ref: 104c218c9b1565a8d4bc403fea1a527c19271935
dir: /libcelt/mdct.c/
/* (C) 2008 Jean-Marc Valin, CSIRO
*/
/*
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*/
/* This is a simple MDCT implementation that uses a N/4 complex FFT
to do most of the work. It should be relatively straightforward to
plug in pretty much and FFT here.
This replaces the Vorbis FFT (and uses the exact same API), which
was a bit too messy and that was ending up duplicating code
(might as well use the same FFT everywhere).
The algorithm is similar to (and inspired from) Fabrice Bellard's
MDCT implementation in FFMPEG, but has differences in signs, ordering
and scaling in many places.
*/
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
#include "mdct.h"
#include "kiss_fft.h"
#include <math.h>
#include "os_support.h"
#include "_kiss_fft_guts.h"
#include "mathops.h"
#include "stack_alloc.h"
#ifndef M_PI
#define M_PI 3.141592653
#endif
void mdct_init(mdct_lookup *l,int N)
{
int i;
int N2, N4;
l->n = N;
N2 = N/2;
N4 = N/4;
l->kfft = kiss_fft_alloc(N4, NULL, NULL);
l->trig = (kiss_twiddle_scalar*)celt_alloc(N2*sizeof(kiss_twiddle_scalar));
/* We have enough points that sine isn't necessary */
#if defined(FIXED_POINT)
#if defined(DOUBLE_PRECISION) & !defined(MIXED_PRECISION)
for (i=0;i<N2;i++)
l->trig[i] = SAMP_MAX*cos(2*M_PI*(i+1./8.)/N);
#else
for (i=0;i<N2;i++)
l->trig[i] = TRIG_UPSCALE*celt_cos_norm(DIV32(ADD32(SHL32(EXTEND32(i),17),16386),N));
#endif
#else
for (i=0;i<N2;i++)
l->trig[i] = cos(2*M_PI*(i+1./8.)/N);
#endif
}
void mdct_clear(mdct_lookup *l)
{
kiss_fft_free(l->kfft);
celt_free(l->trig);
}
void mdct_forward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar *out)
{
int i;
int N, N2, N4;
VARDECL(kiss_fft_scalar, f);
SAVE_STACK;
N = l->n;
N2 = N/2;
N4 = N/4;
ALLOC(f, N2, kiss_fft_scalar);
/* Consider the input to be compused of four blocks: [a, b, c, d] */
/* Shuffle, fold, pre-rotate (part 1) */
for(i=0;i<N/8;i++)
{
kiss_fft_scalar re, im;
/* Real part arranged as -d-cR, Imag part arranged as -b+aR*/
re = -HALF32(in[N2+N4+2*i] + in[N2+N4-2*i-1]);
im = -HALF32(in[N4+2*i] - in[N4-2*i-1]);
out[2*i] = S_MUL(re,l->trig[i]) - S_MUL(im,l->trig[i+N4]);
out[2*i+1] = S_MUL(im,l->trig[i]) + S_MUL(re,l->trig[i+N4]);
}
for(;i<N4;i++)
{
kiss_fft_scalar re, im;
/* Real part arranged as a-bR, Imag part arranged as -c-dR */
re = HALF32(in[2*i-N4] - in[N2+N4-2*i-1]);
im = -HALF32(in[N4+2*i] + in[N+N4-2*i-1]);
out[2*i] = S_MUL(re,l->trig[i]) - S_MUL(im,l->trig[i+N4]);
out[2*i+1] = S_MUL(im,l->trig[i]) + S_MUL(re,l->trig[i+N4]);
}
/* N/4 complex FFT, which should normally down-scale by 4/N (but doesn't now) */
kiss_fft(l->kfft, (const kiss_fft_cpx *)out, (kiss_fft_cpx *)f);
/* Post-rotate and apply the scaling if the FFT doesn't to it itself */
for(i=0;i<N4;i++)
{
out[2*i] = -S_MUL(f[2*i+1],l->trig[i+N4]) + S_MUL(f[2*i] ,l->trig[i]);
out[N2-1-2*i] = -S_MUL(f[2*i] ,l->trig[i+N4]) - S_MUL(f[2*i+1],l->trig[i]);
}
RESTORE_STACK;
}
void mdct_backward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar *out)
{
int i;
int N, N2, N4;
VARDECL(kiss_fft_scalar, f);
SAVE_STACK;
N = l->n;
N2 = N/2;
N4 = N/4;
ALLOC(f, N2, kiss_fft_scalar);
/* Pre-rotate */
for(i=0;i<N4;i++)
{
out[2*i] = -S_MUL(in[N2-2*i-1], l->trig[i]) - S_MUL(in[2*i],l->trig[i+N4]);
out[2*i+1] = S_MUL(in[N2-2*i-1], l->trig[i+N4]) - S_MUL(in[2*i],l->trig[i]);
}
/* Inverse N/4 complex FFT. This one should *not* downscale even in fixed-point */
kiss_ifft(l->kfft, (const kiss_fft_cpx *)out, (kiss_fft_cpx *)f);
/* Post-rotate */
for(i=0;i<N4;i++)
{
kiss_fft_scalar re, im;
re = f[2*i];
im = f[2*i+1];
/* We'd scale up by 2 here, but instead it's done when mixing the windows */
f[2*i] = S_MUL(re,l->trig[i]) + S_MUL(im,l->trig[i+N4]);
f[2*i+1] = S_MUL(im,l->trig[i]) - S_MUL(re,l->trig[i+N4]);
}
/* De-shuffle the components for the middle of the window only */
for(i = 0; i < N4; i++)
{
out[N4+2*i] =-f[2*i];
out[N4+2*i+1] = f[N2-2*i-1];
}
/* Mirror on both sides for TDAC */
for(i = 0; i < N4; i++)
{
out[i] =-out[N2-i-1];
out[N-i-1] = out[N2+i];
}
RESTORE_STACK;
}