[Elecraft] AGC and RTTY Decoding (was [K3] AGC White Paper)

[Fred Jensen]

Having designed digital decoders BR [Before Retirement] on various 
channels including HF, we found exactly what Ed said to be true, both in 
corrected and uncorrected channels.  The less AGC compression on HF 
channels, the better the decode.  However, adjusting AGC parameters on 
many military HF radios was a lot harder than on the K3. [:-)  And, for 
what it's worth, any BW less than about 300 Hz, and better 350 Hz, is 
going to degrade the decoder's capability at 45.5 baud ITA-2.  Wider BW 
in QRM will too.  It's an engineering trade-off.

73,

Fred ("Skip") K6DGW
Sparks NV DM09dn
Washoe County

On 3/8/2017 2:39 PM, john at kk9a.com wrote:
> This is a very interesting post Ed! I will definitely will try these AGC
> settings in the next RTTY contest.
>
> John KK9A - W4AAA
>
>
> Ed Muns w0yk said:
> Tue Mar 7 21:48:52 EST 2017
>
> Below is a thread from 7 March 2016 about AGC usage with RTTY decoders.
> David Wicks, G3YYD, is the author of 2Tone and Kok Chen, W7AY, is the author
> of CocoaModem.
>
> Anecdotally, my experience after 250,000+ RTTY QSOs over the past 15 years
> concurs that minimizing AGC action supports best decoder performance.  If my
> ears, or widely varying signal levels, can't tolerate AGC Off, then I use
> AGC Slow, SLP=0 and THR=14 or higher.
>
> Note also the comments about receiver IF bandwidth of 500 Hz except in
> extreme cases.  Even in big RTTY pileups such as I encounter sometimes in DX
> locations, Again, I've anecdotally found that 500 Hz decodes better most of
> the time.  I seldom go lower.  This also implies turning off the K3
> Dual-Tone filter.
>
> Both of these points (no, or minimal, AGC and moderate IF BW) are not
> intuitive, especially for an experienced CW operator.
>
> Ed W0YK
> __________________________________________________________________
>
> G3YYD, 0210:
>
> 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
> __________________________________________________________________
>