Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!husc6!cmcl2!hanley 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 )