hi,
im tyring to search for algorithms for imdct which can outperform or match the imdct implementation in FAAD2. all the literature for fast and efficient implementations of imdct are on acm or ieee portals which are not available to me

so can u suggest and provide me with links to some literature for the same?

thank u in advance

siddharth

function [ y ] = fastdct4( x )

% make x a real-valued row vector
x = real(x(:))';

% N real values -> N/2 complex values
z = (x(1:2:length(x)) + i*x(length(x):-2:1));

% pre twiddling
z = z .* exp((0.5:2:length(x))./length(x).*(-pi/2).*i);

% FFT
z = fft(z);

% post twiddling
z = z .* exp( (0:(length(z)-1)).*(-pi/length(x)).*i );

% extracting DCT4 coeffs and scaling ...
scale = sqrt(2/length(x));
y = zeros(1,length(x));
y(1:2:length(y)) =  real(z) .* scale;
y(2:2:length(y)) = -imag(z(length(z):-1:1)) .* scale;

return;

Here's described how you can decompose the (i)MDCT into a set of butterflies and a DCT4:
http://www.hydrogenaudio.org/forums/index....showtopic=20449
(the "mdct core transform" I was referring to is actually a type-4 DCT)

A Matlab implemetation for a fast type-4 DCT is following: The length of x should be a power of two and must be even.

Code: [Select]

function [ y ] = fastdct4( x )

% make x a real-valued row vector
x = real(x(:))';

% N real values -> N/2 complex values
z = (x(1:2:length(x)) + i*x(length(x):-2:1));

% pre twiddling
z = z .* exp((0.5:2:length(x))./length(x).*(-pi/2).*i);

% FFT
z = fft(z);

% post twiddling
z = z .* exp( (0:(length(z)-1)).*(-pi/length(x)).*i );

% extracting DCT4 coeffs and scaling ...
scale = sqrt(2/length(x));
y = zeros(1,length(x));
y(1:2:length(y)) =  real(z) .* scale;
y(2:2:length(y)) = -imag(z(length(z):-1:1)) .* scale;

return;

Seb

edit: i just fixed a "sign bug" for odd coefficients