Level 3 Extended Diploma Unit 19 Computer Systems Architecture Data Types Level 3 Extended Diploma Unit 19 Computer Systems Architecture

Previously …. We learnt how values can be represented in: Decimal Binary Hexadecimal Now we look at how other things can be represented in a computer  

Text We need a method of coding the alphabet and other characters This has to be recognised across all computers ASCII (American Standard Code for Information Interchange) is a method of doing this Standardises binary codes eg: 01000001 is uppercase A

ASCII code Char Hex Binary A 41 01000001 N 4E 01001110 a 61 01100001 n 01101110 B 42 01000010 O 4F 01001111 b 62 01100010 o 6F 01101111 C 43 01000011 P 50 01010000 c 63 01100011 p 70 01110000 D 44 01000100 Q 51 01010001 d 64 01100100 q 71 01110001 E 45 01000101 R 52 01010010 e 65 01100101 r 72 01110010 F 46 01000110 S 53 01010011 f 66 01100110 s 73 01110011 G 47 01000111 T 54 01010100 g 67 01100111 t 74 01110100 H 48 01001000 U 55 01010101 h 68 01101000 u 75 01110101 I 49 01001001 V 56 01010110 69 01101001 v 76 01110110 J 4A 01001010 W 57 01010111 j 6A 01101010 w 77 01110111 K 4B 01001011 X 58 01011000 k 6B 01101011 x 78 01111000 L 4C 01001100 Y 59 01011001 l 6C 01101100 y 79 01111001 M 4D 01001101 Z 5A 01011010 m 6D 01101101 z 7A 01111010

Text One bit to switch between upper and lower case No information about font, size, colour (this is the job of scripting languages – HTML etc, or word processors) Similar values for punctuation etc

Exercise Write out your surname in binary and hex Decode this message (x20 is a space) 537475666620666F722073747564656E7473 What are the ASCII codes for ? @ # £ $

Graphics 2 ways to represent graphics Bitmaps Vectors

Bit maps Resolution – horizontal x vertical pixels or bits 640 x 480, 1280 x 1024 etc 1

Colour bit maps Images made up of coloured pixels

Colour bit maps Colour depth – number of colour bits per pixel 1 bit – monochrome 0 = white 1 = black 8 bit – grey scale – 256 shades 00000000 = black 00001111 = mid grey 11111111= white

Colour 24 bit = 8 bits per red/green/blue = 16 million colours Exercise – how many colours in 30 bit Deep Colour

Vector graphics Lines are drawn Can be scaled up or down without loss of quality Allows much smaller file sizes than bitmaps

Vector graphics What is going to be drawn (eg line) Start point (x1,y1) End point (x2,y2) Where x and y are co-ordinates on a grid 0 1 2 3 4 5 Y – axis 0 1 2 3 4 5 X – axis

Draw a square // set the starting point for the line graphics.moveTo(10,10); // move the line through a series of coordinates graphics.lineTo(10,100); graphics.lineTo(100,100); graphics.lineTo(100,10); graphics.lineTo(10,10);

Exercise What applications allow you to create and edit bitmaps or vector graphics? What are the advantages of each method of drawing images?

Sound Sound consists of a continuously varying waveform It is an analogue signal

A-D conversion An A-D convertor (analogue to digital) is used to sample the waveform for amplitude at fixed intervals The values are stored digitally Amplitude Sample period

Sampling rates and bits For CD the sampling rate is 44.1kHz Higher sampling rates give better quality HD-DVD uses 192 kHz The bit depth determines the range of amplitude from loudest to softest 16 bits are used for CD quality 24 bits professionally

Bit rate Bit rate = (sampling rate) x (bit depth) x (number of channels) For a recording with a 44.1 kHz sampling rate, a 16 bit depth, and 2 channels (stereo): 44100 x 16 x 2 = 1411200 bits per second, or 1.4112 Mbit/s

Compression Audio can be compressed to reduce file sizes Uncompressed files contain as many bits for silence as for sound Lossless compression replaces the silence with information about how long the silence lasts. Can reduce file size by half Lossy compression eliminates the less audible sounds and can reduce files to one tenth of the original size

File types Uncompressed Lossless Lossy WAV, AVI FLAC, Apple lossless Mp3, mp4, Vorbis, AAC, ATRAC

D-A conversion To convert a digital file back to sound The bits are fed into a D-A converter This produces a voltage with the amplitude proportional to the value of the 16 bits This is played back at the sampling rate

Video Video is encoded by taking a picture (or frame) at regular intervals The frame size is the width times the height in pixels (eg 1280 x 720) Aspect ratio is the width to depth (eg 16:9) The colour depth is in bits (eg 24 bits) The frame rate is the number of frames per second (eg 25)

File size A 1 hour (3600sec) video with a frame size of 640x480 at a colour depth of 24bits and a frame rate of 25fps has: pixels per frame = 640 * 480 = 307,200 bits per frame = 307,200 * 24 = 7.37Mbits bit rate = 7.37 * 25 = 184.25Mbits/sec file size = 184Mbits/sec * 3600sec = 662,400Mbits = 82.8Gbytes

Compression MPEG -2 can compress video by up to 15-30 times Compares each frame with the previous one Only stores the differences Regularly sends a full frame to avoid gradual deterioration When there is a lot of motion, sends full frames H.264 used for Blu-ray, YouTube and iTunes

Video quality The bit rate can be varied, which affects quality and can produce “blocky” pictures Low bit rates can be used to squeeze more channels into a satellite broadcast Transmit video over low capacity media, such as G3 cellular networks