SWE 423: Multimedia Systems

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SWE 423 Multimedia Systems

• Chapter 7 Data Compression (4)

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Outline

• Source Coding
• Predictive Coding
• Lossless
• Lossy
• DPCM
• DM

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Predictive Coding

• Predictive coding simply means transmitting
  differences
• Predict the next sample as being equal to the
  current sample
• More complex prediction schemes can be used
• Instead of sending the current sample, send the
  error involved in the previous assumption

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Predictive Coding Why?

• The idea of forming differences is to make the
  histogram of sample values more peaked.
• In this case, what happens to the entropy?
• As a result, which is better to compress?

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Predictive Coding Why?
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Lossless Predictive Coding

• Formally, define the integer signal as the set of
  values fn. Then, we predict values fn and
  compute the error en as follows
• when t 1, we get ...
• Usually, t is between 2 and 4 (in this case it is
  called a linear predictor
• We might need to have a truncating or rounding
  operation following the prediction computation

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Lossless Predictive Coding
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Lossless Predictive Coding Example

• Consider the following predictor
• Show how to code the following sequence

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Lossless Predictive Coding

• Examples in the Image Compression Domain
• Differential Coding
• Lossless JPEG

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Lossy Predictive Coding DPCM

• DPCM Differential Pulse Code Modulation
• Form the prediction f n
• Form an error en
• Quantize the error

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Lossy Predictive Coding DPCM

• The distortion is the average squared error
• To illustrate the quality of a compression
  scheme, diagrams of distortion vs. the number of
  bit levels used are usually shown
• Quantization used
• Uniform
• Lloyd-Max
• Does better than Uniform

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Lossy Predictive Coding DPCM
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Lossy Predictive Coding DPCM

• Example
• Show how to code the following sequence

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Lossy Predictive Coding

• DM (Delta Modulation) is a simplified version of
  DPCM that is used as a quick analog-to-digital
  converter.
• Note that the prediction simply involves a delay