[RTTY] Skimmer

[k3mm at verizon.net]

I'm surprised one hasnt been written yet for the digital modes, but I guess there isnt much monetary incentive to do so since you wont sell many copies.
 
FYI, the $500 RFSpace SDR-IQ works fine for 200khz of any single band at a time and the $995 QS1R will decode up to 7x100khz band segments with plenty of DSP to spare.   Both connect on a computer USB port and a modest dual-core computer provides enough power to run it.  There are other less costly solutions that use your sound card to decode up to 96khz of audio bandwidth.
 
Check out reversebeacon.net on the web or Telnet Port 7300 and see it in action.  Most of these guys are using very simple broadband vertical antennas.  I got a QS1R funded by our local packetcluster group and it's tapped into my tribanders and some short beverages when I'm not on the air. It's usually on the reversebeacon.net site or you can try it direct at k3mm.dyndns.org port 7300 . I'm usually decoding 160 thru 20, 15, and 10 meters with good coverage to EU, west, and NNW or SSE directions from MD.

It's fun to get on reversebeacon.net, call CQ, and see who spots you and where.  The spots come with an SNR in db and WPM comment so you can use it for antenna comparisons and other cool stuff like that.

73, Ty K3MM

Jun 15, 2010 01:51:08 PM, chen at mac.com wrote:

>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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