DTMF Tx/Rx
for the TMS320C5x
by Signals and Software Limited

Software Overview

DSP software which implements the DTMF signalling system. DTMF is the most common signalling method 
used within the telephone network, and has now largely replaced loop disconnect ("pulse") dialing. 
DTMF works by employing pairs of tones to code the digits 0–9, #, *, and A-D. DTMF receivers need 
to correctly detect the presence of these tones, while eliminating background noise and allowing 
for distortions introduced by the network. The Mitel test tape exists to verify the performance of 
DTMF receivers.

Features and Benefits

The transmitter software produces the required tones with a frequency tolerance of 0.1%.
The receiver software has been extensively tested and passes all the Mitel tests:
Dynamic range:		In excess of 35 dB
Guard time:			The receiver can detect digits of 28.4-ms duration and greater
Signal-to-noise ratio:	The receiver identifies all digits correctly at an SNR of 12 dB
Mitel talk-off test:	Four false digits were detected during this test; the pass level is 30 false 
digits
Frequency offset:		Passes all 64 frequency offset tests
Twist:			The receiver passes all eight twist tests, with average attenuation at cutoff of 9.8 dB 
for normal twist, and 4.2 dB for reverse twist

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.
DTMF
Program Memory
(Words)
Data Memory (Words)
Processing Load (MIPS)
Code
Data Tables
Transmitter
200
16
16
0.8
Receiver
418 + 71n
18
30 + 29n + m*n
0.87n1
n=number of channels   m=buffer size 
 ¹30 receivers may be accommodated on a 28.5-MIP 'C5x DSP

Usage Limitations or Performance Considerations

The code consists of an initialization routine, which is normally called upon DSP reset, and a 
single macro which is called once for each channel. The audio input/output format is 8-kHz linear 
samples. Additional code may then be required to interface to the host platform, for example to 
extract data from a serial link. Applications include DTMF reception for public or private 
telephone exchanges; telephony and line-test equipment; remote control of computer and telephone 
equipment.

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. 
Support consultancy for code integration is also available. We also have experience of the MF 
signalling system.

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