Chen never ceases to amaze me with his knowledge and his ability to tell it in
terms we can understand. What a valuable asset we
have in Chen! Thanks for all you do for us.
73, Don AA5AU
http://www.aa5au.com
http://www.rttycontesting.com
-----Original Message-----
From: rtty-bounces@contesting.com [mailto:rtty-bounces@contesting.com] On
Behalf Of Kok Chen
Sent: Friday, November 04, 2011 1:11 PM
To: RTTY Reflector
Cc: Jim McDonald
Subject: Re: [RTTY] 45 baud
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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