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Level 3 Extended Diploma Unit 19 Computer Systems Architecture
Data Types Level 3 Extended Diploma Unit 19 Computer Systems Architecture
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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
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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: is uppercase A
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ASCII code Char Hex Binary A 41 01000001 N 4E 01001110 a 61 01100001 n
B 42 O 4F b 62 o 6F C 43 P 50 c 63 p 70 D 44 Q 51 d 64 q 71 E 45 R 52 e 65 r 72 F 46 S 53 f 66 s 73 G 47 T 54 g 67 t 74 H 48 U 55 h 68 u 75 I 49 V 56 69 v 76 J 4A W 57 j 6A w 77 K 4B X 58 k 6B x 78 L 4C Y 59 l 6C y 79 M 4D Z 5A m 6D z 7A
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Text One bit to switch between upper and lower case
A = a = No information about font, size, colour (this is the job of scripting languages – HTML etc, or word processors) Similar values for punctuation etc
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Exercise Write out your surname in binary and hex
Decode this message (x20 is a space) F E7473 What are the ASCII codes for # £ $
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Graphics 2 ways to represent graphics Bitmaps Vectors
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Bit maps Resolution – horizontal x vertical pixels or bits
640 x 480, 1280 x 1024 etc This example is 5x7, a 0 is black and a 1 is white 1
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Colour bit maps Images made up of coloured pixels
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Colour bit maps Colour depth – number of colour bits per pixel
1 bit – monochrome 0 = white 1 = black 8 bit – grey scale – 256 shades = black = mid grey = white
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Colour 24 bit = 8 bits per red/green/blue = 16 million colours
Exercise – how many colours in 30 bit Deep Colour
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Vector graphics Lines are drawn
Can be scaled up or down without loss of quality Allows much smaller file sizes than bitmaps
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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 Y – axis X – axis
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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);
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Exercise What applications allow you to create and edit bitmaps or vector graphics? What are the advantages of each method of drawing images? Consider File size What happens when you scale the image Colour reproduction
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Sound Sound consists of a continuously varying waveform
It is an analogue signal
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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
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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
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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 = bits per second, or Mbit/s
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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
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File types Uncompressed Lossless Lossy WAV, AVI FLAC, Apple lossless
Mp3, mp4, Vorbis, AAC, ATRAC
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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
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