Data Compression

Question 1

what do the terms lossy and lossless describe

Answer 1

whether compressed data is fully recovered when uncompressed

Question 2

what is lossless compression

Answer 2

• when all data that was initially in the file remains when it is uncompressed
• shrinks data and keeps quality

Question 3

what is lossy compression

Answer 3

• when some data is permanently removed

- reduces quality of the file

Question 4

why would you want to compress a file

Answer 4

• makes it easier to store digital files as less space is taken when downloading
• the file can be transmitted quickly

Question 5

what technique can be used to shrink files

Answer 5

compression techniques

Question 6

what is the aim of file compression

Answer 6

making the file smaller than its uncompressed version

Question 7

what does the term ‘smaller’ mean in terms of data compression

Answer 7

less bits (actual bits like 0 and 1) are used to store the file

Question 8

why is the sending of compressed files quicker than an uncompressed files in terms of rate

Answer 8

• transmission time is measured in bits per second

- fewer bits results in fewer seconds for a given rate of transmission

Question 9

how do compression techniques even compress files or what do they look for

Answer 9

• they work by identifying patterns in the binary digits that make up a file
• for example, a number stored 100 times could be stored once with the reference to how many numbers (100)
  are being stored

Question 10

what compression technique stores several bits of data into one file with the reference of how many bits there are

Answer 10

run-length encoding

Question 11

how does huffman coding compress files

Answer 11

• it identifies commonly used sequences of bits
• converts them to shorter bit patterns
• so that they take up less space
• example is like simplifying x x x x x into 5x

Question 12

what file is a common expression of a lossless compression technique

Answer 12

PNG files

Question 13

what file is a common expression of a lossy compression technique and what is specifically compressed

Answer 13

• JPG files
• colour of every pixel isnt stored precisely
• but enough colour data is stored

Question 14

what is zipping

Answer 14

zipping is packing multiple files into a single file

Question 15

what can zipping do

Answer 15

it can take any number of files and folders with any number of levels of sub folder, and convert everything into a single file

Question 16

when is zipping useful and why

Answer 16

• when it comes to emailing files
• because when you send an email, you can attach a file but not a folder
• to email a complex folder structure, you would need to attach files individually

Question 17

what happens after several files are zipped

Answer 17

the new file created from zipping is often compressed and organised into a complete single file

Question 18

what type of compression does a zipped file go through and why

Answer 18

• a lossless compression technique is used

- because you might have saved spreadsheets or a database that you might want to preserve perfectly

Question 19

what are the pros of lossy compression

Answer 19

• greatly reduced file size meaning more files can be stored
• takes up less bandwidth so files can be downloaded quicker
• lots of software can already read lossy files so its commonly used

Question 20

what are the cons of lossy compression

Answer 20

• loses data so the file cant be turned back to the original
• cant be used on text or software files as they need to retain all information
• worse quality than the original file but it is unnoticable usually

Question 21

what are the pros of lossless compression

Answer 21

• data is only removed temporarily so there is no loss of data or reduction if file quality
• lossless files can be decompressed or tuned back to the original
• can be used on text and software files

Question 22

what are the cons of lossless compression

Answer 22

• only a slight reduction in file size so the file still takes up a lot of space on the device