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[linux-dvb] Decoding of data carrousels used for interactive services
Hi everybody,
I'm trying to get information about an interactive service broadcasted on
TPS and CanalSatellite (in France). The service is broadcasted using a data
carrousel.
My goals are to detect the resets of this service (when the information
contained in the service is renewed, the broadcast is resetted (?) ) and to
survey the PIDs for technical parameters (like throughput, broadcast
incidents, carrousel period,...). I don't want to implement a MediaHighway
or OpenTV player...
I modified the DVB/apps/test/test.c program provided with the driver. I
managed to record the bit flow of the PIDs I'm interested in into files (I
won't get applause for that !).
I need now to decode further all this. I've read things about existing
structures to decode a video or audio flow but I couldn't find anything for
data carrousels. Or what I found did stick to my reality (I found the
'packet start code prefix' (0x000001) in the flow but the next byte was not
a valid StreamId !)
I've just looked a dvbtext (mentionned earlier in this mailing list) to see
how 'they' decode teletext. I've noticed the use of mapping tables and
bitewise operations, but I could not relate these operations to something
clear.
The program contains a lot of 'hard coding' (values like "0x3f7f" or "0x80"
in the lines below :
mag->sub=(unham(data[4],data[5])<<8)|(unham(data[2],data[3])&0x3f7f);
mag->flags=unham(data[2],data[3])&0x80; )
and is not fully explaining why these values and why these operations.
[I hope the developper will see no offense...;-) He probably did not mean
to explain DVB/MPEG2 to me in this file]
I would be more than happy if someone could share his knowledge about the
encoding of such data with me... Or share a few pointers on the info I need
to achieve that...
Thanks in advance,
Karim AMRANI
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