V.32/V.32bis Modem
for the TMS320C2x, TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the ITU V.32bis modem standard (4800–14,400 bps), with backwards 
compatibility to V.32 (4800/9600 bps only) via the rate sequence words exchanged in the handshake. 
V.32bis is a full-duplex modem using echo cancelling to separate the two directions. 

Features and Benefits

Modem type:			V.32bis, V.32
Data rates:			V.32bis:	4.8, 7.2, 9.6, 12, 14.4 kb/s full duplex
				V.32:	4.8, 9.6 (uncoded), 9.6 (trellis-coded) full duplex
Symbol rate:		2400 baud (2-6 bits/symbol)
Sample rate:		9.6 kHz or 8 kHz
Carrier frequency:	1800 Hz 
Data rate adaptation:	Can be fixed or automatic according to line conditions. In automatic mode, 
highest speed possible is chosen during handshake; subsequently, during data mode, the data rate 
can be varied to suit changing line conditions via the V.32bis rate-renegotiation sequence.
Tx spectrum:		Energy at 600/3000 Hz is 3 dB down. Out-of-band energy satisfies BS6305 given 
suitable DAC anti-aliasing filtering.
Echo canceller:		Echo span:	12.5 ms near-end, 12.5 ms far-end - `C25 part
						15.0 ms near-end, 15.0 ms far-end - `C52 part
Round trip delay span: 1.7 secs
Phase-roll extraction range for far-end echo: ±2.4 Hz
Receiver:			Equalization: linear (fractionally-spaced) and decision-feedback sections
				Carrier extraction frequency offset range: ±7 Hz
Decision process: 	Viterbi trellis decoder assisted by `sequence estimation'
Rx performance		Flat channel: 24-dB SNR for 1e-6 error rate
@ 14,400 bps:		Poor channel (seg3002):   29-dB SNR for 1e-6 error rate

Processor and System Requirements

V.32/V.32bis
Program
Memory
(Words)
Data Memory (Words)
Processing Load (MIPS)
Internal
External
'C25
'C52
Transmitter + receiver
8000
512
5500
9.5
14.5
The precise processing load will depend on the adaption rates set for the echo canceller and 
receiver equalizer, and the length of the echo canceller.

Usage Limitations or Performance Considerations

The software is in the form of a subroutine. At the 9.6-kHz sample rate it should be called once 
per 2400 Hz Tx symbol (i.e., every four samples). At the 8-kHz sample rate it should be called 
every five samples. In either case, the same sample clock is common to both the Tx and Rx sample 
streams but is synchronous to the Tx function. Rx clock interpolation is done in software. Rx-Tx 
clock slip is automatically processed in receive operation. A control word is used to set modem 
speed, etc, and a status word provides diagnostic information. Note that the samples must be 
double-buffered to cope with clock slip.

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, VideoConferencing, VideoTelephony, 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