V.21 Modem
for the TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the ITU V.21 modem standard. V.21 is a 300-bps full-duplex frequency 
shift keying (FSK) modem that was developed in 1964 as a very-low speed data modem. Today it is 
more commonly used in Group 3 facsimile transmission for passing control information between remote 
machines under the governing protocol T.30. Frequency-division multiplex is used to separate the 
two channels. The modem that originates a call transmits on channel 1 and receives on channel 2; 
the answering modem transmits on channel 2 and receives on channel 1.

Features and Benefits

Modem type:			V.21
Data rate:			300-bps full-duplex
Symbol rate:		300 baud (1 bit/symbol)
Sample rate:		9.6 kHz 
Mean frequency:		Channel 1  1080 Hz       0 = 1180 Hz    1 = 980 Hz
				Channel 2   1750 Hz      0 = 1850 Hz    1 = 1650 Hz
Frequency deviation:	±100 Hz 
Tx power spectrum:	As per V.2 not exceeding -13 dBm
Receiver:			FSK receiver structure based on quadrature baseband time-delay multiply
Rx performance:		Flat channel: 9dB SNR for 1e-5 block error error rate (1000 bits/block)

Processor and System Requirements

V.21
Program
Memory
(Words)
Data
Memory
(Words)
Processing
Load (MIPS)
Transmitter + receiver
500
100
3.5

Usage Limitations or Performance Considerations

The software is in the form of two subroutines: one for the transmitter and one for the receiver. 
The channels for each direction may be set independently. The sample interface is 9.6-kHz linear 
samples, and there are 32 samples per symbol (9600/300). However, both the transmit and receive 
routines process one sample at a time and are therefore called at the sampling rate. The receiver 
is synchronized to the incoming data and uses a data valid flag to indicate that a new output bit 
is available. A carrier-detect output is also produced by the receiver. The modem can be used in 
either synchronous or asynchronous mode.
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