[Elecraft] K3 APF (audio peaking filter) progress report #2

[Kok Chen]

On Oct 27, 2010, at 1:06 PM, Paul Christensen wrote:

> And the reason why some of us have been stating that roofing filters and brick-wall DSP filters, as  good as they are for their intended purpose, cannot equate to the results obtained by the APF.


Indeed.

If you squint a little at the Yaesu APF filter shapes, the "skirts" fall off with a more or less 1/f shape -- perhaps coincidentally, the spectral envelope of a periodic On-Off Keyed signal follows a 1/f profile.

In a sense, that is what a matched filter does in the signal processing world -- and we know that matched filters, in the absence of QRM, optimizes SNR in an AWGN channel.  Although I won't go as far as extend linear matched filter theory to the highly non-linear ear-brain system, it still raised my curiosity when I first saw the Yaesu filter plots which someone kindly posted earlier.  

When there is strong adjacent QRM though, you may still have to "roof" the APF with a filter that has more abrupt skirt -- similar problem exists with matched filters for RTTY: they are great for pulling signals out of the noise.  But you need to engage a more abrupt filter when QRM creeps closer in.

73
Chen, W7AY