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G.726 Speech Coder Software for Texas TMS320C54x/C55x DSPs

The ITU G.726 coder is intended for telephone quality speech signals. The sample rate is 8kHz and the compression is to data rates of (selectably) 16, 24, 32, or 40kb/s. Applications include circuit multiplication, digital telephony, and speech storage. The algorithm uses Adaptive Differential Pulse Code Modulation (ADPCM) in a backwards configuration. Delay is minimal, the encoder output being a code adaptively quantised from the current input sample and the coder memory.

The G.726 software operates in 3 sample modes. In linear mode, the encoder inputs and the decoder outputs 14-bit samples. In PCM mode, the samples are in 8-bit G.711 PCM form, either A-law or u-law. In PCM+SCA mode, the PCM samples at the output of the decoder have synchronous coding adjustment (SCA) applied to prevent cumulative distortion occurring on synchronous tandem connections (ADPCM-PCM-ADPCM, etc.).

The Algotron implementation has been developed to have very low DSP resource usage whilst still passing the G.726 test vectors. Encoder and decoder are operated as separate items with individual per-sample/ADPCM-code subroutine calls. Further subroutine calls can be made for initialisation and/or for changing the data rate. The code routine calls are re-entrant to allow multi-channel operation, the memory block associated with each channel being pointer-addressed. A comprehensive user's guide is provided with the code.

Performance:

Speech conveyed at 40kb/s G.726 is near identical to 64kb/s PCM (G.711) in signal quality, whilst G.726 32kb/s is only marginally lower. Speech quality at G.726 24kb/s and 16kb/s is increasingly impaired, being intended for overload channels in circuit multiplication. As inherently a waveform coder, and having also adaptive quantisation for non-voice signals, G.726 at 32 or 40kb/s can pass standard DTMF signalling and low-speed modem signals up to 2400b/s. G.726 at 40kb/s can convey modem signals eg fax to 12,000b/s.

DSP Load:

Notes: for encoder- or decoder-only implementation, the MIPS count is approximately halved, the data memory / channel is halved, and the program memory is reduced by 30-90 words depending on function

Availability: NOW, with implementer's guide - integration support offered