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eee116j1 1 Digital Information Engineering Science EEE116J1 Prof Paul Maguire w

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eee116j1 2 Examples of Digital Communication Computer to Computer – , web browsing, downloading files Text Messaging Digital TV Within Computer –central processing unit (CPU) to RAM –RAM to hard disk –RAM to CD –to printer, to screen, to camera, to PDA w

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eee116j1 3 Analogue World Most things in our world are ANALOGUE Analogue means Continuously Changing –example: music or speech is due to constantly changing pitch and volume –door opening. We can open it at any width DIGITAL –not Continuous but DISCRETE –certain values only –compare PIANO to VIOLIN –imagine a door with only a few fixed opening positions –compare a RAMP to STAIRS DIGITAL IS APPROXIMATE w

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eee116j1 4 Continuous v Discrete Continuous Ramp Discrete Staircase w

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eee116j1 5 An Analogue System Record and Playback Audio Cassette System Amplifier microphonespeakercassette But what if we want to use a CD player instead of cassette? Amplifier microphone speaker CD Digital to Analogue converter Analogue to Digital converter We need to convert to DIGITAL then record to CD and convert to ANALOGUE to playback from CD w

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eee116j1 6 What is digital information Digital information is REAL WORLD information but converted into specific format CODE The format is chosen so that Electronic Devices can work with it. Digital information is a LIST of NUMBERS Analogue information can be Voltage, Light Intensity, Sound Intensity, Temperature, Colour etc. etc. etc. How do we convert one to other. w

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eee116j1 7 A Picture Each square is called a picture element PIXEL. Each Pixel has a Number (or address) Each Pixel is filled with ONE colour The Picture can be described as a list of {Pixel No, Colour} starting at Top Left, finishing at Bottom Right w

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eee116j1 8 Image Description 1 red 2 blue 3 orange 4 orange 5 pink 6 pink 7 green. 200 green Picture is made up of 20 x 10 elements = 200 pixels w

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eee116j1 9 Colour Codes w

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eee116j1 10 Colour Image Description red 2 blue 3 orange 4 orange 5 pink 6 pink 7 green. 200 green red green blue One long number describes a picture, nine digits for each pixel w

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eee116j1 11 How long is the digital code? So our simple picture requires –200 pixels x 9 digits per pixel –= 1800 digits Full screen picture –say 1024 x 768 pixels for a typical screen –= 786,432 Pixels –= 786,432 x 9 digits –= 7,077,888 digits Now we have a list of numbers. Still not in a suitable format. Simpler example: a voltage varying in time w

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eee116j1 12 SNAPSHOT AUDIO w

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eee116j1 13 Simple Sine Wave Imagine playing a pure note on, say a Flute Capture with microphone which captures the sound waves and converts it to a voltage wave Amplitude of voltage varies with Time V pp w

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eee116j1 14 Convert to Digital In Analogue format we can pick ANY value of TIME and measure ANY value of VOLTAGE For digital format we measure at fixed intervals of time we have a set of allowed voltages w

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eee116j1 15 Setting allowed voltage values Choose no of allowed voltages e.g N = 8 Max voltage range is: -5V to +5V i.e 10V Allowed voltages are given by: V n = n V max /N V n = 1.25 n n = 1, 2, 3…. N so V n = 1.25, 2.5, 3.75, 5 and for negative too w

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eee116j1 16 Measurement Time Interval Say we make a measurement every 5ms Voltage at these points must be converted into allowed points Rule is: Set at closest value of allowed voltage Allocate numbers to voltages starting at -5V = Set of numbers are: w

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eee116j1 17 Sampling Sampling is the name given to the process of converting an Analogue signal to a series of digital values Look at what would happen if we convert digital series back to analogue Accurate? w

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eee116j1 18 Accuracy To improve accuracy: –increase number of measurements –Sampling Rate –increase number of allowed voltages –Sample Resolution Results in a lot more information w

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eee116j1 19 Changing the Grid Size w

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eee116j1 20 Binary Numbers The conversion of analogue information gives us a list of numbers So far, we have used decimal numbers But electronic equipment e.g. computers cannot easily work with decimal numbers BINARY numbers are better BINARY NUMBERS Only 2 values allowed 0 or 1 –decimal | 1000 | 100 | 10 | –binary 1024|512|256|128|64|32|16|8|4|2| = = bit number Max decimal value is 2 11 = 2048 For an N-bit number, max = 2 N w

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eee116j1 21 Why Binary? Hard disk: billions of magnets per square inch. Magnets are either north or south. Call North 1 and South 0 RAM (computer memory) is an array of billions of cells containing 1 transistor and 1 capacitor. Capacitor charged = 1, uncharged = 0 CD Flat disk with a spiral. Along the length of the spiral, it is divided up into small sections. Each section contains either a pit (hole) or a land (no pit) Pit = 1, no pit = 0 Optic Fibres Transmit sequence of light pulses. Pulse present = 1 Pulse absent (no light) = 0 w

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eee116j1 22 Bits and Bytes 1 bit = 1 or 0 8 bits = 1 byte Quality determined by sample rate per second and no of bits per sample example CD: sample rate = 44kHz 12 bit = resolution no of allowed voltages = 2 12 = 4096 How much data on a CD? Each sample requires a 12 bit binary number per sec 12x44000 = 528 kBits/sec = 66 kB/s: B = byte = 8 bits How long is a CD? = 80 mins Total data = 66kB x 80 x 60 = 316 MB anything wrong with this? w

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eee116j1 23 Download a high resolution picture Say 1200 x 800 pixels 16 million possible colours How many bits do we need to represent 16 x N = 16 x 10 6, so N = 24 So 1 picture = 1200x800x24 bits = 23 Mbits = 2.8MB Download at 50 kbits/s = 7.6 mins A digital movie 1 frame = picture = 3MB 25 frames/sec = 75 MB/sec 2 hour movie = 540 GB = DVD download time hours! To download a digital film for showing in a cinema, each frame will be time more detailed So that it can be expanded onto a large screen Better modems! Compression w

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