What is the exact definition of Composite Video Signal?

Answer 1

A composite video signal is one in which several signals are combined. The signals are:

Horizontal synchronising pulse

Vertical synchronising pulse

Luma signal (the brightness of the image)

2 chroma signals (the color information of the image)

The signal is multiplexed so that image information is delivered between the sync signals. The luma signal is mixed with one of the chroma signals on one line and with the other chroma signal on the next line.

The signal refreshes a complete frame by delivering odd numbered lines in the first field and all even numbered lines in the second field. Therefore, two fields are used to generate a complete frame. The use of two fields per frame was an early form of compression and allowed the frame rate to be fixed at 25 or 30 frames per second without a noticeable flicker appearing in the image.

The color encoding system has three variants. North America uses NTSC encoding, Europe uses PAL while France chose a third system called SECAM. All color encoding systems use the composite signal in the same way but carry slightly different chroma signals.

Composite signals are used only for standard definition signals. HD signals do not use encoded color signals and are always carried as three separate signals, whether in analog or digital form.

Frame rates and line counts are not part of the composite signal definition but generally, NTSC encoded signals use 60Hz field rates with 480 visible lines (525 lines including lines that are not used for the image). PAL signals generally use a 50Hz field rate with 576 visible lines (625 lines in total including non-visible lines). SECAM signals are in line with PAL rates and line counts.