Digital communication system. Shannon s separation principle

Transcription

1 Digital communication system Representation of the source signal by a stream of (binary) symbols Adaptation to the properties of the transmission channel information source source coder channel coder modulation binary symbols channel noise information sink source decoder channel decoder demodulation We will be looking at this part digital channel Bernd Girod: EE368b Image and Video Compression Lossless Compression no. Shannon s separation principle Source Assume:. Point-to-point communication 2. Ergodic channel 3. Delay Adapt to source statistics (and distortion measure) Source coder Source decoder Channel Channel Adapt to channel statistics coder decoder Channel Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 2

2 How does compression work? Exploit redundancy. Take advantage of patterns in the signal. Describe frequently occuring events efficiently. Lossless coding: completely reversible Introduce acceptable deviations. Remove information that the humans cannot perceive. Match the signal resolution (in space, time, amplitude) to the application Lossy coding: irreversible distortion of the signal Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 3 Lossless compression in lossy compression systems Almost every lossy compression system contains a lossless compression system Lossy compression system Quantizer Encoder Lossless Encoder Lossless Decoder Lossless compression system Quantizer Decoder We will discuss the basics of lossless compression first, then move on to lossy compression Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 4 2

3 Topics in lossless compression Binary decision trees and variable length coding Entropy and bit-rate Huffman codes Statistical dependencies in image signals Sources with memory Arithmetic coding Redundancy reduction by prediction Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 5 Example: 2 Questions Alice thinks of an outcome (from a finite set), but does not disclose his selection. Bob asks a series of yes-no questions to uniquely determine the outcome chosen. The goal of the game is to ask as few questions as possible on average. Our goal: Design the best strategy for Bob. Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 6 3

4 Example: 2 Questions (cont.) Observation: The collection of questions and answers yield a binary code for each outcome. A B C D E F A B C D E F Which strategy (=code) is better? Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 7 Fixed length codes A B C D E F G H Average description length for K outcomes Optimum for equally likely outcomes Verify by modifying tree l av = log 2 K Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 8 4

5 Variable length codes If outcomes are NOT equally probable: Use shorter descriptions for likely outcomes Use longer descriptions for less likely outcomes Intuition: Optimum balanced code trees, i.e., with equally likely outcomes, can be pruned to yield unbalanced trees with unequal probabilities. The unbalanced code trees such obtained are also optimum. Hence, an outcome of probability p should require about log 2 p bits Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 9 Entropy of a memoryless source Let a memoryless source be characterized by an ensemble with: Alphabet {a,a,a 2,..., a K } Shannon: information conveyed by message : I(a k ) = log(p(a k )) Entropy of the source is the average information contents: H U U Probabilities {P(a ), P(a ), P(a 2 ),..., P(a K )} K ( ) = E{ I( a k )} = P( a k ) log( P( a k )) = P( u ) k = For log = log 2 the unit is bits/symbol log P u u a k ( ( )) Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 5

6 Properties of entropy: Entropy and bit-rate max{h(u )} = log(k) H(U ) with P(a j ) = P(a k ) j, k The entropy H(U ) is a lower bound for the average word length l avu of a decodable variable-length code for the symbols. Conversely, the average wordlength l av can approach H(U ), if sufficiently large blocks of symbols are encoded jointly. Redundancy of a code: R = l av H(U ) Bernd Girod: EE368b Image and Video Compression Lossless Compression no. Encoding with variable word length A code without redundancy, i.e. l av = H(U ) is achieved, if all individual code word length l cw (a k ) = log(p(a k )) For binary code words, all probabilities would have to be binary fractions: P(a k ) = 2 l cw (a k ) a i Example P(a i ) redundant code optimum code a.5 a.25 a 2.25 a 3.25 H(U ) =.75 bits / symbol l av =.75 bits / symbol R = Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 2 6

7 Huffman-Code Design algorithm for variable length codes proposed by Huffman (952) always finds a code with minimum redundancy. Obtain code tree as follows: Pick the two symbols with lowest probabilities and merge them into a new auxiliary symbol. 2 Calculate the probability of the auxiliary symbol. 3 If more than one symbol remains, repeat steps and 2 for the new auxiliary alphabet. 4 Convert the code tree into a prefix code. Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 3 Huffman-Code - Example Fixed length coding: l =3. bits/symbol Huffman code: lav =2.7 bits/symbol Entropy H(U )=2.68 bits/symbol Redundancy of the Huffman code: R=.3 bits/symbol Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 4 7

8 Probability density function of the luminance signal Y Images: 3 EBU test slides, 3 SMPTE test slides, uniform quantization with 256 levels (8 bits/pixel) H(U Y ) = 7.34 bits / pixel Image with lowest entropy: H L (U Y ) = 6.97 bits / pixel Image with highest entropy: H H (U Y ) = 7.35 bits /pixel Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 5 Probability density function of the color difference signals R-Y and B-Y H(U R Y ) =5.57 bits/pixel H L (U R Y ) = 4.65 bits/pixel H H (U R Y ) = 5.72 bits / pixel H L (U B Y ) = 4. bits /pixel H H (U B Y ) = 5.34 bits /pixel Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 6 8

9 Joint sources Joint sources generate N symbols simultaneously. A coding gain can be achieved by encoding those symbols jointly. The lower bound for the average code word length is the joint entropy: H(U,U 2,...,U N ) =... P(u,u 2,...,u N )log(p(u,u 2,...,u N )) u 2 u N It generally holds that u H(U,U 2,...,U N ) H(U ) + H(U 2 ) H(U N ) with equality, if U,U 2,...,U N are statistically independent. Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 7 Statistical dependencies between video signal components Y, R-Y, B-Y Data: 3 EBU-, 3 SMPTE test slides, each component Y, R-Y, B-Y uniformly quantized to 64 levels H = 3x6 bits / sample = 8 bits/sample H(U Y,U R Y,U B Y ) = 9.44 bits/sample H(U Y ) + H(U R Y ) + H(U B Y ) =.28 bits/sample H = 2.74 bits/sample Statistical dependency between R, G, B is much stronger. If joint source Y, R-Y, B-Y is treated as a source with memory, the possible gain by joint coding is much smaller. Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 8 9

10 Markov process Neighboring samples of the video signal are not statistically independent: source with memory P(u T ) P(u T u T,u T 2,...,u T N ) A source with memory can be modeled by a Markov random process. Conditional probabilities of the source symbols u T of a Markov source of order N: P(u T, Z T ) = P(u T u T,u T 2,...,u T N ) state of the Markov source at time T Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 9 Entropy of source with memory Markov source of order N: conditional entropy H(U T Z T ) = H(U T U T,U T 2,...,U T N ) = E{ log(p(u T U T,U T 2,...,U T N ))} =... p(u T,u T,u T 2,..., u T N ) log( p(u T u T,u T 2,...,u T N )) ut u T N H(U T Z T ) H(U T ) (equality for memoryless sources) Average code word length can approach with a switched Huffman code. Number of states for an 8-bit video signal: N = N = 2 N = states states states H(U T Z T ) e.g. Bernd Girod: EE368b Image and Video Compression Lossless Compression no. 2