[Discuss-gnuradio] multiple sources synchronization ?

[jean-michel . friedt]

Possibly a stupid question, but might help me better understand how the 
gnuradio scheduler works:

my objective is to make a low cost phase-referenced radiofrequency 
interferometer using two DVB-T dongles. 
Since I have observed that the PLL inside each dongle induces slow phase drift, 
I want to use an external
RF switch to monitor a known (50 MHz) reference oscillator feeding both 
dongles, and then monitor the
unknown signal. Current switching rate is about 50 Hz triggered by an external 
generator (which also
synchronizes other events of the experiment, not relevant to this post). My 
idea for synchronizing the
post-processing of phase extraction was to record on the one hand the two DVB-T 
dongle data flow
(this I know works), and on the other hand the sound card microphone connected 
to the switch trigger
signal. This process is summarized in the grc flowchart at 
http://jmfriedt.sequanux.org/damien_grc.png

However, the sound card output shows a result completely out of sync with the 
phase measurements. I 
understand that the sound card and DVB-T dongles do not share the same clock 
sources, but considering 
the huge decimation factor (48*32 kHz for the DVB-T, 48 kHz for the sound card, 
and a phase output recorded
at about 0.5 to 5 kHz, not shown on this grc chart), I would have expected the 
trigger signal to be more or 
less synchronized with the DVB-T outputs, which is not at all the case.

Is the gnuradio scheduler unable to interleave two data sources as different as 
a sound card and the
USB data flow from the two DVB-T dongles ? Is there a way I might tune my 
flowchart to achieve
the expected result ?

Thanks, JM

-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe, 25000 
Besancon, France