1. Data Representation

2. Data representation Encoding Analogue v. digital Sampling
File systems (drives, folders, naming)

3. Data representation Humans work with Computers use
Text
Plain
Formatted
Numbers
Integers
Fractions
Big & small
Graphics
Video
Sound
Computers use
1 or 0
How do we resolve this mismatch?

4. Data encoding What does this say? How do we know?
●●● — — — ●●●
How do we know?
Information has been encoded

5. Encoding
Finding a way to represent data in a way that can be handled by computers
Many different encoding methods e.g.
Text – ASCII, Unicode etc.
Numbers – Integers, Fixed point, floating point
Pictures – 2 bit, 8 bit, 16 bit, 24 bit colour & different file formats
Sound – WAV, MP3, OGG etc.
Video – AVI, MPG, MOV etc.

6. Encoding examples - ASCII
American Standard Code for Information Interchange
Represents English language characters using numbers
Originally 7 bits per character -> 128 options
Alphanumeric & control characters (e.g. BS=Back Space)
8th bit used for parity checking
Extended ASCII uses 8 bits = 256 options

7. ASCII code (7 bit)

8. Encoding examples - Unicode
ASCII just one of many encoding schemes
ASCII only supports basic Latin alphabet
Other encoding schemes required to support different alphabets, accented letters, mathematical symbols etc.
Problems of incompatibility etc.
Unicode aims to replace all “legacy” character sets

9. Encoding examples - Unicode
provides a unique number for every character,
no matter what the platform
no matter what the program
no matter what the language
Uses 8, 16 or 32 bits for each character
Currently at version 8.0
Full details available at Unicode website

10. Unicode 8.0 code charts

11. Unicode 8.0 code charts (continued)

12. Unicode 8.0 code charts (continued)

13. Numbers Numbers fundamental part of life Many different forms
1,2,3,4,5 …
£17.99
47%
Earth to Sun 149.6×109 m
½ , ¾ √2
Negative numbers

14. Numbers Computers can only use 0 or 1 => binary (Base 2)
Encoding schemes needed to represent all other types of number in binary
Next week we will look at
Binary Base 2
Decimal Base 10
Hexadecimal Base 16

15. Analogue A continuously varying signal
Varying in terms of frequency, amplitude, or both
A signal that is constantly changing
Analogue can represent many values
Accurate BUT vulnerable to noise

16. Digital A signal with discrete value changes
Signal levels are either on or off
Often thought of as either 1 or 0, especially in computers, but digital can represent many different values using discrete levels
Due to limited possible values, digital signals less vulnerable to noise

17. Analogue to digital conversion
Most things we deal with are analogue e.g.
Colours
Sounds
Time
Weights
Lengths etc.
Computers are digital ...

18. Sampling

19. Sampling Most common way to convert analogue data in digital form
Take measurements at set intervals (time or space)
Measure value of analogue source to a defined degree of accuracy => resolution
Higher resolution gives more accurate result but increases storage
Store digitised values

20. Resolution Two aspects to resolution Sampling rate examples
No. of possible values per data point
Sampling rate examples
Dots per inch -> pictures
Sampling frequency -> sounds
Values per data point examples
16bit, 24bit, 32bit colour -> graphics
Bit rate -> MP3 files

21. End result? All these forms of information end up as 1s and 0s…
which have to be stored or transmitted…

22. File systems
Each operating system has facilities to manage files in various ways e.g.
Create
Move
Copy
Delete
Rename
Open
Execute

23. File systems - Windows Each drive has a letter e.g. C:
Files are organised in folders (used to be called directories)
Top level of each drive is the root folder e.g. C:\
File names are not case sensitive MyFile.txt = myfile.txt
Some characters are not allowed as they have special meanings
\ / ? : * “ > < |

24. File systems - Windows
Files generally have an extension which tells Windows what type of file it is e.g.
myfile.txt = plain text file
myfile.doc = Word document
Windows maintains a database of file extensions and the program that opens each type

25. File systems - Windows
File names must be unique in a folder but can be duplicated if in different folders
Full name of a file includes the path to find it
Starts with drive letter
Folders divided up by \ character
D:\documents\college work\Year 1\CS\PC hardware.pptx
Note: spaces are allowed but can cause problems…

26. File systems – Linux / Android
Based on Filesystem Hierarchy Standard (FHS)
No drive letters
Everything including physical drives appears in directories (folders) under the root directory (/)
Levels in file paths divided by / (where else have you seen this?)
Case sensitive

27. Ubuntu Drives show as folders under /dev or /mnt
e.g. sda is first SATA drive

28. Android Android is a version of Linux Same file system (ish)
Often has sdcard/ as symbolic link to /mnt/sdcard

29. Apple iOS
“To keep the system simple, users of iOS devices do not have direct access to the file system and apps are expected to follow this convention.” (iOS Developer Library)
Each app runs in its own “sandbox”
“an app is generally prohibited from accessing or creating files outside its containers”
Must use operating system to access “things such as the user’s contacts or music”