----- Original Message -----
From: "David Harper"
Subject: FSK technical question


For a communications protocol such as RTTY, I know the mark and space
frequencies indicate 0 and 1 values of a (usually) 5-bit character.
But how does the receiving side synchronize with the transmitting
side? How does the receiver continue to properly allocate the

OK, remember the whole start and stop bit thing?

The line sits at mark when idle. When a character comes, the line drops to
space for one bit time. This is the start bit. Then the 5 or 8 bits are
transmitted, then one, 1.5 or two stop bits, which are really nothing more
than the minimum time between characters. So the receiver is guaranteed *at
least* one bit time of mark followed by exactly one bit time of space
between characters. The receiving side does need to be reasonably accurate,
but only accurate enough to not garble a character. It never has to keep in
sync for more than 10 bits worth (8 data bits plus a start and stop bit).
If the protocol specifies more than one stop bit, from the receiver's
perspective that is nothing more than additional time the transmitter has
allotted to do end of character processing.

On your earlier question about receiving FSK, the various posters answered
what you would do if you wanted to use an SSB or AM rig, or an audio FM rig
to receive FSK. However, a purpose-built FSK receiver would probably use an
FM discriminator, and simply recover data, rather than audio, from the
discriminator. Remember that an FM discriminator has an output that is
related to the frequency. If you fed the discriminator two frequencies, the
output would be two voltages.

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