Theory of Operation
4–6 VITS100 NTSC VITS Inserter Instruction Manual
Diagram 1. U12 and U15 form a state machine to generate the data for shaped
pulse edges. When U15 sees a low-to-high transition, it counts up to six and
stops. When U15 sees a high-to-low transition it counts back to zero and stops.
This count goes to U12, where it is decoded into the data used to produce rising
and falling shaped edges out of the DAC, U65 on schematic 6.
When source ID pulses are called for, the outputs of the Test Signal Data Latch,
U1, become high impedances, and the outputs of U12 are enabled.
The source identification signal is one line of video with up to sixteen pulses on
it that can be recognized by a Tektronix VM 700A Video Measurement Set. The
VM 700A expects a start and a stop bit, but the other 14 bits (pulses) can be used
to identify up to 16,384 sources.
Diagram 3. The 16 bits can be set or cleared by two sets of DIP switches, S5 and
S6, accessible through the rear panel of the instrument. The 16 bits are loaded
into two shift registers, U22 and U23, once every line by a signal decoded from
the Horizontal Counters. The bits are then shifted out by “idclock,” a clock that
is decoded from the Horizontal Counters.
Diagram 1. The serialized ID bits go to U15 where they are gated with
“idclock” and latched by the 8 FSC clock. If Source ID is chosen for a certain
line, U16 activates “iden,” which causes the state machine in U15 to be
controlled by the latched ID signal.
The circuitry that controls signal switching appears on one of the schematic
diagrams located in the Diagrams section of this manual. Key circuitry is located
on diagram 3 (H & V Counters, Source Identification & Bypass Controls).
Diagram 3. REMOTE, through J27, is limited to a ground closure bypass control
signal and three indicator lines: “power,” “unlocked,” and “bypass.” U70, a PAL,
gates the remote bypass line, “rembypass,” with the “locked” line from the
genlock circuitry to control the bypass relay.
The power supply circuitry appears on two of the schematic diagrams in the
Diagrams section of this manual. Key circuitry is located on diagram 8 (Power
Supply) and diagram 9 (Regulator & Decoupling Caps).
Diagram 8. The line current passes through line filter LF1, fuse F1, and power
switch S10, and is applied to full-wave bridge rectifier CR12. Two of the diodes
within CR12 conduct on each half cycle of the AC input, causing charge to build
up on C110. The voltage on C110 will vary anywhere from 120 VDC at low-line
(90 VAC) to 350 VDC at high-line, (250 VAC). R157 discharges C110 when
power is turned off.
Remote Control
Power Supply