Audio

• Sound is a series of waves
• We are intersted in the frequency and amplitude
• We need to digitize this for representation in a computer.
• 
  • We sample on regular intervals.
  • Notice we miss quite a bit of change in sound when we do this.
  • Some people like analog better (live for example)
• The sampling rate is how often we sample a sound
  • More frequent sampling -> better representation of the original
  • But takes more space
  • Lower sampling means less quality
  • But means less space.
  • Like the first images in this chapter, it is a tradeoff and we must consider what is acceptable.
• CD Storage
  • 1s and 0s are encoded on a cd as a series of pits and ridges
  • A laser can detect these pits, (they are not shiny).
  • 
  • 
• Audio Formats.
  • There are many of these.
  • Wav, au, aiff, vqf, mp3
  • Each uses different techniques
    • sampling rates
    • Lossless vs lossy compression methods
  • MP-3, current dominant standard
    • Moving Picture Experts Group (MPEG)
    • Audio version of this standard.
    • Losless and Lossy compression methods.
    • Removes sounds not audiable to humans
    • Uses Huffman encoding to compress data.
    • Very high rates of compression can be achieved using this method

Images

• Black and white
  • 1 is white, 0 is black.

    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0 
    0  1  1  1  1  0  0  1  1  1  1  0  0  1  1  1  1  0
    0  1  0  0  0  0  0  1  0  0  1  0  0  1  0  0  1  0
    0  1  1  1  0  0  0  1  0  0  1  0  0  1  0  0  1  0
    0  1  0  0  0  0  0  1  0  0  1  0  0  1  0  0  1  0
    0  1  0  0  0  0  0  1  1  1  1  0  0  1  1  1  1  0
    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
    	

  • 
• Color is represented as three integers
  • Red, green, blue
  • The higher the value, the brighter the color
  • 0 0 0 -> black
  • 255 255 255 -> white (why 255?)
  • 255 0 0 -> red
  • 16 colors, 246 colors, 16million colors (24 bit), ...
• Pixel
  • Picture Element
  • A dot on the screen.
  • The number of pixels a screen can display is its resolution
  • 1280x1024 for example
  • 
  • 
  • GIF format -
    • Graphics Interchange Format
    • 256 color max.
    • No lossy compression schemes.
  • JPG
    • Many more colors (24 bits)
    • Lossless and lossy compression schemes
• Vector graphics
  • Equivelent to lines
  • These scale much better.
  • 
  • 
  • Nice for fonts and line drawings, not good for pictures.

Video

• CODEC - COmpressor DECompressor
• 30 frames per second at 640x480x24 bits per frame > 200Mbits per second
• or 27 MB per second (uncompressed)
• Two ways to compress video images
  • Temporal Compression
    • Only send the information that has not changed
    • Keyframe - a starting point
    • delta frames - change since last frame.
  • Spatial Compression
    • Remove redundant information within a frame
    • run-length encoding