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ESE 531: Digital Signal Processing ESE 531: Digital Signal Processing Lec 3: January 24, 2019 Discrete Time Signals and Systems Penn ESE 531 Spring 2019 - Khanna Lecture Outline ! Discrete Time Systems ! LTI Systems ! LTI System Properties ! Difference Equations Penn ESE 531 Spring 2019 - Khanna 3 Discrete-Time Systems Discrete Time Systems ! Systems manipulate the information in signals ! Examples " Speech recognition system that converts acoustic waves into text " Radar system transforms radar pulse into position and velocity " fMRI system transform frequency into images of brain activity " Moving average system smooths out the day-to-day variability in a stock price Penn ESE 531 Spring 2019 - Khanna 5 System Properties ! Causality " y[n] only depends on x[m] for m<=n ! Linearity " Scaled sum of arbitrary inputs results in output that is a scaled sum of corresponding outputs " Ax1[n]+Bx2[n] # Ay1[n]+By2[n] ! Memoryless " y[n] depends only on x[n] ! Time Invariance " Shifted input results in shifted output " x[n-q] # y[n-q] ! BIBO Stability " A bounded input results in a bounded output (ie. max signal value exists for output if max ) Penn ESE 531 Spring 2019 - Khanna 6 Stability ! BIBO Stability " Bounded-input bounded-output Stability Penn ESE 531 Spring 2019 - Khanna 7 Examples ! Causal? Linear? Time-invariant? Memoryless? BIBO Stable? ! Time Shift: " x[n]y[n =] x[n-m]= x[n − m] ! Accumulator: " y[n] n y[n] = ∑ x[k] k=−∞ ! Compressor (M>1): y[n] = x[Mn] Penn ESE 531 Spring 2019 - Khanna 8 Non-Linear System Example ! Median Filter " y[n]=MED{x[n-k], …x[n+k]} " Let k=1 " y[n]=MED{x[n-1], x[n], x[n+1]} Penn ESE 531 Spring 2019 - Khanna 9 Spectrum of Speech Speech Penn ESE 531 Spring 2019 - Khanna 10 Spectrum of Speech Speech Corrupted Speech Penn ESE 531 Spring 2019 - Khanna 11 Low Pass Filtering Penn ESE 531 Spring 2019 - Khanna 12 Low Pass Filtering Corrupted Speech LP-Filtered Speech Penn ESE 531 Spring 2019 - Khanna 13 Median Filtering Corrupted Speech Med-Filter Speech Penn ESE 531 Spring 2019 - Khanna 14 LTI Systems Penn ESE 531 Spring 2019 - Khanna 15 LTI Systems ! LTI system can be completely characterized by its impulse response ! Then the output for an arbitrary input is a sum of weighted, delay impulse responses y[n] = x[n]∗h[n] Penn ESE 531 Spring 2019 - Khanna 16 Convolution ! Convolution formula: ! Convolution method: " 1) Time reverse the impulse response and shift it n time steps to the right " 2) Compute the inner product between the shifted impulse response and the input vector " Repeat for evey n Penn ESE 531 Spring 2019 - Khanna Convolution Example ! Convolve a unit pulse with itself Penn ESE 531 Spring 2019 - Khanna 18 Convolution is Commutative ! Convolution is commutative: ! These block diagrams are equivalent ! Implication: pick either h or x to flip and shift when convolving Penn ESE 531 Spring 2019 - Khanna 19 LTI Systems in Series ! Impulse response of the cascade of two LTI systems: Penn ESE 531 Spring 2019 - Khanna 20 LTI Systems in Series ! Impulse response of the cascade of two LTI systems: ! Proof by picture Penn ESE 531 Spring 2019 - Khanna 21 LTI Systems in Parallel ! Impulse response of the parallel connection of two LTI systems: Penn ESE 531 Spring 2019 - Khanna 22 Causal System Revisited ! An LTI system is causal if its impulse response is causal: Penn ESE 531 Spring 2019 - Khanna 23 Causal System Revisited ! An LTI system is causal if its impulse response is causal: ! To prove, note that the convolution does not look into the future if the impulse response is causal Penn ESE 531 Spring 2019 - Khanna 24 Duration of Impulse Penn ESE 531 Spring 2019 - Khanna 25 Duration of Impulse ! Example: Moving average Penn ESE 531 Spring 2019 - Khanna 26 Duration of Impulse ! Example: Moving average Penn ESE 531 Spring 2019 - Khanna 27 Duration of Impulse Penn ESE 531 Spring 2019 - Khanna 28 Duration of Impulse ! Example: Recursive average Penn ESE 531 Spring 2019 - Khanna 29 Duration of Impulse ! Example: Recursive average Penn ESE 531 Spring 2019 - Khanna 30 BIBO Stability Revisited Penn ESE 531 Spring 2019 - Khanna 31 BIBO Stability Revisited ! Bounded input and output: x < ∞ and y < ∞ ∞ ∞ ! Where x = max x[n] ∞ Penn ESE 531 Spring 2019 - Khanna 32 BIBO Stability Revisited ! Bounded input and output: x < ∞ and y < ∞ ∞ ∞ ! An LTI system is BIBO stable if and only if Penn ESE 531 Spring 2019 - Khanna 33 BIBO Stability – Sufficient Condition Penn ESE 531 Spring 2019 - Khanna 34 BIBO Stability – Sufficient Condition Penn ESE 531 Spring 2019 - Khanna 35 BIBO Stability – Necessary Condition Penn ESE 531 Spring 2019 - Khanna 36 BIBO Stability – Necessary Condition Penn ESE 531 Spring 2019 - Khanna 37 BIBO Stability – Necessary Condition Penn ESE 531 Spring 2019 - Khanna 38 Examples Penn ESE 531 Spring 2019 - Khanna 39 Examples Penn ESE 531 Spring 2019 - Khanna 40 Example Penn ESE 531 Spring 2019 - Khanna 41 Example Penn ESE 531 Spring 2019 - Khanna 42 Difference Equations ! Accumulator example n y[n] = ∑ x[k] k=−∞ n−1 y[n] = x[n]+ ∑ x[k] k=−∞ y[n] = x[n]+ y[n −1] y[n]− y[n −1] = x[n] Penn ESE 531 Spring 2019 - Khanna 43 Difference Equations ! Accumulator example n y[n] = ∑ x[k] k=−∞ n−1 y[n] = x[n]+ ∑ x[k] k=−∞ y[n] = x[n]+ y[n −1] y[n]− y[n −1] = x[n] N M a y[n k] b x[n m] ∑ k − = ∑ m − k=0 m=0 Penn ESE 531 Spring 2019 - Khanna 44 Difference Equations ! Accumulator example n y[n] = ∑ x[k] k=−∞ n−1 y[n] = x[n]+ ∑ x[k] k=−∞ y[n] = x[n]+ y[n −1] y[n]− y[n −1] = x[n] N M a y[n k] b x[n m] ∑ k − = ∑ m − k=0 m=0 Penn ESE 531 Spring 2019 - Khanna 45 Example: Difference Equation ! Moving Average System M 1 2 y[n] = x[n − k] M + M +1 ∑ 1 2 k=−M1 ! Let M1=0 (i.e. system is causal) M 1 2 y[n] = ∑x[n − k] M 2 +1 k=0 Penn ESE 531 Spring 2019 - Khanna 46 Big Ideas ! LTI Systems are a special class of systems with significant signal processing applications " Can be characterized by the impulse response ! LTI System Properties " Causality and stability can be determined from impulse response ! Difference equations suggest implementation of systems " Give insight into complexity of system Penn ESE 531 Spring 2019 - Khanna 47 Admin ! HW 1 posted tomorrow " Due 2/3 at midnight " Submit in Canvas Penn ESE 531 Spring 2019 - Khanna 48 .
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