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From: JDurand@cup.portal.com
Newsgroups: comp.dcom.telecom
Subject: Re: Touchtone Detection Question
Message-ID: <10669@accuvax.nwu.edu>
Date: 9 Aug 90 17:46:27 GMT
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X-Telecom-Digest: Volume 10, Issue 556, Message 4 of 9


In article <10356@accuvax.nwu.edu>, gmc@wisvr.att.com (Glenn M Cooley)
writes:

> Some/most systems I've come across which have you enter data through
> TT are able to correctly decode my input, long pulses, short pulses,
> quick pulses, Bell phones, non-Bell phones.  Other systems, such as
> various answering machines are very fickle.  I have to master a
> certain pressing technique and can only use certain phones (non-PBX
> Bell phones are the best) and still need to use several tries.

> Why/comments/etc?

The two problems with detecting tones are that you are not allowed to
detect a digit when none is present _AND_ you may not miss or mis-read
a digit that is present no matter what the customer puts on the line
(a lot of my voice-mail boards have been used in 976-xxxx applications
with all sorts of music playing over voice!).  This is NOT easy.  The
boards I am currently working on use a DSP (Digital Signal Processor
[special fast computer]) to first equalize the line, apply auto gain
control, and remove any correlation in the incoming audio to the
outgoing program.  After you do all this, then you use very good
filters and a voting scheme between different methods of detecting the
tone.  This generally works fairly well.
 
In low cost equipment (less than the cost of the DSP alone), a
hardware chip that was designed for central office use is used.  In a
typical call into a central office, there is never any outgoing
program, the only sounds on the line are static and the tones (your
phone is supposed to turn off the mic when you press a button).  If
these chips hear anything other than the pure tone and dialtone, they
assume you are talking and not pressing a button and disable
detection.

I hope this helps.


Jerry Durand, Durand Interstellar, Inc., jdurand@cup.portal.com