[mythtv-users] Can Mythtv record uncompressed video from PVR-150? (my Comcast solution)

[Devin Heitmueller]

On Fri, Apr 9, 2010 at 12:13 PM, Bill Bogstad <bogstad at pobox.com> wrote:
> Since my original message was unclear, let me clarify:
>
> 1. I don't want to store the uncompressed digitized audio/video
> streams.  I want to do software compression on it with a different
> codec/parameters then the on-board chip allows on my PVR-150s.  This
> is with the goal to get 'better' compression then the onboard chips.
>
> 2. According to the documentation I have already found for the IVTV
> driver, you can in fact get the raw digitized streams for both video
> and audio.   You can go check the MythTV page for the PVR-500 to which
> I provided a link for a hint of this functionality.  I am having
> trouble accessing the IVTV web site at the moment so I can't provide a
> link to a page that I found which documents this functionality.  Or at
> least that is what the pages I read say, I won't know for sure until I
> actually try it.
>
> I've also found references to doing this with Windows software as
> well.   In that case, someone was trying to play a TV based video game
> via a PVR-150 input.   The problem was that due to compression there
> was a noticeable 'lag' between when they would change a control input
> and it would appear on their computer screen.   They apparently found
> software to do this.
>
> If #2 above is correct, then I believe it should be possible to treat
> a PVR-150 as an analog card that requires a software encoder much like
> the bttv based cards operate.   I'll be using an external channel
> changer script to work with my Comcast DTAs anyway so I'll just be
> asking Mythtv to
> accept the raw PCM audio and YUV? video streams that the IVTV driver
> apparently makes available.
>
> I was hoping that someone had tried this out already and could tell me
> if it was worth investigating further.  It looks like I am a pioneer
> on this.  I'll let people here know when I get a chance to try it out
> for myself.
>
> Bill Bogstad

The IVTV driver does have the ability to provide uncompressed video,
but it does so in a way that uses obscure functionality in the V4L2
spec, and as a result applications such as MythTV cannot treat the
PVR-150 as a frame grabber.  In fact, this is why the PVR-150 doesn't
work with applications such as tvtime either.

In theory, in order to make this work the ivtv driver would have to be
converted over to the videobuf framework so that it can provide the
raw video via the MMAP interface.  At that point the 150 would behave
like any other framegrabber.  Also, the PCM device for audio would
have to be converted over to use ALSA, just as we have done for the
cx88, cx18, em28xx, and other drivers.

Be warned though - even though the mythtv backend is capturing raw
video from the framegrabber, it then compresses it and stores it on
disk (and the frontend then uncompresses it to be rendered).  Hence,
even if you bypassed the hardware encoder, there would still be
terrible latency - far too much to use the card under MythTV for
realtime applications such as video games.  If the goal is to do
things like play video games, you would need an application tailored
for low latency (such as tvtime).  Low latency was simply never a
design goal for MythTV and hence has had little to no optimization in
that area..

Devin

-- 
Devin J. Heitmueller - Kernel Labs
http://www.kernellabs.com