1. GCSE COMPUTER SCIENCE
Topic 3 - Data
3.3 Data Storage and Compression

2. This presentation covers the following content from the specification
Students should:
3.3.1 understand how to convert between the terms ‘bit, nibble, byte, kilobyte (KB), megabyte (MB), gigabyte (GB), terabyte (TB)’
3.3.2 understand the need for data compression and methods of compressing data (lossless, lossy) and that JPEG and MP3 are examples of lossy algorithms
3.3.3 understand how a lossless, run-length encoding (RLE) algorithm works
3.3.4 understand that file storage is measured in bytes and be able to calculate file sizes

3. Speed Units Speed units are needed to measure how fast processors are.
Example A 1GHz processor can compute 1 billion instructions a second.
The faster a CPU is, the more computations it can perform a second.

4. The base unit of speed is called a:
Speed Units
The base unit of speed is called a:
Hertz

5. The next unit of speed is:
Speed Units
The next unit of speed is:
Kilohertz
1 Kilohertz is 1024 Hertz

6. Megahertz Speed Speed Units The next unit of speed is:
1 Megahertz is 1024 Kilohertz

7. Gigahertz Speed Speed Units The final unit of speed is:
1 Gigahertz is 1024 Megahertz

8. Storage Units
Storage units are needed measure how much data a device can hold.
Example A 1GB USB stick can hold roughly 230 mp3s, 16 hours of music or 20 music albums. Not enough to hold a movie.
The larger a storage device is, the more data is can hold.

9. The base unit of storage is called a:
Storage Units
The base unit of storage is called a:
Byte

10. The next unit of storage is:
Storage Units
The next unit of storage is:
Kilobytes
1 Kilobytes is 1024 Bytes

11. The next unit of storage is:
Storage Units
The next unit of storage is:
Megabytes
1 Megabyte is 1024 Kilobytes

12. The next unit of storage is:
Storage Units
The next unit of storage is:
Gigabytes
1 Gigabyte is 1024 Megabytes

13. The final unit of storage is:
Storage Units
The final unit of storage is:
Terabytes
1 Terabyte is 1024 Gigabytes

14. You need to remember these, especially the order.
Speed & Storage Units: Recap
Speed Units
hertz (Hz)
Storage Units
bit (b)
Hertz
kilohertz (KHz)
Bytes
byte (B)
megahertz (MHz)
kilobyte (KB)
gigahertz(GHz)
megabyte (MB)
gigabyte (GB)
terabyte (TB)
You need to remember these, especially the order.
ACTIVITY
Write these down in your books under a title of speed and storage units

15. Activity 1 We need a phrase to remember this sequence. Bytes B BIG
Kilobytes K
KANGAROOS
Megabytes M
MAKE
Gigabytes G
GOOD
Terabytes T
TEA
Title: Speed and Storage Units
Your task is to create a scene to remember this phrase!

16. Example Here’s one I made earlier!
However, this is unfinished. It needs to have the phrase and what each word stands for on it.
Now that’s a good cup of tea!

17. Compression Compression is the process of reducing the size of a file
It has two main benefits: 1
It allows us to reduce the amount of storage space we use
(I.e. we can store more data) 2
It increases the transfer rate of files over networks
(I.e. sends and receives data over a network faster)
ACTIVITY
Write this information in your book.

18. Compression
There are two types of compression you need to understand:
Lossless
Reduces the size of a file while retaining all of the original information.
Lossy
Reduces the size of a file by permanently removing some of the data.
ACTIVITY
Write these two definitions in your book.

19. Lossy Compression JPG Images MP3 Audio Type Example Example
(pronounced jpeg)
The most common file format used for images. Most of the images produced by modern digital cameras and mobile phones are jpegs. These images are lower in quality than bitmap images, however have much smaller file sizes.
Audio
MP3
(mpeg
layer 3)
The most common file format used for audio files. Most of the sounds downloaded from the Internet are mp3 files, for example, songs downloaded on iTunes. This audio is lower quality than wave files (.wav), again however have smaller file sizes.

20. Activity 2
Explain the advantages and disadvantages of lossy and lossless compression.
Advantages
Disadvantages
Lossless
Keeps all of the original data
Original can be reproduced exactly
When the file is uncompressed
Reduces the file size
But not by as much as lossy compression
Lossy
Does not keep all of the original data
File can be compressed much more
Than in lossless compression
Some data is permanently lost
Therefore the original cannot be recreated

21. Run Length Encoding
Watch this video to learn more about Run Length Encoding.
Link:

22. Run Length Encoding
Definition
Run length encoding (RLE) is a simple form of lossless compression. Runs of data (sequences in which the same data value occurs consecutively) are stored as a single data value and count, rather than as the original run.
Old uncompressed string
AAAAABBBBCCCDDE 5A 4B 3C 2D 1E
New compressed string
5A4B3C2D1E
ACTIVITY
Write this definition and example in your book.

23. Activity 3 Text string Answer AAAABBBBBBBBBCADDDDEEFFFFFFFF
4A9B1C1A4D2E8F
ABCABCABCABCABC
1A1B1C1A1B1C1A1B1C1A1B1C1A1B1C
BBGGYYAACCFFEEBBGGYYAACCFFEE
2B2G2Y2A2C2F2E2B2G2Y2A2C2F2E
Which one compresses the most?
The first text string.
Why is this?
Lots of repeated letters in the string that are in sequence.
Describe in English the process RLE uses to encode text.
Look for sequences of letters in the string. Count how many letters are in each sequence. Shorten the sequence to the amount and the letter only.