Hello Everyone and Merry Christmas!
Your filters are supposed to be useful for rejecting other signals
when you're using CW or SSB. But it's not clear to me that they
really do much when you're using software defined radio (which of
course this is). SDR can provide software filters that are as close
to perfect as possible. However, the crystal or mechanical filters in
your radio add ringing and non-linearity to the signal, and thus may
actually make things worse when the result is fed to an SDR.
At least for the FT950, the 3khz roofing filters use block out adjacent
strong signals are understood to be pretty useless:
http://ac0c.com/main/page_ft2k_roofing_filters_project_overview.html
That's why I had made the previous comment about maybe considering a
different frequency. Maybe it just a matter of my radio not being up to
handling this duty. Dunno yet but I do know in that specific situation,
the FreeeDV transmission was impacting my Digital SSTV decode on
14.233. I would personally think that an FT950 is maybe a middle of the
road radio and others will probably have similar problems that I did.
Dunno.
So, before we decide to center anywhere, we should determine 1) do
filters consistently help or should they just be defeated and 2) are
filters generally centered at 1 kHz or is that just true for a few radios?
Next time I run into this situation, I'll narrow down the filters as
best as I can to minimize any adjacent interference and see if it works
well enough. Btw, the "sweet spot" of radios can be different on the
make and model so I would encourage the FreeDV group to consider
supporting different center points than just a fixed 1500hz. Here is a
decent post about this:
http://lists.contesting.com/archives//html/RTTY/2011-04/msg00108.html
Btw, per a previous recommendation from David Rowe, I installed FreeDV
for Windows and was able to decode maybe 20% of the signal I captured!
Very slick but I guess signal I captured through my dipole wasn't strong
enough to get a better decode (it was a bit overdriven too). Adam also
posted to the list on using the "ecasound" program which might be able
to post-process the raw files and change the center point of the WAV
files I captured. I looked at doing this with sox and it's "pitch"
feature but that command does everything in ‘cents’ and 'semitones' and
I have NO clue of what those are!
One thing worth noting fro David Witten's fantastic FreeDV UI: I could
never get the Windows program to play back my "raw" audio files until
there where two sound cards installed *and* configured. Without the two
soundcards present, either the FreeDV program complained about an
invalid configuration or it crashed. It would be been nice if FreeDV
for Windows could support just one sound card for test scenarios like
this. It would also be slick if it could handle WAV files.. I can
convert them with Sox but I don't know if the average Windows user would
know how. Maybe Audcity can do it? Dunno.
Anyway.. kudos to everyone on getting the project so far! It's amazing
to see how far it's come and I look forward to playing with it on Linux
when it's ready!
73s
--David
KI6ZHD
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