V.22/V.22bis Modem
for the TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the ITU V.22bis/V.22 modem standards. These are full-duplex 
quadrature-amplitude modulation (QAM) modems using frequency division multiplex to separate the two 
channels. V.22 has data rates of 600 bps and 1200 bps; V.22bis has a data rate of 2400 bps.

Features and Benefit

Modem type:			V.22/V.22bis
Data rates:			V.22:	600/1200-bps full duplex
				V.22bis: 2400-bps full duplex
Symbol rate:		600 baud (1–3 bits/symbol)
Sample rate:		9.6 kHz 
Carrier frequency:	Call channel:      1200 Hz
				Answer channel:    2400 Hz 
Guard tone:			1800 Hz (answer channel only)
Tx spectrum:		In-band response to V.22 requirements (75% roll-off square-root raised cosine)
				Out-of-band energy satisfies UK BS6305 given suitable anti-aliasing filtering
Receiver:			Equalization: fractionally-spaced linear and decision-feedback sections
				Carrier extraction frequency offset range: ±7Hz
Rx performance		Flat channel:   15-dB SNR for 1e-6 error rate
@ 2400 bps:			Poor channel (Conus Poor Voice):  16-dB SNR for 1e-6 error rate

Processor and System Requirements

V.22/V.22bis
Program -Memory
(Words)
Data
Memory
(Words)
Processing
Load
(MIPS)
Transmitter + receiver
2700
300
4

Usage Limitations or Performance Considerations

The V.22/V.22bis software is implemented as a subroutine which is called once per symbol, every 16 
samples (9600/600). The same sample clock is common to both the transmit and receive sample streams 
and is synchronous to the transmitter function. Receive clock interpolation is done in software, 
and receive-transmit clock slip is automatically processed in receive operation. A control word is 
used to set modem speed, and a status word provides signal quality, clear-to-send, and carrier 
detect information. Note that the samples must be double-buffered to cope with clock slip.
Interpolator: Signals and Software can also supply an 8/9.6-kHz sample interpolator to enable modem 
operation over ISDN or other applications that require 8-kHz sampling.

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