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13 - Compression

1. Compression means to ______________ the size of a data file, whilst retaining most, or all, of the original information.

Note: Do watch the video if you can, but it is not specifically necessary to do so in order to answer the question.

gentrify

increase

obliterate

reduce

2. Commonly used file types that can be compressed include documents, music files, video and image files.

TRUE

FALSE

3. Data streaming services may compress their files to ensure _____________________

a higher bit per second transfer rate

a huge decrease in the quality of their files

a lower bit per second transfer rate

None of the above

4. What are main reasons for compressing files?

Less storage space required

All of the above

Smaller attachments for email

Faster downloads and uploads (coping with slow links when streaming)

5. This may sound strange, but compression depends on ____________being present in the information .

Note: This fact (if you get the right 'blank' is true of text, images, music and video data!)

text

patterns

bits

pictures

6. A pattern implies that certain parts of the data are identical but are just located at different places in the file

TRUE

FALSE

7. Consider a text file like this. What statement is true?

www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful

This file is far too small to be compressed

The information is massive, when you convert to binary and therefore compression is not possible

This file could never be compressed

The information, rather than the data, is really just the first sentence (being repeated)

8. A text compression algorithm could do the following with this text file:

www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful
www.testandtrack.io is wonderful

create a file that only contains the first sentence and some instructions about how many times to repeat it

nothing - nothing could be done to compress this!

create a file that converts all of the text to audio - audio (waves) would mean a lower rate of storage

create a file that contains ten times the volume of data in the original, and then save it as bits

9. The efficiency of compression is a simple formula:

Compression ratio = compressed data size/2 x original data size/2

Compression ratio = original data size / compressed data size

Compression ratio = compressed data size + 2 / original data size x 2

Compression ratio = compressed data size / original data size x 2

10. Compression is also used by video and music streaming companies to _____________________

quadruple the bit rate per second and store it as bits, rather than bytes

reduce the bit rate per second. (compression ratio = original data rate / compressed data rate)

increase the bit rate per second

Compresion is never used by video and music streaming services

11. The two types of compression are:

tight, open

lossy, lossless

mossy, mossless

mismatch, match

12. Disregarding some of the original information if file size is an issue is usually referred to as ____________________

tight compression

open compression

lossy compression

lossless compression

13. An example of lossy compression is when a music file…

has sounds at frequencies that the human ear cannot hear (these can be discarded safely)

wishes to retain the highest of quality and every single original bit of data

is too small to compress so is discarded altogether

has sounds at normal frequencies and it is important to retain every original frequency

14. What statement is true about this lossy compression of images?

They are vastly different and hugely matter

Compression has reduced the quality by a significant degree and should not be used

It is slightly more blurry but not enough to matter in most circumstances

None of the above

15. What is the benefit of compressing an image file for a use on a website?

it would render exactly 15 times faster

It takes up less storage space on your website

It may offer the user speed in loading the page along with improved quality

it may offer the user an alternative viewing experience which helps manage expectations

16. A film could take up to an hour to download in uncompressed format whilst a compressed file could take just 12 minutes

FALSE

TRUE

17. Lossless compression is when you are:

reducing file size with a massive reduction in quality

reducing a file's size with NO loss of quality

reduing a file's size in exactly the same was as lossy compression

reducing file size with an increase in quality

18. Lossless compression is _________ effective than reducing file sizes than lossy compression but the trade off is no loss of quality.

Note: …but the trade off is no loss in the quality. Lossless compression is also done using pattern recognition.

None of the above

LESS

FAR MORE

19. Lossless data compression is used in many applications. For example, it is used in the ZIP file format and in the GNU tool gzip

TRUE

FALSE

20. Fill in the blanks for the following excerpt on lossless compression.

Lossless compression is used in cases where it is important that the original and the decompressed data be identical, or where deviations from the original data would be unfavourable. Typical examples are executable _____________________

videos, web images and repetitive text

programs, text documents, and source code

None of the above

video cards, CPUs and operating system

21. Two common lossless compression algorithms are:

minch compression and zilch compression

All of the above

huffman coding and Run length encoding (RLE)

Apple Compression and IBM HoHoMon

22. Wider applications: Genetics compression algorithms (not to be confused with genetic algorithms) are the latest generation of lossless algorithms that compress data

FALSE

TRUE

23. Fill in the blanks for RLE

RLE
===================================================
One of the simplest examples of compression is RLE. 
RLE is a basic form of data compression that converts 
consecutive identical values into a code consisting of
 the character and the number __________________________________.
The more similar values there are, the more values can be compressed. 
The sequence of data is stored as a single value and count.

marking the length of the run.

marking the binary file size

marking the number of bits contained in the file

marking the number of times the file has been compressed

24. In the following picture we compress …

consecutive pixels by only replacing each run with one pixel from it and a counter showing how many items it contains.

random pixels by replacing each run by four pixels from it.

nothing at all

random pixels by replacing each run with several pixels from it

25. Run-length encoding isn’t a very effective option when compressing texts, but for images …

None of the above

where short runs of text bits happen to occur in sequence, it is useful

where long runs of the identical pixels happen to occur it is quite useful.

where there are pixels it is even less useful

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