V.23 Modem
for the TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the ITU V.23 modem standard. V.23 is a 600-/1200- bps half-duplex 
frequency-shift keying (FSK) modem that was developed in 1964 as a low-speed data modem. Today it 
is still used for data transmission where low complexity receivers are needed, such as caller ID. 
The modem is essentially half duplex but it does have a very-low speed backward channel that 
operates at 75 bps, also using FSK modulation.

Features and Benefits

Modem type:			V.23 forward channel
Data rates:			600-/1200-bps half duplex
Symbol rate:		600/1200 baud (1 bit/symbol)
Sample rate:		9.6 kHz 
Mean frequency:		Mode 1 (600 bps)	1500 Hz     1 = 1300 Hz   0 = 1700 Hz
				Mode 2 (1200 bps)	1700 Hz     1 = 1300 Hz   0 = 2100 Hz
Frequency deviation:	Mode 1   ±200 Hz
				Mode 2   ±400 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:		600 bps   Flat channel:     11-dB SNR for 1e-5 block error rate 
(1000 bits/block)		1200 bps  Flat channel:    16.5-dB SNR for 1e-5 block error rate (1000 
bits/block)

Modem type:			V.23 backward channel
Data rate:			75-bps half duplex
Symbol rate:		75 baud (1 bit/symbol)
Sample rate:		9.6 kHz
Mean frequency:		420 Hz     1 = 390 Hz   0 = 450 Hz
Frequency deviation:	±30 Hz

Processor and System Requirements

V.23
Program
Memory
(Words)
Data
Memory
(Words)
Processing
Load
(MIPS)
Transmitter + receiver
600
120
4.5

Usage Limitations or Performance Considerations

The software is in the form of four subroutines: one for the transmitter and one for the receiver, 
for each of the forward and backward channels. The sample interface is 9.6-kHz linear samples, and 
there are eight samples per symbol at 1200 bps, 16 samples per symbol at 600 bps, and 128 samples 
per symbol for the 75-bps backwards channel. However, all 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 forward and backward channels operate 
in the same fashion. 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