[Elecraft] Dual Pass Band

[Bill Coleman]

On Jan 6, 2014, at 5:16 PM, Jim Brown <jim at audiosystemsgroup.com> 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 at arrl.net
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
           -- Wilbur Wright, 1901