[FFmpeg-user] sntsc

[Devin Heitmueller]

On Fri, Dec 22, 2023 at 3:57 PM Mark Filipak <markfilipak.imdb at gmail.com> wrote:
>
> Hi Devin,
>
> On 12/22/23 15:14, Devin Heitmueller wrote:
> > On Fri, Dec 22, 2023 at 2:38 PM Mark Filipak <markfilipak.imdb at gmail.com> wrote:
> >>
> >> This page: https://ffmpeg.org/ffmpeg-utils.html
> >> 'says' this:
> >>
> >> "The following abbreviations are recognized:
> >> "‘sntsc’
> >> "    640x480"
> >>
> >> What is it? I'm not aware that any such thing exists.
> >
> > It's NTSC with square pixels...
>
> How do you go from 720x480 to 640x480 without throwing away 80 pixels per line?
> Normally, 720x480 decodes to 720x540.

I would encourage you to do some reading up on "pixel aspect ratio"
and "non-square pixels"

Briefly, the number of lines is fixed, since they correspond to the
scanlines put out to the CRT raster.  However the width of the video
is determined on how often the analog waveform was sampled.  The
display aspect ratio is 4:3, and if you have square pixels that would
mean you have a resolution of 640x480.  However it was common to
capture 720 samples per line to improve image quality, and as a result
the actual pixels are rectangular (i.e. slightly taller than they are
wide).

If you don't treat the pixels as non-square, you end up with a display
aspect ratio which is greater than 4:3, and the image looks
horizontally stretched.

>
> > (as opposed to the more common pixel
> > aspect ratio of 10:11 found with 720x480).
> >
> > Devin
>
> Umm... 10:11? What is that?

See above.  The pixel isn't square.  It's actually a rectangle which
is 10% taller than it is wide.

Even worse, you can have 720x480 video with a display aspect ratio of
16:9 (i.e. standard definition widescreen).  In this case the pixel
aspect ratio is going to be 40:33.

Yeah, this is all a huge PITA.  I'm very happy that with HD
resolutions they just decided to make all pixels square.

Devin

-- 
Devin Heitmueller, Senior Software Engineer
LTN Global Communications
o: +1 (301) 363-1001
w: https://ltnglobal.com  e: devin.heitmueller at ltnglobal.com