On 4/23/2014 10:13 PM, Larry Lopez wrote:
Some of the wider filters in that paper are pretty good. The narrowest filters are not so good. The flat part of the passband has constant group delay. The skirts are awful
Nothing at all surprising. In general, phase distortion (group delay) goes along with variation in amplitude response, steeper skirts results in more phase distortion in the passband. That means it's greatest with steep filter skirts, AND extends within the passband from the filter skirts. DSP signal processing is a simulation of analog circuits -- when you do filtering in DSP, it's a simulation of an analog filter, and it's far easier to build a filter in DSP with low phase distortion than it is with physical components.
In general, when phase shift is an issue, we want to use DSP for the narrow filtering and fairly wide analog filters (the crystal roofing filters) to protect the DSP from overload. And when phase shift is an issue, wider bandwidth is better.
The current wisdom from those who "get it" is to use a 400 Hz roofer for most narrow digital modes (RTTY, JT65) and set DSP for about the same bandwidth. K1JT, the Nobel laureate who gave us the WSJT modes, says to run the RX broad and let his software do the filtering.
73, Jim K9YC ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
