9-2     Related Technologies

1 The Television Camera

The television camera is the first tool used to produce a television program. Most cameras have three basic elements: an optical system for capturing an image, a pickup device for translating the image into electronic signals, and an encoder for encoding signals so they may be transmitted.

(l) Optical System

The optical system of a television camera includes a fixed lens that is used to focus the scene onto the front of the pickup device. Color cameras also have a system of prisms and mirrors that separate incoming light from a scene into the three primary colors: red, green, and blue. Each beam of light is then directed to its own pickup device. Almost any color can be reproduced by combining these colors in the appropriate proportions. Most inexpensive consumer video cameras use a filter that breaks light from an image into the three primary colors.

(2) Pickup Device

The pickup device takes light from a scene and translates it into electronic signals. The first pickup devices used in cameras were camera tubes. The first camera tube used in television was the iconoscope. Invented in the 1920s, it needed a great deal of light to produce a signal, so it was impractical to use in a low-light setting, such as an outdoor evening scene. The image-orthicon tube and the vidicon tube were invented in the 1940s and were a vast improvement on the iconoscope. They needed only about as much light to record a scene as human eyes need to see. Instead of camera tubes, most modem cameras now use light-sensitive integrated circuits (tiny, electronic devices) called charge-coupled devices (CCDs).

When recording television images, the pickup device replaces the function of film used in making movies. In a camera tube pickup device, the front of the tube contains a layer of photosensitive material called a target. In the image-orthicon tube, the target material is photoemissive, that is, it emits electrons when it is struck by light. In the vidicon camera tube, the target material is photoconductive, that is, it conducts electricity when it is struck by light. In both cases, the lens of a camera focuses light from a scene onto the front of the camera tube, and this light causes changes in the target material. The light image is transformed into an electronic image, which can then be read from the back of the target by a beam of electrons (tiny, negatively charged particles).

The beam of electrons is produced by an electron gun at the back of the camera tube. The beam is controlled by a system of electromagnets that make the beam systematically scan the target material. Whenever the electron beam hits the bright parts of the electronic image on the target material, the tube emits a high voltage, and when the beam hits a dark part of the image, the tube emits a low voltage. This varying voltage is the electronic television signal.

A charge-coupled device (CCD) can be much smaller than a camera tube and is much more durable. As a result, cameras with CCDs are more compact and portable than those using a camera tube. The image they create is less vulnerable to distortion and is therefore clearer. In a CCD, the light from a scene strikes an array of photodiodes arranged on a silicon chip. Photodiodes are devices that conduct electricity when they are struck by light; they send this electricity to tiny capacitors. The capacitors store the electrical charge, with the amount of charge stored depending on the strength of the light that struck the photodiode.[1] The CCD converts the incoming light from the scene into an electrical signal by releasing the charges from the photodiodes in an order that follows the scanning pattern that the receiver will follow in re-creating the image.[2]

(3) Encoder

In color television, the signals from the three camera tubes or charge-coupled devices are first amplified, then sent to the encoder before leaving the camera. The encoder combines the three signals into a single electronic signal that contains the brightness information of the colors (luminance). It then adds another signal that contains the code used to combine the colors (color burst), and the synchronization information used to direct the television receiver to follow the same scanning pattern as the camera. [3]The color television receiver uses the color burst part of the signal to separate the three colors again.

2 Scanning

Television cameras and television receivers use a procedure called scanning to record visual images and recreate them on a television screen. The television camera records an image, such as a scene in a television show, by breaking it up into a series of lines and scanning over each line with the beam or beams of electrons contained in the camera tube.[4] The pattern is created in a CCD camera by the array of photodiodes. One scan of an image produces one static picture, like a single frame in a film. The camera must scan a scene many times per second to record a continuous image. In the television receiver, another electron beam -- or set of electron beams, in the case of color television -- uses the signals recorded by the camera to reproduce the original image on the receiver's screen. Just like the beam or beams in the camera, the electron beam in the receiver must scan the screen many times per second to reproduce a continuous image.

In order for television to work, television images must be scanned and recorded in the same manner as television receivers reproduce them. In the United States, broadcasters and television manufacturers have agreed on a standard of breaking images down into 525 horizontal lines, and scanning images 30 times per second. In Europe, most of Asia, and Australia, images are broken down into 625 lines, and they are scanned 25 times per second. Special equipment can be used to make television images that have been recorded in one standard fit a television system that uses a different standard. Telecine equipment (from the words television and cinema) is used to convert film and slide images to television signals. The images from film projectors or slides are directed by a system of mirrors toward the telecine camera, which records the images as video signals.

The scanning method that is most commonly used today is called interlaced scanning. It produces a clear picture that does not fade or flicker. When an image is scanned line by line from top to bottom, the top of the image on the screen will begin to fade by the time the electron beam reaches the bottom of the screen. With interlaced scanning, odd-numbered lines are scanned first, and the remaining even-numbered lines are scanned next. A full image is still produced 30 times a second, but the electron beam travels from the top of the screen to the bottom of the screen twice for every time a full image is produced.

3. Transmission of Television Signals

The audio and video signals of a television program are broadcasted through the air by a transmitter. The transmitter superimposes the information in the camera's electronic signals onto carrier waves. The transmitter amplifies the carrier waves, making them much stronger, and sends them to a transmitting antenna. This transmitting antenna radiates the carrier waves in all directions, and the waves travel through the air to antennas connected to television sets or relay stations.

(1) The Transmitter

The transmitter superimposes the information from the electronic television signal onto carrier waves by modulating (varying) either the wave's amplitude, which corresponds to the wave's strength, or the wave's frequency, which corresponds to the number of times the wave oscillates each second. The amplitude of one carrier wave is modulated to carry the video signal (amplitude modulation, or AM) and the frequency of another wave is modulated to carry the audio signal (frequency modulation, or FM). These waves are combined to produce a carrier wave that contains both the video and audio information. The transmitter first generates and modulates the wave at a low power of several watts. After modulation, the transmitter amplifies the carrier signal to the desired power level, sometimes many kilowatts (1000 watts), depending on how far the signal needs to travel, and then sends the carrier wave to the transmitting antenna.

The frequency of carrier waves is measured in hertz (Hz), which is equal to the number of wave peaks that pass by a point every second. The frequency of the modulated carrier wave varies, covering a range, or band, of about 4 million hertz, or 4 megahertz (4 MHz). This band is much wider than the band needed for radio broadcasting, which is about 10,000 Hz, or 10 kilohertz (10 kHz). Television stations that broadcast in the same area send out carrier waves on different bands of frequencies, each called a channel, so that the signals from different stations do not mix. To accommodate all the channels, which are spaced at least 6 MHz apart, television carrier frequencies are very high. Six MHz does not represent a significant chunk of bandwidth if the television stations broadcast between 50 and 800 MHz.[5] 

In the United States and Canada, there are two ranges of frequency bands that cover 67 different channels. The first range is called very high frequency (VHF), and it includes frequencies from 54 to 72 MHz, from 76 to 88 MHz, and from 174 to 216 MHz. These frequencies correspond to channels 2 through 13 on a television set, The second range, ultrahigh frequency (UHF), includes frequencies from 407 MHz to 806 MHz, and it corresponds to channels 14 through 69.

The high-frequency waves radiated by transmitting antennas can travel only in a straight line, and may be blocked by obstacles in between the transmitting and receiving antennas. For this reason, transmitting antennas must be placed on tall buildings or towers. In practice, these transmitters have a range of about 120 km (75 mi). In addition to being blocked, some television signals may reflect off buildings or hills and reach a receiving antenna a little later than the signals that travel directly to the antenna. The result is a ghost, or second image, that appears on the television screen. Television signals may, however, be sent clearly from almost any point on earth to any other -- and from spacecraft to earth -- by means of cables, microwave relay stations, and communications satellites.

(2) Cable Transmission

Cable television was first developed in the late 1940s to serve shadow areas -- that is, areas that are blocked from receiving signals from a station's transmitting antenna. In these areas, a community antenna receives the signal, and the signal is then redistributed to the shadow areas by coaxial cable (a large cable with a wire core that can transmit the wide band of frequencies required for television) or, more recently, by fiber--optic cable. Viewers in most areas can now subscribe to a cable television service, which provides a wide variety of television programs and films adapted for television that are transmitted by cable directly to the viewer's television set. Digital data-compression techniques, which convert television signals to digital code in an efficient way, will eventually increase cable's capacity to 500 or more channels.

(3) Microwave Relay Transmission

Microwave relay stations are tall towers that receive television signals, amplify them, and retransmit them as a microwave signal to the next relay station. Microwaves are electromagnetic waves that are much shorter than normal television carrier waves and can travel farther. The stations are placed about 50 km (30 mi) apart. Network television stations use relay stations to broadcast to affiliate stations that are located in cities far from the original source of the broadcast. The affiliate stations receive the microwave transmission and rebroadcast it as a normal television signal to the local area.

(4) Satellite Transmission

Communications satellites receive television signals from a ground station, amplify them, and relay them back to the earth over an antenna that covers a specified terrestrial area. The satellites circle the earth in a geosynchronous orbit, which means they stay above the same place on the earth at all times. Instead of a normal aerial antenna, receiving dishes are used to receive the signal and deliver it to the television set or station. The dishes can be fairly small for home use, or large and powerful, such as those used by cable and network television' stations.

Satellite transmissions are used to efficiently distribute television and radio programs from one geographic location to another by networks; cable companies; individual broadcasters; program providers; and industrial, educational, and other organizations. Programs intended for specific subscribers are scrambled so that only the intended recipients, with appropriate decoders, can receive the program.[6]  

Direct-broadcast satellites are used in Europe and Japan to deliver TV programming directly to TV receivers through small home dishes. The Federal Communications Commission (FCC) has licensed several firms to begin DBS service in the United States; in the early 1990s actual launch of DBS satellites was delayed due to the economic factors involved in developing a digital video compression system. The arrival of digital compression, however, made it possible for a single DBS satellite to carry up to 200 TV channels. DBS systems in North America are operating in the Ku band (12.0-19.0 GHz). DBS home systems consist of the receiving dish antenna and a low-noise amplifier that boosts the antenna signal level and feeds it to a coaxial cable. A receiving box converts the superhigh frequency (SHF) signals to lower frequencies and puts them on channels that the home TV set can display.

WORDS AND PHRASES

affiliate           联络,加入;会员,分部

charge-coupled devices (CCDs)    电荷藕合器件

color burst                       彩色脉冲串,彩色同步信号

data-compression                 数据压缩

direct-broadcast satellite(DBS)   直播卫星

fade                        褪色,消失,凋谢

fucker                   闪烁,闪光,颤动

geosynchronous orbit          地球同步轨道

iconoscope         映像管,光电摄像管

image-orthicon                 移像正析像管,低速电子束摄像放大管

interlace             使交错,隔行扫描

oscillate                 振动,振荡

photoemissive                 光电发射的

pickup                  拾音,检出,电视摄像

prism                  棱镜,棱柱

scramble            混杂,变换频率,加密

slide                     滑动装置,幻灯片

superhigh frequency (SHF)    极高频(段)

superimpose    重叠,加上去

synchronization  同步

target material                 屏幕材料

ultrahigh frequency (UHF)      超高频(段)

very high frequency (VHF)      甚高频(段)

videotape recorder             磁带录像机

vidicon                视像管,光导摄像管

NOTES

[1] The capacitors store the electrical charge, with the amount of charge stored depending on the strength of the light that struck the photodiode.

这些电容器存储电荷，所存储的电荷量取决于照射光电二极管的光线强度。

[2] The CCD converts the incoming light from the scene into an electrical signal by releasing the charges from the photodiodes in an order that follows the scanning pattern that the receiver will follow in re-creating the image.

按照扫描方式依次释放光电二极管的电荷，电荷耦合器件就把从景物投进的光线转换成电信号，接收机再现图像时也将遵循同样的扫描方式。

[3] It then adds another signal that contains the code used to combine the colors (color burst), and the synchronization information used to direct the television receiver to follow the same scanning pattern as the camera.

亮度信号还要加上另一个信号，这个信号包含着彩色同步信号（用来组合色彩的的代码）和扫描同步信号（使电视接收机保持与摄像机相同的扫描方式）。

[4] The television camera records an image, such as a scene in a television show, by breaking it up into a series of lines and scanning over each line with the beam or beams of electrons contained in the camera tube.

电视摄像机把图像，例如电视节目中的一个镜头，分解成很多行，并且用摄像管中的一个或者多个电子束扫描每一行，用这种方法记录图像。

[5] To accommodate all the channels, which are spaced at least 6 MHz apart, television carrier frequencies are very high. Six MHz does not represent a significant chunk of bandwidth if the television stations broadcast between 50 and 800 MHz.

为了容纳所有的频道，而且每个频道间最少还得隔开6MHz，因而，电视载频是很高的。如果电视台在50兆赫到800兆赫之间广播，6MHz的带宽就不显得太大了。

[6] Programs intended for specific subscribers are scrambled so that only the intended recipients, with appropriate decoders, can receive the program.

对具体订户的节目经过加密，因此，只有这些用户用适当的解码器才能够接收这类项目。