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Training target: In this part ， our target is to train your reading speed. You should pay more attention to” word group”. Treating a group of words as a whole unit is a short cut to save your time. We have underlined the word groups in some sentences ， and the rest can be practiced by yourself. If there are some new words ， you may cover the note area with a piece of paper and try to guess their meanings without looking them up in the dictionary.

 Text  Multimedia: Multimedia is the combination of sound （ Audio ）， graphics ， animation ， video and text within an application.

..  Audio: Audio is referring to sound or to things which can be heard; usually ， the human ear can hear a range of frequencies between around 20Hz ～ 20KHz.But some animals can hear higher frequencies.

.  Audio waves can be converted to digital form by an ADC （ Analog Digital Converter ）.To represent this signal digitally ， we can sample it everyΔT seconds ， sampling more often is of no value.

.  Audio CDs are digital with a sampling rate of44 ， 100sample/sec ， enough to capture frequencies up to22 ， 050Hz ， which is good for people ， bad for dogs.

 Computers in software can easily process digitized sound. Virtually all professional sound recording and editing are digital nowadays. MIDI is a representation of many musical instruments now.

.  MIDI music: Musical Instrument Digital Interface abbreviates MIDI. It is the serial interface running at32.5kbps that connects electronic instruments; the MIDI interface can connect up to32different instruments and carries signals from a sequencer or computer that instructs the different instruments ， such as a drum machine or synthesizer ， to play notes together with instructions called control-change messages ， which control the volume ， pitch and type of instrument used.

.  NOTE: a MIDI interface does not carry sounds ， rather it carries notes and instructions to change to different preset sounds; many PCs generate sounds from MIDI data on-board. There are two kinds of MIDI sound generation: FM synthesis simulates musical notes by modulating the frequency of a rectangular base carrier wave ， whereas waveform synthesis uses digitized samples of the notes to produce a more realistic sound. MIDI also allows multiple different voices ， or notes ， to be played back simultaneously （ MPC requires an8-voice synthesizer ， but studio-quality instruments can have up to32MIDI voices ）.

 Video: video can be divided into two patterns: analogical system and digital system.

 1. Analogical system: To represent the two- dimensional image in front of it as a one- dimensional voltage as a function of time ， the camera scans an electron beam rapidly across the image and slowly down it ， recording the light intensity as it goes. The exact scanning parameters vary from country to country. The system used in North and South America and Japan has 525 scan lines ， a horizontal to vertical aspect ratio of 4 ∶ 3 ， and 30 frames/sec. The European system has 625 scan lines ， the same aspect rate of 4 ∶ 3 ， and 25 frames/sec.

 2.Digital system: The simplest representation of digital video is a sequence of frames ， each consisting of a rectangular grid of picture elements ， or pixels. Each pixel can be a single bit ， to represent either black or white. To use8bits per pixel to represent256gray levels. This schedule gives high-quality black-and-white video. For color video ， good systems use8bits for each of the RGB colors. While using24bits per pixel limits the number of colors to about16million ， the human eye cannot even distinguish so many colors ， let alone more.

 To produce the smooth motion ， digital video ， like analog video ， must display at least 25 frames/sec. However ， since good quality computer monitors often rescan the screen from images stored in memory at 75 times per second or more ， interlacing is not needed and consequently is not normally used. Just repainting the same frame three times in a row is enough to eliminate flicker.

 Data Compression: Transmitting multimedia material in uncompressed form is completely out of the question. The only method is that massive compression is possible. All compression systems require two algorithms: one for compressing the data at the source （ encoding ）， and the other for decompressing it at the destination （ coding ）.These algorithms have certain asymmetries ， that is to say ， a multimedia document will be encoded once but will be decoded thousands of times.

 On the other hand ， for the real-time multimedia ， such as video conferencing ， slow encoding is unacceptable. Encoding must happen on the fly in real time. A second asymmetry is that the encode/decode process need not be invertible ， that is ， when compressing a file ， transmitting it ， and then decompressing it. When the decoded output is not exactly equal to the original input ， the system is said to be lossy. If the input and output are identical ， the system is lossless. Lossy systems are important because accepting a small amount of information loss can give a huge payoff in terms of the compression ratio possible.

 The MPEG （ Motion Picture Experts Group ） standards: These are the main algorithms used to compress video and have been international standards since1993.Because movies contain both images and sound ， MPEG can compress both audio and video. The goal of MPEG-1 （ International Standard11172 ） was to produce video recorder-quality output （ 352×240for NTSC ） using a bit rate of1.2Mbps. At this lower resolution ， MPEG-1can be transmitted over twisted pair transmission lines for modest distances.MPEG-1 is also used for storing movies on CD-ROM in CD-Video format.MPEG-2 （ International Standard13818 ） was originally designed for compressing broadcast quality video into4to6Mbps. MPEG-2was expanded to support higher resolutions ， including HDTV. It is likely that in the long run MPEG-1will dominate for CD-ROM movies and MPEG-2will dominate for long-haul video transmission.

 JPEG:JPEG stands for Joint Photographic Experts Group ， which is also sometimes abbreviated JPG. An image-compression standard and file format that define a set of compression methods for high- quality images such as photographs ， single video frames ， or scanned pictures. JPEG does not work very well when compressing text ， line art ， or vector graphics. JPEG uses lossy compression methods that result in some loss of the original data. When you decompress the original image ， you don’t get exactly the same image that you started with ， although JPEG was specifically designed to discard information not easily detected by the human eye. JPEG can store24-bit color images in as many as16million colors; files in Graphics Interchange Format （ GIF ） form can only store 256colors.

.  The End