Compressing Media Types

Question 1

Lossy Compression

Answer 1

Once the file is compressed the original data file can never be recreated (uncompressed)
Data removed lost forever

Question 2

Lossless Compression

Answer 2

Data encoded in a way which allows file to be uncompressed

This recreates original file

Question 3

Run length encoding

Answer 3

Analyses a bit-mapped image for repeating pixel colours
Single value for colour is stored where colour appears more than once in a row
Note of the number of times which the colour is repeated

Question 4

LZW encoding (Give an example of what it is used on as well as explanation)

Answer 4

Looks for repeating patterns of pixels
Dictionary of patterns is created
Dictionary replaces repeating patterns with a code
New data and the dictionary are saved in compressed file
Eg Used on GIF files

Question 5

DCT encoding (Give an example of what it is used on as well as explanation)

Answer 5

Mathematical algorithm applied to groups of 64 pixels
Each group is saved in compressed form
Groups uncompressed when graphic is viewed
This recreates an approximation of the original groups which are joined to complete the image
Eg Used on JPEG files

Question 6

Vector Graphic Encoding (Give an example of what it is used on as well as explanation)

Answer 6

Properties of objects stored using text
Similar objects stored
Repeating patterns of characters in text file
Patterns stored in a shorter form within a dictionary
Eg SVG graphics

Question 7

Perceptual Noise Shaping

Answer 7

MP3 files compressed to a tenth of the original file size

Retains high sound quality because focuses on removing data that the human ear cannot easily hear

Question 8

Techniques used by perceptual noise shaping

Answer 8

Quiet sound next to a loud sound is removed when they are of a similar frequency
Human ear has difficulty with extreme high and low frequencies and so these are removed
Low base sound changed from stereo to a single mono sound as human ear cannot detect direction from which base sounds are coming

Question 9

Inter frame compression

Answer 9

Only stores information changes between each frame of a video
Reference I-frame stored at regular intervals
Algorithm searches for blocks of pixels which stay the same between frames following this
Position of these blocks is stored in the I-frame
Compression achieved determined by number of differences between frames

Question 10

Intra frame compression

Answer 10

Same as inter frame compression
But treats each frame of the video as a separate bit-mapped graphic and
Compresses the frames using similar techniques

Question 11

Codecs

Answer 11

Accompanying sound for a video also compressed
Technique variety used stored in a codec
Video playing software must have the same codec installed to view the compressed video

Question 12

Large file sizes cause the problems of…

Answer 12

Longer download/opening times
Increased storage requirements
More demanding hardware requirements