APF in V1.1.2 does not seem as good

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  • Updated 6 years ago
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The APF function in V1.1.2 does not seem to work nearly as well as it did previously.  The effect of the slider is minimal and the peaking seems significantly diminished over the previous version.

I know there were some complaints that it boosted the signal too much in V 1.1 but it worked much better than V1.1.2 for me. 

I made a video and would like to see if others on V1.1.2 see the same thing.  Also if you are still on V1.1,  can you compare your version with the video?  

Regards, Al / NN4ZZ  
al (at) nn4zz (dot) com


The video shows the APF being turned on and off as well as adjustments being made to the slider.  The audio does not change much and almost sounds better off with some settings.

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Al / NN4ZZ

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Posted 6 years ago

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Steve - N5AC, VP Engineering / CTO

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Official Response
I read ACØC's notes on APF.  Good stuff -- thanks for the link Paul.  

I've not had a chance to do a lot of operating in the last few months, but my impression of the v1.1 APF was that it worked well -- perhaps too well -- but that for me getting the signal into the filter was tedious.  Once I turned APF on I would listen and then if the signal wasn't peaking I would move a little up and down the band trying to get the signal in the filter.  Perhaps this is not a problem.  

The APF filter is a different kind of filter to what we use for bandpass.  The bandpass filters have a rectangular response designed to have a gain of one inside the filter and a gain of zero outside the filter.  Of course this is never achieved and there are skirts, but this is the goal.  The APF is different in that it has more of a triangular response as seen in Jeff's notes.  It is designed to boost a narrow range of frequencies while reducing others (but not totally rejecting them).  The higher the Q (controlled by the slider now), the sharper the filter and the narrower the peak.  

Al asked about widths and we could measure the 3dB width of the filter, but this is not a flat-topped filter.  I agree with Jeff that there should be some gain in the APF to reduce the "the volume went DOWN?" effect.  I recall giving the engineer direction that the gain of the filter should be one and this was a mistake.  Of course, everything is software and we'll tweak things until we get consensus that it's the best stuff out there.

My question is about the width of the filter and if any of you are bothered by it.  There are alternatives to the shape  to make it look more flat-topped so that you get equal response in a range of frequencies that is easy for you to hit.  Another alternative would be to have a control or method to lock to a specific CW signal.  If we did this (somewhat harder) we could make the filter very peak-y since we would know that you would always have the signal where you need it.  My inclination is to:

1. Flat-top the filter for a 10Hz or so range and that this combined with (add a couple of more poles)
2. Set the Q range in the slider from 5-20
3. Include 3-6dB of gain in the passband of the filter

We'll take another pass at this and I'd like to have some of you test it before we release this next time.  What do you think about this set of changes?  Full disclosure -- I've not talked to the engineer yet so I could be off base in some of this.