Main Features
Automatic Gain Control (AGC)
Video Output
The VV5402 automatically increases the
system gain of its output stage if with the
current gain setting and maximum exposure
the image is too dark. Gain can be varied from
x1 to x16 in times-two steps, giving five
different gain settings.
The VV5402 delivers a fully-formatted
composite monochrome video signal. Stand-
ards options include EIA (320 x 244) and
CCIR (384 x 287). On chip signal conditioning
allows user-selection of linear or gamma-
corrected output.
If the scene is too dark and the integration
period has almost reached its maximum
value, the gain value is incremented by one
step (times two). In the same frame period the
exposure value is divided by two, halving the
integration period. The exposure controller
then increases the exposure value as neces-
sary. Similarly if the image is too bright and
the integration period is short then gain will be
reduced by one step (divide by two) and the
exposure value will be doubled. The exposure
controller can then adjust the exposure value
as necessary to provide a correctly exposed
image.
The integrated 75Ω driver eliminates the need
for additional active components to drive
standard loads, including double terminated
lines.
Automatic Exposure and Gain Control
Automatic exposure and gain control are
enabled with AEC=1 (pin 21) and AGC=1
(pin22). However, If AEC is inhibited by pin
21, AGC is also inhibited. Inhibiting AEC or
AGC by taking pin 21 or 22 low freezes the
current value(s) for these.
Automatic Exposure Control (AEC)
Backlit Mode
The VV5402 controls exposure over a range
of 99,000:1 in EIA mode and 146,000:1 in
CCIR mode, and operates at illumination
levels as low as 0.5 lux at standard clock
frequencies. (Thesystem clock frequency can
be reduced to provide increased sensitivity.)
The VV5402 can be configured to operate in
two auto-exposure modes, selected by the
BKLIT pin (pin28) state. The default mode
(BKLIT = 0) provides exposure control for
normally illuminated scenes. For scenes
where a bright background can cause the
foreground subject to be severely under
exposed, the ‘Backlit’ mode (BKLIT = 1) offers
superior performance.
Automatic exposure control is achieved by
varying pixel current integration time
according to the average light level on the
sensor. This integration time can vary from
one pixel clock period to one frame period.
‘Backlit Mode’ (BKLIT=1) operates by using a
higher threshold level for the exposure control
comparator over the central area of an image,
which is therefore exposed for longer and so
enhanced. The area in which the higher
comparator threshold is used when BKLIT=1
is illustrated below:
Pixels above a threshold white level are
counted every frame, and the number at the
end of the frame defines the image exposure.
If the image is other than correctly exposed, a
new value for integration time is calculated
and applied for the next frame. Corrections
are either ±1/8 or ±1/64, depending upon the
degree of over or under exposure.
09/04/97
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