Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rutgers!mit-eddie!genrad!rep From: rep@genrad.UUCP (Pete Peterson) Newsgroups: sci.electronics Subject: Re: DTMF Message-ID: <1319@genrad.UUCP> Date: Wed, 22-Jul-87 07:45:12 EDT Article-I.D.: genrad.1319 Posted: Wed Jul 22 07:45:12 1987 Date-Received: Fri, 24-Jul-87 03:18:29 EDT References: <1377@mit-amt.MEDIA.MIT.EDU> <17966@cmcl2.NYU.EDU> Reply-To: rep@genrad.UUCP (Pete Peterson) Organization: GenRad, Inc., Concord, Mass. Lines: 50 Keywords: dtmf remote control In article <17966@cmcl2.NYU.EDU> hanley@nyu.arpa (John Hanley) writes: #> 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? #> It does work, provided that the 3f, 5f, etc harmonics of the square-wave don't cause you any problems. The square wave has a fundamental frequency content which is about 1.27 times the sine wave giving about 1.6 times the power. Of course the square wave has twice the power of the sine wave, but the power in the harmonics doesn't help your S/N ratio. #> #>Addendum: Bandpass active filter #> #> R R #> +----/\/\/\/-----+-----/\/\/\/-----+ #> | | | #> ----- C ----- C ----- C #> ----- ----- ----- #> | | | #> +----------------+-----------------+ #> | | R/12 | #> Rin | |\ +--/\/\/\/--GND | #>audio in o-----/\/\/\/----+---| -\___________________________+------o Vout #> +-| +/ #> | |/ #> | #> GND #> Simpler circuit with roughly equivalent behavior and many fewer parts: Rin audio in o-----/\/\/\/------+------o Vout | | GND Unless you have a pile of op amps and R's and C's which are burning a hole in your junk box (or other parts storage facility) and you are extremely poverty stricken and take great delight in building and tuning multiple bandpass filters, the integrated DTMF encoders/decoders are definitely the way to go. pete peterson {decvax,linus,wjh12,mit-eddie,masscomp}!genrad!rep