Andrei,

11 years ago I wrote the 80C85 program to decode 2 of 6 frequencies. They values were 700, 900, 1100, 1300, 1500, 1700 Hz. Each decimal digit was coded with 2 frequencies. The MC got the input signal from operational amplifier working like comparator.

I used the modern radioengineering methods to process input signal. The program recognized the frequencies at an input signal/noise ratio not less than minus 4 dB.

Well, to the question. I can send you the code for the 80C85 processor if you know its machine language.

Email me at address: makeev@podolsk.ru

See you,

George