机械数控专业--外文资料及翻译

外文资料及译文

原文：

Television Video Signals

Although 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 horizontal 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 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 television’s 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 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 63.5 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. 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 command 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 one 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 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 signal 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 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 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 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 cannot 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 isn’t properly reproduced, the image looks squashed horizontally or vertically.

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 lines per frame. Just as with NTSC, some of these lines occur during the vertical sync, resulting in about 576 lines that carry picture information. Other more subtle differences relate to how color and sound are added to the signal.

The most straightforward way of transmitting color television would be to have three separate analog signals, one for each of the three colors the human eye can detect: red, green and blue. Unfortunately, the historical development of television did not allow such a simple scheme. The color television signal was developed to allow existing black and white television sets to remain in use without modification. This was done by retaining the same signal for brightness information , but adding a separate signal for color information. In video jargon, the brightness is

called the luminance signal, while the color is the chrominance signal. The chrominance signal is contained on a 3.58 MHz carrier wave added to the black and white video signal. Sound is added in this same way, on a 4.5 MHz carrier wave. The television receiver separates these three signals, processes them individually, and recombines them in the final diplay.

译文：

关键词：核心，合成信号，电压耦合

电视信号

尽管已经拥有50年的历史了,电视信号依然是常用的传递信息的途径之一。图 8.3演示了电视信号如何出现在一个示波器上。这叫做合成信号,意谓有垂直的方向和水平的方向的合成(同步)和真实的图片数据混合的脉冲信号。

这些脉冲被电视接收器同垂直与水平线以及其他歪斜线路配和成信号并被电视显示出来。标准的信号每秒包含30个完整的图像,一般被做成了体格,电视工程师会把每个体格编制成包含525条行（电视专门术语）。因为在这些线中的只有80到486条包含了电视信号的数据；剩余39到45条行被同步脉冲保留用以维持电视能与信号一起同时被使用,所以这一个数字稍微具有一定的迷惑性。

标准的电视信号使用了一个被交织的格式以便减少显示时图像的闪烁。这就意谓着每个体格中的所有奇数的线首先被传输,而那些平坦的线然后跟随着被传输。那群奇数的线被叫做奇数领域, 和另外一群线叫做平坦领域。由于每个体格都是由二个领域组成,并且每秒以60个领域的速度进行信号传送。由一个复杂的连续垂直的同步脉冲长1.3个毫秒领域开始。这与跟随线或电视的平坦或奇数的线相结合。每条线的速度为63.5个微秒,包括一个10.2微秒的水平线以同步脉冲持续,分开并从下一个阶段排成一行。在每条线里面,类比电压符合图像的灰色刻度,由较明亮的线在水平方向中远离同步脉冲。在超过黑色的范围这一个地方同步。在电视的专门术语中，同步脉冲被说成是比黑色的线更具有黑色性。

作为电视的信号类比到转变为传送信号的硬件叫做一个体系的核心。通常是以一张的形式插入到一部计算机中,而且经由一个同桥电缆线连接到一个摄像机的电子学卡片的形式。由来自软件的指令之下，核心等候下一个体格的开始,如垂直的同步脉冲所指出。在下列各项领域的出现的时候，电视的每条线许多次被抽取样品,典型地以每线512,640或720个三种样品,每样品8B。这些样品被储存就像传送图像一样被记忆.

这样获得的传送图像造成在垂直和水平线之间的一种明显的不同方向。每个在数传图像中符合电视的信号排成一行,并因此在电压耦合元件中输出。然而，信号并不是如此垂直。在电压耦合元件中，每排包含在约400和800之间输出,依赖一种被用的特别装置。当从电压耦合元件读出来时,电视的产生线进入平滑的类比信号之内然后被过滤, 如此就如在图 8.3 中所显示的那样. 换句话说，电视信号并不依赖信号在电压耦合元件中存在的多少。水平的方向被限制类比信号有多快的速度决定了其是否允许被改变。这通常是以 3.2个百万赫兹为彩色电视

放置,造成上升时间大约 100个十亿分之一秒,i.e,约 1/53.2 微秒中的第 500个电视信号线。

当电视的信号在核心中被数字化的时候,然而,它被转换返回专栏,被数字化了的图像专栏没有关系到电压耦合元件的专栏。数传图像的专栏数字独自地依赖核心抽取样品许多次电视信号的每条线。举例来说，一个电压耦合元件可能每一排有800得好,当被数字化的图像只可能有每排 512个图素 ( i.e,专栏) 的时候。

被数字化的图像专栏的数字也对另外的一个非常重要的理由。标准电视图像要占3/4,也就是,有些稍微宽有些稍微高一些。体育照片就有9/25的宽度比。作为科学的申请电压耦合元件时常用1：1的宽度比，i.e ，就是一个完美的正方形。无论如何，电压耦合元件的方向比被电极的安置调整,而且不能够再被改变。然而，被数字化的图像方向比依赖每条样品线的数字。当图像在电视监视器上或在显示器中被显示的时候,这就变成了一个问题。如果这方面不能被适当地调整,图像容貌就会水平方向或垂直方向压扁。

信号在这里描述的525行电视信号被称为国际电视系统委员会(国家的电视系统委员会),一个标准一直到1954定义了其方法。这是沿用于美国和日本的系统。在欧洲有二个被称为可程序化行列逻辑(时期交互线)和SECAM 的相似标准。(继续影象和记忆)他们基本的观念是相同的,只是数字不同而已。可程序化行列逻辑SECAM 操作由于25使其交织成了一秒体格,由每体格 625条行。正如国际电视系统委员会所说，一些线在垂直的同步期间发生,大约造成进位画数据的576条行。其他的较敏感的方面不同例如如何把颜色和声音增加到信号之中。

传输彩色电视的最直接的要求要有三个分开的类比信号,一为人类的眼睛能发现的三种颜色:红色,绿色和蓝色。然而，电视的历史发展并不是一个如此简单的方案。彩色电视信号被发展并允许存在于黑白的电视，其设定在没有修正的使用当中保存。这被称为光亮数据并保有相同的信号,是增加一个分开信号为彩色数据。在电视的专门术语中，光亮叫做亮度信号,当颜色成为电视信号的时候。电视信号被包含在能增加到黑白的电视信号的一个3.58百万赫兹的运送波上。声音以相同的方式被增加在一个4.5百万赫兹运送到波上。电视接收器分开这三信号，独立地处理他们，并且最后在显示器中结合他们。