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ADC

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ADC

CHAPTER 20 HOW ENERGY TURNS INTO D

At the bottom of the array, the bottommost strips of red,blue, and green CCDs lead to a circuit where each of thethree streams of analog voltage feed to an analog-to-digital converter. When all the voltages in the bottomrow of each color has been passed to the ACD, all thevoltages of the color-matched diodes move down threerows. Then the new bottommost rows pass off their

charges to the ACDs, and are replaced by the next threerows. This continues until the last three rows havereached the bottom and been sent to the converter. Theresult is continuous streams of analog current represent-ing each color level for the entire surface of the array.The ACDs turn those steams into digital data that recorda color value for each photodiode in the CCD array.

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The diodes in each red, blue, and greenCCD string are coupled as they were inthe scanner. In addition, each diode in,say, the bottom blue strip is coupled ver-tically to next blue diode two stripsaway. This happens for red and yellowstrips as well.

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How Scanners andDigital Cameras See

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The array consists of strips of CCDs thatoverlay the entire picture area like rowsof bricks in a wall. Covering the array arecolored filters that let every third stripepick up only red, green, or blue light.

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A CCD arrayis used in digital stilland video cameras to capture atwo-dimension area struck bylight. Light going though the lensis focused on the array at the backof the camera.

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The analog-to-digital converter expects to receive a sin-gle, continuous stream of electrical current as its analoginput. But what if more than one stream of informationneeds to be recorded at the same time? For example, a

scanner that converts hard copy pictures and pages intodigital images uses a moving scan head that takes in theentire width of a page in a single pass. A digital camerahas even more data to take in at the same time. Not onlydoes it divide light passing through the lens into red,green and blue elements, it must register all the lightthat falls, at the same moment, on an area the size of twopostage stamps.

Both the scanner and camera, along with dozens of othercomputer peripherals, solve the problem with a charge-coupled device(CCD). It is a row of photodiodes connectedlike beads on a string. When the CCD is exposed to a variedlight path, such as that reflected from the page of a book,each photodiode converts the amount light it receives into acorresponding voltage. After the microscopic instant whenthe diodes register the amount of light each receives, thecharge at one end of the string is passed off to a circuit lead-ing to an analog-to-digital converter chip. Instantly, all thecharges in the string of diodes move to the diodes next tothem in the direction of the ADC.

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The process continues until the last voltage level is carried off as current to the ADC, which con-verts the voltages to digital values. In this case, because the original image was black and white,only two voltages are createdon or off, giving a visual representation of the analog signalsthat is squared off as current changes abruptly between on and off.

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