[mythtv-users] External Channel Change Script for DVB cards

Raymond Wagner raymond at wagnerrp.com
Sun Oct 30 17:24:43 UTC 2011

On 10/30/2011 08:29, Simon Hobson wrote:
> Raymond Wagner wrote:
>>   >  But if I spit the input to the cable box then let the cable box and the
>>>   QAM card tune the same channel.  I am only using the cable box to
>>>   request the cable co to send that channel to my house.  Once it is being
>>   >  broadcast I can receive it normally on the QAM card.
>> You would have absolutely no idea where it is sending that channel to,
>> so you would have no way to tune it with the QAM tuner.
> Bear in mind I have no experience whatsoever with cable ...
> Do they encode each channel/mux specially for each subscriber, or do
> they just turn on/off passing of a specific mux to that outlet on the
> distribution unit ?

Digital cable boxes in the US have two independent tuners.  One is the 
standard QAM tuner, tied to a cablecard for decryption, that captures 
video.  The other (Out-Of-Band tuner) is dedicated for communication 
with the headend, and is used to update things like subscription 
information, decryption keys, channel listings, and switched video.

Most cable systems simply transmit all available channels, all the time, 
to the entire broadcast domain, and then segregate their domains by how 
much channels they have in reserve for internet bandwidth.  The more 
domains you have, the more duplicate hardware you need, and the more 
expensive it is to operate.  On the other hand, switched video only 
broadcasts those channels that customers actually want to use at any 
given time, freeing up additional channels for internet, and allowing 
the use of larger domains.  The cable box informs the headend that it 
wants to access a particular channel, and the headend responds with what 
multiplex and stream id to use.

The cablecard spec was not designed with such use in mind, so most 
cablecard tuners like the HDHomeRun, Tivo, and a handful of TVs instead 
have to make use of a tuning adapter.  The cablecard tuner tells the 
tuning adapter to negotiate the switched stream with the headend, and 
then tunes the new channel itself.  Now whether the cablecard tuner 
discovers the new channel directly from the USB link, or queries it from 
the headend using its own OOB tuner, I don't know.

More information about the mythtv-users mailing list