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8.2.3.4 Spatial coherence:
A line of analog video data is sampled (digitized) at the PLL derived pixel clock rate in a
conventional system. The derived pixel clock is not in any way related to the actual pixel data
that was originally used to "create" the line of analog video. Although a reasonable sampled
version of the line may be re-created, the actual spatial coherence to the CCD charge sites is
lost in a conventional system ! A direct correlation and transfer of the gray level information
from the pixel element in the sensor, to a precise memory element within the memory array
exists in a digital camera based image processor system.
8.2.4 Do I need a frame grabber to use digital cameras?
Yes; the output data from a digital video camera is a data stream of parallel 8 or 10 bit words.
Several "handshaking" signals are also provided. A compatible Image Processor can translate
the data streams into "frames" of video by means of storing the video data into the appropriate
locations of a frame buffer, based on the information encoded in the handshaking signals.
8.2.5 How should I go about selecting a frame grabber for my application?
1. Determine the software/hardware requirements of the application. Try to figure out the actual
image processing that will be required. Not all Image Processors are created alike ! Some
are better at image capture and display, others offer better number crunching.
2. Choose the "platform" or the processor bus. (Sometimes this can be a decision based on
experience or familiarity with one platform over others !)
3. Consult the List of Compatible Image Processors
8.2.6 Do I need to cool the camera?
With a signal-to-noise ratio of >62 dB and an equivalent number of bits = 10 at 25°C running at a
real-time rate of 30 frames/sec, most DVC camera applications can achieve high performance
without the cost and complexity of external cooling.
Cooling a CCD sensor reduces flow of the dark current electrons; therefore, cooling a CCD can
improve its dynamic range. However, under normal conditions, the dark current for the TC-245
CCD is lower than one LSB (for 1/60 sec integration) and is therefore not very significant. In non-
standard modes such as long term integration etc., cooling can be useful
There are two ways to cool the CCD in a DVC camera:
• Due to the unique opto-mechanical design of the DVC camera, the CCD is in thermal contact
with the front-plate. Cooling to a temperature that is just above the dew point is possible by
simply attaching a cooler to the outside front metal plate. Below the dew point, condensation
can be a problem ! To eliminate condensation, the camera can be enclosed in a dry
chamber.
• In a vacuum dewar down to -35°C or -40°C. Single stage and multi-stage coolers are
available.
Please contact DVC for information.
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