[linux-dvb] Lost lock problems over extended periods of time

Manu Abraham manu at kromtek.com
Thu Jun 9 21:07:37 CEST 2005


marc abramson wrote:
> Hello Manu and all other
> 
> a quick explanation why switching from a channel to an other chanel  or having a
> signal loss and a signal back on the same chanel is NOT the same
> thing (even if some STB are sending again the CA_PMT when the signal
> come back).
> 
> in a module, when a CA_PMT is received, if the module is smart and
> wellwritten, the module will start by checking if the PMT is the same
> than the previous one or not. If it is the same, it will NOT do
> anything.
> The reason why a good module will do that, instead of handling each
> new CA PMT as if it the first one, is that, in this case, each time a
> CA_PMT will be received, all demux filter (corresponding to the old
> ca_pmt) will be closed, and all descrambler for the "old" elementary
> stream will also be closed. Then, when analysing the "new" ca pmt,
> everything will be reopened.. but , between the two, during a few
> hundred of ms, there will be a black screen, while the descrambler
> will be closed.
> 
> Let's now imagine that the module itself is not checking the PMT
> change, but sending the CA_PMT to the CA_lib (to sum up, a module is
> divided in drivers, in charge of communication with the host, of the
> demux , the descrambler and the smart card part, and in a CA_lib, witch is receiving
> the CA_pmt - through the driver - and which is using the demux, the
> descrambler and  the sc  driver to do the descrambling).
> Then, at the ca_lib stage, the CA lib will also check if everything
> has changed, or if the ES pid and the ECM PID are the same, and, once
> again, if nothing has changed, not reprogram anything.
> 
> It means that, if the driver are blocked by a signal loss (and this is
> often the case with some modules), resending the same CA_PMT when the
> signal come back will NOT correct anything.
> 

Probably that explains why sometimes i had to do a retune before sending 
in the CA_PMT again, recovering after a signal loss. I will try out a 
NULL CA_PMT first before trying again.. in that case..


> during a channel switch, the module will send the new CA_PMT to the
> ca_lib, and the ca_lib will see that the ES are no more the same (then
> , it will close the descrambler for the old ES pid), and that the ECM
> PID have also changed (then, the CA_lib will close the "old" ecm pid
> filters). Then it will analyse the new ca_pmt, and reopen some demux
> filter for the new ecm pid, and some descrambler channel for the new
> ES.
> 
> the only way for a host to restart correctly a module after a signal
> lost will be to send 2 CA_PMT, a first one empty (that will close all
> the "old" things) and then the CA_PMT. Doing that, the CA_PMT will be
> handled as if it would have been the first one.
> 
> this explain why, from time to time, after a signal loss, the
> descrambling is NOT restarting, when there is never a problem when
> switching from a channel to another ?
> 

Yes, clear enough to get things moving i believe.. :-) That was a lot 
informative..

> please don't hesitate to ask me any question when my explanations are
> not clear enough
>

Sure .. Thanks a lot for your input, for clearing that up. I will work 
out next Thursday or so and will post back, how it went..

Thanks again,
Manu





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