P2 T1 L13 - Compression

Question 1

____ is constantly being moved around computer systems and networks.

Transfer is usually ________ and ________.

As distances get longer, transfer is ______ and more susceptible to ____________.

Storage space can be ______.

Answer 1

Data

high-speed accurate

slower interference

limited

Question 2

Text, image and sound data can be significantly reduced in size.

Reducing the amount of data to send or store ensures that…. (4 points)

Answer 2

1. Data is sent more quickly
2. Less bandwidth is used as transfer limits may apply
3. Buffering on audio and video streams is less likely to occur
4. Less storage is required

Question 3

What are the 2 different types of data compression?

Answer 3

1. Lossy

2. Lossless

Question 4

Explain lossy compression

1 point

Answer 4

1. Non-essential data is permanently removed

e. g. different shades of the same colour in an image or frequencies of sound outside the range of human hearing

Question 5

Explain lossless compression

1 point

Answer 5

1. Patterns in the data are spotted and summarised in a shorter format without permanently removing any information

Question 6

Describe lossy compression on a JPG file.

2 steps

Answer 6

1. Removes data permanently to reduce file size

2. Tries to reconstruct an image without the missing data

Question 7

Describe lossy compression on an MP3 file.

2 steps

Answer 7

1. Lossy compression removes the sounds in the frequency ranges that we can’t so easily hear or that least affect the perceived playback quality.
2. Quieter notes played at the same time as louder sounds are also removed.

Question 8

Explain run length encoding (RLE)

2 steps

Answer 8

1. A basic method of compression that summarises consecutive patterns of the same data
2. Works well with image and sound data where data could be repeated many times

Question 9

Describe RLE of sound

3 steps

Answer 9

1. A sound recording could have many thousands of samples taken every second (typically 44,000)
2. The same sound or note played for a fraction of a second could result in hundreds of identical samples
3. RLE records one example of the sample and how many times it consecutively repeats

Question 10

Explain dictionary compression

3 points

Answer 10

1. Spots regularly occurring data and stores it separately in a dictionary
2. The reference to the entry in the dictionary is stored in the main file thereby reducing the original data stored
3. Even though the dictionary produces additional overheads the space saving negates this problem

Question 11

_________ reproduces the original data but doesn’t always compress by a significant factor.

Answer 11

Lossless

Question 12

_______ can never recreate exactly the original data but does reduce file size significantly

Answer 12

Lossy