Video Fundamentals
Active Image
The active video image resulting from the scanning always has an aspect ratio (horizontal/vertical) of 4/3, independent of the video format. The color composite video signal shows that the scanning process requires some additional room on the left and right sides of each line, as well as on the top and bottom of the active video image region. This additional room includes the synchronization signals, color bursts, and other format-specific information, like the ITS, which are not part of the active video image. Approximately 90% of all the lines and 80% of each line can transmit the active image information. The exact values depend on the video format, as shown in the following table.
| Video Format | Lines/Frame | Active Lines | Frame Rate | Line Duration | Active Line Duration |
|---|---|---|---|---|---|
| M-NTSC/M-PAL | 525 | 480/486 | 29.97 frames/sec | 63.55 µs | 52.2 µs |
| All others | 625 | 576 | 25.00 frames/sec | 64.00 µs | 52.0 µs |
Active Lines represents the number of lines that are actually used to transmit the image information. For example, only 480 lines out of 525 lines/frame transmit the image information in NTSC. Likewise, on each line, the image information is transmitted only during the active lines sequence, which is shorter than the entire line duration. For example, of 63.55 µs only 52.2 µs are the active line duration in NTSC. Frame rate is the scanning speed described in Scanning Speed.