[time-nuts] Sound Cards for locking to GPSDO 10 MHz references
rmoncur at bigpond.net.au
Mon Jun 1 22:49:26 UTC 2009
Thanks for your advice which I will follow up - the reason for going for a
USB sound card is that the system must be operated portable with a Laptop -
but perhaps there is a way to use a PCI sound card on a Laptop.
While we use WSJT at present we have a new Mode under development for the
mill-Hz bandwidth. In testing this new mode is acheiveing around 15 dB
better than WSJT with 5 mHz binwidths and should get to 20 dB better with 1
mHz binwidths. It uses M-ary FSK like WSJT but does not need a reference
tone for time or frequency locking on the basis that both soundcards are GPS
locked. Timing errors are not an issue as the tone durations are 16 mins at
1 mHz binwidth. We use around 20,000 separate M-ary tones (cf 64 for WSJT),
which is sufficient to send the first three characters of a call sign in
Clark-Karn source encoded format - thus it requires only two tones to be
sent to receve a full callsign. However at one mHz bandwidth this takes 16
minutes to send a single tone and thus an hour to send two callsigns.
However, we have some shorter techniques for exchanging reports and RRR so a
QSO can be comppleted in around 3 hours, hi. We can fit 20,000 tones spaced
1 mHz apart into just 20 Hz so there is not problem there. We have not yet
added FEC which should allow a further improvement but we would like to
resolve the sound card stablity issues first.
73 Rex VK7MO
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