[Elecraft] K3 filter performance

[David Woolley (E.L)]

Larry Molitor wrote:

> The DSP filters and DSP modems following the IF
> filters cannot re-constitute the "distorted" signal so

Do not, rather than cannot.

Firstly, the Finite Impulse Response (FIR) filters used need not, and 
are almost certainly constructed so that they do not, introduce any 
group delay distortion of their own.  That means that you can still use 
narrow DSP filters, even if you have to compromise the roofing filter.

Moreover, it has been standard practice for at least a couple of 
decades, to use adaptive versions of such filters mitigate group delay 
distortion in telephone modems.  In that context, they generally require 
synchonrous transmission, because the standard adaptive equalizers only 
fully cancel inter-symbol interference at the actual sampling points. 
Although synchronous operation does make the adaptive process easier, 
I'm not sure that the longer filters that can be implemented in modern 
fast DSPs, combined with the relatively fixed distortion the the receive 
filters, cannot compensate for non-synchronous signals.


-- 
David Woolley
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