1.2.5 Compression

Perceptual music shaping

Refers to the process of removing inaudible sounds in order to make a file size smaller.

e.g.

Noises at frequencies that humans cannot hear

Quiet sounds that cannot be heard over louder sounds

Compression

a method or protocol for using fewer bits to represent the original information

Lossy Compression

Reduction of file size by removing certain, redundant information from the file

The eliminated data is unrecoverable.

Tries to recreate an file without the omitted data

Much smaller file sizes but there will be some loss of quality

Lossy benefits

Means the decompressed file is not identical to the original...

...the difference is unlikely to be noticed by humans

Lossy will decrease the file size ...

... so it can be sent via e-mail/sent quickly/uses less bandwidth

Lossy Image Compression

A compression algorithm is used

Permanently deleting some data // file cannot be restored to original

Colour depth / colour palette can be reduced

Resolution can be reduced // number of pixels can be reduced

Lossy text compression

File is compressed some detail / data / quality / is lost...

... would make the text file unreadable / lose meaning or comprehension

Lossless compression

Every bit of the original data can be recovered from the compressed file.

the uncompressed image will be the same as the original with no loss of data

Works by looking for patterns in the data

Larger compressed file sizes than lossy

e.g. Run-length encoding

Lossless image compression

A compression algorithm is used

No data is removed in the compression process

An index/dictionary of pixels is created

The number of times a pixel is repeated in a row is stored

Run length encoding

Reasons to Compress data

Download/buffering times decrease

Smaller files = fewer packets = faster transmission time

Reduces traffic over the Internet

Less chance of collisions or transmission errors

Data allowances do not run out as quickly = Saves money

Voice can be transmitted fast enough to keep up with speech in a video

Saves spending more money on data storage e.g. hard drives, cloud etc

Faster to back-up data

Run Length Encoding (RLE)

Lossless compression technique

Summarises consecutive patterns of the same data

Works well with image and sound data where data could be repeated many times

RLE of sound

Sound recordings can have many thousands of samples taken every second

The same sound or note played for a fraction of a second could result in hundreds of identical samples

RLE records one example of the sample and how many times it consecutively repeats

JPEG

A commonly used file format that uses lossy compression for digital photography

MP3

Lossy compressed audio file format