On Fri, 4 Jul 2008 00:50:36 -0700 (PDT), G4ILO wrote: >Is there any benefit in using such a narrow flter for PSK? The DSP decoder >is only looking at the actual band of frequencies containing the signal so >it is, in effect, a narrow band filter already.
Narrow filters reduce the amplitude of impulse noise, like lightning static and many forms of QRN. Impulse noise can push gain stages, including audio stages and the output transformers, into clip, which can make life more difficult for any decoder. Going from 150 Hz to 50 Hz bandwidth reduces the power of that impulse noise by 4.8 dB. On the other hand, any sharp filter skirt can introduce ringing from that noise, and very narrow filters bring that ringing closer to the signal. AND, any change in amplitude response (i.e., the filter skirts) MUST introduce phase shift, which can get in the way of decoding. In other words, there may be a point of diminishing returns. This is one of those questions that must be answered by experimentation, and will likely vary from one decoder to another. W0YK is a very serious RTTY contester, and has been using K3s since the earliest stages of beta. I'd bet that Ed could tell us a lot about these issues with respect to RTTY decoding. :) 73, Jim K9YC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
