Since MMTTY has both 45 and 45.45 baud rates, I was curious whether my HAL
DXP38 was using 45 or the precise 45.45 baud rate, so I asked Joe K9SZ at
HAL. The manual doesn't say, and neither does the current HAL software for
the DXP38, DXPWin 3.0.5, which works well for me with Windows 7/64, BTW.
Joe just responded:
Hi Jim,
I was curious as well after reading the post on the list.
The DXP38 uses the 22ms bit period. This calculates to 1000ms/22ms =
45.4545. baud
Yes, we have sold out of the DXP38. Thank you very much for owning one.
73
Joe Wittmer K9SZ
Jim N7US
-----Original Message-----
On Nov 4, 2011, N7US 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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