[Elecraft] Re APF vs 10? Hz DSP, why they don't sound the same (K2AV)

[Robert Allbright]

The APF on the 1000D was truly excellent, I owned one many years ago.
The TenTec ORION II, the K3  and other modern radios would benefit from.
I don't know about all this technical stuff, there's sometimes too  
much of it!

At the end of the day, the 1000D was able to copy a weak
160m cw signal whereas the K3 failed - as said in an earlier posting.
(It all boils down to having the two radios side by side, and operating
weak signal 160 meters for instance.  Both receivers hear the signal  
exactly the
same, but a touch of the APF on the old 1000D brings the signal out of  
the
surrounding noise just enough to make copy possible - see message 15  
Vol 64 Issue 21 by Merv KH7C.

that says it all. Why is it that modern designers fail in so many ways?

I also remember building and using a Q multiplier for use with my B40  
and Heathkit RA1 with great success.

Incidentally, I often have the agc turned off for weak signal work.

73 Robert G3RCE
K2 #5219 and K3 2855
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Original message by Guy  K2AV
After watching spectrogram on a noise signal and noting a much sharper  
shape
on 50 Hz DS as an audio peak filter, I began to wonder about some of the
posts I was seeing asking for APF. What was going on? After some
measurements...

1) the 50 Hz DSP bandwidth is *narrower* at the top than any APF I have
used. The 100 or 150 Hz DSP is more like it.

2) the 50/100 Hz DSP has *far* steeper skirts than the APF.

3) the APF *sounds to the ear* to drop off a signal more, but that is
because the signal as it moves outside the APF is still controlling  
the IF
AGC off the APF center, holding the input to the APF constant.  In the
50/100 Hz DSP the AGC opens up the gain when one tunes off the signal,
giving the *appearance* of no skirts. Watching the S meter will show  
what is
going on.  This effect dissappears entirely with the AGC *off*, where  
the
signal falls off the table on the DSP skirts as one tunes away.

If one simply measures the amount of discrimination between two close
signals, the 50 Hz DSP can beat the APF by 10, 20 db or more.  
Listening to
two S9 signals only 150 Hz apart, the 50 Hz DSP can completely isolate
them. In run situation, the shift and 50 Hz DSP would allow me to pick  
up
just one and then the other by remembering their respective tones,  
without
changing my transmit or RX reference.

The SOUND of tuning across a signal with APF in a 500 Hz IF bandwidth is
illusory in close signal discrimination, even if it is one's favorite  
analog
sound. Isn't this just dumbing down a superior DSP function to "sound  
like"
an analog result?

And reversing myself in one of my earlier posts, the modulation "pops"
produced by the sharp skirt drops on 10 Hz shift increments off a strong
signal are annoying, and perhaps the shift can be damped just a tiny  
bit to
remove them.
-- 

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