On Fri, 29 Oct 2004, James Bigler wrote:
> > That should have been 352x576 for 625line systems. the 352xN is
> > a valid DVD frame size that has worked well for me.
>
> Tell me more about this, please. You can encode an MPEG-2 stream with
> half the x resolution? I imagine you have to change the aspect ratio of
> the file as well, right?
No, you don't change the aspect ratio of the file. What changes is
the SAR (Sample Aspect Ratio) - basically each pixel is considered to
be wider.
Think of it this way:
It takes 704 10:11 pixels to display an image. So if you scale to
1/2 the size but want to cover the same area the pixels just need
to be considered as having a 20:11 aspect. The SAR is different for
625 line systems but the arithmetic is similar.
'y4mscaler' even has a "preset" for this builtin. "-O preset=CVD"
since the CVD (China Video Disk) uses the 352x480 frame size (vs
480x480 for SVCD). Even better 'y4mscaler' will "do the right thing"
when presented with DV (720x480) framesize by first cropping 8 pixels
from each side and _then_ performing the 2->1 downscale (other scalers
do 720->352 which is WRONG).
Yes, you lose horizontal resolution BUT for typical broadcast TV
that's been recorded onto VHS you don't have more than a couple hundred
lines of resolution anyhow - so 352xN works out quite well and the
encoder has LOTS of bits to use. I've gotten 3+ hours of reasonable
looking video on a single DVD using the 1/2 D1 size.
Cheers,
Steven Schultz
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