[linux-dvb] Descrambling suddenly stops

Sigmund Augdal Helberg sigmund at snap.tv
Tue Nov 22 18:03:17 CET 2005


On Tue, 2005-10-25 at 21:24 +0400, Manu Abraham wrote:
> Andreas U. Trottmann wrote:
> 
> >On Tue, Oct 25, 2005 at 03:00:21PM +0400, Manu Abraham wrote:
> >
> >  
> >
> >>>Basically, after a certain time (which can range from two minutes
> >>>to several hours), descrambling suddenly stops, and the stream I
> >>>get from /dev/dvb/adapter0/dvr0 contains no sensible audio/video
> >>>data anymore.
> >>>
> >>>Running ca_zap again (with the same parameters as the initial
> >>>run) makes the stream work again.
> >>>      
> >>>
> >>Your descrambling will stop when the CA_PMT changes. We will need to 
> >>create a thread in ca_zap, which keeps sending CA_PMT messages to the CA 
> >>module, maybe something in the order of say 5 minutes.
> >>    
> >>
> >
> >Thank you very much for this answer, which certainly is correct.
> >
> >However, I've now had it happen twice in a relatively short row,
> >and both times both the "PMT Data" as well as the "CA MESSAGE" as
> >output by ca_zap have been identical "before" and "after" the
> >stopping. So maybe there's something else that makes the
> >descrambling stop.
> >  
> >
> 
> What makes the descrambling stop is when your Session Key/Control Word 
> changes. This control word is present in the ECM, and is encrypted and 
> the service keys present in the EMM. Now when these keys do change, you 
> will not see a difference in the CA_PMT message.
> 
> This change need not always be accompanied by a change in the CA_PMT 
> message, as the message itself is not composed of EMM's and ECM's.
> 
> So the solution is to send the message continuously, in a timely manner. 
> say for example once in 5 minutes, to the module.
I've tried various approaces to this:
1: send a capmt update each time the pmt changes. This is not nearly
often enough.
2: send a capmt update each time a pmt is received. Same as 1
3: send a capmt update every 5 minutes as suggested. This seem to give
breaks of up to 5 minutes where the stream is not decrypted
4: send a capmt  very often (every 10 second). Even this does not give
good results. It seem that when the decryption stops it can take up to
20 sec before it start again.

Any further suggestions is appreciated.

Sigmund A Helberg
> 
> 
> Manu
> 
> 
> 
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