Standard
|
NTSC
|
PAL
|
SECAM
|
| Property
|
|||
| images
/ second
|
29.97
|
25
|
25
|
| ms
/ image
|
33.37
|
40.0
|
40.0
|
| lines
/ image
|
525
|
625
|
625
|
| (horiz./vert.)
= aspect ratio
|
4:3
|
4:3
|
4:3
|
| interlace
|
2:1
|
2:1
|
2:1
|
| us
/ line
|
63.56
|
64.00
|
64.00
|
In an interlaced image the odd numbered lines (1,3,5,...) are scanned in half of the allotted time (e.g. 20 ms in PAL) and the even numbered lines (2,4,6,...) are scanned in the remaining half. The image display must be coordinated with this scanning format. (See Section 8.2.) The reason for interlacing the scan lines of a video image is to reduce the perception of flicker in a displayed image. If one is planning to use images that have been scanned from an interlaced video source, it is important to know if the two half-images have been appropriately "shuffled" by the digitization hardware or if that should be implemented in software. Further, the analysis of moving objects requires special care with interlaced video to avoid "zigzag" edges.
The number of rows (N) from a video source generally corresponds one-to-one with lines in the video image. The number of columns, however, depends on the nature of the electronics that is used to digitize the image. Different frame grabbers for the same video camera might produce M = 384, 512, or 768 columns (pixels) per line.