[vorbis-dev] More mdct questions

[Dave, Anish]

Sorry for starting another topic, this is actually a reply to Segher's post
on Sun Oct 22 on the 'mdct question' topic.  I wasn't subscribed properly
and so I didn't get email confirmation and thus can't add to that thread.

So Segher, if the equation is indeed what you say it is, then replacing
mdct_backward with this version should work, but it doesn't.
Am I applying it incorrectly?

void mdct_backward(mdct_lookup *init, double *in, double *out) // I call
this with in != out
{

    // For 256 size blocks, coef are in[0..127]
    // For 2048 size blocks, coef are in[0..1023]
    int N = init->n / 4;
    for( int j = 0; j < 4*N; j++ )
    {
        double sum = 0;
        for( int k = 0; k < 2*N; k++ )
        {
            sum += in[k]*cos(M_PI*(2*j+1+N)*(2*k+1)/8/N);
        }
        out[j] = sum;
    }
}

> It just a normal shifted mdct, like the one used in mpeg layer 3. 
> it's equations are: 
> (time domain: a_j, freq domain: b_k; w_j is the window) 
> forward: 
> b_k = sum(j=0..4N-1) a_j * w_j * cos( (2k+1)(2j+1+-N) / 8N ) // forward
> a_j = w_j * sum(k=0..2N-1) a_j * cos( (2k+1)(2j+1+-N) / 8N ) // backward
(missing PI in the cos, and a_j should be b_k in summation)
> where +- means: I'm not sure whether it is + or -, but I think it is + 
> (forward as well as backward). 
> k is 0..2N-1, j is 0..4N-1. 

---------------------------
Anish Dave
Univ. of Waterloo
Comp Sci.

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