Data compression · GCSE Computer Science

Why compress files?

Compression makes files smaller, so they take up less storage space and are quicker to send or download, using less bandwidth.

Lossy compression permanently removes some data, usually detail that people are unlikely to notice. It greatly reduces the file size, but the original can't be recovered exactly.
It is used for photos (JPEG), music (MP3) and video. It is not suitable for text or program files.

Lossless compression makes a file smaller without losing any data, so the original can be recovered exactly.
It is used for text, program files and some images (PNG). It usually reduces the file size less than lossy compression.

RLE stores each run of repeated data as a count and a value: AAAAABBB becomes 5A 3B. In binary, 0000011100 becomes 5 0s, 3 1s and 2 0s.
It works well when there are long runs of repeated values, but can make data bigger when there are few repeats.

Huffman coding gives shorter binary codes to the characters that appear most often, using a Huffman tree. It is lossless.
Bits needed = the total of (frequency × code length) for each character. A text with 4 As, 2 Bs and 1 C, coded A = 0, B = 10 and C = 11, needs 4 × 1 + 2 × 2 + 1 × 2 = 10 bits, compared with 7 × 7 = 49 bits in 7-bit ASCII.

Compression: Making a file smaller.
Lossy compression: Compression that permanently removes some data.
Lossless compression: Compression that keeps all the data, so the original can be recovered exactly.
Run-length encoding: Storing runs of repeated data as a count and a value.
Huffman coding: Lossless compression that gives shorter codes to more frequent characters.
Bandwidth: The amount of data that can be sent over a network each second.