[NUUG video] Resolution on compressed video (384x288 vs 360x288)?

Herman Robak herman at skolelinux.no
Sun Sep 21 20:37:14 CEST 2008

On Sun, 21 Sep 2008 20:02:05 +0200, Petter Reinholdtsen <pere at hungry.com> wrote:

> [Herman Robak]
>> Since the aspect ratio is supposed to be 4/3, this means 384x288
>> gets square (well, almost...)* pixels.  Square pixels can be
>> displayed without stretching on a computer screen. 360x288 would
>> need to be scaled _up_ to 384x288, or the image would be too narrow;
>> squashed and noticably distorted.
> OK.  Seem like a good argument for keeping the resolution at 384/288.
> I removed the file using 360x288 and is compressing all of 2007 to Ogg
> Theora with the 384x288 resolution now.
> Why is DV using aspect ration of 5/4 and not 4/3?

It doesn't.  The aspect ratio is 4:3, plus a DCT block's worth of padding on each side, for "overscan".

The real question is why it uses 720 samples per line, rather than 788.  This stems from television standards like DMAC (analog, but discrete-sampled) and CCIR 601*.  Digital TV standards from the early 80s did not have general purpose computer screens in mind.  VGA came several years later.

DV has 4/3 display aspect ratio.  It's the pixel aspect ratio that matters here, and it's not 1/1 (square).  That's according to the spec, but somewhat annoying to programmers and users.  Our 384x288 MPEG1 videos are not really kosher, either:  A truly standard compliant half-res MPEG1 shall be 352x288, with the right flags in the header to tell that the video is actually 4/3.  The overscan should have been cropped off first, mind you.

But our target audience is not very (VERY!) old or very strict players.  Some of the players used today may be a little too naive or lacking to honour aspect ratio flags.  They will not abort because of a minor spec violation, but they may fail to follow the spec themselves.

*) http://www.itu.int/ITU-R/index.asp?category=information&link=rec-601&lang=en

Herman Robak

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