General Tips: Background on Video

From The Neuros Technology Wiki

Jump to: navigation, search


Random history of TV and "VGA" resolution

NTSC is a 60 Hz system, so 60 fields (30 frames) per second, 525 lines. The real video signal specification has 482 lines of active video. Because 482 is a "bad" number, computer buffs adopted 480 -- a good number -- instead. Since the screen has a 4:3 aspect ratio, 640 pixels per line are required to make the pixel square. So VGA specifies 640x480. That is a computer term. VGA has no system (NTSC/PAL) because it is a computer term. It is 640x480 in Europe as well.

Well, PAL is a 50 Hz system -- 25 frames per second, 625 lines, of which 576 are active. The CIF line of resolutions is based on this resolution. If there was such thing as VGA for PAL, the it would be 640x576 @ 25 fps.

Broadcast TV is analog. Therefore, while the computer guys were working their VGA (and XGA, and UXGA ...) specs, the video guys were worrying about analog bandwidth. It turns out that it fits about 720 pixels on a analog NTSC studio signal. Since there are 482 active lines, D1 (the first Digital standard developed for video recorders) defines its resolution as 720x482, but everybody drops 2 lines these days, resulting in the familiar 720x480 @ 30 fps.

For PAL, D1 is 720x576 @ 25fps. Again, more lines, fewer frames.

If you are receiving an analog 16:9 signal, the width of it will be displayed across the entire screen, but the height of the video will not fill the height of the screen. Therefore, if the 4:3 equivalent is VGA (640x480), then a 16:9 "VGA" (there is no such thing by definition) would be 640 across by 270 (480*9/16).

Conversely, D1 at 16:9 is 720 x 270 (remember, the pixel aspect ratio remains intact).

Let's say you are playing back a 16:9 movie recorded at 640x270. If your screen is 16:9, then it will be filled up (the same way that a 4:3 screen fills up with VGA). But if your screen is VGA, then there will be 480-270=210 lines missing, therefore the player should blank 105 lines up top and 105 lines in the bottom.

If you are playing a 4:3 movie on a 16:9 screen, the same process happens. This time, however, the blank bars should be on each side. 630984738418944648495319

Personal tools