[linux-dvb] Descrambling suddenly stops

Manu Abraham abraham.manu at gmail.com
Tue Oct 25 19:24:02 CEST 2005

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.


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