Network and multimedia knowledge (3)

10. Computer viruses and Trojans

Viruses ： A program written or inserted into a computer program that destroys the function of a computer or destroys data , A set of computer instructions or program codes that affect the use of a computer and can be copied by itself . The virus has ： Infectious 、 Concealment 、 Latency 、 Destructive 、 pertinence 、 Derivative 、 Parasitism 、 Unknown .

Trojan horse ： It's a backdoor program , The field is used by hackers as a tool to control remote computers , A small program hidden on the controlled computer monitors all computer operations and steals information .

Types of viruses and Trojans ：

System boot virus ;

File shell virus ：

Directory virus ;

worm-type virus （ infection EXE file ）： Panda burning incense , Romeo and Juliet , Evil Eagle , Nimda , Shockwave .

Trojan horse ：QQ Message tail Trojan horse , Trojan horse .

Macro virus （ infection word、excel And other macro variables in the file ）： Beautiful sand , Taiwan 1 Number .

CH Viruses ： The only virus in history that destroys hardware .

Red code ： worm-type virus + Trojan horse .

Denial of service attacks will cause the server to reject legitimate service requests from legitimate users for legitimate resources ： The attacker entered Line denial of service attack , Generally, a large number of users access the server in a short time , Load up the server , No longer receive Other requests , But it has nothing to do with divulging passwords .

11. Basic concepts of multimedia

Media can be divided into the following five categories ：

Feeling media ： It acts directly on human sensory organs , The media that makes people feel directly . Such as ： Vision 、 auditory 、 Tactile, etc .

Presentation media ： It refers to the intermediary media transmitting sensory media , Coding for data exchange . Such as ： written words 、 graphics 、 Animation 、 Audio and video, etc .

Performance media ： A medium for information input and output . Such as ： keyboard 、 Mouse and microphone ; Monitor 、 hit Printing machine and audio, etc .

Storage media ： The physical medium that stores the presentation media . Disk 、 CD and memory, etc . Transmission media ： Transport physical media that represents media . Such as cable 、 Optical fiber 、 Twisted pair, etc .

voice

Measure the size of a sound by its bandwidth , The unit is H Recalling . Sound is an analog signal , To deal with it , It must be converted into digital signals . The conversion process has three steps ： sampling 、 quantitative 、 code .

The frequency range of audio signals that human ears can hear is 20Hz~20KHz.

The sampling frequency of sound is usually twice the highest frequency , To ensure no distortion .

The transmission rate of uncompressed digital audio data is calculated as follows ：

Data transfer rate (bps)= sampling frequency (Hz)× Quantization Bits (bit)× Track number

The calculation formula of the storage space occupied by the digitized sound waveform is as follows ：

The amount of sound signal data (Byte)= Data transfer rate (bps)× The duration of the (s)/8

Digital music synthesis method

Digital frequency modulation synthesis FM: A modulation method that changes the frequency of high-frequency oscillation wave according to the law of modulation signal . Adopt different modulation wave frequency and modulation index , It is convenient to synthesize waveforms with different spectrum distribution , Reproduce some The timbre of the instrument . This method can be used to obtain the unique effect of delicacies " Electronic acoustic simulation “, Create colorful voice , It's a timbre that real instruments don't have .

Wave table synthesis Wavetable: All the sounds that can be made by various real instruments （ Including all ranges 、 tone ） Record it , Stored as a wave table file . When the play , according to M1D The music information recorded in the file is sent to the wave table Instructions , from “ form " Find out the corresponding sound information one by one , After synthesis 、 Playback after processing . Synthetic sound quality Better .

Sound characteristics

The volume ： That's loudness , Indicates the intensity of the sound , It mainly depends on the amplitude of the sound wave

pitch ： Indicates the level of various sounds , It mainly depends on the vibration frequency of the sound wave , The higher the vibration frequency, the higher the sound

tone ： Indicates the pitch of the voice , Determined by the frequency of the sound itself .

timbre ： Also known as sound products , It is determined by the harmonic spectrum and envelope of the sound waveform .

Sound file format ：.wav、snd、.au、.aif、voc、.mp3、.ra、.mid etc. .

Graphics and images

Three elements of color

brightness ： Color shade .

tonal （ red 、 green ）： Color category .

saturation ： The depth of a color .

Color space

That is, the color space used by the device to display pictures , Ordinary computer monitors are RGB Color space , Except red 、 green 、 Outside the three primary colors of blue , Other colors are formed by the superposition of these three primary colors ;

Used in TV YUV Color space , Mainly for compatibility with black-and-white TV , Using the brightness principle , That is, adjust different brightness , Show different colors ;

CMY(CMYK), The color space used in printing books , This uses and RGB The opposite principle of subtraction , Light blue 、 pink 、 yellow The printing pigment of three primary colors actually absorbs other colors besides its own color , therefore , These colors are printed ; HSV(HSB), Artist color space , It is divided from the perspective of art .

The properties of the image ： The resolution of the （ Pixels per inch di)、 Pixel depth （ Stores the number of binary digits used by each pixel ）.

Image file format ：bmp、·gif、jpg、png、.tif、wmf etc. .

Image depth is the number of bits required to record a pixel in an image file . The display depth indicates that the records in the display cache The number of digits of a point on the screen (bit), That is, the number of colors that the display can display .

Horizontal resolution ： The number of pixels the display has in the horizontal direction .

Vertical resolution ： The number of pixels the display has in the longitudinal direction .

The basic unit of vector graph is primitive , The basic unit of a bitmap is pixels , Basic animation and video composition A unit is a frame .

Multimedia Computing

The concept of chromatic number should be understood in the image . To understand the principle of audio capacity calculation , It's every one Sampling times of sampling channel * Number of bits per sample * Total number of channels used

Video is a combination of frames of images , Therefore, the essence is to find the image capacity , Of course, add audio capacity .

Pay attention to the unit B and b The difference and conversion of , Be careful K( Capitalization ,1024, For storage ) and k( A lowercase letter , 000) The difference between , Pay attention to the unit of result .