[mythtv] Re: Adding DVB-T support to MythTV

[Richard King]

On Wed, 19 Mar 2003, Edward Wildgoose wrote:

> > It is a nice feature, its things like this that make me think that doing 
> > DVB support properly is more complicated than it appears and will require 
> > changes to myth. That doesn't change the fairly easy get it working using 
> > pipes method though.
> 
> The backend already has support for multiple tuners so if the
> abstraction was at the "tuner" level then this would appear to fit with
> the current design.  So perhaps create an alternative tuner object which
> can read from the DVB card instead of from a v4l device.  If there are
> multiple streams within a DVB stream, then this is great because we
> effectively have multiple "tuners" available.
The abstraction to tuner level is difficult because DVB cards already have 
a tuner (only one) that tunes into a specific frequency. This then outputs 
an MPEG transport stream with the data for several channels in it.

a quick example for the uk:
BBC Multiplex: BBC1, BBC2, BBC3, BBC4, BBC News 24, etc
ITV Multiplex: ITV1, ITV2, ITN News, etc  

If your card is tuned into the BBC multiplex then you can watch multiple 
channels from that multiplex (pip or on different front-ends). As soon as 
one of the receivers wants to watch a channel on a different multiplex 
then all the receivers will lose their channel.

This problem is made worse in the DVB-S case (satellite). Where you can 
want to receive from multiple satellites (and sharing a single lnb for 
horizontal and vertical phases - i think - my knowledge of satellite stuff 
is very limited) and can only access one at a time.

Myth currently can't express any of these dependancies. As I said in an 
earlier post, implementing this properly requires a reworking of the way 
myth stores channel information in the database. 

> If this tuner object can read a pre-compressed stream, where available,
> then this will save a ton of resources.
The main suggestion being discussed atm reads the pre-compressed stream.  
and this is proven to demand very low resources. people can run VDR (a
program like Myth specifically for DVB) on very low end machines.

-- 
Richard King
E-mail: rak at cs.man.ac.uk