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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
