On Nov 4, 2011, at 10:23 AM, Jim McDonald wrote:
> Isn't 45.45 the standard?
Yes, 45.45 baud is the standard and represents a precise 22ms bit period. The
actual number is 45.454545... baud, but most people stop at the second decimal
place.
However, there are commercial amateur interfaces that can only do 45.0 baud and
if you can switch to 45.0 baud to copy them, you can get a smidgen worth of
improvement (a small fraction of a dB of equivalent SNR).
The improvement is measurable under AWGN conditions using a Matched Filter, but
probably not even noticeable if there is any fading at all, and especially if
your modem does not use a Matched Filter. When the SNR is good, you will not
be able to notice any degradation.
However, the degradation with different transmit and receive baud rate is
noticeable if you are using some kind of a pseudo synchronous clock extraction
method to pull out a really weak signal, e.g., using RITTY in "digital
flywheel" mode.
Most people don't even know whether they transmit at 45.0 baud or 45.45 baud --
this is one of the "hidden" losses that are not usually discussed. Just as the
loss in SNR when people use bit-banging methods to generate FSK signals. The
latter can be quite large, actually.
Transmit with 45.45 baud if it is available to you. Many software modems are
optimized to demodulate the proper baud rate, and are not optimized for 45.0
baud.
73
Chen, W7AY
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