G.726 Audio Coder
for the TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the ITU G.726 audio-coding specification. This codes narrowband audio 
(300–3400 Hz, sampled at 8 kHz) to a selectable range of bit rates from 16 to 40 kb/s. G.726 
supersedes G.721 and G.723, as the G.726 specification at 32 and 40 kb/s is identical to G.721 (32 
kb/s only) and G.723 (32 and 40 kb/s).

Features and Benefits

The G.726 specification uses an Adaptive Differential Pulse Code Modulation (ADPCM) algorithm. An 
important feature is that the coding/decoding delay is essentially zero, as the algorithm processes 
one sample at a time. The performance at the various bit rates for a single encode/decode is judged 
to be as follows:
40 kb/s	Identical to 64-kb/s PCM (G.711) for audio–passes 9.6-kb/s modem signals
32 kb/s	Identical to 64-kb/s PCM for audio
24 kb/s	Some degradation from 64-kb/s PCM for audio
16 kb/s	Considerable degradation from 64-kb/s PCM for audio but still intelligible

The software passes the ITU G.726 test vectors at all bit rates.

Processor and System Requirements

G.726
Program Memory (Words)
Data Memory (Words)
Processing Load (MIPS)
Permanent
Temporary
Encoder + decoder
2000
81
15
15.9
Encoder only
1140
49
15
8.5
Decoder only
1040
32
14
7.4

Usage Limitations or Performance Considerations

•	The software consists of four subroutines: an initialization routine and a processing routine for 
each of the encoder and decoder functions. The initialization routines, which also select the bit 
rate, are normally called only on DSP reset or change of coding rate. The audio input and output 
format is 8-kHz linear, and the processing routines are called once per sample. The encoder 
converts each input sample into a codeword (2 to 5 bits long, according to the rate), which is used 
by the decoder to reconstruct the sample.
•	Applications of the G.726 algorithm feature speech compression for speech storage, digital 
circuit multiplication, telephony multiplexing applications where the delay must be kept low to 
avoid sidetone echo problems.

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 that includes audio/speech 
coding, modems, acoustic echo cancellation, video coding, and general telecommunications functions. 
Key technology areas include: GSM/PCN mobile comms, video conferencing, video telephony, and 
PSTN/ISDN voice and data transmission.

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