Learning Event 3:
DESCRIBE THE SIN2 WINDOW SIGNAL
1. The sin2 window signal is normally accompanied by a half-line and half-field
window pulse, which is sometimes termed a "bar". Figure 3-10 shows a basic block
diagram of a generator which also includes the modulated 20T pulse.
a. The timing circuit driver (monostable multivibrator) is triggered from the
leading edge of sync and generates a rectangular pulse of about 16 microseconds
duration. The trailing edge of this pulse initiates the operation of the pulse and
window timing circuit, which positions the pulse and window leading and trailing
edges relative to sync.
Blanking pulses are used to inhibit the timing circuit
action during field blanking.
The output of the impulse generator is an 18-
nanosecond spike which becomes the Tpulse after shaping in the T-pulse shaping
network.
A switch is normally provided so that either 2T, T, or T/2 pulses are
available.
Note also that the leading and trailing edges of the window signal,
since they pass through the same shaping filter, have the same rise and fall times
as the associated T pulse.
b. The 20T pulse is shaped by appropriate sin2 filters and applied to a double-
balanced 3.58-MHz modulator in a manner similar to that in which chroma information
modulates the color subcarrier in an encoder. Thus, both the 3.58-MHz carrier and
the original 20T pulse are cancelled, and the output is the modulated sidebands of
the carrier. This produces the modulated T pulse envelope (fig 3-10). Finally the
original 20T pulse is linearly (resistively) added to the modulated pulse,
producing the symmetrical pulse with a base line.
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