[mythtv-users] quality

[Cory Papenfuss]

> > Standard definition (as opposed to HD - High Definition like 720p or 1080i
> > which are 1280X720 and 1920X1080 resolution respectively).
> 
> Additionaly, SD isn't defined by a fix resolution, though NTSC is
> generally accepted digitally to be 480 lines (thus 640x480), and PAL
> as 576 lines. When encoded though, they can have varying sizes (like
> through satellite systems).
> 
> 
	You are right and wrong here.  The 480 "lines" is not the same 
thing as 480 lines.  The 480 horizontal scanlines that make up the picture 
*absolutely* is part of the NTSC spec... well, 525 *total* and 480-483 of 
which are visible in two interlaced fields running at slightly under 60Hz 
refresh each.  The 480 "lines" which refers to the analog resolution of 
NTSC is not the same thing as the 480 scanlines.  It's a term called 
"analog lines of resolution," and refers to the horizonal analog bandwidth 
of the signal.  i.e. the number of black and white vertical lines that can 
be drawn on the screen without them blurring together.  This is actually 
done "per-picture-height" as well.

	A cable NTSC signal that has 330 "lines" amounts to 330 * 4/3 = 
440 resolvable b&w vertical lines on the screen.... i.e. 440x480 
resolution (ignoring Kell factor).

	640x480 is convenient because it provides enough resolution to 
capture NTSC, and has square pixels for a 4:3 aspect ratio.  720x480 is 
the D1 standard for SD signals.

-Cory

-- 

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* Cory Papenfuss, Ph.D., PPSEL-IA                                       *
* Electrical Engineering                                                *
* Virginia Polytechnic Institute and State University                   *
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