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75 lines
1.9 KiB
75 lines
1.9 KiB
6 years ago
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function y = mdct4(x)
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% MDCT4 Calculates the Modified Discrete Cosine Transform
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% y = mdct4(x)
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%
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% Use either a Sine or a Kaiser-Bessel Derived window (KBDWin)with
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% 50% overlap for perfect TDAC reconstruction.
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% Remember that MDCT coefs are symmetric: y(k)=-y(N-k-1) so the full
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% matrix (N) of coefs is: yf = [y;-flipud(y)];
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%
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% x: input signal (can be either a column or frame per column)
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% length of x must be a integer multiple of 4 (each frame)
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% y: MDCT of x (coefs are divided by sqrt(N))
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%
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% Vectorize ! ! !
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% ------- mdct4.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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% Make sure length is multiple of 4
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if (rem(flen,4)~=0)
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error('MDCT4 defined for lengths multiple of four.');
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end
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% We need these for furmulas below
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N = flen; % Length of window
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M = N/2; % Number of coefficients
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N4 = N/4; % Simplify the way eqs look
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sqrtN = sqrt(N);
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% Preallocate rotation matrix
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% It would be nice to be able to do it in-place but we cannot
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% cause of the prerotation.
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rot = zeros(flen,fnum);
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% Shift
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t = (0:(N4-1)).';
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rot(t+1,:) = -x(t+3*N4+1,:);
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t = (N4:(N-1)).';
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rot(t+1,:) = x(t-N4+1,:);
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clear x;
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% We need this twice so keep it around
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t = (0:(N4-1)).';
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w = diag(sparse(exp(-j*2*pi*(t+1/8)/N)));
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% Pre-twiddle
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t = (0:(N4-1)).';
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c = (rot(2*t+1,:)-rot(N-1-2*t+1,:))...
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-j*(rot(M+2*t+1,:)-rot(M-1-2*t+1,:));
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% This is a really cool Matlab trick ;)
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c = 0.5*w*c;
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clear rot;
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% FFT for N/4 points only !!!
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c = fft(c,N4);
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% Post-twiddle
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c = (2/sqrtN)*w*c;
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% Sort
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t = (0:(N4-1)).';
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y(2*t+1,:) = real(c(t+1,:));
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y(M-1-2*t+1,:) = -imag(c(t+1,:));
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