[mythtv-users] Question about Air2PC HDTV capture

Cymen Vig cymenvig at gmail.com
Wed Aug 29 05:27:51 UTC 2007

On 8/28/07, Matthew Schroeder <blaupolisboxen at yahoo.com> wrote:
> I was wondering if it was possible for the linux drivers to capture/record
> the HDTV content for transcoding purposes, even if the capturing machine
> would be incapable of actually playing these HDTV files on its own?  I
> would think with a frontend/backend system, for strict ATSC capturing the
> backend shouldn't need much horsepower to actually save the stream, since
> it will not actually view the content.  Feel free to correct me on this.
> I'm looking for something that looks reasonable on a regular 27" analogue
> CRT television (S-Video) after some modest transcoding to smaller
> resolution H.264, Xvid, or MPEG2 for later viewing.  My local PBS station
> has both HDTV and SD subchannels broadcasting, but they do not always
> simulcast the same content.  Sometimes I want to pull a program from the
> HDTV stream, other times the SD.

Here is my setup:

27" TV (SD)

Celeron D 340 (2.93 Ghz)
Nvidia 6200 LE
2 x Air2PC PCI cards (ATSC)

The TV is connected to the computer by S-Video. The Nvidia card has a
virtual screen size of 1024x768 so when I play back HD content it is
automatically resized to that and displayed without issue (expected
black bars at top and bottom of picture).

(II) NVIDIA(0): Virtual screen size determined to be 1024 x 768

Some SD channels display full screen. Some display in a box (black
bars all around) and I have to toggle to "fill" mode in MythTV to get
full screen.

I am very happy with this setup. The transmitters are about 35 miles
away and this setup gives me a superb picture. I can see the
difference between HD and SD content but it isn' the "night and day"
difference that those with HD displays talk about (one day, one day).
Occasionaly when there is a strong thunderstorm I get drop outs in the
recordings. This is like static on the analog feeds but much worse in
terms of disrupting the viewing experience. This is likely caused by
the distance to the transmitters and the tall trees around my antenna.
This is a rare problem though in my area.

I did have to do a far amount of tweaking to get a backend and a
frontend in one box that can do live HD tv on a Celeron without any
issues. I ran 2.6.21-ck1 and use schedtool and ionice to lower the CPU
and IO priority of non-playback related processes (ie Apache, MySQL,
Samba, etc). With these tweaks I can browse MythWeb running on the
same machine while HD content is playing without any problems.

With a separate frontend and backend you shouldn't have to do much
tweaking. I know 2.2 Ghz is a bit border line for HD playback and you
might have to use XvMC for it to work (especially when using a PCI
card with the lower bus bandwidth and if that bandwidth is also shared
by NIC and IDE components).

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