[RTTY] Some basic RTTY radio questions

[G3YYD]

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. 

They compare the individual tone amplitudes with their amplitude over about
one character time before and after the character being decoded. They then
combine the tones together before the final decision is made based on their
individual signal to noise ratio. Sudden changes to receiver gain will
provide less than optimum performance as it will alter the amplitude
relationship and noise over much less than 3 character times (about half a
second).

For those older decoders that use a FM demodulation system fast or slow AGC
makes no difference so set the AGC time constant as you would for SSB rag
chewing - slow.

As for bandwidth do not set it below 350Hz as Chen W7AY indicated earlier
this can cause distortion across the bandwidth by delaying some parts of a
RTTY signal more than others. This blurs one bit of the RTTY signal into the
adjacent bits. This is the signal causing QRM to itself. I personally tend
to use 500Hz on my K3 and only reduce to 350Hz in extremis. The filters in a
modern decoder are very narrow. 2Tone for instance uses a filter for each
tone that are just 45.45Hz wide and at 90Hz wide have more attenuation than
the receiver's dynamic range. Reducing RX bandwidth below 350Hz is for human
hearing limitations not that of the decoder.

73 David G3YYD

-----Original Message-----
From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of Bill Turner
Sent: 07 March 2016 06:10
To: RTTY Reflector
Subject: Re: [RTTY] Some basic RTTY radio questions

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

On Sun, 6 Mar 2016 21:26:21 -0600, Michael, KT5MR wrote:

>
>My Yaesu FT-950 has two pre-amp settings. 

REPLY:

I read this in the ARRL handbook 50 years ago and it still holds true
today:  With the antenna disconnected, observe the background noise level.
With any modern receiver it should be very low. Then connect the antenna and
observe the noise increase. The background noise should increase
significantly, and if it does you don't need a preamp.
What is "significantly"? Well, if you have to listen carefully to hear the
increase, you probably do need a preamp. If it jumps out at you, you don't. 

If you're not sure you can always verify this empirically. Find a weak RTTY
signal and switch the preamp in and out. Does the print improve either way?
There's your answer. If there is no difference, leave the preamp off. This
will help prevent overload from nearby strong signals While we're talking
about receiver settings, the question sometimes arises about AGC speed.
Fast, medium or slow? For a continuous carrier mode like RTTY, AM or FM, use
fast. When the carrier takes a quick fade, you want the AGC to follow
quickly. Medium and slow are fine for CW and SSB. 

Welcome to RTTY. Hope to see you on the air!

73, Bill W6WRT
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