4.5.6 Representing images, sound and other data

Question 1

Why may files be compressed?

Answer 1

Files may be compressed to reduce their size.

Question 2

What is an advantage of smaller files?

Answer 2

• Smaller files may be transferred faster between storage devices or over the internet.

Question 3

What are the two kinds of compression?

Answer 3

• Lossy

* Lossless

Question 4

What kinds of files are compressed?

Answer 4

• Images
• Sound files
• Text files

Question 5

What happens to the information with lossy compression?

Answer 5

• Some information is lost in the process of reducing the file’s size.

Question 6

What occurs in lossy compression?

Answer 6

• Some information is lost in the processing of reducing a file’s size
• This could be reducing the resolution of an image or lowering the sample resolution of a sampled audio file.

Question 7

What is an advantage of lossless compression?

Answer 7

There is no loss of information when using lossless compression, the size of a file can be reduced without decreasing its quality.

Question 8

What are the two methods of lossless compression?:

Answer 8

• Run length encoding

* Dictionary based

Question 9

How does RLE work?

Answer 9

Reduces the size of a file by removing repeated information and replacing it with one occurrence of the repeated information followed by the number of times it is to be repeated

Question 10

Why may RLE not be suitable?

Answer 10

where information has no repeated values, it cannot be compressed by RLE.

Not all data is suitable for compression by RLE.

Question 11

How does dictionary based methods work?

Answer 11

• When a file is compressed with a dictionary based method, a dictionary containing repeated data is appended to the file.

Question 12

How is file size affected by dictionary based methods?

Answer 12

• Results in a reduction in size, however the dictionary has to be present in the file in order for the files to be reproduced, resulting in the file size increasing.

Question 13

What are RLE and dictionary based methods most effective on?

Answer 13

Files that contain a lot of repeated data.

Question 14

What is the advantage of lossy compression?

Answer 14

• Extent to which file size can be reduced is not limited.

Question 15

What are the disadvantages of lossy compression?

Answer 15

• Quality of file is reduced

* Some information is lost in the compression process.

Question 16

What is the advantage of lossless compression?

Answer 16

• No loss of information

* No loss of quality

Question 17

What is the disadvantage of lossless compression?

Answer 17

There is a limit to how much a file can be compressed