1. Component 1.9 Security and Data Management
Compression

2. Complete the Entry Ticket on Compression
Starter: Entry Ticket
Complete the Entry Ticket on Compression
Extension:
Explain using examples how video and image files can be compressed using lossy and lossless compression.

3. Learning Intentions and Outcomes
To develop an understanding of how data is compressed to reduce the filesize
Grade 4
Explain the term compression and its purpose
Identify appropriate methods of compression for various digital products
Grade 5-6
Explain using examples how lossy and lossless compression are used
Explain using examples whether lost or lossless compression is the most appropriate for reducing the filesize of given digital products
Grade 7
Analyse the importance of compression
Evaluate scenarios to identify the best method of compression for a given digital product

4. Assessment Outcomes
Explain how lossy and lossless data compression algorithms are used
Calculate compression ratios

5. Starter Discussion Two types of compression: Lossy Lossless
Lossy compression is used when it doesn’t matter if some of the quality is lost. For example a photograph or a video – you can lose a little quality and the product will still fulfil its intended purpose.
Lossless compression is used when no information can be lost. For example when compressing the source code of a program! If you lost some of the source code the program may not function!

6. Starter Discussion
The purpose of compression is to reduce the file size of a digital product. This may be so that it can be sent over a network quicker or so that it takes up less space on a hard drive.

7. Compression Purpose: Reduce file size
Mostly used with sound, image and video file types
Two types:
Lossy compression (JPG, GIF, MP3)
Lossless compression (PNG, TIF, SVG)

8. Lossy compression Permanently removes some data
Recreates the file using the remaining data and uses algorithms to guess the removed content
Uncompressed data is not the same as the original
Would this technique work for compressing a computer program?

9. How lossy compression works
Similarly coloured pixels are all made the same

10. Lossless image compression
Finds groups of repeating data and records the data only once along with the number of times it was repeated
= 12 x x
When data is uncompressed it is restored exactly as it was in the original

11. Lossless text compression
Finds patterns in the original text
Encodes each pattern in a dictionary
An eye for an eye,
a tooth for a tooth .
0000
An_ 1
0001
eye 2
0010
_for_ 3
0011
an_ 4
0100 ,_ 5
0101 a_ 6
0110
tooth 7
0111

12. Lossless text compression
38 Characters including spaces = 38 bytes (assuming an 8-bit ASCII table is used)
48 bits = 12 bytes = 32% of original size (plus codes) .
0000
An_ 1
0001
eye 2
0010
_for_ 3
0011
an_ 4
0100 ,_ 5
0101 a_ 6
0110
tooth 7
0111 1 2 3 4 5 6 7
0001
0010
0011
0100
0101
0110
0111
0000

13. Transmission of data over IP
Why use compression?
Download speeds are increased
Data allowances are reduced
Voice can be transmitted fast enough to keep up with speech

14. Downloading a music track
Dancing Queen by ABBA = 3m 51sec = 231 seconds
MP3 quality = 128kbps
CD quality = 1411kbps
231 x 128kbps = 29,568kbs / 1024 / 8 = 3.6MB
OR:
231 x 1411kbps = 325,941kbs / 1024 / 8 = 39.79MB
11.5 times faster with a compressed file
36MB less download data used

15. Benefits of compression
Smaller files = fewer packets = faster transmission
Quicker to complete
Reduces traffic over the Internet
Less chance of collisions or transmission errors
Improves download speed of video, sound (including speech used for VOIP systems) and image files
Speeds up download of webpages that use images
Reduces space on disk / servers
Enables better streaming of music and video

16. Common file standards
PDF – Fixed layout document that maintains its original appearance regardless of the software used to view it
JPG – Lossy compressed files commonly used for website images
GIF – Lossy compressed images using only 256 colours, used for simple web graphics
PNG – Lossless compressed format using an alpha channel to preserve transparent backgrounds
MP3 – Lossy music format

17. Grade 4: Compression Define the term compression
Put the subtitle “Compression” in your book and answer the Grade 4 Exercises
Extension
Move on to the exam questions at the front of the room
Define the term compression
Explain the difference between the two compression techniques (lossy and lossless)
Identify which compression technique is most appropriate for the following, giving examples to explain why for each:
Source Code
Word Document containing a Script for a film
Database containing customer information
Family photo from holiday – to be printed and put on a wall
Selfie to upload to Facebook
Short video clip to share on Instagram

18. Grade 5-7 Exam Questions Complete them Self Assess them Stick them in
Bring them to me

19. Grade 8 Extension
Look in the A-Level folder and complete the Grade D exercises about disaster planning and recovery.

20. Exit Ticket
Complete the Exit Ticket and Stick it in your books.

21. Homework Graded Exercises
Complete up to your Year 11 target grade in the graded exercises.
Pre-Reading
Pages Lossy and Lossless compression