Video bandwidth

The frequency of the video waveform is determined by the change in the brightness of the electron beam as it scans the screen line by line. Maximum video frequency is obtained when adjacent pixels are alternately black and peak white; this represents the maximum definition of a TV image.

For PAL, this means 414,720 alternating black and peak white pixels and for NTSC, 345,600 pixels. When an electron beam scans a line containing alternate black and white pixels, the video waveform is that shown in Figure 1.7, representing the variation of brightness along the line. As can be seen, for any adjacent pair of black and white pixels, one complete cycle is obtained. Hence, for the 10 pixels shown, five complete cycles are produced.

It follows that, for a complete picture of alternate black and white pixels, the number of cycles produced is given by 1/2 X number of pixels:

Since there are 25 (PAL) complete pictures per second, then the number of cycles per second, i.e. the maximum video frequency is

cycles/picture x 25 = 207,360 x 25 = 5,184,000Hz = 5.184MHz

NTSC, with 30 pictures per second, has the same maximum video frequency as PAL namely,

cycles/picture X 25 = 172,800 x 30 = 5,184,000Hz = 5.184MHz

The minimum video frequency is obtained when the electron beam scans pixels of unchanging brightness. This corresponds to unchanging amplitude of the video waveform, a frequency of 0 Hz or d.c. The video bandwidth is, therefore, 0–5.184MHz.

Notice here that the specifications outlined by the SD common interface format, results in a common video bandwidth for both systems. That is of course is the purpose of the CIF.