Computing Academy GCSE Computer Science

ApplicationsOff the ShelfIf you need to word process an essay or report you’ll use a word processor application; ifyou need to conduct some research on the Internet you’ll use a web browser application;if you want to enhance your holiday photos you’ll use a photo editing application.These applications already exist and you can purchase them from both high street andonline shops. They are called off the shelf applications as the user has no input into thedesign or features of the software; the user simply buys the application as it is ‘off theshelf’. 50

CustomSometimes, off the shelf software is too generic and doesn’t quite work in the way you’dlike it to. Some companies have very specific requirements and need software to work in avery specific way, look a certain way or integrate with existing computer systems in acertain way. If off the shelf software does not give businesses the control they need overthe software, they can commission a developer to build a new, custom written application.There is often a significant cost involved in this, and as the software needs to be designed,built and tested - it is not available immediately.Software as a Service (SaaS)Traditionally, if you wanted an application to perform a certain job you would go to a highstreet or online store and buy it. Once installed on your computer it was yours to use. Aftera period of time a new version of the software with new features might have been releasedand you could choose to go andbuy the latest version, orcontinue to use the (slightly outof date) version you alreadyowned.Increasingly, users are nowturning to Software as a Service(SaaS) as an alternative. SaaSinvolves users accessingapplications via the Internet(usually using a web browser).The software is not installed onthe users’ computer but isinstead hosted centrally on aserver in the cloud.Unlike traditional software wherethe user purchases theapplication once, SaaS is usually sold on a subscription basis with users paying a monthly 51

fee to access this on-demand software. Although some users may prefer to ‘own’software rather than ‘rent’ it, SaaS does mean that users always have access to the latestversion of the software and can access it from any computer with an Internet connection.Some SaaS is actually free; examples include Google Drive and Apple iCloud which allowusers to access word processing, spreadsheet and presentation applications over theInternet which are not installed on their home computers.SaaS does rely on a stable, reasonably fast Internet connection and as such has onlybecome a possibility in recent years as higher bandwidth broadband services havebecome available.Proprietary vs Open SourceProprietary software is developed commercially to be sold in order to make money. Thedeveloper owns the software and they allow customers to use it by selling them a license.Only the owner of the software - the intellectual property - has the right to sell it andmodify it.As proprietary software is sold, customers can expect a reasonable level of service andsupport; there is usually a help website, telephone number or email address.Open source software is not owned by anyone. Developers write the software eitherindividually or as part of a community and make it freely available. Anyone can then usethe software as it is, or edit and the source code to make new versions of the software. 52

UtilitiesWe know that the operating system (supported by device drivers) take care of thehardware in a computer system and provide a platform on which applications can run. Inaddition to application software, computer systems often run small maintenance programsor utilities.Anti-virus software, firewall software, disk management software and backup software areexamples of utilities. Often these programs run in the background and perform their jobswithout us even knowing that they are there. 53

4 Representing data within computer systems is done byData Representation turning switches on and off

BinaryWhat is Binary?We need to be able to store data within a computer system and transfer it from onecomputer to another, for example when sending emails, storing images, and playingsound.Electronic devices such as computers consist of thousands of tiny electronic switches.The basics of any electronic device means either electricity is flowing or it is not i.e. aswitch is either on or off. Representing data within a computer system is done by turningswitches on and off.How on earth can we represent letters, reports and essays, videos and sounds with just 1and 0? That’s exactly what computers do and it’s called Binary. 55

ExampleSo, data is passed through and stored in computers by turning on and offlots of tiny switches.Switches are either ON (1) or OFF (0) therefore data in a computer canonly be represented by two numbers...1 and 0.The normal number system that we use everyday and is very familiar to us has 10 differentsymbols – 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. We use these symbols to write numbers, so 629means 6 lots of 100, 2 lots of 10 and 9 1s. In primary school we may have seen thiswritten in a table:Hundreds (100) Tens (10) Units (1) 6 2 9When learning basic maths at school, decimal tables are sometimes used like this one(below). If we had one unit and we multiply it by 10 we get one ten. If we multiplied one tenby 10 we’d end up with one hundred and so on.Thousands Hundreds Tens Units 56

The column values are 10 times the value in the previous column moving from right to left.This is called the base 10 system, or the denary or decimal system.Computers do not use the decimal system. They use a system based on just twonumbers: 1 and 0. This is called the binary or base 2 system.In the same way that we multiply by 10 in the denary system to move across from right toleft, when using the binary system, multiplying by 2 moves across to the left.See how we move across from right to left with 1, 2, 4, 8, 16, 32, 64, 128.128 64 32 16 8 4 2 1Binary UnitsWhen using the denary (base 10) system we assign certain numbers special names, forexample 10 × 10 × 10 is 1000 which we call a thousand. 1 thousand multiplied by 1thousand is called 1 million etc.Binary also has some special names to refer to certain values.The basic unit is 0 or 1 – this is a binary digit or bit.When talking about storing and transferring data within a computer system we talk about‘bits’ and ‘bytes’. ‘Bit’ is short for ‘Binary Digit’ and a byte contains 8 bits. A ‘nibble’ ishalf of one byte (4 bits). 57

Binary to Denary128 64 32 16 8 4 2 100101001In the above example, we can say that we have no 128s, no 64s, 1 lot of 32, no 16s, 1 lotof 8, no 4s, no 2s and 1 lot of 1.So we have 32 + 8 + 1 = 41.00101001 is therefore the binary representation or equivalent of the decimal number 41.Here are some more numbers represented using the binary system.128 64 32 16 8 4 2 1001000000101000100101101 00100000The first number has a ‘1’ for 32 and everything else has a ‘0’, so 00100000 is binary for32. 01010001The second number has a ‘1’ for 64 and a ‘1’ for 16 and a ‘1’ for 1, so we have64 + 16 + 1 = 81, so 01010001 is binary for 81. 00101101The third binary number is 00101101 so there is a ‘1’ for 32, 8, 4 and 1.32 + 8 + 4 + 1 = 45, so 00101101 is binary for 45! 58

Denary to BinaryPerforming conversation from denary to binary is also really simple.Let’s say we’d like to convert the denary number 37 into binary. First we’ll need our binarycolumn headers.128 64 32 16 8 4 2 1Now, looking at our ‘target’ number of 37 we can ask ourselves, do we need 128 to get to37?The answer is of course, no. 128 is too big. We can put a ‘0’ down in the 128 column.128 64 32 16 8 4 2 10We can then ask, do we need 64 to get to 37? The answer is again no as 64 is bigger than36. We put down another 0 in the 64 column.128 64 32 16 8 4 2 100Next, we have 32. Can we use 32 to get to 37? Yes! 32 is smaller than our target of 37 sowe can use it. We’ll put a ‘1’ in the 32 column to show that we’re using it.128 64 32 16 8 4 2 1001We wanted 37, so far we have 32, so we’re still in need of 5. Can we use 16 to get 5? No:too big. Can we use 8 to get 5? No: too big. We can put ‘0’ down in the 16 and the 8column. 59

128 64 32 16 8 4 2 100100Can we use 4 to get 5? Yes! So we can put down a ‘1’ in the 4 column. Now all we ned isa 1, so we put down a ‘0’ in the 2 column and a ‘1’ in the 1 column.128 64 32 16 8 4 2 1 10010010Lets check our answer by adding together those values represented by a 1:32 + 4 + 1 = 37 So we are correct!The denary number 37 is represented as 00100101 in binary. 60

TaskWork out what numbers are being represented here as binary and writedown the decimal equivalent. The first one has been done for you.128 64 32 16 8 4 2 10 0 1 0 0 0 1 0 34?0 0 0 0 0 0 0 1?0 0 0 0 0 0 1 0?0 0 0 0 1 0 1 0?1 0 0 0 0 0 0 0 61

TaskHere are some more binary numbers for you to convert to decimal but it’sa little more tricky as you don’t have the headings to show you what eachbit represents. The first one has been done for you. Binary Decimal1 10000000 1282 001010013 101010104 10000115 11111111TaskWork out what numbers are being represented here in decimal and writedown the binary equivalent. The first one has been done for you. Decimal Binary11 00000001263 24495 131 62

Binary AdditionWhen using the decimal system to add two numbers together such as 26 plus 36 wewould do: Tens Units 26 +361. To add these two numbers, we first consider the ‘units’ column and calculate 6 plus 6, which results in 12.2. Since 12 is greater than 9 (remembering that base 10 operates with digits 0-9), we ‘carry’ the 1 from the ‘units’ column to the ‘tens’ column and leave the 2 in the ‘units’ column. Tens Units 26 +36 123. Considering the \"tens\" column, we calculate 1 + (2 + 3), which results in 6. Since 6 is less than 9, there is nothing to \"carry\" and we leave 6 in the \"tens\" column. Tens Units 26 +36 62Binary addition works in the same way, except that only 0's and 1's can be used. Thisactually makes binary addition much simpler than decimal addition. Here’s an example. 63

8421 0101 +01111. To add these two numbers, we first consider the ‘1’ column and calculate 1 + 1 which equals 2. In binary ‘2’ is written as 10. We carry the 1 to the ‘2’ column and leave the 0 in the ‘1’ column. 8421 0101 +0111 102. Moving on to the ‘2’ column, we calculate 1 + (0 + 1), which again in binary is 10. We carry the 1 to the ‘4’ column and leave the 0 in the ‘2’ column. 8421 0101 +0111 1 003. We now look at the ‘4’ column and calculate 1 + (1 + 1). This equals 3 which in binary is written as 11. We carry 1 to the ‘8’ column and leave 1 in the ‘4’ column. 8421 0101 +0111 1 100 64

4. Looking at the ‘8’ column we see that there are no further 1s to add together which leaves us with an answer of 1100, or 1 lot of 8, 1 lot of 4, 0 lot of 2 and 0 lot of 1. 1100 is the binary equivalent of 12. 8421 0101 +0111 1100Here is another example, this time adding together the binary numbers:0110101 plus 0001101.64 32 16 8 4 2 10110101+0001101 10 1 + 1 = 10 (carry the 1) 10 1 + (0 + 0) = 1 1 010 1 + 1 = 10 (carry the 1) 1 + (0 + 1) = 10 (carry the 1) 1 0010 1 + (1 + 0) = 10 (carry the 1)1 00010 1 + (1 + 0) = 10 (carry the 1)1 000010 1 + (0 + 0) = 11000010So the answer is 1000010 (the binary equivalent of 66). 65

TaskWork out the answers to the following binary additions. Remember, youranswer should also be written in binary.10001101 + 00101010 = ____________________________________10100011 + 00101100 = ____________________________________00110111 + 01011001 = ____________________________________01010110 + 00001111 = ____________________________________ 66

Two’s ComplementTwo's complement is a methodology used by computers to represent and processnegative integers, and it’s a simple way of performing binary subtraction.To get the two's complement notation of an integer:1. Write out the number in binary2. Invert the digits3. Add one to the resultTwo’s Complement ExampleLet’s turn the decimal integer ‘59’ into two’s complement negative notation.1. First we write out the binary equivalent of 59.128 64 32 16 8 4 2 1 0011 1 0 1 12. Next, we invert the digits – so we turn 1s into 0s and 0s into 1s128 64 32 16 8 4 2 1110001003. Finally, we add one to the result128 64 32 16 8 4 2 111000101So, 59 is represented as 00111011 in binary and 11000101 in two’s complement notation. 67

Binary SubtractionNow we’re comfortable with two’s complement we can use it to perform binarysubtraction. Let’s say we want to subtract 9 from 49.1. First, we write out these numbers in binary128 64 32 16 8 4 2 100110001- 00001001We now convert the number to be subtracted (the 9 or 00001001) into two’s complement.2. We invert the second number128 64 32 16 8 4 2 100110001111101103. We then add one to the second number128 64 32 16 8 4 2 10011000111110111 68

• We wanted to subtract 9 from 49.• We’ve converted the 9 into two’s complement• Now in order to perform the subtraction we actually add together the numbers.256 128 64 32 16 8 4 2 1 00110001+11110111 10 1 00 1 000 1000 1 01000 1 101000 1 01010001 00101000This addition results in an overflow (e.g. the result has an extra bit on the left hand side –we started with 128, 64, 32, 16, 8, 4, 2, 1 – but we’d now need to have 256, 128, 64, 32,16, 8, 4, 2, 1 in order to fit the result of this addition.)We don’t add an extra bit, we ignore and discard the overflow.So...256 128 64 32 16 8 4 2 1100101000…becomes.. 128 64 32 16 8 4 2 1 00101000 69

If we convert the result back to decimal form....128 64 32 16 8 4 2 100101000…we have 1 lot of 32 and one lot of 8 which is 40.Our original problem was 49 - 9 which is 40. So it worked! 70

HexadecimalLarge binary numbers are difficult for programmers to remember and work with, and theconversion process between decimal and binary is time consuming. We can thereforeconvert these into smaller, hexadecimal (or hex) numbers.The base 10, or denary number system uses 10 symbols: 0-9; the hexadecimal systemuses 16 symbols: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7 8 9 ABCDEFHex values are represented using two numbers and / or digits, for example: 3D = 61, F7 =247 and BB = 187. 71

Binary to HexadecimalThe binary number for 143 is: 10001111.128 64 32 16 8 4 2 110001111This uses 8 bits or 1 byte of data. In order to covert this binary number into hex, we firstsplit the 1 byte into 2 nibbles (each of 4 bits).8421 84211000 1111Notice that we also change the column headers so that each nibble uses. 8, 4, 2, 1. Wecan now work out the value of each nibble.8421 84211000 1111 =8 8 + 4 + 2 + 1 = 15The symbols used in hexadecimal are 0-9 and then A-F. So, any number larger than 9 willbe represented by a letter. 15, is therefore represented by F.8421 84211000 1111 8 F10001111 in binary is 8F in hex. Here is another example. 72

The binary number for 111 is: 01101111 84 21 11 11 128 64 32 16 0110 84 21 11 11 8421 8 + 4 + 2 + 1 = 15 (F) 0110 4+2=601101111 in binary is 6F in hexTaskConvert the following binary numbers to hexadecimal. The first one hasbeen done for you.Decimal Binary Working Out... Hex 54 00110110 36 0011 0110 3 6195 1100001199 01100011101 01100101200 11001000 73

Hexadecimal to BinaryTo convert a hexadecimal number back to binary we reverse the process. Let’s convert thehex number C9 into binary.First we can write out the number and include the nibble headers: C9 84218421C represents the number 12 and we can represent 12 in binary using 8 + 4, so C = 1100 C 8 9 1 8421 42 11009 is represented in binary as 8 + 1 = 1001 C9 1 8421842 1 1100100So, C9 in hex is 11001001 in binary. Here are a couple more examples: FF 84218421 11111111F represents 15 which in binary is 8 + 4 + 2 + 1 or 1111. FF in hex is therefore 11111111 inbinary. 74

3A 84218421 001110102 + 1 = 3 which gives is 0011. In hex, A represents 10 which we get from 8 + 2. 3A in hexis therefore 00111010 in binary.TaskConvert the following hexadecimal numbers to binary. The first one hasbeen done for you.Hex Working Out... Binary 9D9D 9 13 10011101 1001 110177F2A013 75

Hexadecimal to DenaryRather than using the column headers for denary: 1000 100 10 1...or for binary:128 64 32 16 8 4 2 1...we use just: 16 1If we then wish to convert the hex number 9F into denary we can note this down below thecolumn headers. 16 1 9FWe now need to multiply each value by the column header as follows 16 1 9F 9 x 16 = 144 F x 1 = 159 x 16 = 144, and F (which represents 15) x 1 = 15. We then add together the two values:144 + 15 = 159. So, 9F in hex is the equivalent of the denary number 159. 76

Here are some more examples: 1 D 16 D x 1 = 13 5 5 x 16 = 80 1 65D in hex = 93 in denary 6x1=6 16 1 7 3 7 x 16 = 112 3x1=376 in hex = 118 in denary 16 C 12 x 16 = 192C3 in hex = 195 in denary 77

TaskConvert the following hexadecimal numbers to denary. The first one hasbeen done for you. 16 1 247 F 715 x 16 = 240 7x1=7 16 1 A 916 17A16 131 78

Denary to HexadecimalShould we wish to convert a denary number such as 235 into hexadecimal we divide by16 and record the remainder. 235 / 16 = 14 r11In hex 14 = E and 11 = B. So, 235 in denary is EB in hex.Note that you need to use long division here, not just use the divide function on acalculator as this will give a decimal number rather than a remainder.Here are a couple more examples: 110 / 16 = 6 r14110 in denary = 6E in hex. 25 / 16 = 1 r925 in denary = 19 in hex. 80 / 16 = 5 r080 in denary = 50 in hex. 79

TaskConvert the following denary numbers to hexadecimal. The first one hasbeen done for you. 15 / 15 = 0 r15 1615 in denary = 0F in hex.101 / 101 = r 16101 in denary = _______ in hex. 64 r 64 / 16 =64 in denary = _______ in hex.205 / 205 = r 16205 in denary = _______ in hex. 80

ASCIIIn 1960 the American Standards Association developed a standard set of codes to beused in telegraph systems to represent the main characters used in communications; thisset of codes is known as ASCII (American Standard Code for Information Interchange).ASCII allows us to represent all alphabetic upper and lower case characters, all numericcharacters, 32 punctuation and other symbols including ‘space’ and 32 reserved non-printable codes. 81

ASCII TableASCII Char ASCII Char ASCII Char ASCII Char 0 NULL 32 65 A 97 a 1 SOH 33 ! 66 B 98 b 2 STX 34 “ 67 C 99 c 3 ETX 35 # 68 D 100 d 4 EOT 36 $ 69 E 101 e 5 ENQ 37 % 70 F 102 f 6 ACK 38 & 71 G 103 g 7 BEL 39 ‘ 72 H 104 h 8 40 ( 73 I 105 i 9 BS 41 ) 74 J 106 j 10 HT 42 * 75 K 107 k 11 LF 43 + 76 L 108 l 12 VT 44 , 77 M 109 m 13 FF 45 - 78 N 110 n 14 CR 46 . 79 O 111 o 15 SO 47 / 80 P 112 p 16 SI 48 0 81 Q 113 q 17 DLE 49 1 82 R 114 r 18 DC1 50 2 83 S 115 s 19 DC2 51 3 84 T 116 t 20 DC3 52 4 85 U 117 u 21 DC4 53 5 86 V 118 v 22 NAK 54 6 87 W 119 w 23 SYN 55 7 88 X 120 x 24 ETB 56 8 89 Y 121 y 25 CAN 57 9 90 Z 122 z 26 EM 58 : 91 [ 123 { 27 SUB 59 ; 92 \ 124 | 28 ESC 60 < 93 ] 125 } 29 FS 61 = 94 ^ 126 ~ 30 GS 62 > 95 _ 127 31 RS 63 ? 96 ` 128 DEL US 64 @ 82

While ASCII was useful for textual messages, it did not allow non-English languagecharacters or mathematical symbols to be represented. Today we use the Unicode systemwhich allows us to encode all characters. ASCII is now a subset of Unicode, so ’78’represents the letter ‘N’ in ASCII and in Unicode.When typing the words ‘Sheffield Wednesday’ into a computer, the following sequence ofASCII codes is generated.She f f i e l d We d n e s d a y83 104 101 102 102 105 101 108 100 32 87 101 100 110 101 115 100 97 121Remember, that computers cannot deal with decimal numbers, so these ASCII codes thenneed to be converted into binary to be processed by the CPU or stored. 83

Character ASCII Binary Character ASCII Binary S 83 01010011 W 87 01010111 h 104 01101000 e 101 01100101 e 101 01100101 d 100 01100100 f 102 01100110 n 110 01101110 f 102 01100110 e 101 01100101 i 105 01101001 s 115 01110011 e 101 01100101 d 100 01100100 l 108 01101100 a 97 01100001 d 100 01100100 y 121 01111001 32 00100000 SpaceTaskWrite a phrase or short sentence and convert this into ASCII codes. Passyour code (without the original) to a friend and ask them to convert thisback into English.You could even convert the phrase or sentence into ASCII codes and theninto binary and ask a friend to work out what the original message said. 84

ImagesWe know that in order to represent numbers computers use binary, or hexadecimal; torepresent letters they use ASCII which is converted into binary. Images are the same. 85

This image of a cat is made up of an arrangement of pixels which can be seen when weenlarge the image. The second enlargement focuses on the cat’s eye and here we canclearly see the pixels.The eye consists of 10 x 10 pixels or dots. Each pixel is signalled to be on (black) (1) or off(white) (0) using binary bits. The table below shows how the data used to represent thecat’s eye would look. 0000011100 0000100010 0001000001 0010000001 0100000001 1000110010 1000110010 1000000100 0100001000 0011110000The image of a cat uses one bit per pixel (bpp); this allows the computer to store either a 1or 0 (black or white) for each pixel. In order for us to store more data about each pixel (e.g.colour) we’ll need more bits. Using 2 bits per pixel allows us to represent up to 4 colours,3 bits gives us 8 colours, 8 bits gives us 256 colours. 1 bit 2^1 colours 2 colours High Colour2 bits 2^2 colours 4 colours True Colour3 bits 2^3 colours 8 colours8 bits 2^8 colours 256 colours16 bits 2^16 colours 65536 colours24 bits 2^24 colours 16777216 colours 86

This photograph of the Sacré-Cœur Basilica de Montmartre in Paris originally used 293KBof space to store. The colour depth of the image has been reduced below. Notice whathappens to the image and the amount of memory used to store the image.1 bit colour (2 colours) [12KB] 2 bit colour (4 colours) [18KB]3 bit colour (8 colours) [28KB] 8 bit colour (256 colours) [95KB]The more colours used, the greater the colour depth, and the more space required to storethe data. 87

In addition to colour depth, the resolution of an image will also affect file size.Resolution does not mean the size (height and width) of an image.Resolution is measured in dots (pixels) per inch (dpi). The more pixels per inch, the higherresolution the image. By looking at the image size properties of our 2 bit Sacré-Cœur Basilica de Montmartre image we can see that the dimensions are 17.64 cm (width) x 11.75 cm (height); the resolution is 500 x 333 at 72 pixels per inch (dpi).We can increase the dimensions of theimage (to make it appear bigger) but thiswill not increase the resolution (the numberof pixels per inch). The resolution remains at500 x 333 but as the canvas or page islarger, there will now only be a dpi of18.143.When increasing the dimensions (enlarging)an image, each pixel is simply stretchedwhich will look blotchy or pixelated. 88

If we zoom in on the 2 bit (4 colour) version of the Sacré-Cœur Basilica de Montmartreimage we can clearly see the 4 colours used.The computer uses 2 bits to store the four colours; each colour is assigned a binary value. 00 01 10 11As images are stored as 1s and 0s, the computer needs to know how to interpret thesebinary numbers and display the image. Metadata means ‘data about the data’ and isincluded with each image. The metadata specifies the colour depth (e.g. 2 colours, 4colours, 256 colours etc.) and the height and width of the image in pixels 89

SoundAs with numbers, text and images, sound is also stored and processed by computers asbinary. If you open any sound file using an audio editor you’ll see something that looks likethe waveform below.Sound waves are analogue; this means that they change continuously and do not have aspecific, measurable value. Unfortunately, specific, measurable values are exactly whatcomputers need as they deal with digital, not analogue information. In order to store or 90

process analogue sound waves we need to sample them and store a digitalrepresentation of the original sound.If we zoom in and take a closer look at the previous waveform, we can see the peaks andtroughs more clearly; these represent the high and low tones within the piece of music. Inorder to sample the waveform, the computer takes measurements at various intervalsduring the wave.Sampling the waveform creates a representation of the original sound but usingmeasurable, specific value.In the following sample waveform, at 1 second in, the value of the curve is 30, at 2seconds the value is 20 and so on. 91

X (Time) Y (Frequency) 0 25 1 30 2 20 3 15 4 30 5 5 6 10 7 5 8 25 9 0 10 10Once a sound wave has been sampled and the tone recorded as a sequence of numbervalues, it is easy to convert these values into binary for processing and storage.If we increase the sample frequency and record the values of the curve more often thedigital representation of the audio will sound much closer to the original analogue versionas more tones (high and low sounds) can be recorded as illustrated in the following chart. 92

A higher sample rate produces better quality sound, but takes more storage space asmore data is being recorded.48 kHz is the recommended sample rate for most purposes, which means audio issampled 48,000 times per second. Human ears cannot benefit from sampling rates higherthan about 50  kHz; audio could be sampled at a higher rate than this but we (humans)would not be able to tell the difference.So, sample rate is important as it determines how many different tones can berepresented; the higher the sample rate, the larger the range of tones. A piece of musicwith lots of high frequency and low frequency notes needs a high sample rate, otherwisethose high and low parts of the music may be lost.Bit rate is also important when thinking about digital audio. Bit rate refers to the number ofbits you have available to capture audio (remember how when talking about images, morebits allows us to record more colours).  With sounds, more bits allows us to record morepressure levels (volume). 16 bit audio has 2^16 (65,536) different pressure levels that canbe recorded, 24 bit audio records 16.7 million different pressure levels. The more pressurelevels that can be recorded, the greater the resolution of the audio and the more accuratethe sound will be. 93

Recording 16 bit audio sampled at 44,100Hz means that the audio will play at1,411.2kbps; this is standard CD quality sound300kbps MP3s are compressed audio files with a reduced bit rate and reduced samplefrequency. MP3s will have a reduced range of tones due to the lower sample rate, and areduced range of pressures due to the reduced bit rate. 94

5 Networks allow us to pass information betweenNetworking computer systems in different locations

What Are Networks?Stand-alone computers are devices that are not connected to any other computersystem. A user can interact with a stand-alone computer and input data; the computermay then process the data and output information back to the user, but no data isexchanged with any other systems.While stand-alone computers were once extremely useful, computer scientists wanted tocoordinate and pass information between computer systems in different locations and sobuilt networks; today we could not live without them. 96

Key Point A network is a collection of two or more computers connected to one another in order to exchange information and share resources.The earliest example of communication between two computer systems is in 1940 whencomputer scientist, George Stibitz sent instructions from one computer in NewHampshire to another, 270 miles away, in New York.Joseph Carl Robnett Licklider is also considered one of the most important figures incomputing history for his early vision of a world-wide computer network in 1962, longbefore it was built.Originally, computer networks were developed for military purposes. In 1969 the AdvancedResearch Projects Agency deployed ARPANET (Advanced Research Projects AgencyNetwork). 97

The network was designed to allow communications between military installations aroundthe the United States; data on the network did not have to travel one particular route fromsource to destination; it could instead travel via any other nodes on the network. This wasimportant, as ARPANET was designed to still function even if large portions of it weredestroyed.It is said that ARPANET was designed to ensure messages could be sent even if someareas of the country were destroyed due to nuclear war. Considering the previous map, if,for example, HARVARD needed to send a message to UTAH, this could go from HARVARD-> BBN -> MIT -> ILLINOIS -> UTAH. In the event that ILLINOIS was destroyed, however,the message could instead take the route: HARVARD -> BBN -> RAND -> SDC -> UTAH.Although originally conceived for military purposes, computer scientists quicklyrecognised non-military application for computer networks. Today, we use networks on adaily basis; examples include mobile phone networks, home computer networks and ofcourse, the Internet. 98

Backup Advantages of Networks Communications Device Sharing Backing up data is much easier when all files are stored centrally on a Distributed Systems networked file server.File / Database Sharing Whether it’s email, video conferencing, instant messaging or file sharing - it wouldn’t be possible without networks. The physical infrastructure (cables and hardware) allow us to communicate instantly with people around the world. Without networks communications would have to take place face-to-face which would be much more costly and time consuming. Businesses often need to print and scan documents, but not every single computer in the office needs a printer and a scanner. These peripherals can be connected to the network so that any computer on the network can access them. When connected via a network it is possible to use several computers to process the same data in order to speed up the process. The Search for Extraterrestrial Intelligence (SETI) project distributes massive amounts of data collected by radio telescopes via the Internet to the computers of 3 million volunteers for processing. Networks allow files and databases to be shared and accessed by more than one person in more than one location. All users access the same, up-to-date / live version of the data which makes collaboration possible.Security Networks usually have a central server which is responsible for managing permissions and access rights. The network administrator can allow and revoke access to files and resources and easily review server logs should a problem or security breach occur. It is much easier to investigate a problem when this can be done from the central server without needing to visit several stand alone computers.Software Sharing When several computers require the same software (such as word processing software and an email client), networks make it possible to remotely install this software from the server. It is not necessary to go to each computer individually to install the software, Maintaining the software and installing updates is also far easier over the network. 99