero after expansion, and are ignored during compression.

This application note describes subroutines for performing mu-law and A-law companding using Texas Instruments (TI)(tm) TMS320C2xx ('C2xx) DSP.

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