A PC screen may have a resolution of 800 x 600 which means 800 pixels
(dots) going across horizontally (width) and 600 pixels going down vertically
(height).
TV's engineers, however, only speak about TV resolutions in terms of
the number of lines going across (resolution width) not down vertically
(resolution height)! Why? Because all TV's have exactly the same amount
of lines going down (resolution height), but not all TV's have the same
amount of decernable dots going across. For example, an American TV picture
will always scan (project) 480 lines horizontally (resolution height), but
the number of lines going across (resolution width) will always depend
on the quality of the TV and the signal broadcast to it.
A VHS video will only offer about 210 dots across while a TV station may
offer about 330 dots across!
TV engineers use a test patten to determine a TV's resolution. This test
pattern has lots of vertical lines like this:
The engineer increases the lines until it is impossible
to see any lines because they have all blurred into each other. When the
lines cannot be seen any more the maximum resolution of the TV has been
reached. These test lines are stacked from left to right as seen in the
picture above. Because the lines are stacked from left to right, the number
of dicernable lines across on the TV screen is called the horizontal
resolution!
So when we say a TV has 485 lines we mean it has a maximum resolution
of 487 dots across. But to say a TV has 487 dots across is never correct
since it will always be less unless the signal quality is perfect . .
. If we take into account signal loss and low broadcast quality we are
looking at something like 330 lines.
TV screens have an aspect ratio of 1.33:1 and are slightly
oblong.
Video Format.............................. Horizontal Resolution (resolution
width)
Standard VHS............................. 210 Horizontal Lines
Hi8...............................................400 Horizontal Lines
Laserdisc......................................425 Horizontal Lines
DV...............................................500 Horizontal Lines
DVD............................................540 Horizontal Lines??
[ actual: 720x480, 704x480 or 352x480 ]
Typically, for actual NTSC signals, 485 lines are used for displaying
the picture (because real NTSC signals are interlaced, that equals 242.5
lines for each of the two fields making up the frame).
"We suggest capturing at a resolution that most closely matches
the resolution of the video source.
For video sources from VHS, Hi8, or Laserdisc, SIF resolution of 352x240
will give good results.
For better sources such as a direct broadcast feed, DV, or DVD video,
Half D1 resolution of 352x480*is fine."
*352 matches the actual visual resolution width of about 330 for tvs;
480 provides that max theoretical visual resolution height.
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Comments: |
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Name: John Love-Jensen
Comment Date: Wednesday, May 16th 2007 - 05:41:12 AM
E-mail address: eljay a t adobe d o t com
Regarding (subject or URL):: http://lyberty.com/encyc/articles/tech/vid_horizontal_resolution.html
Comments:
NTSC has 486 visible scan lines.
The "485 visible scan lines" is only if one prefers to count the two half-wide scan lines
(line 263 at the bottom of field 1, and line 283 at the top of field 2) as 0.5
instead of counting them integrally as a scan line (albeit ones that are only half-wide).
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