[Elecraft] 2.13 beta

[Jim Brown]

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