Cecil Moore wrote in message ...
Reg Edwards wrote:
How much, if any, is digital filtering better than the old-fashioned Xtl
filter.

What makes DSP filtering better than Xtal filters is that it is
programmable, not hardwired. The parameters of a DSP filter are
continuously variable although not usually implemented that way
in most amateur radio receivers. Given the processing power
necessary for a filter, one can theoretically have any of a
near-infinite number of filter configurations.

Reg, you could probably program a DSP audio filter that runs on
a PC and outputs audio through the sound card like the ClearSpeech
speaker systems. PSK-31 software packages for the PC certainly contain
a number of DSP filters and other DSP algorithms.

I' ve got 2-3 of those programs. Probably more than that out
there...SBFFT is a DOS DSP program that works real well for a CW or
bandpass/bandstop filter. You can notch, etc , up to six signals at
once, within the stock passband width you select. It's real good for
notching heterodynes, etc...You can cut just a tiny sliver, taking out
the signal, but leave all the rest. I use that for MW sometimes. I
have another DSP program for windows that the guy that wrote MMSSTV
did...It has NR modes, which SBFFT doesn't. Chromasound is pretty
good, but payware...If I use a radio which has DSP in it, such as my
mk2g, I have dual DSP if I want it. Even dual auto-notch, etc. But the
software on the puter is much more powerful overall than the simple
DSP in the 706. All the 706 has is auto notch and NR. Neither of which
I use much at all. Only bandpass filtering is really useful to me, and
most radios don't have it. BTW, all those programs have the various
spectrum graphs, waterfalls, etc... You can see what to notch, and
what freq , etc..Kinda like PSK...
One thing though...DSP is nice and all, and the filters are brick wall
steep, but you can still have blowby problems with near strong
signals, etc...DSP is not near as "bulletproof" as a real hardware
filter. I use both. Get the best of both worlds...As an example...My
830 is pretty good as it is. Variable bandwidth, CW filters, etc...Add
the puter DSP, and it's killer. But in reality, the 830 is good enough
as it is...I have DSP but I don't use it all that much. It's not that
practical to use. For example, my 706 doesn't have a CW filter yet,
and the DSP would be real handy. But the sidetone delay on *sending*
makes it nearly useless. Doesn't do any good to have a 200 cycle
filter if I get tripped up sending due to the delay...:/
I'd have to rig up a separate real time sidetone. And that then means
I would prefer to mute the puter audio when sending...All that mess is
too much hassle...I just get on my old 830 with it's rock solid old
fashioned filters, and be done with it...And the receiver is usually
cleaner to boot, as no artifacts or blowby..MK