[mythtv] Relationship of XVideo to V4L

Michael J. Hammel mythtv at graphics-muse.com
Tue Jul 8 16:15:15 EDT 2003


First off - Woohoo!  This is just the kind of response I hoped to get. 
Thanks.

On Tue, 2003-07-08 at 14:24, Joseph A. Caputo wrote:
> Be prepared to get a lot of responses to this, as you seem to have some
> fundamental concepts confused.

Understood.  Still, its better to appear a fool for the moment than
clueless for a lifetime.  So I had to ask.

> Myth works as follows:  video is captured from a tuner/capture card using
> either the V4L or ivtv interface.  This video is encoded (ivtv is already
> hardware-encoded) and written to disk, either in a ringbuffer (for LiveTV)
> or to a named file.  

Ah-ha!  The magic ringbuffer.  I was wondering what that was being used
for.

> When you want to view the recorded video (immediately
> for LiveTV), the file is read and decoded.  The decoded video stream is
> displayed using X11 with Xv extensions.  Myth can also output video using
> XShm-MIT extensions if Xv is not available, but this is more taxing on the
> system as the scaling has to be done in software rather than hardware.

So far so good.  I follow all of it up to here.

> At some point, someone might get around to writing alternate video output
> support, say, for SDL or framebuffer device.  Then we might have the option
> of running Myth without any X11 at all.  Until then (unless you're
> volunteering to write driver code) you need a tuner card supported by V4L
> (check the list archives to see if anyone has the SAA7114 working) 

The People and Places page didn't list it.  I'm subscribing to the V4L
list right now and will check the archives.

> and a
> video (display) card that supports Xv extensions under Linux/XFree86 (again,
> check the mailing list for your Trident/CyberBlade i1).

This already works.  Its the backend input support that is missing, I
think - or at least I don't have it installed if it exists.

> Your MPEG decoder board is currently useless under Myth.  Support for
> hardware MPEG decoding has been a frequent question/request, but so far
> nothing has been promised.

Actually, I have that working already.  I'm able to play VOB files (and
others, including streaming) using that hardware and the RealMagic (re:
NetStream) drivers.  All I have to do is configure an external player
and I have source for one of those that came with the RealMagic stuff.
However, I'm not sure if its open source code.  It came with a version
of software I got (via the STB maker) from Sigma Designs.  Anyway,
that's a future issue since the first step was trying to get the basic
MythTV features working with this hardware.

> The VIA C3 800MHz (Samuel) CPU should be enough to do either recording or
> playback, but would probably be hard put to it to do both simultaneously.
> Even the folks with the VIA C3 1GHz (Nehemiah) have been hard-pressed; a
> bunch seem to have opted for the PVR-250, which takes the encoding load off
> of the CPU.

We might get away with both if the EM8470 can be brought into play for
decoding the recorded MPEG-2 streams.  Again, that's a future issue.  I
can't add (I don't think) the 250 since this STB has only one PCI slot
and the card would have to be teeeny to fit in there.  It would also be
cost prohibitive to add components to the box as is.  We're trying to
avoid that.

Anyway, you got me started.  I'll dig into the V4L side for a bit and
see what I can come up with.

Thanks!!
-- 
Michael J. Hammel <mythtv at graphics-muse.com>
Graphics Muse


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