> http://abelian.org/vlf/tmp/170617a.gif

> Yes that's it!

Then there's a high pass getting in the way, must be
the input circuit of the Octo.   You'll have to
use a shorter PPS.   2mS to 5mS ought to do it.

> Might be easier to user a stand-alone GPS

Yes, Linux can use any GPS for timing, the daemon
gpsd can communicate with just about every GPS using
either NMEA or native protocols.  If you get a GPS
with PPS and USB serial, just plug the USB into the
RPi and run gpsd and ntpd. gpsd will find the GPS and
detect the protocol and get the time from it.   ntpd
reads the time from gpsd and syncs the system clock.
vtcard uses that time to roughly timestamp the sound
card data.  All you need are a few magic words in
/etc/ntp.conf to set it up:

 server 127.127.28.0 minpoll 4 maxpoll 4
 fudge 127.127.28.0 time1 0.051 refid GPS

and you can play with the time1 parameter to tweak
away the comms latency on the GPS serial data.

The nice thing about ublox GPS is that their comms
protocol is very well documented and you can adjust
things in great detail, firing commands at it via
the program gpsctl.

--
Paul Nicholson
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