Compression (Copy)

Compression Types

• Compression
  • Reduces file size
  • Saves storage
  • Reduces time taken to stream or transmit data
• Lossless File Compression
  • Data of the original file can be reconstructed at uncompressed
• Lossy file compression
  • Unnecessary data is removed
  • Such as pixels that human eye can’t detect.
  • Impossible to reconstruct the original file

File Compression Applications

• MPEG-3 and MPEG-4 (MP3/ MP4)
  • MPEG-3 uses audio compression
  • Reduces original file size by 90 percent
  • It makes downloading and streaming easy
  • Full version on the CD can not be constructed again.
  • Perpetual Music Shaping
    • Certain sounds removed
      • Frequencies outside human hearing range
      • Quieter of the two simultaneously playing sounds removed
  • Certain parts removed without damaging quality too much.
  • Bit rate
    • Number of bits per second used when creating the file
    • Bit rates can be between 80 and 320 kilobits
    • Normal CD quality is matched by 200 kilobits and higher.
  • MPEG-4
    • It is a storage formats for multimedia
      • Not just sounds
    • No real loss in discernable quality
• Photographic Bitmap Images
  • Compression reduces size and quality
  • JPEG
    • Lossy file compression
    • Original file can not be reconstructed
    • Raw image bit map factor between 5 and 15
    • Depending on original sound quality
  • Vector Graphics
    • Some form of file compression
    • XML files can be compressed as .svg files.
• Run-length encoding (RLE)
  • Can compress different formats
  • Encoded in two values of repeated strings
  • Number of Identical data items represented by first value
  • Second value represents the code of the data item, for example the ASCII code of the keyboard character
  • RLE useful when a long run of repeated units/ bits.
• RLE on text data
  • Can be used with images as well

General Methods of Compressing Files

• Movie files
  • Reduce sampling rate, resolution or frame rate
• Image files
  • Crop images, decrease color/ bit depth or resolution