[mythtv-users] Resolution

[papenfuss at juneau.me.vt.edu]

> not pixel-based and do not scale NTSC or PAL video. The "resolution" of a 
> good consumer TV is approximately equivalent to 352x480, which is considered 
> the number of pixels needed to represent a SVHS quality image. The signal 
> quality of most analog cable TV sources in the US is about VHS quality or 
> slightly better, commonly about 280 lines. So, 352x480 is actually overkill 
> for my cable source, which is about 352x280. It's certainly a waste to use 
> more than half my bitrate to encode at 720x480!

Not quite... your cable source should probably be captured at 
(280*4/3=)372x480... read on...

	I got off on this tangent yesterday, and from what I was able to dig up 
after a great deal of google research and critical reading is that an analog 
video signal's "lines of resolution" is almost always misunderstood.  This the 
distilled result of the research:

- The most common misconception of a sources "lines of resolution" (e.g. VHS is
240 lines, cable TV is 330 lines, etc), is that it refers to the vertical
resolution.  That would be the number of scan lines, and is *fixed* for a
particular video format (NTSC, PAL, etc).  For NTSC, the number of lines is
fixed at 525, about 480 of which are viewable picture lines. Since a VHS VCR
has no temporal storage (a head swiping a scan on the tape is simply drawn as a
line on the TV in real-time), it can't have 240 lines of vertical resolution.  
If this were the case, there would either be black bars on the top and bottom, 
or only every other line would be drawn... think about it and it makes sense.

- The *actual* meaning of "lines of resolution" is (loosely) the number of 
lines that can be seen *horizonally*.  In other words, how many vertical lines 
can be put on the screen before they cannot be individually distinguished and 
blur together.  Thus, it is a measurement of the analog bandwidth capability of 
the recording medium.

- More accurately, the horizontal lines is defined in terms of a 1:1 aspect
ratio chunk of the screen.  It's weird to think about from a computer
standpoint, but works like this:  Make the largest *square* you can on the
screen, and put vertical lines closer together until you can't make them out.  
Then count these lines.... that's the lines of resolution as defined by analog
video folks.  Note that not quite all the screen is used in this, since
standard TV is 4:3.  So, in order to convert the number of "lines of 
resolution" into something useful for capturing purposes, the aspect ratio also 
needs to be taken into account.

SO, with this in mind, take a few data points.  NTSC VHS has 240 lines.  
Formatted at 4:3, this translates into 480 vertical pixels (NTSC viewable scan 
lines), and horizonally (240 * 4/3) = 320.  So, to capture VHS-quality video, 
you need a capture resolution of 320x480... 1/2 D1 is close at 352x480.  To 
capture high-quality broadcast at 330 lines, (330 * 4/3) = 440... SVCD is close 
at 480x480.  Capturing any more than that wastes encoding time, space, encoding 
quality, etc.

	Hope that helps... I know when I figured it out it made a lot more 
sense.  Makes better video with less CPU useage, too... :)

-Cory

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