[mythtv-users] pcHDTV with PVR-350 for output

Daniel Thor Kristjansson danielk at mrl.nyu.edu
Sat Oct 23 22:12:06 UTC 2004

]Hmm.  From reading the hardware requirements documentation I had gotten the
](mis?)impression that even machines slower than "state of the art" could
]handle a dumb capture card and software display decoding at the same time.
]That's the same number of MPEG encode/decode steps.  I don't have a good feel
]for the cost of resampling, but I would think that going from 480p to 480i
](which should take care of any SD stream?) would not require much computation
]since it's only re-arranging scan lines.  Am I missing something?

Resampling is not very expensive. But decoding HDTV in software requires 
about a P4 3Ghz and encoding a 640x480 MPEG2 stream requires about the 
same amount of hardware. So you are looking at a P4 6Ghz just to avoid 
using a $35 video card with TV-out. MPEG decoding is relatively simple 
and quick (although we do struggle with it at 1080i), but encoding in 
real-time is still generally done with specialized hardware.

]|There's also the small matter that what Myth stores for HDTV is an 
]|MPEG-TS (transport stream) file, not an MPEG-PS (program stream) as 
]|expected by the PVR-350.
]I'm not clear on how the on-disk format enters into it.  Is there some
]architectural reason that there can't be any post-processing after reading
]the data?  In any case, storing the whole transport stream might be something
]to rethink eventually, especially if multicasting pay services take off.  No
]sense saving stuff you can't even watch...

It wouldn't be very difficult to save a PES stream instead of a TS 
stream. But then we couldn't implement some of the nice features coming 
to ATSC soon. And because the PVR-350 hardware isn't capable of decoding 
HDTV sized frames there is little incentive to save PES streams. We 
already save only the pid's we need so the space savings would only be 
about 2%.

-- Daniel

More information about the mythtv-users mailing list