[mythtv-users] XvMC, Plextor Question, and Sound

Nathan Lutchansky lutchann-mythtvusers at litech.org
Fri Feb 10 23:11:39 UTC 2006


On Fri, Feb 10, 2006 at 03:24:30PM -0700, Brian Wood wrote:
> On Feb 10, 2006, at 2:12 PM, Jesse Guardiani wrote:
> > Brian Wood wrote:
> > In addition to XvMC support, recording to Mpeg-2
> > would allow
> > for DVD burning via MythBurn.
> 
> Well the world is going the other way, towards mpeg4. I heard that  
> DirectV's newest satellite is putting out mpeg4 streams, with  
> Echostar to follow soon.

Actually, the world is going to H.264, which isn't supported in any
video card yet as far as I know.  MPEG4 isn't used for much commercial
content at all aside from what the DivX folks are able to get their
hands on.

> Well you could always transcode mpeg4 to mpeg2 and then burn, would  
> take a lot of CPU cycles though. In fact you'd have to transcode  
> anyway, as the Plextor only supports 640x480, unless you recorded at  
> 320x240.

The Plextor supports any resolution up to D1; does MythTV not offer the
option of 720x480 (720x576 for PAL)?

> > Maybe I'm being naive, but I don't think it will be all that difficult
> > to add support for mpeg-2.

The IVTV hardware outputs an MPEG2 program stream, which MythTV writes
to disk directly.  The Plextor only outputs an elementary stream, so
you'd probably have to put a program stream mux into Myth to get it to
work.  I'm not sure though; maybe MythBurn can remux for you.

> >> The Plextor has been working surprisingly well, but if anyone is
> >> considering one, be aware that it does not embed the sound in the
> >> MPEG stream, the way a PVR does. Rather it sends uncompressed sound
> >> to ALSA as PCM, which requires a sound card input.
> >
> > MythTV then compresses this audio to mp3 using your system's CPU (by
> > default - you can
> > turn it off in the recording profiles), so the plextor will actually
> > cause mythbackend to burn a
> > good bit more CPU than a true PVR card.
> 
> That's my point, and I wonder why Plextor went the way they did.  
> AFAIK there are no MPEG2 encoder chips that won't do audio, so  
> there's no money to be saved there.

Probably 90% of these chips go into video surveillance systems, so
adding half again more to the unit cost for an on-board audio encoder
wouldn't have made any sense for that market.  As for why Plextor chose
that chip, well, there weren't a whole lot of MPEG4 hardware encoders on
the market four years ago.  -Nathan
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