1、1UNIT 8READINGPassage ANOTES1. PDAA personal digital assistant (PDA) is a handheld computer also known as palmtop computers. Newer PDAs also have both color screens and audio capabilities, enabling them to be used as mobile phones, (smartphones), web browsers, or portable media players. Many PDAs ca
2、n access the Internet, intranets or extranets via Wi-Fi, or Wireless Wide-Area Networks (WWANs). Many PDAs employ touch screen technology.The term was first used on January 7, 1992 by Apple Computer CEO John Sculley at the Consumer Electronics Show in Las Vegas, Nevada, referring to the Apple Newton
3、. In 1996 Nokia introduced the first mobile phone with full PDA functionality, the 9000 Communicator, which has since grown to become the worlds best-selling PDA and which spawned a catetory of phones called the smartphone. Today the vast majority of all PDAs are smartphones, selling over 150 millio
4、n units while non-phone (“stand-alone“) PDAs sell only about 3 million units per year. The RIM Blackberry, the Apple iPhone and the Nokia N-Series are typical smartphones. However, many PDAs (typically those used primarily as telephones) may not have a touch screen, using softkeys, a directional pad
5、 and either the numeric keypad or a thumb keyboard for input. Software typically required to be a PDA includes an appointment calendar, a to-do list, an address book for contacts and some sort of note program. Connected PDAs also typically include E-mail and Web support.2. CPUA central processing un
6、it (CPU) is a machine that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term “CPU“ ever came into widespread usage. The term itself and its initialism have been in use in the computer industry at least since the early
7、 1960s. The form, design and implementation of CPUs have changed dramatically since the earliest examples, but their fundamental operation has remained much the same. Early CPUs were custom-designed as a part of a larger, sometimes one-of-a-kind, computer. However, this costly method of designing cu
8、stom CPUs for a particular application has largely given way to the development of mass-produced processors that are suited for one or many purposes. This standardization trend generally began in the era of discrete transistor mainframes and minicomputers and has rapidly accelerated with the popular
9、ization of the integrated circuit (IC). The IC has allowed increasingly complex CPUs to be designed and manufactured to tolerances on the order of nanometers. Both the miniaturization and standardization of CPUs have increased the presence of these 2digital devices in modern life far beyond the limi
10、ted application of dedicated computing machines. Modern microprocessors appear in everything from automobiles to cell phones to childrens toys.3. RAM (random-access memory)Random-access memory is a form of computer data storage. Today it takes the form of integrated circuits that allow the stored da
11、ta to be accessed in any order (i.e., at random). The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data. This contrasts with storage mechanisms such as tape
12、s, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.4. CRT (cathode-ray tube) The cath
13、ode ray tube is a vacuum tube containing an electron gun (a source of electrons) and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to form images in the form of light emitted from the fluorescent screen. The image may represent electrical wav
14、eforms (oscilloscope), pictures (television, computer monitor), radar targets and others.The single electron beam can be processed in such a way as to display moving pictures in natural colors. The CRT uses an evacuated glass envelope which is large, deep, heavy, and relatively fragile.5. liquid-cry
15、stal display (LCD)A liquid crystal display is an electro-optical amplitude modulator realized as a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source (backlight) or reflector. It is often utilized in battery-powered electronic devices bec
16、ause it uses very small amounts of electric power.Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual liquid cr
17、ystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer.The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This t
18、reatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).Before applying an
19、electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces. In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arra
20、nge themselves in a helical structure, or twist. Because the liquid crystal material is birefringent, light passing through one polarizing filter is rotated by the liquid crystal helix as it passes 3through the liquid crystal layer, allowing it to pass through the second polarized filter. Half of th
21、e incident light is absorbed by the first polarizing filter, but otherwise the entire assembly is reasonably transparent.When a voltage is applied across the electrodes, a torque acts to align the liquid crystal molecules parallel to the electric field, distorting the helical structure (this is resi
22、sted by elastic forces since the molecules are constrained at the surfaces). This reduces the rotation of the polarization of the incident light, and the device appears grey. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwist
23、ed and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal l
24、ayer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.When a large number of pixels are needed in a display, it is not technically possible to drive each directly since then each pixel would require independent electrodes. Instead, the
25、 display is multiplexed. In a multiplexed display, electrodes on one side of the display are grouped and wired together (typically in columns), and each group gets its own voltage source. On the other side, the electrodes are also grouped (typically in rows), with each group getting a voltage sink.
26、The groups are designed so each pixel has a unique, unshared combination of source and sink. The electronics, or the software driving the electronics then turns on sinks in sequence, and drives sources for the pixels of each sink.6. hard driveA hard disk drive (HDD), commonly referred to as a hard d
27、rive, hard disk, or fixed disk drive, is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, “drive“ refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its fl
28、oppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.7. DVD-ROMDVD (also known as “Digital Versatile Disc“ or “Digital Video Disc“) is a popular optical disc storage media format. Its m
29、ain uses are video and data storage. Most DVDs are of the same dimensions as compact discs (CDs) but store more than six times as much data. Variations of the term DVD often describe the way data is stored on the discs: DVD-ROM has data that can only be read and not written, DVD-R and DVD+R can reco
30、rd data only once and then function as a DVD-ROM. DVD-RW, DVD+RW and DVD-RAM can both record and erase data multiple times. The wavelength used by standard DVD lasers is 650 nm, and thus the light has a red color.“DVD“ was originally used as an initialism for the unofficial term “digital visual disc
31、“. It was reported in 1995, at the time of the specification finalization, that the letters officially stood for “digital versatile disc“ (due to non-video applications). However, the text of the press release announcing the specification finalization only refers to the 4technology as “DVD“, making
32、no mention of what (if anything) the letters stood for. A newsgroup FAQ written by Jim Taylor (a prominent figure in the industry) claims that four years later, in 1999, the DVD Forum stated that the format name was simply the three letters “DVD“ and did not stand for anything. The official DVD spec
33、ification documents have never defined DVD. Usage in the present day varies, with “DVD“, “Digital Video Disc“, and “Digital Versatile Disc“ being the most common.8. combo DVD/CD-RW driveA Combo drive is a type of optical drive that combines CD-R/CD-RW recording capability with the ability to read (b
34、ut not write) DVD media. The term is used almost exclusively by Apple Computer as a name for the low-end substitute for their high-end SuperDrive, which was designed to both read and write DVD and DVD recordable media. The device was created as a mid-range option between a CD burner and a DVD burner
35、. The cost difference between a Combo drive and DVD burner has been steadily declining in recent years, so most new PCs (except for the low-end budget computers) have a DVD burner (which also reads and writes CDs).9. USBIn information technology, Universal Serial Bus (USB) is a serial bus standard t
36、o interface devices to a host computer. USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the Plug and Play capabilities by allowing hot swapping, that is, by allowing devices to be connected and disconnected without rebooting the
37、computer or turning off the device. Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.USB is intended to re
38、place many legacy varieties of serial and parallel ports. USB can connect computer peripherals such as mice, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives. For many of those devices USB has become the standard connection method
39、. USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs and video game consoles, and as a bridging power cord between a device and an AC adapter plugged into a wall plug for charging purposes.10. FireWireThe IEEE 1394 interface is a serial bu
40、s interface standard for high-speed communications and isochronous real-time data transfer, frequently used by personal computers, as well as in digital audio, digital video, automotive, and aeronautics applications. The interface is also known by the brand names of FireWire (Apple Inc.), i.LINK (So
41、ny), and Lynx (Texas Instruments). IEEE 1394 has replaced parallel SCSI in many applications, due to lower implementation costs and a simplified, more adaptable cabling system.IEEE 1394 has been adopted as the High Definition Audio-Video Network Alliance (HANA) standard connection interface for A/V
42、(audio/visual) component communication and control. FireWire is also available in wireless, fiber optic, and coaxial versions using the isochronous protocols.5Since the mid 1990s, consumer grade camcorders had included a four-circuit 1394 interface, though, except for premium models, this is becomin
43、g less common. It remains the primary transfer mechanism for almost all high end professional audio and video equipment. Since 2003 many computers intended for home or professional audio/video use have built-in FireWire/i.LINK ports, especially prevalent with Sony and Apples computers and the older
44、iPods. The legacy (alpha) 1394 port is also available on premium retail motherboards.11. ModemModem (from modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The go
45、al is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.Modems are generally classified by the amount of data they can send in a given time, normally mea
46、sured in bits per second, or “bps“. They can also be classified by Baud, the number of times the modem changes its signal state per second. Faster modems are used by Internet users every day, notably cable modems and ADSL modems.12. EthernetEthernet is a family of frame-based computer networking tec
47、hnologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model, through means of network access at the Media Access Control (MAC) /Data Link Layer, and a common
48、addressing format. Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been in use from around 1980 to t
49、he present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET.13. WLANA wireless LAN or WLAN or wireless local area network is the linking of two or more computers or devices using spread-spectrum or OFDM modulation technology based to enable communication between devices in a limited area. This gives users the mobility to move around within a broad coverage area and still be connected to the network. For the home user, wireless has become popular due to ease of installation, and location freedom with the gaining popularity of laptops. Public businesses such a
