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

[Devin Heitmueller]

On Fri, Apr 9, 2010 at 12:28 PM, Devin Heitmueller
<dheitmueller at kernellabs.com> wrote:
> 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..

After re-reading your original message, a couple of other points
probably worth clarifying:

Suggesting that you can beat the hardware encoder's quality by basing
your assumption on the quality of SD ClearQAM is a bit misguided.  In
many cases, the SD ClearQAM feed is the result of downscaling an HD
digital feed.  As a result, the quality will be *much* better than
anything you could possible achieve by attempting to compress the feed
off of the PVR-150, regardless of whether you use the onboard hardware
MPEG encoder or some software solution.

Also, in cases where the original source material is analog, the
analog feed the cable company is converting to MPEG is studio quality
and they use an expensive high quality analog video decoder.  Note,
the term "video decoder" is not synonymous with "MPEG encoder" - a
video decoder is what converts the analog signal into an uncompressed
digital representation .

You will never be able to reproduce that level of quality based on the
broadcast feed and by using a consumer grade video decoder.  There is
just too much signal loss and your capture hardware is just not going
to be anywhere near the quality.

Devin

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