Datasheet

Jon Waddington
17
3.2 PAL Video
Video signals are displayed on a television screen by drawing each line from left to right,
from top to bottom. The amplitude of the waveform represents the brightness of the
screen with a high voltage being white and a low voltage being black. The video signal
drops below the black level after each line has been displayed. This is the horizontal sync
pulse, used to inform the television that a new line is to be drawn [25]. After all the lines
have been drawn, the signal issues pulses to inform the television that the whole frame
has been displayed and that the first line is to be drawn again.
In PAL (Phase Alternation Line) video systems, after the line has been displayed, the
signal drops to the black level for 1.5μs. This is known as the front porch. The signal
then drops for 4.7μs before rising to the black level for 5.8μs. These portions of the
signal are known as the horizontal sync pulse and the back porch respectively. The back
porch also contains a colour burst, a 4.43Mhz signal [26] which the television uses as a
reference to display the correct colours. The full horizontal synchronisation signal is seen
in figure 3.2.1. After this horizontal blanking, the active video begins. Each line of active
video is 52μs in duration.
The vertical blanking consists of five equalizing pulses, five broad pulses and another five
equalizing pulses. This is shown in figure 3.2.2. This informs the television that the
whole frame has been drawn and the next line will be the top line [27].
Figure 3.2.1 Horizontal Blanking Waveform
Front Porch
Colour Burst
Horizontal
Sync
Back Porch