Operating and Technical Descriptions
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8.4.
Video Processing
Two stages of amplification are used with a combined gain of
approximately 25. The first stage utilises an LM733 [NE592] video
amplifier which has a gain of 5 and also acts as a phase splitter giving
outputs of both positive and negative phase. The video phases are
further amplified (x 5) and then recombined in a balanced video
selection, each phase being switched synchronously with a signal
derived from the shutter blade. Circular blanking is then added to the
video waveform before being passed to the flicker suppressor. Balance
between the two phases is achieved by comparing the signals in the
video averaging comparator, V204 [IC3], whose output acts on the
negative phase. A clamp pulse is produced from the line blanking
waveform using V106, delayed and timed by V202 [IC4], and used to
sample the video black level. The resulting signal is compared with the
required black level, set by RV204 (and RV200 in 'pan' [automatically
in SMT version]), in the comparator V204 and the feedback loop
completed by causing the comparator output to control the dc level in
the main amplifier V201. Shading correction signals are taken from the
circle generator and added to the black level circuit.
The video signal level varies between the pan and chopped modes of
operation. To correct this the shutter switch also modifies the channel
gain and black level by switching transistor Q204 [TR6] and resistors
R227, R228 and RV200 [R34, R35].
8.5.
Circular Blanking Generator (Video Board)
The circle is produced using the principle of intersecting parabolas.
Line and field blanking waveforms are separately double integrated,
the first stage producing ramp waveforms and the second parabolas.
A crossing detector (V209) [IC12] then produces the circle which
operates on both positive (Q205)[TR8] and negative (Q206) [TR9]
video signals.
The shape of the circle is controlled by the field parabola amplitude,
RV207, and the size by applying a dc shift to the line parabola, RV206.
The ramp and parabolic waveforms are used for shading correction
and are added to the video signal via the black level circuit.
8.6.
Flicker Suppressor (Video Board)
Flicker is removed by delaying one field of video information and
adding it to the next; thus the flicker, which inverts every field, is
cancelled and the signal, which is of the same polarity on all fields, is
doubled. In order to achieve the delay, the signal is digitised, written
into RAM, and read out on the following field. The data is then
converted to analogue form and added to the then present signal which
comes directly from the video input.
Meanwhile the present signal is digitised and written into the memory
location which has just been read to provide the delayed data. The
address then moves to the next location. A short delay (V211) is
needed in the direct path to balance propagation delays encountered in
reading the store.
