Predictive Coding i

predictive coding
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  PREDICTIVE CODING I  Predictive Coding  Makes use of the past history of the data being encoded.  Principally used for text and images  JPEG  –   2000 : Lossless method  –   Predictive coding  In a message, if certain symbols occur with much higher  probability than others (skewed)  –   more compression can  be achieved  One way  –   transform the sequence into another sequence that has the desired property- If the transformation is based on the history of the sequence  –   such schemes are called  predictive schemes.  Predictive Coding - Example  1, 2, 5, 7, 2, -2, 0, -5, -3, -1, 1, -2 ,-7, -4, -2, 1, 3, 4  Entropy is 4 bits/symbol  If Huffman or arithmetic  –     Min. 4 bits/symbol   needed  If we form another sequence by adding „ 2 ‟  to the previous no. in the sequence and send the difference between the current element in the sequence and this  predicted value   Transmitted sequence will be  1, -1, 1, 0, -7, -6 , 0, -7, 0, 0, 0, -5, -7, 1, 0, 1, 0, -1  If this rule is known to the decoder, decoding can be done from the above residual sequence whose entropy is very much less than the srcinal sequence. Hence more compression can be achieved  Prediction Schemes  Use the preceding symbols to predict the  probability model and with this probability model (Cum.Distr.Function) arithmetic coding can be done.  Adaptive Arithmetic Coding : Probability model changes while encoding process.  Preceding symbols form the context.
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