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
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
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