[linux-dvb] Descrambling suddenly stops

Andreas U. Trottmann andreas.trottmann at werft22.com
Thu Nov 24 20:32:08 CET 2005


On Tue, Nov 22, 2005 at 06:03:17PM +0100, Sigmund Augdal Helberg wrote:

> > 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.

I've tried to do this, too - indeed, "sometimes" sending CA_PMT
does "not work" as well, and it has to wait for the next message.

But, it appears that using the non-CVS drivers (plain Linux
2.6.14.2) gives significantly better results. Of course it is very
difficult to "prove" that a problem that "sometimes" happens just
went away, but I now have watched about 60 "channel-hours" with
2.6.14.2 drivers without any problems (i.e. without the
descrambling suddenly stopping), while just loading CVS driver
modules makes the problem appear within minutes.

Unfortunately, I am not able to pinpoint any specific diff right
now - I'm not even able to see which module might be the culprit.
Time permitting, I'll try to narrow the field further, and will
inform the mailing list of my findings. But for now, indeed I'm
an extremely happy user.


FWIW, I'm using Technisat SkyStar 1 CI cards (Twinhan clones,
dvb-bt8xx module), Viacrypt official "red" CAMs, Swiss National
TV keycards, dvbstream and the latest CVS ca_zap.

-- 
Andreas Trottmann
Werft22 AG
Tel    +41 (0)56 210 91 37
Fax    +41 (0)56 210 91 34
Mobile +41 (0)79 229 88 55



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