[Elecraft] RX Mush, why the fuss?

[David Gilbert]

That isn't actually true.  Threshold and slope combine to form a point 
of non-linearity that can cause all sorts of in-band mixing products 
when multiple signals exist at roughly the same levels.  I and others 
have experienced that first hand in the past.  I can running stations in 
a contest with a fairly narrow passband and if I get more than a few 
callers at roughly the same level, and if that level is in the vicinity 
of the knee in the gain curve formed by the onset of the AGC, the A-2B 
and 2A-B (etc) products generated by the nonlinearity create all sorts 
of mush that muddles the copy.  Noise at that point in the curve can 
perform similar dirty deeds.

I use as little AGC slope as possible with a fairly high threshold as a 
result ... tough on the ears sometimes but helps the rate dramatically.

If I remember correctly, even without the AGC there is a knee at the low 
end of the response curve (but still above the noise limit) in the 
original synths.  I recently purchased the new synths for my K3 and 
supposedly they help significantly on that score, but I haven't had the 
opportunity to install them yet.

73,
Dave   AB7E



On 2/28/2017 4:37 PM, dave wrote:
>
> My thoughts on this are that those who are concerned about the slope 
> and threshold settings are barking up the wrong tree. The mush would 
> result if you have your hold time or hang time or decay set too short. 
> With a brief hold time the weaker signal pops up to the level of the 
> stronger on as soon as the stronger one disappears. A longer hold time 
> keeps the relative level of the two signals at the correct 
> relationship. The answer is hold time, not threshold or slope.
>
> Those of us in the southeastern US may have a problem with all the 
> lighting we get. The lightning spikes tend to drive the desired signal 
> too low. So we kinda have to keep hold time short if we are to hear 
> anything. But there should be some level of decent compromise in there 
> somewhere. IIRC the K3 has an AGC setting that helps with this but 
> does not eliminate it.
>
> 72 de dave
> ab9ca/4
>
>
>
> On 2/28/17 2:53 PM, Wes Stewart wrote:
>> I must confess to some bewilderment about the seemingly endless
>> discussion about the adjustment, or mis-adjustment, of AGC slope,
>> threshold or "RF" gain.
>>
>> Claims are made that one's favorite settings cause signals within the
>> passband to retain their relative amplitudes thus allowing the
>> discrimination between them, while less favorable settings compress
>> them into "mush."
>>
>> I will confess that my experience with the design of AGC systems is
>> limited to analog receivers and perhaps there is some digital magic
>> that makes DSP radios act differently from analog ones in this case.
>> But in my experience, AGC control is derived from the stronger signal
>> received.
>>
>> After the SNR is adequate (delayed AGC in 1960s terms, above threshold
>> today), the overall gain is reduced by some amount to maintain a
>> desired output or to prevent overload, and any other signals present
>> suffer the same gain reduction.  Hence a signal 30 dB stronger than
>> another is still 30 dB stronger even after the application of AGC.  If
>> it isn't then we have a very nonlinear receiver, which is desirable if
>> we're receiving FM but highly undesirable otherwise.
>>
>> My reading between the lines suggests that the "mush" proponents think
>> that after achieving threshold, changing the slope somehow changes the
>> ratio between signals, i.e. there is less gain for strong signals than
>> there is for weaker ones.
>>
>> Frankly, after 60 years of listening to shortwave noise and in my
>> youth working in a machine shop and hanging around too many alcohol
>> and nitro burning race cars, my tinnitus practically drives me nuts at
>> times; I welcome a flat AGC slope.
>>
>> If I'm all wet with this, I'd like to be enlightened.
>>
>> Wes N7WS
>>
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