Analog Television - Digital Video and Audio Broadcasting Technology - page 12

served in TV receivers; and even there the technology is changing to LCD
and plasma screens.
The result of scanning the original is the luminance signal where 0 mV
corresponds to 100% black and 700 mV is 100% white. The original pictu-
re is scanned line by line from top to bottom, resulting in 625 or 525 active
lines depending on the TV standard used. However, not all lines are vi-
sible. Because of the finite beam flyback time, a vertical blanking interval
of up to 50 lines had to be inserted. In the line itself, too, only a certain
part represents visible picture content, the reason being the finite flyback
time from the right-hand to the left-hand edge of the line which results in
the horizontal blanking interval. Fig. 2.4. shows the original to be scanned
and Fig. 2.5. shows the associated video signal.
2.5 lines
Vertical sync puls
1 line
Begin of line 1 of 1st field
Center of line 313 of 2nd field
Fig. 2.6. Vertical synchronization pulse
2.2 Horizontal and Vertical Synchronization Pulses
However, it is also necessary to mark the top edge and the bottom edge of
the image in some way, in addition to the left-hand and right-hand edges.
This is done by means of the horizontal and vertical synchronization pul-
ses. Both types of pulses were created at the beginning of the television
age so as to be easily recognizable and distinguishable by the receiver and
are located in the blacker than black region below zero volts.
The horizontal sync pulse (Fig. 2.5.) marks the beginning of a line. The
beginning is considered to be the 50% value of the front edge of the sync
pulse (nominally -150 mV). All the timing within a line is referred to this
time. By definition, the active line, which has a length of 52 Μs, begins