ntly less information content. tortion specifications. Motion compensation takes ad- Two useful information sources are used in modeling vantage of the significant information redundancy in the video encoders; the discrete memoryless source (DMS), temporal domain by creating current frame predictions and Markov sources. VLC coding is based on the DMS based upon block matching motion estimates between model, and the predictive coders are based on the the current and previous image frames. Motion com- Markov source models. The DMS is simply a source in pensation generally achieves a significant increase in the which each symbol is generated independently. The video coding efficiency over pure spatial encoding. symbols are statistically independent and the source is completely defined by its symbols/events and the set of probabilities for the occurrence for each symbol, i.e., 7.4.5.1 Entropy and predictive coding E={e1, e2,...,en} and the set {p(e1), p(e2),...,P(en)}, where n is the number of symbols in the alphabet. It is Entropy coding is an excellent starting point in the useful to introduce the concept of entropy at this point. discussion of coding techniques because it makes use of Entropy is defined as the average information content of Video compression CHAPTER 7.4 the information source. The information content of probability of occurrence of event ei conditioned on the a single event or symbol is defined as, occurrence of the last m events, i.e., p(lilei-1, ej-2, ei-m). The Markov source is made up of all possible nm I(ei) = logpley (7.4.4) states, where n is the number of symbols in the alphabet. Each state contains a set of up to n conditional proba- bilities for the possible transitions between the current The base of the logarithm is determined by the number of symbol and the next symbol. The differential pulse code states used to represent the information source. Digital modulation (DPCM) predictive coder makes use of the information sources use base 2 in order to define the Markov source model. DPCM i