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AN4229

Vocoder overview and comparison

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Vocoder overview and comparison

A vocoder is a speech encoding, decoding and filtering application used to reproduce the

human voice.

In this application note, four speech codecs are implemented: G.711, G.726, IMA-ADPCM

and Speex. In the following sections, a short overview of these codecs is presented.

1.1

G.711

The G.711 codec is an ITU-T (telecommunication section of the international

telecommunication union) standard for the compression of voice frequency signals. Its

principal features are:

•

Sampling frequency: 8 kHz

•

64 Kbit/s bitrate (8 kHz sampling frequency, 8-bit samples)

•

Compression ratio: 1:2

•

Used with ITU-T recommendations G.726

G.711 defines two main compression algorithms, the µ-law algorithm (used in North America

and Japan) and A-law algorithm (used in Europe and the rest of the world). Both are

logarithmic, but A-law was specifically designed to reduce computer processing

requirements.

G.711 is primarily used in telephony, but it can also be used for fax communication over IP

networks.

1.2

G.726

G.726 is an ITU-T ADPCM speech codec standard covering the transmission of voice at

rates of 16, 24, 32, and 40 Kbit/s; actually, it supersedes both G.721 (ADPCM at 32 Kbits/s)

and G.723 (ADPCM at 24 and 40 Kbits/s). It also introduces a new 16 Kbit/s rate (Figure 1).

G.726 main features are:

•

Sampling frequency: 8 kHz

•

Bitrates: 16 Kbit/s, 24 Kbit/s, 32 Kbit/s and 40 Kbit/s

•

Uses adaptive differential pulse code modulation (ADPCM)

In addition to DECT (digital enhanced cordless telecommunications) telephony, where

G.726 is widely used, it is also used in several fields such as data and voice carrying

(depending on the selected channel bitrate).