[linux-dvb] Re: Suggestion for channel tuning

[Johannes Stezenbach <js@convergence.de>]

Dave Chapman wrote:
> 
> I've written a small program that tunes a channel based on only the
> frequency, polarity, symbol rate and program number.  This works well, and
> the scanning of the PAT (to get the PID of the PMT), PMT (to get the ES PIDs)
> and SDT (to get the name of the channel and broadcaster - possibly
> unneccesary) adds between 0.1 and 0.5 seconds to the tuning time.  This
> depends on luck and how many channels are on the transponder.

In my experience PAT and PMTs are broadcast quite frequently, but the
SDT can take a second or two (Astra 19.2E; depends on the transponder).

> Has anyone else had any experience of tuning like this?  It seems to me that
> ISO-13818 is implying that the only parameters needed to tune a channel is
> the service_id.  This makes sense to me - at least on Astra 28, the channels
> change PIDs quite frequently.

Everyone I talked to wants zapping to work as fast as possible. Our
strategy is to set the PIDs immediately after tuning from stored
values, and check them afterwards. This gives you fast access plus
automatic update in case of changes. Also of importance is that some
services have multiple audio channels, which you can select only after
a PMT/SDT scan since the number of channels depends on the event
currently broadcast.


Regards,
Johannes


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