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Slightly improving the accuracy of the fixed-point MDCT downscale

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Also simplifying the code
This commit is contained in:
Jean-Marc Valin 2013-12-28 19:10:44 -05:00
parent e0c00e27d8
commit cc344fb8ff
1 changed files with 9 additions and 13 deletions
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22
celt/mdct.c
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@ -119,15 +119,20 @@ void clt_mdct_forward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar
VARDECL(kiss_fft_cpx, f2); VARDECL(kiss_fft_cpx, f2);
const kiss_fft_state *st = l->kfft[shift]; const kiss_fft_state *st = l->kfft[shift];
const kiss_twiddle_scalar *trig; const kiss_twiddle_scalar *trig;
opus_val16 scale;
#ifdef FIXED_POINT #ifdef FIXED_POINT
/* FIXME: This should eventually just go in the state. */ /* FIXME: This should eventually just go in the state. */
opus_val16 scale;
int scale_shift; int scale_shift;
scale_shift = celt_ilog2(st->nfft); scale_shift = celt_ilog2(st->nfft);
if (st->nfft == 1<<scale_shift) if (st->nfft == 1<<scale_shift)
scale = Q15ONE; scale = Q15ONE;
else else
scale = (1073741824+st->nfft/2)/st->nfft>>(15-scale_shift); scale = (1073741824+st->nfft/2)/st->nfft>>(15-scale_shift);
/* Allows us to scale with MULT16_32_Q16(), which is faster than
MULT16_32_Q15() on ARM. */
scale_shift--;
#else
scale = st->scale;
#endif #endif
SAVE_STACK; SAVE_STACK;
@ -195,28 +200,19 @@ void clt_mdct_forward(const mdct_lookup *l, kiss_fft_scalar *in, kiss_fft_scalar
kiss_fft_scalar re, im, yr, yi; kiss_fft_scalar re, im, yr, yi;
t0 = t[i]; t0 = t[i];
t1 = t[N4+i]; t1 = t[N4+i];
#ifdef FIXED_POINT
t0 = MULT16_16_P15(t0, scale);
t1 = MULT16_16_P15(t1, scale);
#else
t0 *= st->scale;
t1 *= st->scale;
#endif
re = *yp++; re = *yp++;
im = *yp++; im = *yp++;
yr = -S_MUL(re,t0) + S_MUL(im,t1); yr = -S_MUL(re,t0) + S_MUL(im,t1);
yi = -S_MUL(im,t0) - S_MUL(re,t1); yi = -S_MUL(im,t0) - S_MUL(re,t1);
yc.r = yr; yc.r = yr;
yc.i = yi; yc.i = yi;
#ifdef FIXED_POINT yc.r = PSHR32(MULT16_32_Q16(scale, yc.r), scale_shift);
yc.r = SHR32(yc.r, scale_shift); yc.i = PSHR32(MULT16_32_Q16(scale, yc.i), scale_shift);
yc.i = SHR32(yc.i, scale_shift);
#endif
f2[st->bitrev[i]] = yc; f2[st->bitrev[i]] = yc;
} }
} }
/* N/4 complex FFT, down-scales by 4/N */ /* N/4 complex FFT, does not downscale anymore */
opus_fft_impl(st, f2); opus_fft_impl(st, f2);
/* Post-rotate */ /* Post-rotate */