1 Lecture 14: More on finite-length discrete transforms Instructor: Dr. Gleb V. Tcheslavski Contact: gleb@ee.lamar.edu Office Hours: Room 2030 Class web site: http://ee.lamar.edu/gleb/ds p/index.htm Smiley face ELEN 5346/4304 DSP and Filter Design Fall 2008 2 Orthogonal transforms Frequently, it is beneficial to convert a finite-length time-domain sequence xn into a finite-length sequence in other domain and vice versa: N 1 Analysis: X k xn *k ,n ,0 k N 1 (14.2.1) n 0 Synthesis 1 xn N N 1 X k 0 k k ,n ,0 n N 1 (14.2.2) We restrict our discussion to the class of orthogonal transforms. For such transforms, the basis sequences (functions) k,n satisfy the following properties: 1,l k 1 N 1 * k ,n l ,n N n 0 0,l k ELEN 5346/4304 DSP and Filter Design Fall 2008 (14.2.3) 3 Orthogonal transforms For the orthogonal basis functions, we can verify that (14.2.2) is indeed an inverse of (14.2.1): N 1 x n n 0 N 1 1 N 1 * 1 N 1 X k k ,n l ,n X k k ,n *l ,n X l n 0 N k 0 n 0 N k 0 N 1 * l ,n (14.3.1) An important consequence of orthogonality is the energy conservation property of such transforms that allows to compute the energy of a time-domain sequence in the transform domain (Parseval’s relation): N 1 xn n 0 2 1 N 1 1 N 1 * 1 N 1 N 1 * 1 N 1 * x x X k k ,n xn X k xn k ,n X k X k X k N k 0 n 0 N k 0 n 0 n 0 N k 0 N k 0 N 1 N 1 * n n 2 (14.3.2) In some applications, it may be important to use a transform that decorrelates the transform coefficients. In other applications, energy compaction (concentration of most of signal energy in few transform coefficients) is highly desirable. ELEN 5346/4304 DSP and Filter Design Fall 2008 4 DFT revisited Basis functions: k ,n e j 2 kn N (14.4.1) Therefore, the N-point DFT pair: N 1 X k xn e n 0 j 2 kn N N 1 xnWNkn ,0 k N 1 n 0 1 N 1 1 N 1 j 2 kn N xn X k e X kWN kn ,0 n N 1 N k 0 N k 0 WN e j 2 where (14.4.2) N (14.4.3) (14.4.4) As a result, even for real xn, its DFT Xk is generally complex: X k X re,k jX im,k ELEN 5346/4304 DSP and Filter Design Fall 2008 (14.4.5) 5 DFT symmetry relations Real and imaginary parts of the DFT sequence can be found as: 1 X k X k* 2 1 X k X k* 2 X re,k (14.5.1) X im ,k (14.5.2) Assuming that the original time-domain signal is complex: xn xre,n jxim,n (14.5.3) its DFT can be found as: 2 k 2 k X k xre,n jxim,n cos j sin N N n 0 N 1 2 k 2 k N 1 2 k 2 k xre,n cos xim,n sin j x cos x sin im , n re , n N N N N n 0 n 0 N 1 ELEN 5346/4304 DSP and Filter Design Fall 2008 (14.5.4) 6 DFT symmetry relations Therefore, real and imaginary parts of the DFT sequence are: 2 k 2 k N 1 j kn X re,k xre,n cos xim,n sin x e cs,n N N n 0 n 0 N 1 2 k 2 k N 1 j kn X im,k xim,n cos xre,n sin x e ca,n N N n 0 n 0 N 1 (14.6.1) (14.6.2) Here xcs,n and xca,n are circular conjugate-symmetric and circular conjugateantisymmetric parts of xn: xn xcs,n xca,n ,0 n N 1 xcs ,n 1 xn x* n ,0 n N 1 N 2 1 xn x* n ,0 n N 1 N 2 xcs ,n circular shift ELEN 5346/4304 DSP and Filter Design Fall 2008 (14.6.3) (14.6.4) (14.6.5) 7 DFT symmetry relations Therefore, for complex sequences: 1 X k X * k N N DFT 2 1 j xim ,n X ca ,k X k X * k N N DFT 2 xcs ,n X re ,k xre,n X cs ,k N DFT (14.7.3) (14.7.4) x* n X * k (14.7.5) N DFT x* n DSP and Filter Design (14.7.2) xca ,n j X im ,k N DFT ELEN 5346/4304 (14.7.1) N N X *k N DFT Fall 2008 (14.7.6) 8 DFT symmetry relations real For real sequences: Therefore: xcs , n xev , n xca , n xod ,n (14.8.3) xod ,n j X im,k (14.8.4) N DFT X k X * k (14.8.5) N X re,k X re, k (14.8.6) N X im ,k X im , k Xk X k Fall 2008 (14.8.7) N (14.8.8) N X k X DSP and Filter Design (14.8.2) xev ,n X re,k N DFT ELEN 5346/4304 (14.8.1) k (14.8.9) N 9 DFT symmetry relations If N (the length of the sequence) is even, the DFT samples X0 and X(N-2)/2 are real and distinct. The remaining N – 2 samples of DFT are complex: a half of them are distinct and the rest are their complex conjugates. If N (the length of the sequence) is odd, the DFT sample X0 is real and distinct. The remaining N – 1 samples of DFT are complex: a half of them are distinct and the rest are their complex conjugates. It is frequently desired to have an orthogonal transform that represents a real time-domain sequence as another real sequence in the transform domain. ELEN 5346/4304 DSP and Filter Design Fall 2008 10 Type 2 DCT There are several (8?) types of Discrete Cosine Transform (DCT), for the development of which Dr. Rao (UT Arlington) is credited… The most commonly used is DCT 2: Basis functions: k ,n k (2n 1) k ( n 1 2 cos Re W 2N 2N (14.10.1) The DCT pair: k (2n 1) X DCT ,k 2 xn cos ,0 k N 1 2N n 0 1 N 1 k (2n 1) xn k X DCT ,k cos ,0 n N 1 N k 0 2N N 1 1 2,k 0 1,1 k N 1 k where ELEN 5346/4304 DSP and Filter Design Fall 2008 (14.10.2) (14.10.3) (14.10.4) 11 Type 2 DCT DCT Properties: 1. Linearity: g n hn GDCT ,k H DCT ,k (14.11.1) g *n G * DCT ,k (14.11.2) DCT 2. Symmetry: DCT N 1 3. Energy preservation: n 0 gn 2 1 N 1 k GDCT ,k 2 N k 0 2 (14.11.3) DCT of a real sequence is another real sequence. DCT has very good energy compaction properties: most of the signals energy is confined to several first DCT coefficients. Therefore, DCT is used for lossy data compression: JPEG, MPG, mp3… ELEN 5346/4304 DSP and Filter Design Fall 2008 12 Computation of type 2 DCT DCT is related to DFT X DCT ,k k 2 2 N 1 2 Re W2 N xnW2nkN ,0 k N 1 n 0 (14.12.1) Therefore, the N-point DCT can be computed (by utilizing FFT algorithms) as follows: 1. Extend xn to a length-2N sequence xe,n by zero-padding and compute its 2N-point DFT Xe,k; 2. Extract the first N samples of Xe,k and multiply each sample by W2kN2 ; 3. Extract the real part of the above samples and multiply each by 2. ELEN 5346/4304 DSP and Filter Design Fall 2008 13 The Haar transform The transform pair: X Haar = H N x (14.13.1) x = H N1X Haar (14.13.2) T Haar transform coefficients: X Haar X Haar ,0 X Haar ,1... X Haar , N 1 Time-domain samples: x x0 x1... xN 1 (14.13.4) Normalized Haar transform matrix: 1 H2 1 (14.13.5) (14.13.3) T H 2v1 2 2 2 1 2 1 H v 1 2 1 2 2 ,v 0 I 1 2 1 2 2v Where denotes the Kronecker product and Ik is a k x k identity matrix. ELEN 5346/4304 DSP and Filter Design Fall 2008 (14.13.6) 14 The Haar transform Properties: 1. Orthogonality: which, for the inverse transform, leads to: H N1 x n 0 (14.14.1) 1 t H N X Haar N N 1 2. Energy conservation: 1 t HN N xn 2 1 N 1 H Haar ,k N k 0 (14.14.2) 2 (14.13.3) The Haar transform represents a real time-domain sequence as another real transform-domain sequence. Used in data compression algorithms. ELEN 5346/4304 DSP and Filter Design Fall 2008