1.3 Input/Output Systems

1.3.1 Text

Keyboard

If you are familiar with a typewriter, you’ll find the layout of the computer keyboard very similar. You can use your keyboard for many purposes:

· Typing information

· Entering numbers with the numeric keypad

· Requesting specific functions

· Performing system functions with key combinations

· Moving around the computer screen

The keyboard has letter keys, punctuation keys, and a spacebar. It also has functions, numeric, and arrow keys. How you use the keys depends on the software installed on your computer. The documentation that comes with your software has information about specific key functions. You will probably notice a difference between the touch (response) on a computer keyboard and the response of a typewriter. A computer keyboard is so responsive that you can type using a light touch. When you hold down a character key, the character continues to type. This is called the typematic effect of a computer keyboard.

Monitor

Monitors maybe are one of the most important output devices. Computers only use monitors to show you exciting operation results or marvelous and vivid pictures. Monitors also are the best windows for conversation between users and computers. So, many users select monitors carefully. Which parameters or indexes ought be paid attention to when you select a monitor? We provide some here for your reference.

Element Distance: The distance between two picture elements in horizontal direction is called element distance here and its current value in most PC monitors is 0.28mm. If the value is smaller, the screen is more distinct.

Video Bandwidth: It is an important concept in monitor technology. It is related to the highest work frequency of the monitor. It is from tens MHz to hundreds MHz.

Solution: It is another important parameter of a monitor. It’s higher, the view on a screen is clearer. Solution means the sum of all picture elements on a screen.

Scan Style: The scan style of a electron gun in a tube is divided into two styles: interlace and non-interlace. In interlace style, electron-beam sweeps elements in odd rows first time and does elements in even rows second time. A frame to be renewed needs sweeping two times. In non-interlace style, electron-beam sweeps all elements only in one time. In non-interlace work style, the monitor works better and gives clear pictures without flash.

Mouse

The interface between a mouse and a system can take one of two forms: the mouse either generates a series of pulses when it is moved (using the LED and detector to generate the pulses), or it increments and decrements counters. The processor can periodically read these counters, or count up the pulses, and determine how far the mouse has moved since it was last examined. The system then moves the cursor on the screen appropriately. This motion appears smooth because the rate at which you can move the mouse is slow compared with the rate at which the processor can read the mouse status and move the cursor on the screen.

Most mice also include one or more buttons, and the system must be able to detect when a button is depressed. By monitoring the status of the button, the system can also differentiate between clicking the button and holding it down. Of course, the mapping between the counters and the button position and what happens on the screen is totally controlled by software. That’s why, for example, the rate at which the mouse moves across the screen and the rate at which single and double clicks are recognized can usually be set by the user. Similarly, software interpretation of the mouse position means that the cursor doesn’t jump completely off the screen when the mouse is moved a long distance in one direction.

Optical Disks

An optical disk is a disk on which data are encoded for retrieval by a laser. Optical disks offer information densities far beyond the range of current magnetic mass-storage devices. Similar devices have been on the market for several years in the form of laser videodisks and audio compact disks (CDs) for consumer use. These laser videodisks are analog, that is, the disk contains one spiral track, like the track on a phonograph record. Optical disks for computer applications are digital and store their information on concentric tracks, like their magnetic cousins. Currently, three versions of optical disk technology are competing for the mass-storage market, they are read-only optical disks, write-once optical disks, and erasable optical disks.

Unlike conventional magnetic disks, read-only optical disks cannot be written on and so have the functional equivalence of read-only memory. The most popular version of read-only optical disks employs the same technology as the CD that has become popular for audio recording. The technology is digital and based on a 4 3/4 inch optical disk that can store 540MB on a single side. The devices are called Compact Disk Read-Only Memories (CD-ROM).

Write-once optical disks (also called write-once, read-mostly, or WORM) are blank disks that are recorded on by the user. To write data, a powerful beam of laser light burns tiny spots or pits into the coating that covers the surface of these disks. Once burnt in, the spots are not erasable. To retrieve the data, a less powerful laser is used to read the pattern of spots and convert the patterns into audiovisual signals that can be played back on a television set. Write-once optical disks are being used to replace microfilm storage. Because optical disks have the ability to store images as well as sound, their use is quite versatile. Anything that can be digitized, such as documents, pictures, photographs, line drawings, and music, can be recorded and stored on an optical disk.

Erasable optical disks use lasers to read and write information to and from the disk but also use a magnetic material on the surface of the disk and a magnetic write head to achieve erasability. To write on such as disk, a laser beam heats a tiny spot on it; then a magnetic field is applied to reverse the magnetic polarity of the spot. Erasable optical disk systems offer the same storage capabilities of the non-erasable optical disks, along with the same reusability capabilities of conventional magnetic disks, such as Winchester systems.

Bus

A bus is a shared communication link, which uses one set of wires to connect multiple subsystems. The two major advantages of the bus organization are versatility and low cost. By defining a single connection scheme, new devices can easily be added, and peripherals can even be moved between computer systems that use the same kind of bus. Furthermore, buses are cost effective, because a single set of wires is shared in multiple ways. The major disadvantage of a bus is that it creates a communication bottleneck, possibly limiting the maximum I/O throughput. When I/O must pass through a single bus, the bandwidth of that bus limits the maximum I/O throughput.

One reason bus design is so difficult is that the maximum bus speed is largely limited by physical factors: the length of the bus and the number of devices. These physical limits prevent us from running the bus arbitrarily fast. Within these limits, there are a variety of techniques we can use to increase the performance of the bus; however, these techniques may adversely affect other performance metrics. For example, to obtain fast response time for I/O operations, we must minimize the bus latency by streamlining the communication path. On the other hand, to sustain high I/O data rates, we must maximize the bus bandwidth. The bus bandwidth can be increased by using more buffering and by communicating larger blocks of data, both of which increase the bus latency! Clearly, these two goals, low latency and high bandwidth can lead to conflicting design requirements. Finally, the need to support a range of devices with widely varying latencies and data transfer rates also makes bus design challenging.

A bus generally contains a set of control lines and a set of data lines. The control lines are used to signal requests and acknowledgments, and to indicate what type of information is on the data lines. The data lines of the bus carry information between the source and the destination. This information may consist of data, complex commands, or addresses. For example, if a disk wants to write some data into memory from a disk sector, the data lines will be used to indicate the address in memory in which to place the data as well as to carry the actual data from the disk. The control lines will be used to indicate what type of information is contained on the data lines of the bus at each point in the transfer. Some buses have two sets of signal lines to separately communicate both data and address in a single bus transmission. In either case, the control lines are used to indicate what the bus contains and to implement the bus protocol.

Key Words

acknowledgement 承认，确认

appropriately 适当地

bandwidth 带宽

blank 空白的

cursor 光标

documentation 文档，文件

interface 接口，界面

latency 潜伏，潜在

layout 布局，分布

marvelous 令人惊异的，了不起的

minimize 最小化

mouse 鼠标

non-interlace 非隔行

optical 光学的

parameter 参数

periodically 定期地

punctuation 标点符号

responsive 响应，应答的

reusability 可重复使用的

solution 分辨率

spacebar 空格键

streamline 流线，流线型

subsystem 子系统

sustain 承受，支持

typewriter 打字机

video 视频

1.3.2 Exercises

1.Translate the following phrases into English

（1）通信瓶颈

（2）总线协议

（3）像素距离

（4）只读光盘

（5）电子枪

（6）输出设备

（7）水平方向

（8）总线带宽

2.Translate the following phrases into Chinese

（1）numeric keypad

（2）electron-beam

（3）typematic effect

（4）scan style

（5）data transfer rate

（6）source and the destination

（7）magnetic polarity

（8）disk sector

3.Identify the following to be True or False according to the text

（1）The two major advantages of the bus organization are versatility and low cost.

（2）The mouse can use LED to generate pulses.

（3）Keyboard, mouse and monitor are all input devices.

（4）The bandwidth of a bus may limit the minimum I/O throughput.

（5）A monitor is the best window for conversation between users and a computer.

（6）Peripherals can be moved between computer systems that use the same kind of bus.

（7）When we design a bus, we do not need to consider the bus speed.

（8）A bus generally contains a set of control lines and data lines.

4.Reading Comprehension

（1）Which is wrong in below four items? ______________

a.The keyboard has letter keys and punctuation keys.

b.The keyboard has spacebar and punctuation keys.

c.The keyboard has function keys and numeric keys.

d.The keyboard has not arrow keys.

（2）______________ is the distance between two picture elements in horizontal direction.

a.Element Distance

b.Scan Style

c.Solution

d.Vertical Scan Rate

（3）The _______________ is used to implement the bus protocol.

a.data bus

b.control bus

c.address bus

d.signal bus

（4）Video Bandwidth is related to the highest work frequency of the monitor. It is from___________________.

a.five MHz to hundreds MHz

b.tens MHz to twenty MHz

c.tens MHz to hundreds MHz

d.tens MHz to thousands MHz

1.3.3 Reading material

How MP3 Files Work

The MP3 format is a compression system for music. The MP3 format helps reduce the number of bytes in a song without hurting the quality of the song’s sound. The goal of the MP3 format is to compress a CD-quality song by a factor of 10 to 14 without noticeably affecting the CD-quality sound. With MP3, a 32-megabyte song on a CD compresses down to about 3MB. This lets you download a song in minutes rather than hours, and store hundreds of songs on your computer’s hard disk without taking up that much space.

Is it possible to compress a song without hurting its quality? We use compression algorithms for images all the time. For example, a GIF file is a compressed image. So is a JPG file. We create Zip files to compress text. So we are familiar with compression algorithms for images and words and we know they work. To make a good compression algorithm for sound, a technique called perceptual noise shaping is used. It is “perceptual” partly because the MP3 format uses characteristics of the human ear to design the compression algorithm. For example:

· There are certain sounds that the human ear cannot hear.

· There are certain sounds that the human ear can hear much better than others.

· If there are two sounds playing simultaneously, we can hear the louder one but cannot hear the softer one.

Using facts like these, certain parts of a song can be eliminated without significantly hurting the quality of the song for the listener. Compressing the rest of the song with well-known compression techniques shrinks the song considerably—by a factor of 10 at least. When you are done creating an MP3 file, what you have is a “near CD quality” song. The MP3 version of the song does not sound exactly the same as the original CD song because some of it has been removed, but it’s very close.

From this description, you can see that MP3 is nothing magical. It is simply a file format that compresses a song into a smaller size so it is easier to move around on the Internet and store.

There are literally thousands of sites on the Web where you can download MP3 files. Go to one of these sites, find a song and download it to your hard disk. Most songs range between 2 and 4MB, so it will take 10 to 15 minutes unless you have a high-speed Internet connection. Once the song has finished downloading, try to double-click on the file and see what happens. If your computer plays it, then you are set.

If you find that you cannot play it，then you need to download an MP3 player. There are dozens of players available, and most of them are free or shareware. One of the most popular is WinAmp, which you can download from www.winamp.com.

You are now ready to begin collecting MP3 files and saving them on your computer. Many people who start collecting MP3 files find that they want to listen to them in all kinds of places. Small, portable MP3 players answer this need. These players plug into your computer’s parallel, FireWire or USB port to transfer the data, and a software application lets you transfer your MP3s into the player by simply dragging the files.

If you have a CD collection and would like to convert songs from your CDs into MP3 files, you can use ripper and encoder software to do just that. A ripper copies the song’s file from the CD onto your hard disk. The encoder compresses the song into the MP3 format. By encoding songs, you can play them on your computer or take them with you on your MP3 player.

You can write the MP3 files themselves onto a data CD in order to save them and clear some space on your hard disk. You can then listen to the files on any computer. Some car stereos and DVD players now let you play data-encoded MP3s, too.

If you are an artist who is recording music at home or in small studio, you can use MP3 files and the Web to distribute your music to an extremely large audience. The first step is to create a song, either on a cassette tape, minidisk or CD. If it is on a CD, you can use the ripper and encoder tools described in the previous section to create an MP3 file. If it is on a cassette, you can connect the output of your cassette deck to the line-in or microphone jack of your sound card and record the music digitally on your computer. Then you can encode that file to create the MP3.