Hi Doug,

Are you sure about that? Most systems that I have seen introduce some
offset (at least a few percent of the frequency) between one tone's
harmonic and the other tone. I don't know if this is the reason, but I
always surmised that it was to prevent problems being caused by the signal
going through a channel with some amount of distortion (which might cause
the system to produce harmonics of the tones).

One example of this is the Bell 202 standard (1200 bps), which uses 1200 Hz
and 2200 Hz, not 2400. Another (more dramatic) example is the Bell 103 (300
bps) standard, 2025 Hz and 2225 Hz.

Sean

At 10:13 AM 4/8/02 -0400, you wrote:
>Those seem like odd specs for an FSK system.  I have used 5kHz/10kHz
>5kbps for oceanographic work, and 1200Hz/2400Hz 1200bps for computer
>work.  Always the high frequency was exactly twice the low.
>Unfortunately I didn't do software on those projects.
>
>Sherpa Doug

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