User`s manual

STH-MDCS USER’S MANUAL 2001 VIDERE DESIGN
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in low-light areas. You can add gamma correction to the displayed image
by choosing an appropriate gamma value in the slider under the right
display window (Figure 6-2).
6.3 Video Digitization Parameters
The CMOS imagers have electronic exposure and gain controls to
compensate for varying lighting conditions. The exposure can vary from a
maximum of a full frame time to a minimum of one line time. Gain is an
additional amplification of the video signal, for low-light situations. It is
settable from 0 to 22 dB.
Both imagers are treated in exactly the same manner. It is not possible to
set a different exposure or gain on each imager.
Digitization control can operate in either manual or automatic mode. Refer
to Figure 6-1 for the controls in the video capture program. Manual mode
is the only currently supported mode for the STH-MDCS/C.
In manual mode, the user program sets the exposure and gain. The
exposure and gain are based on a 0 to 100 scale. Here are some tips for
setting exposure and gain.
• In general, keep the gain as low as possible, since it introduces
additional noise into the system. Use it only if the exposure is set
to maximum, or if the exposure must be kept low to minimize
motion blur.
• Adjust the manual iris of the lens to as small an opening as
possible for your application, without having to use gain. This
will increase the depth of field and give better optical performance.
6.4 Subsampling
In many applications it is not necessary to work with the the full 1280 x 960
image. The CMOS imagers are capable of sampling the pixels in the array.
Sampling allows the video stream to send less data, for faster frame rates or
less bus activity. A sampled image shows the same scene as the original
image, but it uses fewer pixels to do so, and has less detail. Sampling
differs from subwindowing, which picks a rectangular portion of the image,
but doesn’t change its resolution.
Binning is a subsampling technique in which several adjacent pixels are
averaged into one. Binning reduces video noise, sometimes quite
substantially. Binning is available both on-chip and on the host PC. On-
chip binning allows the STH-MCDS/-C to convert the entire 1280x960
image into a 640x480 image before it is sent down the IEEE 1394 bus. The
binning is color-aware for the STH-MDCS-C, so that pixels of like color
are combined.
Binning can also take place in the host computer, giving an additional x2
subsampling of the image. Subsampling is always done in both the vertical
and horizontal direction. Subsampling x2 means that an image of size H x
V will be transformed into an image of size H/2 x V/2. Subsampling x4
transforms it to an image of size H/4 x V/4.
Figure 6-2 shows the frame size and subsampling controls on the video
capture application. Any x2 or x4 subsampling is allowed, including
combined binning and decimation.
6.5 Frame Rates
Frame rates from the STH-MDCS/-C depend on the frame size. Table 3
shows the frame rates available for each of the frame sizes.
Binning Frames per Second
x1 (1280 x 960) 3.75, 7.5
x2 (640 x 480) 3.75, 7.5, 15, 30
x4 (320 x 240) 3.75, 7.5, 15, 30
Table 3 Frame rates at different decimations.