“television”的概念、定义、翻译、参考文献-科学参考

television

Physics

The transmission and reception of moving images by means of radio waves or cable. The scene to be transmitted is focused onto a photoelectric screen in the television camera. This screen is scanned by an electron beam. The camera produces an electric current, the instantaneous magnitude of which is proportional to the brightness of the portion of the screen being scanned. In Europe the screen is scanned by 625 lines, and 25 such frames are produced every second. In the USA 525 lines and 30 frames per second are used. The picture signal so produced is used to modulate a VHF or UHF carrier wave and is transmitted with an independent sound signal, but with colour information (if any) incorporated into the brief gaps between the picture lines. The signals received by the receiving aerial are demodulated in the receiver. Until recently, the demodulated picture signal controlled the electron beam in a cathode-ray tube, on the screen of which the picture was reconstructed. However, televisions based on cathode-ray tubes have now largely been replaced by televisions based on LCDs or plasma displays. See also colour television.

Electronics and Electrical Engineering

A telecommunication system in which both visual and aural information is transmitted for reproduction at a receiver. The basic elements of a traditional television system are as follows.
Television cameras and microphones convert the original visual and aural information into electrical signals, i.e. into video signals and audio signals respectively.
Amplifiers and control and transmission circuits transmit the information along a suitable communication channel. Broadcast television uses a modulated radiofrequency carrier wave; in digital television systems, the picture information is encoded into digital form at the transmitter and decoded at the receiver (see digital codes).
A traditional television receiver detects the signals and produces an image on the screen of a specially designed cathode-ray tube and a simultaneous sound output from a loudspeaker. In modern televisions, cathode-ray tubes have largely been replaced by LCDs or plasma displays (see flatscreen display).
The information on the target in the television camera tube is extracted by scanning and the spot on the screen of the receiver tube is scanned in synchronism with it to produce the final image. A process of rectilinear scanning is used in which the electron beam traverses the target area in both the horizontal and vertical directions. The horizontal direction is termed the line and the vertical direction is the field. Sawtooth waveforms are used to produce the deflections of the beam and in both the camera and receiver the flyback period is blanked out.
The synchronizing pulses synchronize the camera and the receiver and are transmitted during the flyback. The line synchronizing pulses are transmitted during the line flyback period and the field synchronizing pulses during the field flyback. A combination of overscanning (see scanning) and blanking is often used in order to allow a sufficiently long interval for synchronization without loss of picture information. During the blanked interval the level of the signal is held at a reference value, termed the blanking level, that represents the blackest elements of the picture except for the synchronizing pulses. This allows easy recognition of the synchronizing pulses at the receiver. The interval immediately preceding the synchronizing pulse is termed the front porch and the interval immediately succeeding the sync pulse is the back porch. The synchronizing pulses are extracted from the video signal by means of a sync separator in the receiver.
For the maximum information to be obtained from the target area of the receiver the number of horizontal scans is made larger than the number of vertical scans so that as much of the target area is covered as possible. The number of lines traversed per second is the line frequency; the number of vertical scans per second is the field frequency. The scanning process is most important since imperfections in scanning or synchronization between transmitter and receiver can result in geometric distortion of the picture or in other faults, such as pairing. A method of scanning that produces the entire picture in a single field is termed sequential scanning. The pattern of horizontal scanning lines is called the raster.
Most broadcast television systems use a system of interlaced scanning. In this system the lines of successive rasters are not superimposed on each other but are interlaced; two rasters constitute a complete picture or frame. The number of complete pictures per second is the frame frequency, which is half the number of rasters per second, i.e. half the field frequency. The field frequency needs to be relatively slow to allow as many horizontal lines as possible but sufficiently fast to eliminate flicker. Various compromises are used. European television systems use a 50 hertz field frequency (25 Hz frame frequency) system with 625 lines per frame. American television uses a 60 Hz field frequency and 525 lines per frame.
Definition in television is a measure of the resolution of the system, which in turn depends on the number of lines per frame. High-definition systems have more lines. Some closed-circuit television systems use as many as 2000 lines per frame. The relationship between the total number of scanning lines per field and the corresponding bandwidth of the video signal is given by the Kell factor.
Positive or negative transmission may be employed for transmitting the video signals, positive transmission being most often used. In positive transmission an increase in amplitude or frequency above the reference black level of the received signal is proportional to the light intensity. The peak value of the video signal that corresponds to the lightest area of the picture is the white peak. In negative transmission the carrier wave value decreases below the black level in proportion to the light intensity.
The basic television system transmits images in black and white only (monochrome television). In colour television the broadcast signal is received on colour receivers (see colour picture tube). Monochrome receivers use the brightness information transmitted as part of the colour signal – the luminance signal – but the image produced is black and white.
The transmitted signal contains both video and audio information. In analogue television systems the picture carrier is the carrier wave modulated by the video information. The audio signal modulates a second carrier wave, termed the sound carrier. The sound-carrier frequency differs from that of the picture carrier and is chosen so that both signals fall within a designated radiofrequency band but do not overlap each other.
http://www.tvhistory.tv/index.html A fully illustrated history of the television set

单词	television
释义	television Physics The transmission and reception of moving images by means of radio waves or cable. The scene to be transmitted is focused onto a photoelectric screen in the television camera. This screen is scanned by an electron beam. The camera produces an electric current, the instantaneous magnitude of which is proportional to the brightness of the portion of the screen being scanned. In Europe the screen is scanned by 625 lines, and 25 such frames are produced every second. In the USA 525 lines and 30 frames per second are used. The picture signal so produced is used to modulate a VHF or UHF carrier wave and is transmitted with an independent sound signal, but with colour information (if any) incorporated into the brief gaps between the picture lines. The signals received by the receiving aerial are demodulated in the receiver. Until recently, the demodulated picture signal controlled the electron beam in a cathode-ray tube, on the screen of which the picture was reconstructed. However, televisions based on cathode-ray tubes have now largely been replaced by televisions based on LCDs or plasma displays. See also colour television. Electronics and Electrical Engineering A telecommunication system in which both visual and aural information is transmitted for reproduction at a receiver. The basic elements of a traditional television system are as follows. Television cameras and microphones convert the original visual and aural information into electrical signals, i.e. into video signals and audio signals respectively. Amplifiers and control and transmission circuits transmit the information along a suitable communication channel. Broadcast television uses a modulated radiofrequency carrier wave; in digital television systems, the picture information is encoded into digital form at the transmitter and decoded at the receiver (see digital codes). A traditional television receiver detects the signals and produces an image on the screen of a specially designed cathode-ray tube and a simultaneous sound output from a loudspeaker. In modern televisions, cathode-ray tubes have largely been replaced by LCDs or plasma displays (see flatscreen display). The information on the target in the television camera tube is extracted by scanning and the spot on the screen of the receiver tube is scanned in synchronism with it to produce the final image. A process of rectilinear scanning is used in which the electron beam traverses the target area in both the horizontal and vertical directions. The horizontal direction is termed the line and the vertical direction is the field. Sawtooth waveforms are used to produce the deflections of the beam and in both the camera and receiver the flyback period is blanked out. The synchronizing pulses synchronize the camera and the receiver and are transmitted during the flyback. The line synchronizing pulses are transmitted during the line flyback period and the field synchronizing pulses during the field flyback. A combination of overscanning (see scanning) and blanking is often used in order to allow a sufficiently long interval for synchronization without loss of picture information. During the blanked interval the level of the signal is held at a reference value, termed the blanking level, that represents the blackest elements of the picture except for the synchronizing pulses. This allows easy recognition of the synchronizing pulses at the receiver. The interval immediately preceding the synchronizing pulse is termed the front porch and the interval immediately succeeding the sync pulse is the back porch. The synchronizing pulses are extracted from the video signal by means of a sync separator in the receiver. For the maximum information to be obtained from the target area of the receiver the number of horizontal scans is made larger than the number of vertical scans so that as much of the target area is covered as possible. The number of lines traversed per second is the line frequency; the number of vertical scans per second is the field frequency. The scanning process is most important since imperfections in scanning or synchronization between transmitter and receiver can result in geometric distortion of the picture or in other faults, such as pairing. A method of scanning that produces the entire picture in a single field is termed sequential scanning. The pattern of horizontal scanning lines is called the raster. Most broadcast television systems use a system of interlaced scanning. In this system the lines of successive rasters are not superimposed on each other but are interlaced; two rasters constitute a complete picture or frame. The number of complete pictures per second is the frame frequency, which is half the number of rasters per second, i.e. half the field frequency. The field frequency needs to be relatively slow to allow as many horizontal lines as possible but sufficiently fast to eliminate flicker. Various compromises are used. European television systems use a 50 hertz field frequency (25 Hz frame frequency) system with 625 lines per frame. American television uses a 60 Hz field frequency and 525 lines per frame. Definition in television is a measure of the resolution of the system, which in turn depends on the number of lines per frame. High-definition systems have more lines. Some closed-circuit television systems use as many as 2000 lines per frame. The relationship between the total number of scanning lines per field and the corresponding bandwidth of the video signal is given by the Kell factor. Positive or negative transmission may be employed for transmitting the video signals, positive transmission being most often used. In positive transmission an increase in amplitude or frequency above the reference black level of the received signal is proportional to the light intensity. The peak value of the video signal that corresponds to the lightest area of the picture is the white peak. In negative transmission the carrier wave value decreases below the black level in proportion to the light intensity. The basic television system transmits images in black and white only (monochrome television). In colour television the broadcast signal is received on colour receivers (see colour picture tube). Monochrome receivers use the brightness information transmitted as part of the colour signal – the luminance signal – but the image produced is black and white. The transmitted signal contains both video and audio information. In analogue television systems the picture carrier is the carrier wave modulated by the video information. The audio signal modulates a second carrier wave, termed the sound carrier. The sound-carrier frequency differs from that of the picture carrier and is chosen so that both signals fall within a designated radiofrequency band but do not overlap each other. http://www.tvhistory.tv/index.html A fully illustrated history of the television set
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