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

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

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.

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

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

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

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

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.

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

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

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

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

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

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

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!

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!

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.

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.

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.

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

Run Length Encoding Watch this video to learn more about Run Length Encoding. Link: https://www.youtube.com/watch?v=ypdNscvym_E

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.

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.