Compression

Question 1

Define ‘lossy compression’

Answer 1

Compression where non-essential data is removed to free up space, for example:

• Different shades of the same colour in an image
• Frequencies of sound outside the human hearing range

Question 2

Define ‘lossless compression’

Answer 2

Compression where the computer analyses patterns in the data, and summarises it in a shorter format - without removing any information.

Question 3

How does lossy compression work in images?

Answer 3

• Different shades of certain colours are removed
• Picture is reconstructed with less coloured pixels

Question 4

How does lossy compression occur in sounds?

Answer 4

• Sounds with frequencies outside the range of human hearing are removed from the file
• Quieter notes playing at the same time as louder notes are also removed from the file

Question 5

What is the difference between lossy and lossless types?

Answer 5

Lossy: Is most effective at reducing file size

Lossless: Maintains the information of the original data

Question 6

What is lossy compression typically used for?

Answer 6

Audio files and pictures

Question 7

State the effect of using lossy compression on pictures and audio

Answer 7

Pictures files: Would lose their original resolution, as there are less individual colours to make up the picture

Sound files: Would lose their sharpness

Question 8

State one method of lossless compression and where it is used

Answer 8

Run Length Encoding: Used in pictures and sound files.

Dictionary Compression: Used in word files

Question 9

Explain how Dictionary Compression works:

Answer 9

Repeating words / characters are added to a dictionary, and assigned a [shorter] binary value