[Elecraft] APF vs 10 Hz DSP, why they don't sound the same

[Brian Alsop]

Unlike the phone guys, most CW guys could care less about audio 
distortion or quality for that weak one.  They just want to be able to 
copy it.  The information is in the off's and on's, the rest doesn't 
matter.

The old QF-1 Q-muliplier had an advertised Q of 4000 @ 455KHz.  That's a 
bandwidth of about 100Hz.  There was some skill requiring adjusting 
several knobs to make it work for you.  Of course, any K3 implementation 
would have to be knobless.

73 de Brian/K3KO

Bill Tippett wrote:
>  > More generally, the APF proponents seem to have the problem that they
> treat it almost as magic.  If they could characterise what it does in
> terms of filtering, AF gain adjustment, distortion etc. it should be
> possible to simulate it in DSP, subject only to having enough processing
> power.  In particular, peaking just seems to me to mean turning up the
> AF gain in parallel with reducing the bandwidth. If that is all that it
> is doing, and if the K3 has soft AF gain controls, a plugin box could
> issue both commands in parallel.
> 
>          The circuit in question is on page 54 of the FT1000 schematic below:
> 
> <http://www.radioamateur.eu/schemi/FT1000_user.pdf>http://www.radioamateur.eu/schemi/FT1000_user.pdf
> 
> The APF is around Q3016-2 (including Rs 3062, 3065, 3067 per the 
> "un-modification" sheet on N1EU's website below).  The circuit 
> schematic above has *not* been modified to the (incorrect) values 
> shown in the Yaesu service bulletin.
> 
> http://n1eu.com/Mods/apf.pdf
> 
> The FT-1000's APF is generally regarded by many as the best APF ever 
> done.  It should be possible to plug this into a circuit simulator 
> and generate a frequency response plot that Lyle can emulate in the 
> DSP.  I have a feeling that this is a very narrow and high-Q response 
> that is on the verge of ringing, but it should be possible to 
> duplicate the circuit in the DSP.
> 
>                                          73,  Bill  W4ZV
> 
>
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