[mythtv-users] Virtual Tuners / Channels ?

mythtv-users at spam.dragonhold.org mythtv-users at spam.dragonhold.org
Wed Apr 19 13:25:26 UTC 2006


On Wed, Apr 19, 2006 at 02:07:06PM +0100, Derek Conniffe wrote:
> I was hoping that the "Set Top boxes" could be MythTV boxes with both a
> frontend & a back-end together and then in the room where all the TV
> cabling comes in I'd just have one or more PCs with a tuner card for
> each channel (actually I was hoping I could be smarter with a multiple
> channels from a DVB-S / DVB-C card with many channels from the one tuned
> transponder so even less than one tuner per channel).
AFAIK myth doesn't support stripping more than one channel per transponder so you could not 
drop below 1 tuner per channel.  I think it was discussed not too long ago as a feature that 
VDR(?) has, and would be nice - but I don't think it's there at the moment.

There's no point in putting anything more than a minimal front end on each box.  Since 
everyone is just going to be watching a recording (that's the way live TV works in 0.19), 
the front ends just need to have access to the program listing. :)

> This way I would only have 1 tuner per channel streaming out to all
> MythTV front/back ends in each room with each of them doing their thing.
The option of one tuner per channel rather than one per room is certainly there - the issue 
then becomes which is the higher number...  I know that when I stay in a hotel in the UK I 
only get about 8 channels - but on other places people are probably used to getting a full 
cable feed with 100+ channels (USA?).

> It would be great to be able to offer the live TV pausing features, etc
That's where the throughput of the backends starts to be an issue.  How many rooms are you 
looking at, and how many channels?

There's no reason (that I can think of) that theoretically if the backends can stream enough
data, you couldn't do this - it's just that the numbers start to add up mighty fast.

If you set the backends up to record everything, and (for example) store the last 24 hours
(well, use auto expiry and 24 hours of the maximum bandwidth) - then there's no reason why
the front ends shouldn't be able to play it.

To put some numbers to this - imagine that 20 rooms contain football supporters that watch
the match in the bar (one frontend), and then go back to their rooms.  Because they've each
drunk a different amount of beer (and have a different distance to go to get to their rooms)
they each start watching the match again (it was a GREAT match) at different times.  Your
backend now needs to stream 20 copies of the match instead of one - and those are being
watched from slightly different offsets.

Assuming a stream of 4MBit/s (I think that's a reasonable starting point), you're looking at 
needing to maintain 80MBit/s from the backend... And that sounds doable.

But when you then think about the fact that this is going to be coming from a single file, 
the disk/disks behind it are going to be /hammered/ - it's the seek time that's going to be 
the killer....   It's also likely to kill the disks if it happens much :)

It must be doable - since homechoice (I think that's what it's called) have to do something 
similar for their TV on demand service, and movies on demand via satellite/cable have to do 
the same things...


Graham


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