Video Display
As we have discussed this before in my previous posts, that the process of video field production and the process of television image generation, are like as motor and generator. Your home TV set works in much the same way as the camera image sensor, except it reverses the process. That is, instead of turning light into electrical energy as the CCD and pickup tube does, your receiver turn electrical energy into light. This is accomplished by scanning the television picture tube with an electron beam. At the back of the picture tube in your television or receiver set is an electron gun, which shoots an electron beam at the inside of the face of the picture tube. The picture tube is coated with the photosensitive material that glows when it is hit by the beam of electrons. A large blast of electrons causes it to glow a lot; a small blast causes little action.
Lines and Frames of Television Scanning
The video image is composed of a number of frames and lines. Because we normally think of the TV process as existing in time, we can measure the frame rate of television rather easily, or you can understand this as that these frames are units of projection and that how much the clear view result is. In many countries, there are 30 frames of video information in per second, and each one of those frames is composed of 525 lines of information. Starting with line number 1, the beam moves across the picture tube until it gets to the end of that line. Then the beam automatically shuts off, returns to the other side of the picture tube, drops down to the line number 3, and scans across that. Once again, when it reaches the end of the line, it automatically shuts off, drops down to line number 5, and repeats the process all over again. When it gets down to the bottom of frame or last line, it shuts off, returns to the top of the picture, and scans the even number lines like 2, 4, 6, … and so on. This process is known as 2:1 interlaced scanning. First the odd number lines are scanned and then the even ones. Each time gun reaches the bottom of the picture, it has completed one field, or 262.5 lines, of information. A complete frame of information (525 lines) is composed of two individual fields: one is of odd numbers and second is of even numbers. The reason of this system of interlaced scanning is simple. The photosensitive surface on your picture tube glows for a very short time when the electron beam hits it. If beam started scanning from the top of the frame and continue down to the bottom, by the time it got there the top of the picture would already have faded to black. To keep part of the picture from fading out or flickering, the lines are interlaced. As a result, the picture maintains its brightness throughout the program. Incidentally, the 525 lines per frame standard characteristic of U.S. television is an arbitrary standard. That is, the system could have more or less lines still function. Indeed many other countries use 625 line systems, which actually provide greater detail than 525 lines. The system used in U.S. and Canada was adopted as the represented the major U.S. electronic manufactures at the time TV was invented. The Federal Communications Commission (FCC) adopted the NTSC recommendations for the 525 line 30 frame television standard in 1941.
Controlling of Video Signal
Horizontal and Vertical Synchronizing
As we have discussed, each frame of video information is constructed by combining picture information and synchronizing information. Among the most important synchronizing control pulses are horizontal and vertical sync and blanking pulses. These pulses are generated by a sync generator that can be located as an integral component inside the camera or as a separate component outside the camera. The horizontal sync and blanking pulses control the timing of each line of video information, the vertical sync and blanking pulses controls the timing of each field and frame of information. Essentially, each line of information begins with a horizontal sync pulse and ends with a horizontal blanking pulse. Similarly, each field begins with a vertical sync pulse and ends with a vertical blanking pulse. Thus you can see that for each frame of video information there are 525 lines of information and 525 horizontal blanking and sync pulses. These 525 lines are arranged in two fields of information along with two vertical blanking and sync pulses. The sync pulses not only allow the system to work but also this information becomes extremely important when we get to the area of video tape editing.
Internal and External Sync
When sync pulses are generated within the camera, we refer to the sync as internal sync. When sync pulses are generated outside the camera, we refer to the sync as external sync. When a single camcorder is used for video field production, the horizontal and vertical sync pulses are produced internally in the camcorder itself. Most single-camera field production units fall into this category. In more complex multiple camera field production system, which include a video switcher and several cameras operating simultaneously, all camera must scan synchronously. To accomplish this, they all must have the same reference to horizontal and vertical sync. In such a situation, an external sync generator is used to regulate the timing of the all the camera source. Sometimes, the sync pulses generated by one camera can be used to drive the signal of another camera through a process called gen-lock. In this process, the second camera senses the incoming sync pulses from the first camera and then creates its own video signal synchronously with the other camera. The Video Waveform As this discussion has already indicated, the video signal is somewhat complex because it contains not only picture information but also synchronizing information. The picture information alone is referred to as a noncomposite signal. When the video information and sync are both present in a signal, it is referred to as a composite signal. Let’s continue our discussion of the video signal by talking more specially about the black and white picture signal. Black and white television presents a range of brightness only, elements in the picture are somewhere between white and black. This range of variation between white and black, or between the brightest and darkest parts of a scene, can be seen in the video waveform, which shows us what the video signal actually looks like. Figure below shows the components of one line of information of a typical video waveform.
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