On 1/27/2021 3:06 PM, David Woolley wrote:
Having the roofing filter too close to the DSP filter is not necessarily a good thing, as the roofing filters are likely to have worse passband ripples and will have non-linear phase responses, which can compromise digital modes. At least some of the DSP filters are finite impulse response, meaning they are also linear phase, which means that pulses will not get smeared out.
K1JT strongly urges 3kHz or greater IF bandwidths for his modes, and for exactly that reason. And it's why top RTTY contesters have abandoned the K3's dual-peak filter in favor of 500 Hz IF bandwidth. My professional life in audio system design taught me that speech intelligibility is degraded by time/phase distortion and suspected the result would be the same with RTTY, but I was derided when I started preaching that to RTTY guys. Several years later, author of the 2Tone RTTY software G3YYD said the same thing, and folks started believing it. It's also why I find that the 2.1 kHz 8-pole provides better speech intelligibility than the 1.8 kHz filter.
Years ago, I tried using narrow SSB realignments of the K2's CW crystal filters in contests. I had carefully tweaked them per the build instructions, noting that their amplitude response looked like the profile of a mountain range. I wasn't surprised that those settings made signals much harder to copy. The SSB TX filter sounded fine on RX.
When I switched from the 2.7 kHz 5-pole filters to the 2.8 kHz filters for TX in one of my K3s I noticed considerably less incidental AM on RTTY, and immediately converted the other two. I didn't do that in the 2nd RX, because I only use it for weak signal CW work on the lower bands.
73, Jim K9YC ______________________________________________________________ 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 Message delivered to [email protected]
