Posted by
Guy, K2AV on
Feb 10, 2010; 1:58pm
URL: http://elecraft.85.s1.nabble.com/K3-in-a-cw-pileup-needs-work-tp4523884p4548005.html
Traversing the knee does not cause distortion, because the first
transition around the knee is *** HELD *** and only released many,
many cycles later even at the fastest possible FAST AGC, and only
released if the level has subsided.
This is not at all the same thing as having an amplifier device curve
of that shape and subjecting every single audio cycle to such an
amplification curve which would create tons of distortion.
What can happen at a much slower rate (below audio frequencies so
technically not causing distortion), is that the very fastest AGC,
with its quick release, combined with an ambient noise amplified (by
inappropriate PRE/ATT/RFGAIN settings) above the AGC threshold, will
effectively push down every signal to the noise level, presenting to
the ear what sounds like all signals are at the noise level.
In a crowded room our brain neatly interprets stereophonic sound to
separate out a single spatially diverse conversation from dozens of
others at essentially the same level. In research using a monophonic
recording of the multiple conversations from a single microphone in
the same room, very few can discern individual conversations, and most
find any length of listening to the recording irritating and tiring.
The monophonic muddle created by self-defeating PRE/ATT/RFGAIN/AGC
combinations is probably worse than the monophonic room recording, as
we are intending to sit there for a while, like for a whole contest.
At some point the stressed-out brain is emotionally looking for a
scapegoat, and will fasten on the closest object.
Those of us who had Klingon pain-stick training doing traffic nets in
80 meter summer QRN can will ourselves through silly control setting
combinations without learning how to use them. The rest of us will
need to learn how to use them, or go buy a box that decides all our
choices for us.
I decidely prefer to keep the choices. Despite all claims to
contrary, PRE/ATT/RFGAIN/AGC is not rocket science.
73, Guy.
On Wed, Feb 10, 2010 at 1:38 AM, Dave - AB7E <
[hidden email]> wrote:
>
>
> I also think that the current max AGC threshold is too low, but I have another question. Why is it necessary for the K3 AGC to have such a sharp elbow at the point where the AGC kicks in? The Clifton Labs plots show such a sharp bend that it seems multi-signal in-band intermod is inevitable, especially with higher SLP settings. Wouldn't it be feasible, and more desirable, to have some more gradual inflection point? I would think that something like a parabolic shape would be better, and the DSP AGC is just the implementation of an algorithm, correct? I suppose that would tend to blur the distinction between different THR settings, but so what?
>
> Maybe somebody can educate me ...
>
> 73,
> Dave AB7E
>
>
>
>
> ------Original Mail------
> From: "Don Wilhelm" <
[hidden email]>
> To: "Joe Subich, W4TV" <
[hidden email]>,
> <
[hidden email]>
> Sent: Wed, 10 Feb 2010 01:06:09 -0500
> Subject: Re: [Elecraft] K3 DSP AGC Question
>
> Joe,
>
> Can you elaborate your reasons for this suggestion. I had come to
> believe that the best situation is to place attenuation before the first
> mixer to give the greatest amount of dynamic range (headroom).
>
> Yes, I agree that the AGC Threshold could be raised in the K3 and would
> be beneficial. I have felt the default of 005 was too low.
>
> 73,
> Don W3FPR
>
> Joe Subich, W4TV wrote:
>> Perhaps one should get the ambient well down in the ADC operating
>> range but killing gain (adding loss) before the first mixer and
>> the roofing filter is not the way to do it. If you want to
>> better align the dynamic range of the receiver (or specifically
>> the ADC) to conditions, the attenuation should come in the
>> IF - preferably divided between the first IF and 2nd IF.
>>
>> However, there would still be no apparent reason that the K3
>> should not be capable of sustaining an AGC threshold at least
>> 10 db higher than currently set with AGC THR = 008 whether
>> that be reached with preamp on, preamp off or attenuator on.
>>
>> 73,
>>
>> ... Joe, W4TV
>>
>>
>
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