Vocoder overview and comparison

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DocID024112 Rev 1

Figure 1. G.726 versus G.711

1.3

IMA-ADPCM

ADPCM (adaptive differential pulse code modulation) is a variant of DPCM that varies the

size of the quantization step, to allow further reduction of the required bandwidth for a given

signal-to-noise ratio.

IMA-ADPCM is a legacy audio codec developed by interactive multimedia association (IMA)

with the following characteristics:

•

Sampling frequencies: 8 (only frequency supported here), 11.025, 22.05, or 44.1 kHz.

•

Compression ratio: 1:4

The main advantage of the IMA ADPCM compression algorithm reside in its simplicity. The

algorithm is not limited to voice signals and can operate at any input sampling rate thus

enabling compression of high quality audio as well.

1.4

Speex

The Speex codec is an open-source, patent-free and royalty-free software dedicated to

speech compression and decompression.

Speex is based on CELP (code excited linear prediction) and designed to compress voice at

bitrates ranging from 2 to 44 Kbit/s.