Service manual

STP 11-25R13-SM-TG
S - 5
S-1. Functions.
a. The composite video signal includes the image signal obtained from the camera pickup,
blanking pulses, and synchronizing pulses. The system uses blanking pulses added to the camera signal
to prevent signals from appearing on the receiver screen during the retrace. It uses synchronizing pulses
to synchronize transmitter and receiver scanning.
b. A function of the synchronizing and scanning sections is the generation of the blanking signals.
This system uses the blanking signals to turn off the electron beam in the camera or in the receiver
picture tube during retrace periods. This allows the beam to be repositioned for the start of a new line or
new picture. The blanking signals originate in the scanning and synchronizing section of the transmitter.
S-2. Picture Signal.
The amplitude of the video signal divides into two sections. The active camera signal is the upper
75 percent and the sync pulses are in the lower 25 percent. Any signal amplitude lower than the black
level is blacker-than-black. The black level is at a constant percentage of the peak signal amplitude in
order to maintain a brightness level in the television system.
S-3. Synchronizing, Blanking, and Equalizing Pulses.
a. Horizontal Synchronizing Pulses
(Horizontal Sync). In Figure S-4, horizontal sync pulses are
rectangular and occur at the end of each horizontal scanning line. The purpose of the horizontal sync
pulses is to synchronize the horizontal scanning motion in the receiver precisely with the horizontal
scanning motion of the camera at the picture point of origin. The horizontal sync pulses are part of the
transmitted composite video signal and occur at intervals of 63.5 microseconds (usec). This is the time
between the start of one scanning line to the start of the next scanning line.
(1) Each horizontal sync pulse rides on a corresponding blanking pulse, but has duration less than
that of a blanking pulse.
(2) In Figure S-4, the leading edge is delayed with respect to the leading edge of the blanking pulse
upon which it rides. This delay forms a step in the composite signal known as the front porch.
(3) Similarly, the step formed by the difference between the trailing edges of the sync pulse and the
blanking pulse is called the back porch. The front porch is of very short duration and ensures that the
scanning beam of the receiver extinguishes completely before the start of horizontal retrace. The back
porch eliminates unwanted disturbances in the video signal after the sync pulse, before the scanning
beam turns on.
b. Horizontal Blanking Pulse
. The horizontal blanking pulse has a duration that is sufficient to
extinguish the scanning beam at the receiver during horizontal retrace time. Since the horizontal retrace
is delayed for some time after the leading edge of the blanking pulse, and retrace decays sometime
before the trailing edge of the blanking pulse, video information is not visible during these periods. These
delays, before and after the horizontal sync pulse, make up the front and back porch. They eliminate the
horizontal retrace lines and any disturbances that follow the sync pulse at the receiver.