17 - Compression

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





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



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

  a huge decrease in the quality of their files

  a lower bit per second transfer rate

  None of the above

  a higher bit per second transfer rate

 4. What are main reasons for compressing files?

  Less storage space required

  Smaller attachments for email

  All of the above

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

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





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



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

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

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

  This file is far too small to be compressed

  This file could never be compressed

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

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

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

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

  nothing - nothing could be done to compress this!

 9. The efficiency of compression is a simple formula:

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

  Compression ratio = original data size / compressed data size

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

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

  increase the bit rate per second

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

  Compresion is never used by video and music streaming services

 11. The two types of compression are:

  lossy, lossless

  mismatch, match

  mossy, mossless

  tight, open

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

  tight compression

  open compression

  lossless compression

   lossy compression

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

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

  is too small to compress so is discarded altogether

  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

 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

  None of the above

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

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

  It may offer the user speed in loading the page along with a definite upgrade on quality

  It will offer the user an entirely unexpected viewing experience, which always goes down well

  it would render exactly fifteen times faster

  It takes up less storage space on your website

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



 17. Lossless compression is when you are:

  reducing file size with an increase 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 a massive reduction in quality

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



  None of the above


 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



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

  None of the above

  programs, text documents, and source code

  video cards, CPUs and operating system

  videos, web images and repetitive text

 21. Two common lossless compression algorithms are:

  Apple Compression and IBM HoHoMon

  minch compression and zilch compression

  All of the above

  huffman coding and Run length encoding (RLE)

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



 23. Fill in the blanks for RLE

  marking the number of times the file has been compressed

  marking the binary file size

  marking the number of bits contained in the file

  marking the length of the run.

 24. In the following picture we compress …

  random pixels by replacing each run with several pixels from it

  nothing at all

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

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

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

  where there are pixels it is even less useful

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

  None of the above

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