Data compression

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Notes de leçon Diapos Questions d'entraînement

Notes de leçon

Why compress?

Compression makes files smaller: less storage, faster transfer and lower bandwidth costs.

Packing and restoring files

Lossy compression

Lossy compression permanently discards information, so the exact original cannot be reconstructed from the compressed representation alone.
Examples include standard lossy JPEG images, MP3 audio and lossy H.264 video. MP4 is a container; its codec determines compression.

Lossless compression

Lossless reduces size without losing any data; the original can be perfectly restored. Used for PNG, ZIP, FLAC and text where accuracy matters.

Lossless and lossy compression

Run-length encoding (RLE)

Replaces repeated values with a count and the value. AAAABBBCC → 4A3B2C. Works well when many repeats occur (simple images). May not help if there are few repeats.

Run-length encoding

Huffman coding

Huffman coding builds variable-length prefix codes using symbol frequencies, normally giving common symbols shorter codes.
Encoded symbol data can require fewer bits than a fixed-length code; any codebook or tree overhead also contributes to the stored file size.

Choosing

Use lossy for photos, music and video where small quality loss is acceptable; use lossless where exact data matters (documents, program code).

Diapos

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Questions d'entraînement

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1.What is compression?
Easy
- Areducing the size of a file
- Bencrypting a file
- Ccopying a file
- Ddeleting a file
2.What does lossy compression do?
Easy
- Apermanently removes some data
- Bkeeps all data
- Cencrypts data
- Dcopies data
3.What does lossless compression do?
Easy
- Areduces size without losing data
- Bremoves data permanently
- Cincreases size
- Dhides data
4.Which file type uses lossy compression?
Easy
- AMP3
- BZIP
- CPNG
- DFLAC
5.Which file type uses lossless compression?
Easy
- APNG
- Bstandard lossy JPEG
- CMP3
- Duncompressed PCM audio
6.What is the RLE of WWWWBBBWW?
Easy
- A4W3B2W
- B4W3B
- CWBW
- D9W
7.What does 3A2B decode to?
Easy
- AAAABB
- BAB
- C32AB
- DAAAB
8.What does Huffman coding use?
Easy
- Ashorter codes for common characters
- Bequal codes
- Crandom codes
- Dno codes

Plus de questions

9.When is lossless needed?
Easy
- Awhen exact data must be kept
- Bfor music
- Cfor movies
- Dfor photos only
10.Why do JPEG images take less space than uncompressed ones?
Easy
- Asome detail is removed
- Bthey have fewer colours always
- Cthey are smaller pictures
- Dthey are encrypted
11.Which compression is best for a document where every letter matters?
Medium
- Alossless
- Blossy
- Cnone
- Dboth
12.Which method is commonly used for music streaming when reducing data transfer is a priority and some loss of fidelity is acceptable?
Medium
- Alossy
- Blossless only
- Cnone
- Dhash
13.Why might RLE give a bigger file?
Medium
- Athere are few repeated values
- Bthere are many repeats
- Cthe file is empty
- Dit is lossless
14.What is the RLE of ABCD?
Medium
- A1A1B1C1D
- B4ABCD
- CABCD
- DA4
15.What is a disadvantage of lossy compression?
Medium
- Aquality is reduced permanently
- Bthe file is larger
- Cit is not smaller
- Dit is slower
16.What is an advantage of lossless compression?
Medium
- Athe original can be perfectly restored
- Bthe file is smallest
- Cquality is lost
- Dit is lossy
17.A website needs a small photographic image and accepts some loss of fine detail. Which option suits that requirement?
Medium
- Alossy such as JPEG
- Blossless only
- Cneither
- Dboth identical
18.A string of 100 identical letters is stored using RLE. How is it represented?
Medium
- A100 followed by the letter
- B100 letters
- C1 letter
- D100 zero
19.Why does Huffman coding reduce the average bits per character?
Medium
- Afrequent characters use fewer bits
- Ball characters use more bits
- Cit uses 8 bits
- Dit removes characters
20.Why can't lossy compression be reversed to the original?
Medium
- Ainformation has been discarded
- Bit is encrypted
- Cit is stored
- Dit is copied
21.What is bandwidth?
Easy
- Athe amount of data that can be transferred per second
- Bstorage size
- Cfile type
- Dscreen size
22.What does ZIP do?
Easy
- Acompresses files without loss
- Bdeletes files
- Cencrypts files
- Dcopies files
23.What is an MP4 file?
Medium
- Aa container that can hold audio and video streams
- Ba plain-text document format
- Can executable program instruction
- Da compression method guaranteed to be lossless
24.What is a frequency tree used for?
Medium
- Abuilding Huffman codes
- Bsorting
- Csearching
- Dencrypting
25.Lossy compression permanently removes data.
Easy
True or false?
26.Lossless compression can restore the original.
Easy
True or false?
27.PNG uses lossy compression.
Easy
True or false?
28.RLE replaces repeated values with counts.
Easy
True or false?
29.Compression always makes files larger.
Medium
True or false?
30.Which use lossy compression in the forms specified here?
Medium
- Astandard lossy JPEG images
- BMP3 audio
- Clossy H.264 video
- Dlossy AAC audio
- EZIP archives
31.Which can be benefits of successfully reducing a file’s size by compression?
Medium
- Aless storage needed
- Bshorter transfer time at a fixed data rate
- Cfewer bytes transmitted
- Dreduced storage or transfer cost
- Ehigher resolution automatically
32.Apply run-length encoding to the string WWWWWBBBWWW.
Medium

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