2.3.4 Data Compression

Question 1

Data compression

Answer 1

Process used to reduce the storage requirements of a file.

Process used algorithms to look for repeating patterns that can be stored with less data.

Question 2

Reasons to compress files

Answer 2

• reduce amount of storage space needed for storage devices

- reduce bandwidth needed to upload/ download file.

Question 3

Types of compressions

Answer 3

LOSSY

LOSSLESS

Question 4

Lossy compression

Answer 4

Some original data will be lost during compression

Notice difference in quality.

Question 5

Lossless

Answer 5

Algorithms are used but little/ no data lost.

Impossible to notice difference in quality.

Question 6

Zipping

Answer 6

Procedure for compressing data files

Uses an algorithm called LZW

Question 7

LZW - In zipping

Answer 7

• looks for recurring patterns with data.
• can be placed with a single character.
• less bits are used to represent original data.
• when zipped file decompressed by zipping software the reverse procedure happens.

Question 8

WAV waveform audio file format

Answer 8

• portable- played by most audio software.
• uncompressed but can be used to store compressed sounds.
• used by audio profession to record sounds.

Question 9

Txt

Answer 9

• only holds letters and numbers.
• no formatting or images.
• no compression as already small file.
• May contain a special symbol to signify the end of file.
• sometimes used to send basic data between larger applications.

Question 10

Analogue and digital sounds.

Answer 10

• humans hear in analogue- continuously varying signal.

- computers digital- need to convert sound to digital data.

Question 11

Converting sound to digital data

Answer 11

• audio captured via input device ie mic.
• sound travels down wire as an electrical signal.
• computer changes signal to binary data ADC
• software saves the data as sound files with the quality being determined by sample rate and sample resolution.

Question 12

BMP

Answer 12

• map of all the bits
• details of all the pixels that make up and image are stored.
• each pixel has the digital representation of its individual colour stored separately.
• colours that fade into other colours can be accurately represented.

Question 13

JPEG

Answer 13

• looks for sequences of pixels that repeat and only store data once.
• used on webpages
• supports 16million colours.
• LOSSY compression but amount of detail lost can be varied by user.
• not support transparency/ animation.

Question 14

GIF

Answer 14

• restricted to 8 bit colour depth 256 colours.
• LOSSLESS compression when used on simple images with blocks of colour.
• LOSSY compression for real or lifelike images.
• small file sizes
• can contain transparency pixels and supports animation.

Question 15

MPEG

Answer 15

• include MP3 audio and MPEG4 video
• discards sounds that typical human air does not notice for music.
• for video 24/25 FPS- full frames only stored occasionally- only store data different to previous frame.
• heavily compresses the source by only storing data about non changing pixels once.
• designed for online streaming.
• LOSSY compression but can maintain v high image quality.
• can contain subtitles
• default format used by mobile phones and digital cameras.

Question 16

AVI audio video interleave

Answer 16

• portable and be played by most video software
• Does not compress video and therefore keeps full quality.
• can contain multiple streams
• multiple camera angles
• v high file sizes.