1.3.3-1.3.4 Data Compression

Compression

Process whereby algorithms reduce file size.

Decompression

Process whereby algorithms restore compressed data to their original format so we can access the data stored in compressed files.

Compression is heavily used with , __, and __ files. There are 2 different types of compression: ___, and _.

sound, image, video, lossy, lossless

Lossy compression

Irreversible compression, lossy compressed files permanently lose some data that cannot be restored during file decompression. However, it does decrease file size more than lossless compression can.

Lossy compression is mostly used for multimedia like , , ___, etc. Its image/graphic file formats include __ and __, whereas its sound/audio file formats include __ and __.

audio, video, images, JPG (or JPEG), GIF, MP3, AAC

Digital artefacts

Noticeable distortion of media (images, audio, and video), caused by the application of lossy compression. Examples include blurred/lowered image or video quality, glitchy audio sounds, etc.

Lossless compression

Reversible compression, lossless compressed files can restore and rebuild file data during decompression to result in the exact same-quality data as the original, uncompressed file. However, it decreases file size less than lossy compression can.

Lossless compression is mostly used for __, __, ____, etc, where data loss is ______. Its image/graphics file formats include __ and __, whilst its sound/audio file formats include __, , and . Lossless compression algorithms finds groups of repeating data and records them at , along with the number of _.

text, data files, programs, undesireable, PNG, TIFF, FLAC, ALAC, WMA, once, repetitions.

RLE

Run length encoding, lossless compression method where sequences that display redundant data are stored as 2 values: 1 value represents the number of repetitions/frequency the other value represents the actual data being repeated.

RLE is often written as 2 numbers: the first number represents the _ or , i.e. , while the second number represents the , i.e. __.

count, frequency, number of repetitions, data value, what is being repeated

Runs of data

Sequences where the same data value occurs consecutively in many data elements

Runs of data are stored as a __ representing frequency and the actual (what is being repeated), rather than as the original __. This helps save lots of __ and therefore bits as well as file size! However, in files without many data , RLE can ____ the file size.

count value, data value, run, characters, repetitions, increase

It's possible to reduce _ and ___ of images or ______ and ___ of sound files. This leads to as well as a file size at the cost of _ image/sound quality.

image resolution, colour depth, sample rate, sample resolution, permanent data loss, smaller, lower

Explain the uses of compression.

Compression is often used for files and data that are sent via the internet, they lead to reduced transmission time (hence faster downloading), further data allowances, less bandwidth required, and less storage space needed to save files as compression reduces file size.

Transmission

Transfer of data from 1 digital device to another.

Data allowances

Maximum amount of data usage (uploads and downloads) that you're allowed to use without any extra cost because it's pre-paid under your monthly billing plan (e.g. mobile data plan).

Bandwidth

Maximum amount of data transmitted over an internet connection in a given amount of time.

Storage space

Mechanism allowing data storage on your computer either temporarily or permanently to protect it from drive failures.

Downloading data/file size for music tracks (kilobits) =

Downloading speed (kilobits per second) * Duration of music track (seconds)

What are the benefits of compression?

1. Smaller file size → Fewer bits → Faster transmission time and reduced internet traffic.
2. Reduced download time for video, sound (including speech used for VOIP - Voice Over Internet Protocol - systems), and image files.
3. Streaming is possible as the data can be sent as fast as the rate at which it's played.
4. Images in web pages appear faster.
5. Takes up less space on disks/servers.

Internet traffic (a.k.a. network traffic)

Amount of data moving across a network during any given time.

Disks

Computer data storage device

Servers

Computer program/device providing service to another computer program and its user (client).

Buffering

When your device pauses mid-task. This occurs when download speed is slower than the playback speed. It can be reduced by closing other programs/applications, reducing video quality, or speeding up internet connections.

How does video streaming work?

1. Files for video streaming are compressed.
2. The TV or computer needs to decompress the data as the video is played.
3. Before the video is played, it will buffer a sufficient amount to allow for occasionally slow network.
4. If the website or server detects slow connection, it may start sending alternatively lower-quality (hence lower file size) files.

JPEG

Lossy file format for photos.

PNG

Lossless file format for images/photos, may include transparency.

ZIP

Lossless file format for document compression.

GIF

Lossless file format used for simple images, may include animation.

MP4

Lossy file format used for videos.

MP3

Lossy file format used for music/audio.

FLAC

Free lossless file formats for music.

ALAC

Apple lossless file format for music.

AVI

Lossless file format for audio and/or video playback on discs as well as DVDs.