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6.9 DISPLAY AND KEYPAD
TECHNICAL DESCRIPTION
whether they will be keyed into a contrasting background. The on screen
programming menu accomplishes character style selection.
6.8.3
Character Position and Size (6231-33)
The pixel oscillator consists of monostable U11b and associated
components. The oscillator frequency, which determines horizontal size, of
the characters is adjusted by the digital trimpot (NVPOT) U13 and
associated components. The MCU writes different values to the NVPOT
which control the adjustment input to voltage regulator U12, which in turn
sets the voltage present for the RC timing network of the monostable. The
starting position of the characters at the left of the screen is controlled by
U11-A.
Resistor R27 may be replaced to alter the left position of the
characters. Starting with board revision 6231A1, trim pot VR3 can be also
used to adjust the character left edge.
6.8.4
Video Circuits (6231-31)
Composite video, which comes up the submodule header from the main
board (J1 pins 1 and 2) is AC coupled and buffered by Q1 before going
into the sync separator LM1881 U4. The sync separator provides H Sync,
V Sync, a Frame pulse (active low for field 1) and a back porch clamp
pulse to drive the DC restorer circuitry U1, Q2, and Q3 and associated
components. The back porch clamp pulse allows U1 to compare the actual
DC level of the video to ground potential.
generates an error signal that adjusts the bias point of Q2 thus ensuring
proper operation of the video keyer with varying video and sync levels.
The Video keyer U2 is controlled by the KEY signal generated in the LCA.
KEY switches U2 between the program video path and the KEYFILL data.
When the KEY signal is LOW the active video is passed through the keyer.
When the KEY signal is HIGH the KEYFILL data is added to the black level
of the video.
To calibrate the video keyer, connect colour bars from your sync generator
to the Video input loop, of the 4025TR, and to channel A of your
oscilloscope, and terminate it. Connect the video output to channel B of
your scope and terminate it. Adjust the GAIN trimpot (VR2) so that the
output amplitude matches the input. Adjust the LEVEL trimpot (VR1) so
that the inserted VITC/characters are approximately 550 millivolts above
video black level.
6.9.1
LED Displays (7025-31)
A 16 digit alphanumeric display and a 28 button keypad are contained on a
separate circuit card (7025) which is connected to the 632 VCG module via
a 20 conductor ribbon cable.
Model 4025TR Film Footage Encoder Manual
If they are not equal, U1
Page 6-23
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