Digitisation

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Digitisation

• The real world is smooth and continuous.
• The digital world is made of little chunks.
• Smaller chunks lead to a better illusion of
  smoothness.
• For example A screen with small pixels can
  display a smoother image than a similar screen
  with larger pixels.

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Quantisation

• To quantise an image or sound we approximate the
  analogue signal with a digital signal.

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Aliasing - a common problem

• There are many ways for aliasing to occur - but
  what is aliasing?
• T e e a e m n a s f r a i s n o o c r - b t w
  a s a i s n ?
• There many for to - but
  is ?

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Aliasing - a common problem

• Aliasing is a problem associated with sampling.
• Discrete samples only ever give an approximation
  to the original analogue signal.
• Ever wondered why wheels sometimes seem to rotate
  backwards or very slowly on TV?

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Experiments in aliasing

• Try listening to music in bursts of 1 second,
  with 1 second of silence in between.
• Find something with stripes (clothes, a fence,
  blinds) and then sample it every 10cm.
• If the stripes are exactly 10cm apart, you might
  never sample one!

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Different sampling rates
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Aliasing and anti-aliasing

• Problem an 8kHz tone cannot be accurately
  digitised using an 8kHz sampling rate.
• In general you need to sample at twice the rate
  of the most rapidly changing feature.
• This is true for images, sounds and movies.
• Anti-aliasing is a method that tries to reduce
  the effects of aliasing on a signal.

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Anti-aliasing

• Both images are half the size of the original
• Image 1 has 88 colours (same as original)
• Image 2 has 240 colours (extras used for
  smoothing)

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Practical digitisation techniques

• Still images such as drawings and photographs can
  be digitised using a flat-bed scanner
• Images must be scanned at a high enough
  resolution (sampling rate) to give acceptable
  quality
• Images must be scanned at an appropriate colour
  depth - at least 256 colours, usually 16.7M
  colours (or True Colour)

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Practical digitisation techniques

• As with images, sound recording should be done at
  a higher quality than will be required
• Grab sound at 22kHz or 44kHz, 16 bits per sample.
  You can always reduce this later, if quality
  guidelines allow
• Speech can usually be recorded at 22kHz with only
  8 bits per sample
• Beware of noise and insensitive microphones

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Practical digitisation techniques

• Video digitisation is often limited by the
  throughput of the computer
• What style of image are you capturing?
• Think very carefully about the size and colour
  depth of your movie well before you start
• Watch out for disc space problems
• Try to capture compressed data if possible,
  unless you have a lot of storage space
• Raw data may take too much bus bandwidth and too
  much storage space to be practical

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Sound Editing

• Sound editing tools tend to look quite similar
• Waveform view
• Mark sections of waveform for cut, copy, paste,
  clear, amplify

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• CoolEdit2000 provides
• a waveform view
• a spectral view
• Use waveform mode!

• Can you see the relationship between the two
  displays?
• They show the same sound

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What not to do with a sound editor

• Remember the first time you used a DTP or
  presentation graphics system?
• You were tempted to use every colour
• Every font
• Every transition effect
• Every piece of clip-art
• Use special effects sparingly!
• Echo, reverb, distortion and so on are for experts

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Image Editors

• Adobe Photoshop is the industry standard tool
• Full featured
• Powerful and extensible
• Versatile
• Lots of good documentation available
• Excellent at manipulating, combining and
  retouching
• But not great for creating artwork from scratch
• Paint Shop Pro is similar but easier to learn

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Digital Video

• You may be familiar with sound and image editing
• But you are much less likely to have experience
  of digital video editing
• Not enough time to do video editing