   Next: Vector Quantisation Up: Source Coding Techniques Previous: The Discrete Cosine Transform

## Differential Encoding

Simple example of transform coding mentioned earlier and instance of this approach.

Here:

• The difference between the actual value of a sample and a prediction of that values is encoded.
• Also known as predictive encoding.
• Example of technique include: differential pulse code modulation, delta modulation and adaptive pulse code modulation -- differ in prediction part.
• Suitable where successive signal samples do not differ much, but are not zero. E.g. Video -- difference between frames, some audio signals.
• Differential pulse code modulation (DPCM) simple prediction:

fpredict(ti) = factual(ti-1)

i.e. a simple Markov model where current value is the predict next value.

So we simply need to encode: If successive sample are close to each other we only need to encode first sample with a large number of bits:

Actual Data: 9 10 7 6

Predicted Data: 0 9 10 7 : +9, +1, -3, -1.

• Delta modulation is a special case of DPCM: Same predictor function, coding error is a single bit or digit that indicates the current sample should be increased or decreased by a step.

Not Suitable for rapidly changing signals.

• Adaptive pulse code modulation -- Fuller Markov model: data is extracted from a function of a series of previous values: E.g. Average of last n samples. Characteristics of sample better preserved.   Next: Vector Quantisation Up: Source Coding Techniques Previous: The Discrete Cosine Transform
Dave Marshall
10/4/2001