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61 lines
1.2 KiB
61 lines
1.2 KiB
6 years ago
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function y = imdct4(x)
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% IMDCT4 Calculates the Modified Discrete Cosine Transform
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% y = imdct4(x)
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%
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% x: input signal (can be either a column or frame per column)
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% y: IMDCT of x
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%
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% Vectorize ! ! !
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% ------- imdct4.m -----------------------------------------
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% Marios Athineos, marios@ee.columbia.edu
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% http://www.ee.columbia.edu/~marios/
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% Copyright (c) 2002 by Columbia University.
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% All rights reserved.
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% ----------------------------------------------------------
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[flen,fnum] = size(x);
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% Make column if it's a single row
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if (flen==1)
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x = x(:);
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flen = fnum;
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fnum = 1;
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end
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% We need these for furmulas below
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N = flen;
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M = N/2;
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twoN = 2*N;
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sqrtN = sqrt(twoN);
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% We need this twice so keep it around
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t = (0:(M-1)).';
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w = diag(sparse(exp(-j*2*pi*(t+1/8)/twoN)));
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% Pre-twiddle
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t = (0:(M-1)).';
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c = x(2*t+1,:) + j*x(N-1-2*t+1,:);
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c = (0.5*w)*c;
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% FFT for N/2 points only !!!
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c = fft(c,M);
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% Post-twiddle
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c = ((8/sqrtN)*w)*c;
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% Preallocate rotation matrix
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rot = zeros(twoN,fnum);
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% Sort
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t = (0:(M-1)).';
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rot(2*t+1,:) = real(c(t+1,:));
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rot(N+2*t+1,:) = imag(c(t+1,:));
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t = (1:2:(twoN-1)).';
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rot(t+1,:) = -rot(twoN-1-t+1,:);
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% Shift
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t = (0:(3*M-1)).';
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y(t+1,:) = rot(t+M+1,:);
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t = (3*M:(twoN-1)).';
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y(t+1,:) = -rot(t-3*M+1,:);
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