1.3 Data storage & compression

How computers represent text sounds and images

Data is measured in the following units:

  • Bit
  • Nibble
  • byte (1 byte= 2 nibbles)
  • Kibibyte
  • Mebibyte
  • Gibibyte
  • Tebibyte
  • Pebibyte
  • Exbibyte

Calculating the size of an image file:

  • resolution x colour depth (answer is in bits)

Calculating the size of a sound file:

  • sample rate(Hz) x sample resolution x length (seconds)

Compression

What is compression?

  • The reduction of FILE size

Why should we compress a file?

  • Less bandwidth will be required
  • Less storage will be required on the device
  • Shorter transmission time (less time to download/upload)

Difference between Lossy and Lossless Compression

  • Lossy compression is permanent, lossless can be recovered to its original form
  • Lossy compression reduces the file size more than lossless compression

When to use Lossless or Lossy Compression?

Lossless

  • Used when File quality is more important than file size
  • Used when file needs to be recoverable to original form (eg. Text file or source code)

Lossy

  • Used when file size is more important than quality
  • Considerably smaller file size than lossless, quality may be adaquate
  • used for streaming as transmission of smaller files is faster

Lossless compression of files

How is a text file compressed with lossless?

  • Compression algorithm is used
  • ..such as RLE(run length encoding)
  • Repeating words/characters/phrases are identified
  • …and indexed
  • …With the number of occurances
  • …With their position

How is a video file compressed with lossless?

  • Compression algorithm is used
  • ..such as RLE(run length encoding)
  • Repeating frames/pixels are identified
  • …and indexed
  • it records the changes between frames/pixels
  • No data is permanently removed

How is a sound file compressed with lossless?

  • Compression algorithm is used
  • ..such as RLE
  • Repeating patterns in the music are identified (or repeated sounds/notes)
  • ..and indexed
  • List Item
  • No data is permanently removed

How is an image file compressed with lossless?

  • Compression algorithm is used
  • ..such as RLE
  • Repeating colours are identified
  • ..and indexed
  • ..with the number of occurrences
  • No data is permanently removed

Lossy compression of files

Video file

  • Compression algorithm is used
  • Reduce colour depth
  • Reduce image resolution
  • Reduce frame rate
  • Reduce frame rate

Image file

  • Compression algorithm is used
  • Reduce colour depth
  • Reduce image resolution
  • Data is permanently removed

Sound file

  • Compression algorithm is used
  • Reduce sampling rate
  • Reduce sound resolution
  • Data is permanently removed