JimB said:
A waveform with a 10% duty cycle does not translate to 10bit digital data!
You are right, I don't know what I was thinking...
Regarding "harmonically related tones", again you are right, I didn't think of it.
So, if 4kHz is too high a frequency, 4 good frequencies to use would perhaps be 2000Hz, 2300Hz, 2600Hz and 2900Hz? Or are these in harmonics? There has to be a big gap between the frequencies so the decoder can easily distinct them from each other.
Perhaps I could use 4 duty cycles instead of 4 frequencies, and the decoder (PIC processor) could measure the duty cycle and determine the command or bit from that. How about that?