Data Compression

What does lossy compression do?

It removes sounds and details not easily heard/ seen so don’t affect the overall perceived playback quality + permanently loses some data

Disadvantage of lossy compression

It leaves out some data so can potentially negatively affect sound quality

What does lossless compression do

Lossless compression formats can reduce file size when compressed but don’t lose any info (good for text - can’t afford to lose data) 

Reasons why data is compressed + often used

• To reduce bandwidth so files download faster

• Reduce file size as smaller files have faster transmission time

• Reducing the file size reduces the memory space taken on discs/servers

When is RLE not a suitable method for compression?

 It isn’t suitable when the string doesn’t contain many runs, and is very random as it would increase the file size and take more space than originally

State a feature of data that allows it to be compressed effectively using RLE

Has many runs and not very random

Describe how RLE works

• States the frequency of a character or coloured pixel consecutive appearance

• Then, the value of the data to losslessly compress the original data

• Won’t lose any of the original data

Three tests to improve the RLE code

• Test it with an input bit pattern of only 1s, 0ss

• An input bit pattern of a longer length

• An invalid input like ‘abc’

When is huffman not suitable

It is not efficient for short bits of text

How to follow a huffman tree?

left = 0, right = 1