Compression

Types of Compression

Representing audio, images, and especially video take up a lot of secondary storage space. Compression helps to reduce the size of these files so that we can store more data.

Lossy compression

An algorithm is applied to remove unnecessary detail from the original file.
Some data is permanently lost, but enough remains so that the file is still useful and there is barely a noticeable difference.
Lossy compression results in dramatic file size reduction.

Lossless compression

None of the original data is lost.
An algorithm can be used to perfectly restore the original file when needed.
This is especially useful for executable files, where all of the data is necessary.
Lossless compression causes file size to reduce moderately.

Run Length Encoding

One compression algorithm you need to learn is run length encoding (RLE).

Run length encoding

Run length encoding (RLE) is a form of lossless compression that replaces repeating sequences of 0s and 1s with more efficient representations.
Each repeating string will be replaced by a code which represents the character and the amount of times it is to be repeated.

In images

In images, adjacent pixels are likely to be similar colours but slightly different.
Image compression algorithms often group these pixels together, and give them an 'average' colour.
The RLE algorithm can then run on the new image.
This technique then becomes lossy.

Dictionary Coding

Dictionary coding is used to store text in a lossless format.

Dictionary coding

The coder scans through a body of text.
Each word is replaced by the binary number of the word in a dictionary.
Each time it finds a new word which is not in its dictionary, it will add it to the dictionary and give it a binary number.
The word in the actual text is replaced with the binary number.

Benefits of dictionary coding

Dictionary coding is a type of compression. It takes less bits to store a 2 digit binary number than we can a 10 letter word.
No data is lost, so the coding is lossless.
The encoder may not always build its own dictionary. A pre-existing dictionary may be used to encode the text.

1Components of a Computer

1.1Structure & Function of the Processor

1.1.1Structure of the CPU

1.1.2Fetch, Decode, Execute Cycle

1.1.3Factors Affecting CPU Performance

1.1.4CPU Architecture

1.2Types of Processors

1.2.1CISC & RISC

1.2.2Multicore & Parallel Cores

1.3Input, Output & Storage

1.3.1Computer Systems

1.3.2Magnetic Storage

1.3.3Optical Storage

1.3.4Types of Optical Storage

1.3.5Types of Memory

1.3.6Flash Memory

1.3.7Virtual Memory

2Software & Software Development

3Exchanging Data

4Data Types, Data Structures & Algorithms

4.1Data Types

4.1.1Data Types

4.1.2Arrays & Strings

4.1.3Representing Numbers

4.1.4Binary Arithmetic

4.1.5Hexadecimal

4.1.6Converting Binary Numbers

4.1.7Floating Points in Binary

4.1.8Floating Point Arithmetic

4.1.9Bitwise Manipulation

4.1.10Character Sets

4.2Data Structures

4.2.1Arrays, Linked Lists & Queues

4.2.2Graphs, Hash Tables & Stacks

4.2.3Trees

4.3Boolean Algebra

4.3.1Computational Logic

4.3.2Boolean Logic

4.3.3Karnaugh Maps

4.3.4Boolean Identities

4.3.5Flip Flops & Adder Circuits

5Legal, Moral, Cultural & Ethical Issues

5.1Computing Related Legislation

5.1.1Data Protection Act (1998)

5.1.2Computer Misuse Act (1990)

5.1.3Copyright Designs and Patents Act (1988)

5.1.4Regulation of Investigatory Powers Act (2000)

5.2Moral & Ethical Issues

5.2.1Online Activity Tracking

5.2.2Censorship

5.2.3Cultural Issues

5.2.4Environmental Issues

5.2.5Layout, Colour Paradigms & Character Sets

6Elements of Computational Thinking

6.1Thinking Abstractly

6.1.1Abstraction

6.2Thinking Procedurally

6.2.1Decomposition

6.3Thinking Logically

6.3.1Pseudocode & Flow Diagrams

7Problem Solving & Programming

7.1Programming Techniques

7.1.1Programming Fundamentals

7.1.2Modularity

7.2Programming Construction

7.2.1Integrated Development Environments

7.2.2Testing

8Algorithms

8.1Algorithms

8.1.1Interpreting, Correcting & Completing Algorithm

8.1.2Sort Algorithms

8.1.3Insertion & Merge Sort

8.1.4Search Algorithms

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