VoIP Voice and Fax Signal Processing

3.4 G.726 OR ADPCM COMPRESSION

G.726 is an ITU-T-G.726 recommendation (1990) known as ADPCM. This waveform-based compression does not assume any vocal tract models [URL (Cisco-coding)]. It works on a sample basis by making use of a short-term correlation of samples. ADPCM is used in PSTN for overloaded digital circuit multiplication equipment (DCME) channels. In DCME, more T- or E-carrier channels are multiplexed on the same digital channels. Cordless and DECT phones use ADPCM for communication between the base station and multiple cordless units. A modified version of ADPCM is used in wideband G.722 [ITU-T-G.722 (1988)] with two sub-bands that deliver a higher quality than PSTN. In packet voice, G.726 was initially included in voice over asynchronous transfer mode (VoATM). In the migration of VoATM to VoIP, several compression codecs were considered that provide comparable quality at a lower bit rate than G.726. Hence, G.726 is not preferred in many VoIP deployments. In some VoIP-to-PSTN voice gateways, G.726 is supported for better interoperability.

ADPCM is easy to implement on dedicated chips. The processing is mainly through arithmetic and logical operations than through multiplications. The codec takes about 7 to 8 million cycles per second (MCPS) processing on a single multiplier accumulator-based generic digital signal processor (DSP) like TI-54x [URL (TI-54x)] and ADSP-218x [URL (ADI-218x)], which occupies much lower memory when compared with low-bit-rate codecs.

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