[mythtv-users] what width & height for V4L input using aspect 16:9

Nick knowledgejunkie at gmail.com
Wed Oct 4 07:14:30 UTC 2006


On 04/10/06, Paul <mylists at wilsononline.id.au> wrote:
> How do I figure out what width and height I use for a V4L capture device.
> I am configuring the Recording profile of  a V4L device and the setup
> screen is asking dimensions of capture.
>
> The Default is 480 X 480 but this seems to capture 4:3 not 16:9 and also
> the picture is surrounded by black or non image data?
> is this a trail and error or is there a easy way to determine this?

When using a V4L capture device with composite/S-Video inputs, it is
normal to capture everything in 4:3, regardless of the capture
resolution used. Whether you capture at 480x480 (although doesn't
Australia use PAL 576i?) or 720x576, the aspect ratio of capture will
normally always be 4:3, the difference being the "squareness" (or lack
thereof) of the pixels captured in the captured image.

The extra black bars surrounding the captured video is a result of the
capture card capturing more of the video signal than your TV displays
- you normally don't see this (and probably won't when watching
recordings/LiveTV due to the overscan on your TV) and can't really be
eliminated from your captures.

If your STB is sending out a widescreen signal (fullscreen video looks
squashed horizonally when captured at 4:3, and 4:3 content always has
black bars at the sides), you can alter the aspect ratio when watching
LiveTV/recordings within MythTV and this should fix things up. There
is also a widescreen flag that can be embedded in an MPEG stream which
should also help with other media players. There's another current
thread that mentions STB's normally signal to the TV that a video
signal is widescreen so the TV can display it properly, but this does
not apply when using capture cards and composite/S-Video connections.

In the past when using V4L framegrabbers, I have captured the
letterboxed widescreen picture (black bars top and bottom) within the
4:3 capture, and then transcoded using VirtualDub using filtering to
remove the black bars to give a true 16:9 output file - when capturing
initially at 640x480 the output file would have a resolution of
640x360, which has a 16:9 aspect with square pixels.

Nick

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