David Harper wrote:



Is anyone familiar with the hardware details of the FSK process?
Specifically what components are involved in transforming a frequency
into a bit?

Thanks!
Dave

I think you already have a couple of messages on how the frequency can be
generated from a bit.

It appears to me you are asking for the opposite.

Basically, the modem runs the receive frequencies through narrow filters. If
the frequency received matches the passband of the filter, the audio tone
gets through the filter. The tone can then drive something as simple as a
transistor amplifier or darlington pair from one state to another (e.g.
from Vc=5v to Vc=0.5v). These outputs can then drive gates in the right
combination to get your serial output.

What has changed over the past two decades is how the filters have been
implemented and how the serial signal is generated.

What used to be narrow filters built with big inductors and critical
capacitors became filters implemented using op amps which, in turn, became
filters using dsp techniques.

Serial signal generation started out with straight off and on pulses to rtty
ksrs, moved to simple uarts built with logic gates, which became uarts on
ic's.

The basics are the same, however. Narrow filters for the tones and logic to
generate the serial signal.

73,

tim a0bwr