2.6 Compression

Question 1

lossless compression

Answer 1

• reduces file size without deleting any data

- nothing is lost

Question 2

lossy compression

Answer 2

• reduces file size by deletin some data

- original can never be reconstructed, it has been irreversibly changed

Question 3

how lossless compression works

Answer 3

looks for redundancy where the same data is stored many times and groups this data into one reference

Question 4

how lossy compression work

Answer 4

alogrithms analyse the image and find areas with only slight differences
these are given the same value, which takes fewer bits

in sound recordings very small variations in frequency, tone and volume are removed to reduce the file size as the human ear cannot detect the small differences

Question 5

uses of lossless

Answer 5

• text files

- graphics files with a low colour depth

Question 6

use of lossy

Answer 6

• image files

- digital sound recordings

Question 7

less successful use for lossless

Answer 7

• audio files

- 24 bit colour files

Question 8

less successful use for lossy

Answer 8

• text files

- executable files

Question 9

examples of lossless

Answer 9

• compressed text files
• GIF and PNG image files
• free lossless audio (FLAC) and Apple lossless (ALAC) files

Question 10

examples of lossy

Answer 10

• MP3 audio files

- JPG image files

Question 11

need for compression

Answer 11

• less internet bandwidth is used when files are up or downloaded
• transfer speed is quicker
• less storage space is needed
• smaller files reduce congestion on the internet
• audio and video files can be streamed

Question 12

Describe the difference between lossy and lossless compression and give an example where each would be used [4]

Answer 12

when data is uncompressed again in lossy compression, it is not exactly the same as the original but the difference is so small it cannot normally be noticed (e.g. MP3 audio and JPG image files)

when data is uncompressed in lossless compression, it is restored completely to the original file (e.g. compressed text files)

Question 13

compression

Answer 13

reduces the file size so more data can be stored

Question 14

Explain the important of compressing files when they are transported over the internet [2]

Answer 14

reduces the size of the file that needs to be transmitted

shortens download time

reduces internet traffic

allows multimedia files to be streamed