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

2. 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

3. 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

4. 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

5. 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

6. Exercise Write out your surname in binary and hex
Decode this message (x20 is a space)
F E7473
What are the ASCII codes for # £ $

7. Graphics
2 ways to represent graphics
Bitmaps
Vectors

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

9. Colour bit maps
Images made up of coloured pixels

10. 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

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

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

13. 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

14. 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);

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

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

17. 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

18. 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

19. 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

20. 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

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

22. 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

23. 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)

24. 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 = Mbits/sec
file size = 184Mbits/sec * 3600sec = 662,400Mbits = 82.8Gbytes

25. 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

26. 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