[Elecraft] RX Mush, why the fuss?
ab2tc
ab2tc at arrl.net
Thu Mar 2 15:37:26 EST 2017
Hi,
Where in Smith's article does it say that AGC with the slope set for 15 acts
as a hard limiter? There is a huge difference between AGC action (which is
simply a reduction in gain with linearity retained) and hard limiting. I
have read the article and agree that his measurements are very helpful. The
"waviness" he observes he correctly attributes to the log function in the
DSP being less than accurate. This has since been corrected as far as I
know. Whatever some people at the receiving end of a CW pileup hear is not
caused by hard limiting. Why can't we ever see some hard evidence of this
phenomenon (like a video with quality audio or at least an audio only
recording)?
AB2TC - Knut
alorona wrote
> It isn't so much that 'the strongest signal in the passband determines the
> gain of the receiver', it's that once that strong signal sends the
> receiver into AGC, additional signals in the passband do not increase the
> audio output power when the Slope is set at or near its extreme. This is a
> form of gain compression, which is distortion, strictly speaking.
>
> The 'landmark' paper on this was written by Jack Smith, K8ZOA, available
> at: http://www.cliftonlaboratories.com/elecraft_k3_agc_and_s-meter.htm .
> In it, Jack showed that the K3's curve of Audio Output vs. RF Input at
> Slope = 15 acts as a hard limiter. He follows that with a table of
> measurements where a 10 dB change in RF signal level results in virtually
> no increase in audio output. This 10 dB change in RF signal level could
> come from a single signal that increases by 10 dB, or additional signals
> in the passband that add 10 dB of RF input to an existing 'strong' signal
> -- the receiver doesn't care which.
>
> For many listeners, more signals added to the passband that don't result
> in any more audio is a condition that confuses and fatigues the brain and
> can make it difficult to decode the relationship between signals. I
> believe this is the 'mush' that has been reported here. The effect occurs
> on CW, too, though I believe that the effect is made even worse by the
> heavily compressed phone signals that many contesters generate.
>
> I completely agree with Dave AB7E that the use of as little slope as
> possible (lower values of the AGC Slope parameter), coupled with higher
> values of AGC Threshhold allows the receiver to sound very natural, or
> open, or clean, and preserves as much as possible the relationship between
> multiple signals, which makes it easier for your brain to copy them. (I'm
> sorry for using such non-scientific terms, but it's the best I can do to
> describe it.)
>
>
>
> Al W6LX
> <snip>
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