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1、 外文资料及译文 原文：Television Video SignalsAlthough over 50 years old , the standard television signal is still one of the most common way to transmit an image. Figure 8.3 shows how the television signal appears on an oscilloscope. This is called composite video, meaning that there are vertical and horizon
2、tal synchronization (sync) pulses mixed with the actual picture information. These pulses are used in the television receiver to synchronize the vertical and horizontal deflection circuits to match the video being displayed. Each second of standard video contains 30 complete images, commonly called
3、frames , A video engineer would say that each frame contains 525 lines, the television jargon for what programmers call rows. This number is a little deceptive because only 480 to 486 of these lines contain video information; the remaining 39to 45 lines are reserved for sync pulses to keep the telev
4、isions circuits synchronized with the video signal.Standard television uses an interlaced format to reduce flicker in the displayed image. This means that all the odd lines of each frame are transmitted first, followed by the even lines. The group of odd lines is called the odd field, and the group
5、of even lines is called the even field. Since each frame consists of two fields, the video signal transmits 60 fields per second. Each field starts with a complex series of vertical sync pulses lasting 1.3 milliseconds. This is followed by either the even or odd lines of video. Each line lasts for 6
6、3.5 microseconds, including a 10.2 microsecond horizontal sync pulse, separating one line from the next. Within each line, the analog voltage corresponds to the gray scale of the image, with brighter values being in the direction away from the sync pulses. This place the sync beyond the black range.
7、 In video jargon, the sync pulses are said to be blacker than black.The hardware used for analog-to-digital conversion of video signals is called a frame grabber. This is usually in the form of an electronics card that plugs into a computer, and connects to a camera through a coaxial cable. Upon com
8、mand from software, the frame grabber waits for the beginning of the next frame, as indicated by the vertical sync pulses. During the following two fields,each line of video is sampled many times, typically 512,640 or 720 samples per line, at 8bits per sample. These samples are stored in memory as o
9、ne row of the digital image.This way of acquiring a digital image results in an important difference between the vertical and horizontal directions. Each row in the digital image corresponds to one line in the video signal, and therefore to one row of wells in the CCD. Unfortunately, the columns are
10、 not so straightforward. In the CCD, each row contains between about 400 and 800 wells (columns), depending on the particular device used. When a row of wells is read from the CCD, the resulting line of video is filtered into a smooth analog signal, such as in Figure 8.3. In other words, the video s
11、ignal does not depend on how many columns are present in the CCD. The resolution in the horizontal direction is limited by how rapidly the analog signal is allowed to change. This is usually set at 3.2 MHz for color television, resulting in a rise time of about 100 nanoseconds, i.e, about 1/500th of
12、 the 53.2 microsecond video line.When the video signal is digitized in the frame grabber, it is converted back into columns, However, these columns in the digitized image have no relation to the columns in the CCD. The number of columns in the digital image depends solely on how many times the frame
13、 grabber samples each line of video. For example, a CCD might have 800 wells per row, while the digitized image might only have 512 pixels (i.e , columns) per row.The number of columns in the digitized image is also important for another reason. The standard television image has an aspect ratio of 4
14、 to 3, i.e. , it is slightly wider than it is high. Motion pictures have the wider aspect ratio of 25 to 9. CCDs used for scientific applications often have an aspect ratio of 1 to 1, i.e , a perfect square. In any event, the aspect ratio of a CCD is fixed by the placement of the electrodes, and can
15、not be altered. However, the aspect ratio of the digitized image depends on the number of samples per line. This becomes a problem when the image is displayed, either on a video monitor or in a hardcopy. If the aspect ratio isnt properly reproduced, the image looks squashed horizontally or verticall
16、y.The 525 line video signal described here is called NTSC (National Television Systems Committee), a standard defined way back in 1954. This is the system used in the United States and Japan. In Europe there are two similar standards called PAL (Phase Alternation by Line) and SECAM (Sequential Chrominance And Memory). The basic concepts are the same , just the numbers are different. Both PAL and SECAM operate with 25 interlaced frames per second, with 625 lin
