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

[Merv Schweigert]

David Woolley (E.L) wrote:
> Eric Swartz -WA6HHQ, Elecraft wrote:
>   
>> If this is set well above what is really needed, the DSP may generate 
>> some distortion on really strong signals. We usually recommend +4 dB max.
>>     
>
> I wonder if distorting the audio is really what APF is about. With a 
> narrow enough filter, the distortion products will only be based on the 
> signal.  Alternatively, and more worrying, it may be about defeating the 
> ear's defences against load sounds, by making the spectrum too narrow, 
> in which case there may be a risk of burning out the hair cells for the 
> chosen frequency.
>   
APF does not distort the audio at all,  its extremely simple,  all it 
does is peak
an audio freq by X amount.  In older radios its done with a analog 
circuit of
one or two chips and resistor capacitor sets to determine the freq of 
the peak.
I assume there is some feedback loop to increase the audio gain at the 
peaked
freq.  
> 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.
>   
It does nothing as far as filtering,  years ago it was used in cheap 
receivers as a CW filter
but that is not the purpose in a radio that has xtal filters or DSp filters.
Yes it is like turning up the AF gain some amount, but at one particular 
freq,  usually the
freq you have the pitch set at on CW.  but you do not want to reduce the 
bandwidth,
many time you want to use the APF at wider bandwidth settings due to the 
different
sound of the background noise at the wider bandwidth makes copy easier 
than with
the narrow sound of narrow filters.
> PS the relevance of AGC here is whether or not it is applied pre- or 
> post- the ultimate selectivity.  From what the APF people say, it is 
> normally applied pre-.
>   
Since APF is totally an audio function the gain and AGC is not effected 
by its use
at all,  unless you have a radio that has audio derived AGC. 
As I stated before APF is for very weak signal detection,  one would not 
usually
use it on signals that are copyable with normal filters etc.   It is 
used for copying
signals you normally tune across because they are too weak to copy or you
perhaps dont even hear them.  
It will take a signal that is at the noise level,  and peak it to the 
point of being
able to copy.  
Some ops like it so much they use it for louder signals as well. 
The closest thing is to outboard a audio filter like the Datong,  I have 
tried the
Autek and its not up to par, the Datong is fair but again not the same 
as the
circuit in the 1000D radios.  If there was a comparable outboard that worked
I would put one in the P3 empty space and go away laughing.
73 Merv KH7C