Interlacing

Lines 1 – 525 are not scanned in sequence, but rather, every other line is scanned. In the first 1/60th of a second, lines 1, 3, 5, 7, etc. (through line 525) are scanned and shown, then lines 2, 4, 6, 8, etc. (through line 524) are scanned and shown in the next 1/60th of a second. This method of display is called interlacing and is done so that the eye will not see any flicker (your brain interprets 60 flashes of image per second as continuous). Thus, an entire single image (frame) is shown in two interlaced scans (fields) of 1/60th plus 1/60th of a second, which equals 1/30th. During this 1/30th of a second, the image is not moving, but is like a snapshot picture.

Normally, one frame is shown in two fields (two interlaced scans) and, thus, for live or videotaped broadcasts, we see 30 frames (60 fields) per second, giving the illusion of movement. Motion pictures on film are made at 24 snapshots (frames) per second. If we were to show the 24 frames of film per second on TV in the usual two fields per frame, we would end up with only 48 fields per second, which would result in 12 fields per second (6 frames) being left over from the 60 fields per second (30 frames) of TV scanning. Fitting these 24 frames of film per second into television which is displayed at 30 frames per second on the TV monitor is tricky. The problem is solved by showing every other movie frame in three fields rather than two (the third field is usually a repeat of the second field). Thus, movie frame 1 would be shown in two fields, movie frame 2 would be shown in three fields, movie frame 3 would be shown in two fields, movie frame 4 in three fields, and so on. This adds up to having the 24 frames per second of the film being spread out over 30 frames per second of the television.

Since television has to be divided into 60 fields per second (2 fields per frame) to eliminate visible flicker, you might wonder why you don’t see flicker at the theater with 24 frames per second. Flicker is eliminated by a spinning plate in the projector which breaks each frame of film into two flashes on the screen.