G.728 Audio Coder
for the TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the fixed-point variant of the ITU G.728 speech-coding specification. 
This code's narrowband speech (300–3400 Hz, sampled at 8 kHz) to a rate of 16 kb/s. The G.728 
specification for floating-point DSPs has been in existence for some time. The fixed-point 
algorithm interworks directly with the floating-point version and enables the standard to be 
implemented on low-cost fixed-point DSPs. G.728 is primarily intended for Digital Circuit 
Multiplication Equipment (DCME) and so features a low transcoding delay in order to avoid the 
creation of telephony sidetone echo. It is also becoming a de-facto standard for video conferencing 
audio.

Features and Benefits

The G.728 specification describes a Low-Delay Code-Excited Linear Prediction (LD-CELP) algorithm 
which employs backward-adaptive Linear Predictive Coding (LPC) analysis in order to achieve a low 
transcoding delay. The decoder also incorporates an adaptive postfilter to enhance performance for 
multiple transcodings. The postfilter function can be omitted for single coder-decoder operation to 
reduce the processing power required on the DSP. The performance of the G.728 algorithm may be 
summarized as follows:
Bit rate				16 kb/s
Audio quality
•	Single transcode		•	Comparable to 64-kb/s PCM (G.711), identical to G.726 at 32 kb/s
•	Multiple transcodes	•	Degradation only apparent after 6 tandem transcodes
•	Errored transmission	•	Better or equivalent to G.726 at 32 kb/s
•	Transcoding delay		•	2 ms
•	Music coding		•	No detrimental effects
Transmission properties
•	DTMF/network signalling tones	•	Equivalent to 64-kb/s PCM
•	Modem signals		•	Passes modem signals up to 2400 bps (note that the encoder perceptual weighting 
filter must be disabled)

The software passes the full set of ITU G.728 (fixed-point) test vectors.

Processor and System Requirements

G.728
Program Memory (Words)
Data Memory (Words)
Processing Load (MIPS)*
Code
Data Tables
Permanent
Temporary
Encoder + decoder + postfilter
5206
1102
1840
160
37.5
Encoder + decoder
4285
1075
1219
160
33.5
Encoder only
2894
1075
754
160
23.0
Decoder + postfilter
2780
1020
1086
160
14.5
Decoder only
1859
993
465
160
10.5
* The MIPS figures above are worst-case values over a frame of 20 speech samples. 

Usage Limitations or Performance Considerations

The software consists of six subroutines: an initialization routine and a processing routine for 
each of the encoder, decoder, and postfilter functions. The audio input and output format is 8-kHz 
linear samples, and the frame size used is 20 samples. However, the processing routines operate on 
a subframe basis and are called every five samples. The encoder converts each subframe into a 
10-bit codeword, which is used by the decoder to reconstruct the signal. The postfilter operates on 
the five sample decoder output subframe.
Applications include:
•	Speech compression for DCME equipment
•	Coding of the audio channel for video conferencing and desktop terminals where transmission 
bandwidth is limited
•	Speech storage

Availability

Now, under licence, for a one-off payment and/or royalties depending on the commercial application. 
An Application Note is available. Support consultancy for code integration is also available.

Company Background and Contact Information

Signals and Software Limited (SASL), based in Harrow, Middlesex, UK is a design consultancy 
specializing in the area of Digital Signal Processing (DSP). From concept and algorithm design 
through to real-time DSP implementation, SASL is able to offer its clients fast and cost-effective 
solutions to their DSP needs. Services include: feasibility studies, DSP software to order, DSP 
research and algorithm design, computer simulations (C or PASCAL), and hardware design. In support 
of these services, SASL offers a range of "off-the-shelf" software.
Contact:	David Morley
		3 Jardine House
		Bessborough Road
		Harrovian Business Village
		Harrow, Middlesex, HA1 3EX
		United Kingdom
		+44 (0) 181 426 9533
		Fax: +44 (0) 181 869 1182
		e-mail: davem@sasl.demon.co.uk