We now have our basic APF function in place, and I'm amazed at how well it works. Lyle (KK7P, our DSP guy) surprised even himself :) It improves signal to noise ratio significantly, as evaluated both by measurement (S+N/N increase of ~3 to 5 dB using the K3's built-in dBV meter) and by ear (makes many weak signals buried in noise easier to copy). Because we're using DSP, we've been able to improve on analog implementations by providing constant gain over the full pitch range and ideal passband characteristics.
One explanation for why APF improves copy is that it is *not* a brick- wall filter: it has a narrow peak, but the skirts are very broad, leading to a more "natural" sound. It has fewer of the artifacts you get with a really narrow filter. To borrow terminology from our existing DUAL PB function, the K3's first and second I.F. filters provide the "context" bandwidth (along with a very high level of ultimate attenuation), while APF provides the "focus." The combination of the two sometimes works better than brick-wall filters at an equivalent narrow bandwidth, at least in some QRN/QRM scenarios. For example, I was listening to 40 meters last night with S5 background noise. Copy on many signals was better with WIDTH=400 and APF on, than with WIDTH set to 100 or 50 Hz and APF OFF. Many of you sent suggestions for how APF should be turned on and controlled (thanks). We're weighing various alternatives. Wayne N6KR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
