[Al Skierkiewicz]

Marshall,
You will find in DTMF that searching for frequency by amplitude will get you unusual results. The DTMF signaling system uses eight distinct tones mixed in groups of two to represent 16 states. The tones are divided into a high and low group, centered around 1kHz. The tones are high group, 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz. The low group is 697 Hz, 770 Hz, 852 Hz, and 941 Hz. these tones were specifically chosen to center up in the passband of the phone which has a high frequency limit of 3kHz and a low frequency limit of 300Hz. When a DTMF encoded signal is generated, one high signal and one low signal are mixed. The resulting waveform varies in amplitude as the high frequency signal is AM modulated by the low frequency signal. Effectively, the two signals are algebraically summed. Simple DTMF decoders use Phase Locked Loops to recognize the frequencies present and output a one of sixteen tally signal. By using this PLL method, noise, amplitude variations and distortion in the signal is minimized resulting in less errors. You likely will need to use some analog circuitry ahead of the digital implementation to bring the original signal to a level and shape that can be used by the control device. There is a ton of info available on this subject. You might try looking into Ham Radio books for various discussions and implementations. Hams have been using DTMF since it was invented for station control, remote signaling and phone dialing from radios.