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From: jon_sree@world.std.com (Jon Sreekanth)
Newsgroups: sci.electronics
Subject: Re: Programmable Multi-Tone generator wanted
Message-ID: <JON_SREE.90Nov13012237@world.std.com>
Date: 13 Nov 90 09:22:37 GMT
References: <15621@slice.ooc.uva.nl>
Sender: jon_sree@world.std.com (Jon Sreekanth)
Organization: The World
Lines: 40
In-Reply-To: ropg@ooc.uva.nl's message of 12 Nov 90 03:45:56 GMT

In article <15621@slice.ooc.uva.nl> ropg@ooc.uva.nl (Rop Gonggrijp) writes:


   For a phone switch I am currently developing I am looking for a chip that:

   Will generate two sine-wave tones simultaneously

   Is computer programmable (height of tones, and preferebly amplitude)

   Works from 100 to 3000 Hz

If you're trying to generate DTMF ("touch tone"), it may be better to buy
a DTMF chip. Anywhere from $1 to $3, because they're made in large quantities,
(nearly) every telephone has one. 

For other sine waves, I don't know of any good simple analog methods.
(In fact, I'd like to know some ...) It might sound like overkill, but it's
not particularly difficult these days to put a small eprom with an 8 bit 
dac. Especially if you have a ucontroller on your board anyway, with an
eprom and everything. 

If the two frequencies are oddball numbers, you may be stuck with needing 2
dac's. If not, you could store the waveform of the composite (combined)
signal, and re-create it. Of course, if you need independent control of
amplitudes of each component, it'll have to be 2 dacs, with some way of
scaling (either the reference, or by digital arithmetic multiplication)

For one frequency, or a composite of frequencies that have common factors, 
it works out pretty reasonable. For a 50mS DTMF burst, at 8K sampling
rate, 8 bit per sample, that's less than 512 bytes of  eprom per digit, 
and about 4K bytes for all 10. An eprom that size, and an 8 bit dac, 
are quite affordable. You'll have ultra stable frequency, known low
distortion, stable amplitude, etc. 

Regards, 
-- 

/ Jon Sreekanth

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