On 1/6/2014 10:34 AM, Joe Subich, W4TV wrote:
That information is based on input from K7AY (cocoaModem) and G3YYD
(2-Tone). The software decoders do a better job when supplied with
audio at least 400 Hz wide without frequency shaping from the IF (DSP)
filter. The Dual Band Pass filters in general introduce significant
phase delays and amplitude inequality which results in poorer decoding
accuracy in modern FFT based demodulators - although the frequency
shaping may be of value in older amplitude (filter/detector) style
demodulators.
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.
73, Jim K9YC
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