Generated millions of times every day by our telephone keypads, the eight DTMF frequencies were chosen so that the harmonics and intermodulation do not generate significant in-band signal levels. The signal is encoded as a pair of sine waves, ensuring that no frequency is a multiple of the other and the sum and difference between two frequencies does not match any single tone — and that’s why DTMF sounds so ugly!
The DTMF encoder circuit shown here is based on the HT9200B tone generator device produced by Holtek and distributed by Futurlec (w w w.futurlec.com) among others. The encoder is complemented by a decoder elsewhere in this publication. The HT2900B is supplied as a nice old fashioned 14-pin device. It can be instructed by a microcontroller to generate 16 dual tones and (in serial mode only) 8 single tones from the DTMF pin output. Its 8-pin ‘younger brother’ the HT9200A provides a serial mode only whereas the HT9200B contains a selectable serial/parallel mode interface for various applications such as security systems, home automation, remote control through telephone lines, communication systems, etc.
A 74HC148 8-to-3 priority encoder is used to convert the ‘keypad’ information from S1–S8 into 3-bit tone selection words the HT9200B wants to see at its input. The ninth switch, S9, is connected to input D3 on the encoder chip. Pressing one of the switches S1–S8 generates a complimentary 3-bit binary word at outputs A0, A1, A2 of IC1. IC2 then generates the dual tones accordingly to these binary codes.