[mythtv-users] Optimal capture resolution for DVD burning

Cory Papenfuss papenfuss at me.vt.edu
Thu Jul 27 15:12:55 UTC 2006


> According to this (the font of all (mis)information)
> 
>   http://en.wikipedia.org/wiki/Display_resolution
> 
> NTSC only has a horizontal resolution of about 270 lines.
> 
> Ultimately you'll have to suck it and see - it will depend on the
> quality of the MPEG encoder, your TV reception etc!
> 
> 
	Yeah... be careful on quoting wikipedia as gospel.  You've gotta 
have a pretty finely-tuned internal bullshit detector to really trust 
anything thats there.  

	This particular subject is mis-understood more often than not.  
If you look into the term "lines of resolution" as related to analog 
video, you'll discover that it has a very specific (and unusual) meaning.  
People often tend to think it's the number of horizontal lines on the 
screen... i.e. vertical resolution.  Not true... that's fixed at 480 
interlaced lines for SDTV.

	The number of "lines of resolution" for analog video is related 
the *horizontal* bandwidth.  Think of it as the number of alternating 
black and white vertical lines on the screen.  Keep putting them closer 
and closer together until you cannot distinguish one from the other.  Now 
count them up, but here's where it gets tricky....

	The more correct terminology would be "analog lines of resolution 
per picture height."  In other words, take a chunk of the screen that's as 
wide as it is tall, and count how many black/white lines that can be 
distinguished in that screen subsection.  THAT's the number of "lines of 
resolution."  For the oft-cited "lines of resolution" for SDTV of 
somewhere between 270-330, the 4:3 aspect ratio means you'd need (270 to 
330) * (4/3) = (360 to 440) black and white lines to get all the way from 
side to side on the screen.  Thus the total number of vertical lines 
across the screen would be between 360 to 440, depending on whos numbers 
you want to believe.  Now, when you sample that digitally, you'll need at 
least that many, and often a bit more to get the same perceived sharpness 
(see Kell factor).... often cited as approximately 0.7.  So, a 480x480 
digital image has 480 "digital horizonal lines of resolution," across 
the screen which is perceived as about (0.7*480 = 336) "lines of 
resolution," which is termed (336 / (4/3) = 252) "analog lines of 
resolution per picture height" for a 4:3 screen.... about the same as VHS 
quoted at 240.

	Consider the example of DVD.... often quoted as having 540 "lines 
of resolution"... that number comes from assuming a 4:3 screen.  On a 16:9 
screen, it's only got 405 "lines of resolution" even though it all started 
as the same 720-pixel video.

	To summarize:  Best possible SDTV resolution taking into account 
aspect ratio of 4:3 and the conservative Kell factor of 0.7 results in:
Device		"lines"		Necessary capture resolution
VCR		240		240 * (4/3) / 0.7 => 457x480
OTA-SDTV	270		270 * (4/3) / 0.7 => 514x480
HQ-SDTV		330		330 * (4/3) / 0.7 => 628x480

	Those numbers are on the outside (with the conservative Kell 
factor).  Absolutely no point in capturing higher than that for SDTV 
(other than direct DVD compatibility), and generally lower is adequate.

-Cory

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



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