Another little piece of the story is that in start-stop operation the
receiver samples the incoming signal at the place where it expects the
center of the bit to be. Thus it is tolerant of signals that are too
fast or too slow. Of course today it is easy to get the speed very
precise; but in the early days it was a matter of motors with centrifugal
speed governors.

With synchronous transmission the receiver knows where the bit boundaries
are going to be, so it is possible to sample near the end of each bit time
when all of the energy in the received signal has come in. Start-stop
has to throw away roughly half of the energy in each bit because of the
center sampling. Hence synchronous transmission has an advantage in
signal-to-noise ratio.

In the days of mechanical teleprinters, synchronous operation had a much
greater advantage. A mutilated STOP pulse would allow the receiving
shaft to continue rotating, and then several characters would be
received in error as a result of that single bit error. With electronic
reception there is no rotating shaft, so it is possible to reset the
receiver to the starting position instantly.

It is also possible with electronics to achieve a quasi-synchronous
operation with start-stop signals. The idea is that instead of having
the STOP pulse be arbitrarily long, it is of fixed length and an idle
character is sent if there is nothing to send from the keyboard. This
is usually called "diddle". With the incoming data stream being a
steady stream of printable characters and nonprinting idle characters
it is synchronous for the duration of the transmission. The detector
can synchronize to this signal and take advantage of all of the energy
in each signal pulse. The K6STI RITTY software (no longer marketed)
operates on this principle.
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jhhaynes at earthlink dot net