Composite video waveform
The picture information and the sync pulse of a single line constitute what
is known as the composite video waveform. Each line is made up of the active video portion and the horizontal blanking portion. As shown in
Figure 1.5, the picture information is represented by the waveform between
the two-line sync pulses and thus may acquire any shape, depending on
the varying picture brightness along the line. The waveform shown represents
a line at peak white (maximum brightness).
Before and after every sync pulse, the voltage is held below the black
level as shown for short periods of time, known as the front porch and the
back porch, respectively. The front porch has a duration of 1.55 us; it
ensures the video brightness is completely blocked before the sync pulse
is applied. The back porch has a longer duration of 5.8 us; it provides time
for the flyback to occur before the application of the video information.
The back porch is also used for black level clamping. As can be seen, the
front porch, the sync pulse and the back porch are at or below the black
level. During this time, a total of 12.05 s, the video information is completely
suppressed; this is known as the line blanking period.
The total available peak-to-peak voltage of 1V is divided into two
regions:
The duration of one complete line of a composite video may be calculated
from the line frequency:
Hence, the active video duration = 64 - 12 = 52 us.
The composite video waveform consists of video, blanking and sync
pulse, hence it is more commonly known as composite video, blanking
and sync, CVBS.
The equivalent CVBS waveform and timings for the 525-line NTSC
system is shown in Figure 1.6 with a line duration of 63.5 us and active
video duration of 52.6 us.
