[RTTY] Some basic RTTY radio questions

[G3YYD]

I see Chen got into this one before me. Chen and I both design and make
available RTTY decoders. The internal workings of those decoders for best
performance requires that the AGC does not change the receiver gain abruptly
and it stays reasonably constant over several character times. Fast AGC will
cause problems with the automatic threshold "circuit" (software actually),w
which can be avoided by using slow AGC.

As Chen says manual gain control that is set and left is best of all but in
something like a RTTY contest not practical as the human also wants to hear
the signal at a reasonable level. So the compromise is slow AGC. Hang AGC
can also work well if the parameters are set correctly.

73 David G3YYD

-----Original Message-----
From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of Bill Turner
Sent: 07 March 2016 11:50
To: RTTY Reflector
Subject: Re: [RTTY] Some basic RTTY radio questions

------------ ORIGINAL MESSAGE ------------(may be snipped)

On Mon, 7 Mar 2016 10:09:39 -0000, G3YYD wrote:

>Actually with RTTY the AGC setting should be slow. 
>
>The reason for this is the best decoders decode each tone separately 
>and make use of the signal amplitude and  measured noise over time.

REPLY:

There seem to be mixed opinions on AGC speed for RTTY. 

If the AGC is set to slow, that means the overall receiver gain is
relatively constant over time and therefore can not respond to rapid
variations in signal strength due to fading or flutter. The result is the
amplitude of the decoded tones will vary greatly instead of being held
relatively constant, which is the purpose of AGC in the first place. 

Isn't constant better? Are you saying a decoder does a better job when the
tone amplitude is varying rapidly? Hard to believe but I am willing to
listen to arguments. 

73, Bill W6WRT
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