On Jun 15, 2010, at 6/15 9:15 AM, Bill, W6WRT wrote:
> Actually, for some CW contests that has been done. The famous
> "Skimmer" has been outlawed in some categories.
I suspect that one will eventually be publicly available for RTTY ops
to use. Or perhaps not, after the CW Skimmer experience with contest
sponsors.
Anyone who writes a skimmer for RTTY will have to think twice about
making it public and have it banned from even their own personal use.
Notice that one form or other of a "skimmer" is already available in
pretty much all public PSK31 software. I haven't heard of such tools
being banned in the PSK31 world, but I don't pay close attention to
contesting to know for sure.
RTTY skimmers are just a tad tougher to implement than the other
skimmers, since unlike CW and PSK31, RTTY is not a single carrier
system, and identifying whose mark and space some signal on the
spectrum belongs to in a crowded band requires more temporal and
spectral information to sort out.
The only real impediment to a good general RTTY skimmer is the lack of
bandwidth in most transcievers. You can't fit enough RTTY signals
into a single 2.4 kHz bandpass to make it worthwhile. In an RTTY
contest, you need to use an "skimmer" that can watch 50 kHz at a
time. With PSK31, a 2 kHz passband sufficient for daily QSOs; an 8
kHz swath is sufficient for just about any PSK31 pileup (I have seen
wider PSK31 piles than 8 kHz, like the Swains Island case on 30m, but
they are rare).
In addition to wide passbands, needless to say you need a receiving
chain with good dynamic range. But this is true whether it is a CW
skimmer, a PSK31 "skimmer" or an RTTY "skimmer."
The combination of a K3, followed by an LP-PAN, followed by an E-MU
0404 is good and wide and has decent dynamic range. A Flex-5000 is
actually a bit better (123 dB of blocking dynamic range instead of
about 113 dB from the K3/LP-PAN/E-MU combination). Short of those two
rigs, you would need a Perseus or some other SDR like a SoftRock to
get enough bandwidth.
If you have a loud contester living nearby, even 123 dB of blocking
dynamic range might not be sufficient -- but that is true with the CW
Skimmer, too.
73
Chen, W7AY
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