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G.728 Speech Coder TMS320C5000 DSP Code

Description: the ITU G.728 speech coder is intended for telephone-bandwidth signals. The sample rate on nominally 13-bit samples is 8kHz with the signal bandlimited to the range 300Hz to 3.4kHz. Compression is by Low Delay Code Excited Linear Prediction (LD-CELP) to a data rate of 16kb/s.
         The principal feature of G.728 is low throughput delay. Each set of 5 input samples is encoded to a 10-bit codeword within gain and shape tables. The linear predictive filter is 50th order and is adapted backwards, enabling the algorithmic delay to be only 5 samples, ie 0.625ms. Thus in circuit multiplication over 2-wire analog lines, no echo cancellation is required. Decoded speech quality is enhanced by the use of a post-filter. G.728 passes low bit rate modem signals up to 2.4kbit/s, DTMF dialling digits, and network signalling. It is also a standard for video conferencing audio, and is often used for speech storage. Mean Opinion Score (MOS) is 3.61.
         The Algotron implementation has been developed to minimise DSP resource usage whilst still passing the G.728 test vectors. Encoder and decoder are operated as separate items. With the assembler interface, each has individual 5-sample/codeword subroutine calls. Further subroutine calls can be made for initialisation. The code routine calls are re-entrant to allow multi-channel operation, the memory block associated with each channel being pointer-addressed. A C-callable wrapper file can also be provided, allowing individual channels to be called from C for processing of multiple 5-sample/code blocks. A comprehensive user's guide is provided with the code.

Processor Load:

- #figures for assembler interfaces with TMS320C54x operation
- program memory figures include read-only tables

Availability: NOW - sale is under licence - integration support offered