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

[Joe Subich, W4TV]

 > Wider BW in QRM will too.

Only to the extent that the QRM causes AGC pumping or overdrives the
signal chain (receiver IF, detector, sound card) causing IMD.  RTTY
decoding software like Chen's cocoaModem, David's 2-Tone and even
Mori-san's MMTTY have extremely tight MARK/SPACE filtering capable
of rejecting signals even 100 Hz away and nothing (not even tight IF
filters) will reject an overlapping signal.

73,

    ... Joe, W4TV


On 3/8/2017 7:35 PM, Fred Jensen wrote:
> 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
>> __________________________________________________________________
>>
>
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