[linux-dvb] Decoding of data carrousels used for interactive services

["Karim Amrani" <kamrani@jdcv.com>]

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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