Exposure Control
VV5430
7. Exposure Control
Automatic exposure and gain control ensure operation of the VV5430 over a wide range of lighting conditions. Automatic black
level control and optional ‘Backlit’ mode further ensure consistent picture quality. The devices control 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.
Note: The System Clock can be divided by up to eight times to further increase sensitivity by extending the exposure time. This,
of course, also reduces the frame rate to non-standard values.
Automatic exposure and gain control are enabled with AEC=1 (pin 21) and AGC=1 (pin22), but can be inhibited via the serial
interface (Setup Code_1). However, If AEC is inhibited by pin 21, AGC is also inhibited and the serial interface has no control.
Inhibiting AEC or AGC via the serial interface, or by taking pin 21 or 22 low, freezes the current value(s) for these, which can then
be altered by writing to the exposure and gain control registers. (See Serial Interface for details.)
Note: The timing of exposure and gain control messages on the serial interface is very important. External values for exposure
and gain are only applied at the start of a frame, and the serial interface must be paused until the new values are installed—no
further communications will be accepted during this time.
7.1 Automatic Exposure Control (AEC)
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.
Pixels above a threshold white level are counted every frame, and the number at the end of the frame defines the image as
overexposed, above average, correctly exposed, below average or underexposed. 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. If the exposure value is close to its limit (12% below max. or 25% above min.), then gainis
increased or decreased by one step and exposure is set to midway in its range. Exposure is then controlled as normal.
7.2 Automatic Gain Control (AGC)
The VV5430 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.
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 necessary. 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.
Increasing gain is limited to a programmable upper limit, for which the default value is x8. The gain upper limit is programmed by
setting bits [0..3] with header code 0101, when AGC=1. If Automatic Gain Control is inhibited (AGC=0), these registers are used
instead to select a gain setting up to x16.
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