what does the multimedia actually mean? 
　　literally, multimedia means two or more media. if the publisher of this book wanted to join the current hype about multimedia, he could advertise the book as using multimedia technology. after all, it contains two media：text and graphics(the figures). nevertheless, when most pepole refer to multimedia, they generally mean the combination of two or more continuous media, that is, media have to be played during some well-defined timed times interval, usually with the users interaction. in practice, the two media are normally audio and video, that is, sound plus moving pictures.
　　in the multimedia environment, we have graphics and text at the same time, we can also add the photograph, animation, good-quality sound, and full motion video. all of the technologies make computers more interesting and much easier to use. for example, a multimedia program can play a segment of movie：a cat was playing a reel of thread, with "mewing…" the tableau can a be immediately captured and pasted to a text at the side of a paragraph of words.

cd－rom
  cd－rom is an optical technology that is used to make compact disks for cd stereo systems；it has now been adapted to store data for computers. cd disk is an optical disc, on which data is recorded as a set of dips in the surface. when a laser beam shines across the dips, the reflected distortion represents the data. optical disc has a 600 megabyte capacity and are well suited for the computer applications needing the vast storage. the playback effect of optical discs for music is very perfect, no hiss and crackle as with other photo-records. a computer system must have a cd drive that is designed to read the disks and translate the data into a form it can process. cd disks can store huge volumes of data and are very useful for reference material an encyclopedia, for example. however, as the name(read-only memory)implies, cd disks cannot be written on or changed in any way.

musical instrument digital interface
　　musical instrument digital interface, or midi, is a communication interface provided on virtually every synthesizer made today. it provides a standard that allows products by different companies to communicate with each other. midi’s original purpose as to allow a musician to control several synthesizers from one keyboard rather than connecting a number of keyboard-equipped synthesizers, to produce the multilayered sound. the midi standard provides for 16 channels. you can assign each channel to a specific synthesizer or synthesizer voice. if you record a sequence and assign it to channel 5, for example, only an instrument set to listen on channel 5 plays back the sequence, this feature enables midi networks to sound like orchestras.

audio
　　an audio(sound) wave is a one-dimensional acoustic(pressure) wave. when an acoustic wave enters the ear, the eardrum vibrates, causing the tiny bones of the inner ear to vibrate along with it, and sending nerve pulses to the brain. these pluses are perceived as sound by the listener. audio waves can be converted to digital form by an adc (analog digital converter). an adc takes an electrical voltage as input and generates a binary number as output. music, of course, is just a special case of general audio, but an important one. the representation, processing, storage, and transmission of such audio signals are a major part of the study of multimedia systems.

virtual reality－enter a fancy space
　　in the science fiction "neuromancer", william gibson describes a space, cyberspace, controlled by a computer. once his brain was linked with the computer, a man would undergo all experiences in the space. his various senses in the realistic world would be replaced with a series of new electric stimuli. the cyberspace is regarded as a goal of future virtual reality. 
　　in fact, virtual reality is exploiting software and hardware of the computer to generate a simulation of an environment, such as an easeful classroom, a breathtaking action and so on. in such a simulated environment, one would have an immersed sense. for example, in a virtual space, students can "dissect" a human body, "visit" ancient battlefields, or "talk" with shakespeare, …