FR-GSM Speech Coder
for the TMS320C2x, TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the speech-coding functions of the ETSI full-rate GSM digital-mobile 
telephone system. This codes narrowband speech (300–3400 Hz, sampled at 8 kHz) to 13 kb/s, and 
includes voice-activity detection, error concealment, and discontinuous-transmission facilities. 
The following ETSI specifications are implemented: 06.10 (speech transcoding), 06.11 (substitution 
and muting of lost frames), 06.12 (comfort noise), 06.31 (discontinuous transmission), and 06.32 
(voice-activity detection).

Features and Benefits

The speech coder uses a Regular Pulse Excitation with Long-Term Prediction (RPE-LTP) algorithm. The 
audio performance for a single encode/decode is judged to be that some degradation is present 
compared to standard 64-kb/s PCM (G.711); the degradation is only really noticeable over good 
loudspeaker systems. The software passes all the ETSI GSM test vectors.

Processor and System Requirements

The receiver code is highly configurable for a particular application. Output decisions are held in 
buffers of user-selectable size, allowing infrequent polling for results by the host.
Full-Rate GSM
Program Memory
(Words)
Data Memory (Words)
Processing Load (MIPS)
Permanent
Temporary
Encocer + decoder
5873
602
305
4.7
Encoder only
3293
353
305
3.5
Decoder only
2624
249
175
1.2

Usage Limitations or Performance Considerations

The code consists of four subroutines: an initialization routine and a processing routine for each 
of the encoder and decoder functions. The initialization routines are normally called only on DSP 
reset. The audio input and output format is 8-kHz linear samples, and the processing routines 
operate on 160-sample (20-ms) frames. Each input frame of speech is converted by the encoder into a 
260-bit parameter block. The decoder uses this data to reconstruct the speech frame. Applications 
include full-rate GSM, speech compression for speech storage, digital circuit multiplication, 
telephony multiplexing applications.

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