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STATE / IDENTIFY

Brief, factual answer — no explanation needed

1State the number of bytes in 1 KiB
  • 1024 bytes  (2¹⁰)
2Identify the two values stored for each run in RLE
  • The count of identical items and the code of the item
3State the reduction factor range of JPEG compression
  • Between 5 and 15
4State the approximate size reduction achieved by MP3
  • About 90%
5Identify the unit equal to 4 bits
  • A nibble

DESCRIBE

Say what happens or how it works (what / how)

1Describe the difference between lossy and lossless compression
  • Lossless: no information lost, reversible
  • Lossy: some information lost, original unrecoverable
  • Lossless suits text; lossy suits images/sound
2Describe how Huffman coding compresses text
  • Frequency analysis finds the most used characters
  • Frequent characters get the shortest codes
  • The byte stream becomes a shorter variable-length bit stream
3Describe how RLE compresses data
  • Finds runs of adjacent identical data
  • Stores a count followed by the code of the repeated item
  • Only effective where long runs exist
4Describe how perceptual music shaping works
  • Removes frequencies outside the human hearing range
  • When two sounds play together, removes the softer one
  • Quality appears retained although data is lost
5Describe how to convert 6 000 000 bits to MiB
  • Divide by 8 → 750 000 bytes
  • Divide by 1024 → 732.4 KiB
  • Divide by 1024 again → ≈0.72 MiB

EXPLAIN

Give reasons — say why something happens

1Explain why compression is used
  • Larger files need larger storage capacity
  • Larger files have lower transmission/download rates
  • Compression reduces size, saving space, time and bandwidth
2Explain why RLE may not reduce a bitmap's size
  • An image may have few same-colour sequences
  • Each single item then needs a count of 1
  • Storing colour + count 1 adds data — e.g. Red 1 Green 1 Blue 1
3Explain why lossless compression is essential for text files
  • Every character matters — loss would change meaning
  • Lossless can be reversed to re-create the original exactly
  • Lossy would destroy data permanently
4Explain why JPEG is suitable for photographs
  • Photographs have detail the eye can spare
  • JPEG removes information the viewer hardly notices
  • A factor of 5–15 gives big savings in storage and download time
5Explain why MP4 suits video streaming
  • MP4 stores multimedia — video, music, photos, animation
  • Compression reduces the data rate for real-time playback
  • Videos stream without losing any real noticeable quality

EVALUATE / DISCUSS

Weigh up pros and cons with a justified conclusion

1Discuss whether an exam-board archive should use lossy compression
  • Archive documents must be recoverable exactly
  • Lossy compression permanently destroys data
  • Lossless (RLE/Huffman) is the appropriate choice despite smaller savings
2Discuss the trade-offs of a lower MP3 bit rate
  • Lower bit rate = smaller file, faster download
  • More perceptual removal → audible quality loss
  • The producer must balance fidelity against size for the audience
3Discuss whether RLE or JPEG better serves a website thumbnail
  • JPEG gives the largest saving — fastest download
  • At thumbnail size the lossy detail loss is invisible
  • RLE preserves exactness but saves less on photos
4Discuss the impact of compression on a customer who streams the same video weekly
  • Less bandwidth used each week
  • Faster start, less buffering
  • Slight quality loss if lossy — usually imperceptible
5Discuss why the 'best' compression method depends on the situation
  • Exactness requirement forces lossless (text/records)
  • Perceptual content allows lossy (photos/music) for bigger savings
  • Data structure matters too — RLE fails without runs