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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