[mpeg2] Re: Why is horizontal res. set to 704?

[Warren Young <warren@etr-usa.com>]

Marcel van der Boom wrote:
> 
> Why is the full D1 case horizontal width set to 704 (like 720 minus 16 ?) and
> not to 720? There must be a reason for this.

There's not supposed to be any video data in the 8 pixels on either
side, so a lot of video systems just clip them.  Others give you the
option of leaving them in or clipping them off.

I have yet to see an MPEG decoder that cares whether you use 720 or 704
pixels wide.  Also, the higher-end Sigma REALmagic DVR (also based on
the Kfir chip) uses 704 pixels wide exclusively, so this might be a
limit of the encoder chip.  Since the card also comes with Sonic's
DVDit! software, I imagine that 704xNN DVDs are legal, too.

> The same question applies to the 352 width case instead of 360.

That's a simpler issue: 352 is divisible by 16, and 360 is not.  In
MPEG, the horizontal dimension must be divisible by 16, to account for
macroblocks, which are 16x16.

It's also helpful that there's a long-standing format along the lines of
"D1" for 352xNN, called CIF.  (Common Interchange Format)
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