Digital Signal Processing Reference
In-Depth Information
Color sampling is achieved by a variety of methods, but
a common method is to use Bayer filters, which are color filter
arrays. These can filter the incoming light at each pixel location
into different colors
and record the
pixel intensities of each color separately. Newer methods build
color sensitive sensors into the CMOS technology. There aremany
other factors affecting this process, such as aperture, shutter
speed, zooming and focusing, which are not discussed here.
To explain some of the concepts of sampling and aliasing, we
will consider a one-dimensional signal for simplicity. The ADC
will measure the signal at rapid intervals, and produce samples
(pixels in the image and video domain). It will output a digital
signal proportional to the amplitude of the analog signal at that
instant. This can be compared to looking at an object with only
a strobe light for illumination. You can see the object only when
the strobe light flashes. If the object is not moving, then every-
thing looks pretty much the same as if we used a normal,
continuous light source. Where things get interesting is when we
look at a moving object with the strobe light. If the object is
moving rapidly, then the appearance of the motion can be quite
different from that when viewed under normal light. We can also
see strange effects even if the object is moving fairly slowly, or we
reduce the rate of the strobe light enough. Intuitively, we can see
that what is important is the rate of the strobe light compared to
the rate of movement of the illuminated object. As long as the
light strobes fast enough compared to the movement of the
object, this movement looks very fluid and normal. When
the light strobes slowly compared to the rate of object movement,
the movement looks odd, often like slow motion, as we can
see the object is moving, but we miss the sense of continuous,
fluid movement.
Let's mention one more example: a simple animated movie
such as sketching a character on index cards. To depict this
character moving, perhaps jumping and falling, we might sketch
20 or 40 cards, each showing the same character in sequential
stages of this motion, with just small movement changes from
one card to next. Then, by holding one edge of the deck of index
cards and flipping through it quickly by thumbing the other
edge, the character appears to continuously jump and fall. This
is the process used for video
such as red, green and blue
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the screen is being updated at
about 30 times per second, which is rapid enough for us not to
notice the separate frames, so the motion appears to be
continuous.
So it makes sense that if we sample a signal very fast compared
to how rapidly the signal is changing, we get a pretty accurate
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