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this is my first post. i want to encode the characters as well as digits in the "qwerty" keyboard to DTMF tones to send through telephone wire.please provide me with the idea for the encoding as well as decoding operation for such circuit.
For sure you will use a lot of frequencies, and you also have to calculate their harmonics, DTMF's of phone, specifically the frequencies were not just picked up randomly but calculated so that their harmonics would not interfer and be recognized as any other signal.
Well using crystal 3.579545MHz will produce the standard tones which can be decoded easily. A simple solution is to use crystals of different values for producing different tone frequencies and using the same values on the decoder side.