[Jack-Devel] Variable delay line problem

Hello,

I don't know if there are DSP gurus in there, but I have a little problem with basic flanger effect.
I read a few articles / book pieces about this.

I understood that a flanger is just an extremely short delay line (max 15ms) with a variable delay length.
The variable length can follow an LFO wave, I tried with a simple cosine here.

So here is what I do in a loop through all jack buffer samples :

    max = "number of delay buffer samples";
    for( int i=0; i<size; i++)
    {  
        sample = buffer[i];

    // Get current delay buffer index
        n = (float)this->dl.getCurrent() / max;
    // Compute an index between 0 and max based on a LFO, in this case simple cos(2*PI*n)
        dn = max*0.5f - this->depth * cos( 2*PI*n);
    // Return the index of delayed sample index "dn" * decay value between 0 and 1
        temp = this->dl.process( sample, dn);
        //temp = this->dl.process( buffer[i], 300);
    // Output original sample + delayed one
        buffer[i] = sample + temp;
        //buffer[i] = temp;
    }   

First of all, I'm not sure about what should be 'n' here. Then I found formulas for dn that use a frequency like :
dn = D/2 * (1 - cos(2*PI*n*F));
I don't know either what should be F in my code.

But I have to specify that I get dn indexes that seem to evolve like a Cosine between 0 and max sample of the delay buffer.
So it would "seem" to work in that area.

In the meantime, all I hear is the original signal + a not nice to hear distorted version of the original sound (the temp part as I verified byt outputting it alone).
I tried with a constant dn value like 300 ms and I get a normal delay.
I basically round dn by casting float to an int but I even tried to add interpolation between sample dn and dn+1 in cases where dn is not integer. Nothing changed.

I've spent a few hours trying to understand the concepts and to try variants looking for what could be wrong. I'm lost for now.

Regards

1334323258.18854_0.ltw:2,a <84C79E00-C43F-4689-A0CA-2B1436CE6DC0 at free dot fr>