CCD and CMOS
Oct 1, 2008 12:00 PM, By Steve Mullen
Understand old and new camera sensor technology.
Image readout
Figure 6
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A CMOS device has the unique capability to employ both row and column addressing to create a window within the photodiode matrix. A small window can be used to capture slow-motion video. For example, a 960 × 360 pixel window within a 1920 × 1080 sensor requires less than 17 percent of the entire sensor be read out, allowing the readout to occur up to 6X faster.
For a CMOS camera to support interlace video, its chips are clocked at the field rate. Then:
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An external circuit performs line-pair summation that converts each frame to a field. Through this process, sensitivity is increased +6dB, and the interlace coefficient drops to 0.75, thereby minimizing flicker and twitter.
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Cameras that employ vertical pixel-shift or vertical interpolation can simply discard alternate fields and record the field not discarded. Line-pair summation is not required because the image is inherently slightly soft. Sensitivity is the same for both interlace and progressive operation.
Figure 7
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All column signal voltages are loaded into an analog shift register where pixel information is shifted to output port(s). In this design, analog-to-digital (A/D) conversion is external to the CMOS chip. (See Figure 5, previous page.) To decrease image noise, internal A/D conversion can be employed. In this design, a CMOS chip's output path is digital rather than analog. (See Figure 6.)
Sony Exmor technology employs a massive amount of circuitry between each column bus OP amplifier and the chip's output port(s). (See Figure 7.) Each column has its own A/D converter. From this point forward, the data path is digital — not analog — thereby reducing image noise.
Key CCD/CMOS differences
The obvious difference between these technologies is that CMOS sensors inherently have no vertical smear. Therefore, when using CMOS-based cameras you can shoot just as you would were you shooting film. However, the latest CCD chips significantly decrease the appearance of vertical smear. (See Figure 8.)
Figure 8
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Clearly, rolling-shutter artifact is the Achilles' heel of CMOS technology. This artifact can be eliminated by implementing a global shutter. However, this requires an additional transistor within each pixel that decreases a chip's fill factor — thereby reducing its sensitivity or forcing a reduction in the number of pixels.
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