跪求一篇英语口语材料大概能说3分钟,描述一项家用电器.是大学里用的.

跪求一篇英语口语材料大概能说3分钟,描述一项家用电器.是大学里用的.
跪求一篇英语口语材料
大概能说3分钟,描述一项家用电器.是大学里用的.

跪求一篇英语口语材料大概能说3分钟,描述一项家用电器.是大学里用的.
The origins of what would become today's television system can be traced back as far as the discovery of the photoconductivity of the element selenium by Willoughby Smith in 1873 and the invention of the scanning disk by Paul Nipkow in 1884. All practical television systems use the fundamental idea of scanning an image to produce a time series signal representation. That representation is then transmitted to a device to reverse the scanning process. The final device, the television (or T.V. set), relies on the human eye to integrate the result into a coherent image.
Electromechanical techniques were developed prior to World War II, most notably by Charles Francis Jenkins and John Logie Baird. Baird gave the world's first public demonstration of a working television system that transmitted moving images with tone graduation (grayscale) on 26 January 1926 at his laboratory in London. Baird further demonstrated the world's first color television transmission on 3 July 1928.
Completely electronic television systems relied on the inventions of Philo Taylor Farnsworth, Vladimir Zworykin and others to produce a system suitable for mass distribution of television programming. Farnsworth gave the world's first public demonstration of an all-electronic television system at the Franklin Institute in Philadelphia on 25 August 1934.
Regular broadcast programming occurred in the United States,[1] the United Kingdom,[2] Germany,[3] France,[4] and the Soviet Union[5] before World War II. The first regular television broadcasts with a modern level of definition (240 or more lines) were made in England in 1936, soon upgraded to the so-called "System A" with 405 lines. Large scale network broadcasting began in the United States in 1946, and television became common in American homes by the middle 1950s. While North American over-the-air broadcasting was originally free of direct marginal cost to the consumer (i.e., cost in excess of acquisition and upkeep of the hardware) and broadcasters were compensated primarily by receipt of advertising revenue, increasingly United States television consumers obtain their programming by subscription to cable television systems or direct-to-home satellite transmissions. In the United Kingdom, on the other hand, the owner of each television must pay a licence fee annually which is used to support the British Broadcasting Corporation.
Elements of a television system
The elements of a simple television system are:
An image source - this may be a camera for live pick-up of images or a flying spot scanner for transmission of films
A sound source.
A transmitter, which modulates one or more television signals with both picture and sound information for transmission.
A receiver (television) which recovers the picture and sound signals from the television broadcast.
A display device, which turns the electrical signals into visible light.
A sound device , which turns electrical signals into sound waves to go along with the picture.
Practical television systems include equipment for selecting different image sources, mixing images from several sources at once, insertion of pre-recorded video signals, synchronizing signals from many sources, and direct image generation by computer for such purposes as station identification. Transmission may be over the air from land-based transmitters, over metal or optical cables, or by radio from synchronous satellites. Digital systems may be inserted anywhere in the chain to provide better image transmission quality, reduction in transmission bandwidth, special effects, or security of transmission from reception by non-subscribers.
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Display technology
Thanks to advances in display technology, there are now several kinds of video displays used in modern TV sets:
CRT: The most common displays are direct-view CRTs for up to 40 in (100 cm) (in 4:3) and 46 in (115 cm) (in 16:9) diagonally. These are still the least expensive, and are a refined technology that can still provide the best overall picture quality. As they do not have a fixed native resolution, in some cases they are also capable of displaying sources with a variety of different resolutions at the best possible image quality. The frame rate or refresh rate of a typical NTSC format CRT TV is 60 Hz, and for the PAL format, it is 50 Hz. A typical NTSC broadcast signal's visible portion has an equivalent resolution of about 640x480 pixels. It actually could be slightly higher than that, but the Vertical Blanking Interval, or VBI, allows other signals to be carried along with the broadcast.
Rear projection: Most very large screen TVs (up to over 100 inch (254 cm)) use projection technology. Three types of projection systems are used in projection TVs: CRT-based, LCD-based, and DLP (reflective micromirror chip) -based. Projection television has been commercially available since the 1970s, but at that time could not match the image sharpness of the CRT; current models are vastly improved, and offer a cost-effective large-screen display. A variation is a video projector, using similar technology, which projects onto a screen.
Flat panel LCD or plasma: Modern advances have brought flat panels to TV that use active matrix LCD or plasma display technology. Flat panel LCDs and plasma displays are as little as 1 inch thick and can be hung on a wall like a picture or put over a pedestal. Some models can also be used as computer monitors.
See also: Liquid crystal display television
Each has its pros and cons. Flat panel LCD display can have narrow viewing angles and so may not suit a home environment. Rear projection screens do not perform well in natural daylight or well lit rooms and so are best suited to dark viewing areas. A complete run down of the pros and cons of each display should be sought before purchasing a single television technology.
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Terminology for televisions
Pixel resolution is the amount of individual points known as pixels on a given screen. A typical resolution of 800x600 means that the television display has 800 pixels across and 600 pixels on the vertical axis. The higher the resolution on a specified display the sharper the image. Contrast ratio is a measurement of the range between the brightest and darkest points on the screen. The higher the contrast ratio, the better looking picture there is in terms of richness, deepness, and shadow detail.
The brightness of a picture measures how vibrant and impacting the colours are. Measured in cd / m2 equivalent to the amount of candles required to power the image.
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Transmission band
There are various bands of frequencies on which televisions work depending upon the country. The VHF and UHF signals in bands III to V are generally used. Lower frequencies do not have enough bandwidth available for television. Although the BBC initially used Band I VHF at 45 MHz, this frequency is no longer in use for this purpose. Band II is used for FM radio transmissions. Higher frequencies behave more like light and do not penetrate buildings or travel around obstructions well enough to be used in a conventional broadcast TV system, so they are generally only used for satellite broadcasting, which uses frequencies around 10 GHz. TV systems in most countries relay the video as an AM (amplitude-modulation) signal and the sound as a FM (frequency-modulation) signal. An exception is France, where the sound is AM.
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Aspect ratios
Aspect ratio refers to the ratio of the horizontal to vertical measurements of a television's picture. Mechanically scanned television as first demonstrated by John Logie Baird in 1926 used a 7:3 vertical aspect ratio, oriented for the head and shoulders of a single person in close-up.
Most of the early electronic TV systems from the mid-1930s onward shared the same aspect ratio of 4:3 which was chosen to match the Academy Ratio used in cinema films at the time. This ratio was also square enough to be conveniently viewed on round cathode-ray tubes (CRTs), which were all that could be produced given the manufacturing technology of the time. (Today's CRT technology allows the manufacture of much wider tubes, and the flat-screen technologies which are becoming steadily more popular have no technical aspect ratio limitations at all.) The BBC's television service used a more squarish 5:4 ratio from 1936 to 3 April 1950, when it too switched to a 4:3 ratio. This did not present significant problems, as most sets at the time used round tubes which were easily adjusted to the 4:3 ratio when the transmissions changed.
In the 1950s, movie studios moved towards widescreen aspect ratios such as CinemaScope in an effort to distance their product from television. Although this was initially just a gimmick, widescreen is still the format of choice today and square aspect ratio movies are rare. Some people argue that widescreen is actually a disadvantage when showing objects that are tall instead of panoramic, others say that natural vision is more panoramic than tall, and therefore widescreen is easier on the eye.
The switch to digital television systems has been used as an opportunity to change the standard television picture format from the old ratio of 4:3 (1.33:1) to an aspect ratio of 16:9 (approximately 1.78:1). This enables TV to get closer to the aspect ratio of modern widescreen movies, which range from 1.66:1 through 1.85:1 to 2.35:1. There are two methods for transporting widescreen content, the better of which uses what is called anamorphic widescreen format. This format is very similar to the technique used to fit a widescreen movie frame inside a 1.33:1 35mm film frame. The image is compressed horizontally when recorded, then expanded again when played back. The anamorphic widescreen 16:9 format was first introduced via European PALPlus television broadcasts and then later on "widescreen" DVDs; the ATSC HDTV system uses straight widescreen format, no horizontal compression or expansion is used.
Recently "widescreen" has spread from television to computing where both desktop and laptop computers are commonly equipped with widescreen displays. There are some complaints about distortions of movie picture ratio due to some DVD playback software not taking account of aspect ratios; but this may subside as the DVD playback software matures. Furthermore, computer and laptop widescreen displays are in the 16:10 aspect ratio both physically in size and in pixel counts, and not in 16:9 of consumer televisions, leading to further complexity. This was a result of widescreen computer display engineers' uninformed assumption that people viewing 16:9 content on their computer would prefer that an area of the screen be reserved for playback controls, subtitles or their Taskbar, as opposed to viewing content full-screen.
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Aspect ratio incompatibility
The television industry's changing of aspect ratios is not without difficulties, and can present a considerable problem.
Displaying a widescreen aspect (rectangular) image on a conventional aspect (square or 4:3) display can be shown:
in "letterbox" format, with black horizontal bars at the top and bottom
with part of the image being cropped, usually the extreme left and right of the image being cut off (or in "pan and scan", parts selected by an operator)
with the image horizontally compressed
A conventional aspect (square or 4:3) image on a widescreen aspect (rectangular with longer horizon) display can be shown:
in "pillar box" format, with black vertical bars to the left and right
with upper and lower portions of the image cut off (or in "tilt and scan", parts selected by an operator)
with the image horizontally distorted
A common compromise is to shoot or create material at an aspect ratio of 14:9, and to lose some image at each side for 4:3 presentation, and some image at top and bottom for 16:9 presentation. In recent years, the cinematographic process known as Super 35 (championed by James Cameron) has been used to film a number of major movies such as Titanic, Legally Blonde, Austin Powers, and Crouching Tiger, Hidden Dragon (see also: List of top-grossing films shot in Super 35). This process results in a camera-negative which can then be used to create both wide-screen theatrical prints, and standard "full screen" releases for television/VHS/DVD which avoid the need for either "letterboxing" or the severe loss of information caused by conventional "pan-and-scan" cropping.
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Sound
Further information: NICAM
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Television add-ons
Today there are many television add-ons including Video Game Consoles, VCRs, Set-top boxes for Cable, Satellite and DVB-T compliant Digital Television reception, DVD players, or Digital Video Recorders (including personal video recorders, PVRs). The add-on market continues to grow as new technologies are developed.
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New developments
Ambilight™
Blu ray
Broadcast flag
CableCARD™
Digital Light Processing (DLP)
Digital Rights Management (DRM)
Digital television (DTV)
Digital Video Recorders
Direct Broadcast Satellite TV (DBS)
DVD
Flicker-free (100 Hz or 120 Hz, depending on country)
HD DVD
High Definition TV (HDTV)
High-Definition Multimedia Interface (HDMI)
IPTV
Internet television
LCD and Plasma display Flat screen TV
SED display technology
OLED display technology
P2PTV
Pay-per-view
Picture-in-picture (PiP)
Pixelplus
Remote controls
Video on-demand (VOD)
Ultra High Definition Video (UHDV)
Web TV