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From: hanley@cmcl2.NYU.EDU (John Hanley)
Newsgroups: sci.electronics
Subject: Re: DTMF
Message-ID: <17966@cmcl2.NYU.EDU>
Date: Tue, 21-Jul-87 20:35:36 EDT
Article-I.D.: cmcl2.17966
Posted: Tue Jul 21 20:35:36 1987
Date-Received: Thu, 23-Jul-87 05:46:04 EDT
References: <1377@mit-amt.MEDIA.MIT.EDU>
Reply-To: hanley@nyu.arpa (John Hanley)
Organization: New York University
Lines: 66
Keywords: dtmf remote control
To: jh@mit-amt.MEDIA.MIT.EDU

In-Reply-To: <1377@mit-amt.MEDIA.MIT.EDU>


Using audio sounds good, especially since for debugging purposes you can just
listen to a radio receiver.  However, especially since you don't have them
readily at hand, I'm not convinced that DTMF chips are the way to go.
You can easily get VCO's to hum at any frequency you want by tweaking
discrete resistors, and further you can build a frequency detector with
a single 747 dual op amp: one side accepts audio input and acts as a
bandpass filter, the other side acts as a comparator and just cleans
up the analog so it's TTL on the output (or it could act as a Schmitt
trigger, which still requires but a single op amp).  This might be simpler
than DTMF because now you've only got one frequency to worry about at a
time instead of two.  Of course, I'm blindly assuming that you've got enough
bandwidth and steep enough filters for all these frequencies to give each
other enough breathing room.  If need be you can make the filters steeper by
adding another stage or two, and the whole thing would still fit on a single
(quad) chip.  Of course, if some DTMF chips drop into your lap, by all means
use them, but I tend to think in terms of what I can build _now_.  What?!?
You don't have a drawer full of 741's?  Shame on you!

If for some reason you're not real big on op amps, you can use a 567 tone
decoder to do the same thing, but I'm guessing you don't have a whole lot
of those lying around or you'd be using those instead of posting to the net.

P.S.: I almost recommended the use of 555 timers to generate your tones but
      then caught myself because I think you want sinusoidal tones to
      transmit.  I did this because I recall reading something on modems
      that mentioned that mod/demod is done because the phone system has
      low bandwidth and is optimized for the human voice, so sine waves
      make it through the phone system _much_ better than square waves.
      Why is this?  I understand that the rapid hi-lo transitions count
      as high frequency transitions and thus will be attenuated, but it
      seems to me that the worst case should be that you put a square wave
      in and get the corners lopped off so the output looks kind of
      sinusoidalish.  But since an X volt p-p square wave carries more
      power than an X volt p-p sine wave, you should get a better S/N ratio
      at the output using square simply because you applied more signal.
      Why doesn't this work?

                   --John Hanley,
 /  /   ____ __  __  System Programmer, Manhattan College [ ..cmcl2!mc3b2!jh ]
/__/ /__ /  /-< /-/  Researcher, NYU Ultracomputer Labs   [  Hanley@NYU.arpa ]

"The Ultracomputer: to boldly go in log N time where no N processors have
 gone before."



Addendum:  Bandpass active filter  (40 db/decade, I think -- it's 2nd order)

                                   R                 R
                           +----/\/\/\/-----+-----/\/\/\/-----+
                           |                |                 |
                         ----- C          ----- C           ----- C
                         -----            -----             -----
                           |                |                 |
                           +----------------+-----------------+
                           |                |    R/12         |
                  Rin      |   |\           +--/\/\/\/--GND   |
audio in  o-----/\/\/\/----+---| -\___________________________+------o Vout
                             +-| +/
                             | |/
                             |
                            GND

where Rin is whatever input impedance you would like to present to your
RF reciever circuitry, and RC determines the detection frequency:
 f = sqrt(3) / ( 2 pi R C )