Hi, I'll chance another entry on this topic. I am sure that those who are experts in statistical communications theory can prove analytically that the optimal filter ahead of the decoder in the presence of plain white Gaussian noise is some form of Gaussian response with bandwidth slightly wider than the frequency shift. The presence of "QRM" is a different matter. It's hard (impossible?) to quantify mathematically and it is quite possible/likely that the double hump filter can help in those cases.
AB2TC - Knut Bill Coleman-2 wrote > On Jan 6, 2014, at 5:16 PM, Jim Brown < > jim@ > > wrote: >> >> Actually, it's based on things a lot more fundamental than that. In the >> world of pro audio, we learned nearly four decades ago that non-flat >> amplitude response is accompanied by phase shift (distortion). The ear >> does not like phase distortion, and neither do decoders. Filters produce >> distortion, not only when their amplitude response is changing (the >> slopes), but also in the passband. >> >> When i first encountered the dual-filter approach separate from the >> decoder itself, it looked to me like a bad idea for this reason. It was >> good to see it confirmed by G3YYD in the doc for his 2Tone RTTY decoder. >> BTW -- Nobel Laureate Joe Taylor, K1JT, author of the WSJT digital >> protocols, said exactly the same thing during a lecture I heard him do at >> Pacificon several years ago. > > That said, the dual pass band filter is often useful for squashing QRM > that also confuses the decoder. Like, when someone starts tuning up > between your mark and space frequency…. > > Just don’t run it all the time. Use it for those brief times when you need > it. > > Bill Coleman, AA4LR, PP-ASEL Mail: > aa4lr@ > Web: http://boringhamradiopart.blogspot.com > Quote: "Not within a thousand years will man ever fly!" > -- Wilbur Wright, 1901 > <snip> -- View this message in context: http://elecraft.365791.n2.nabble.com/Dual-Pass-Band-tp7582564p7582630.html Sent from the Elecraft mailing list archive at Nabble.com. ______________________________________________________________ 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
