[mythtv] Relationship of XVideo to V4L

[Kristof Pelckmans]

Great explanation !

It took me a much longer time to figure this all out, especially since
information on the XVideo extension seems to be scarce. (So maybe include in
the Howto as extra info ?)

However, I still have one question : I saw the following line in a XF86Config
file :

Load "v4l"

What does this do ? As far as I understand, the v4l driver is loaded as a kernel
module, so why this extra line ? In the XFree86 documentation they speak about
'server extension modules', so is this a special X-version of v4l ? I found
that commenting it out does not seem to make a difference.

Keep up that great info !

Kristof

> V4L is a kernel driver subsystem for video INPUT devices (capture & tuner
> cards).  Myth uses the V4L interface for most analog tuner cards.  The
> exception is the new PVR-250/350 tuner cards, which are supported by the
> (still alpha) ivtv driver.
>
> Xv (Xvideo) is a video OUTPUT mechanism for X11 that supports (among other
> things?) hardware scaling (so you can display, for instance, a 480x480 video
> in full screen on an 800x600 resolution display by letting your Xv-capable
> graphics card do the scaling, instead of the XFree86 software, thereby
> leaving your CPU available to do other things (like decode MPEG video).
> Think of Xv as analogous to OpenGL or SDL (though SDL does more than just
> video output)
>
> 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.  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.
>
> 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) and a
> video (display) card that supports Xv extensions under Linux/XFree86 (again,
> check the mailing list for your Trident/CyberBlade i1).
>
> 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.
>
> 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.
>
> Good luck!
>
> -JAC
>
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