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ac3enc: do not right-shift fixed-point coefficients in the final MDCT stage.

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This increases the accuracy of coefficients, leading to improved quality.
Rescaling of the coefficients to full 25-bit accuracy is done rather than
offsetting the exponent values. This requires coefficient scaling to be done
before determining the rematrixing strategy. Also, the rematrixing strategy
calculation must use 64-bit math to prevent overflow due to the higher
precision coefficients.
This commit is contained in:
Justin
2011-03-11 13:03:26 -05:00
committed by Justin Ruggles
parent 6e7cf13b6b
commit 323e6fead0
7 changed files with 65 additions and 39 deletions
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49
libavcodec/ac3enc_fixed.c
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@ -131,10 +131,10 @@ mdct_alloc_fail:
/** Complex multiply */
#define CMUL(pre, pim, are, aim, bre, bim) \
#define CMUL(pre, pim, are, aim, bre, bim, rshift) \
{ \
pre = (MUL16(are, bre) - MUL16(aim, bim)) >> 15; \
pim = (MUL16(are, bim) + MUL16(bre, aim)) >> 15; \
pre = (MUL16(are, bre) - MUL16(aim, bim)) >> rshift; \
pim = (MUL16(are, bim) + MUL16(bre, aim)) >> rshift; \
}
@ -195,7 +195,7 @@ static void fft(AC3MDCTContext *mdct, IComplex *z, int ln)
p++;
q++;
for(l = nblocks; l < np2; l += nblocks) {
CMUL(tmp_re, tmp_im, mdct->costab[l], -mdct->sintab[l], q->re, q->im);
CMUL(tmp_re, tmp_im, mdct->costab[l], -mdct->sintab[l], q->re, q->im, 15);
BF(p->re, p->im, q->re, q->im,
p->re, p->im, tmp_re, tmp_im);
p++;
@ -234,7 +234,7 @@ static void mdct512(AC3MDCTContext *mdct, int32_t *out, int16_t *in)
for (i = 0; i < n4; i++) {
re = ((int)rot[ 2*i] - (int)rot[ n-1-2*i]) >> 1;
im = -((int)rot[n2+2*i] - (int)rot[n2-1-2*i]) >> 1;
CMUL(x[i].re, x[i].im, re, im, -mdct->xcos1[i], mdct->xsin1[i]);
CMUL(x[i].re, x[i].im, re, im, -mdct->xcos1[i], mdct->xsin1[i], 15);
}
fft(mdct, x, mdct->nbits - 2);
@ -243,7 +243,7 @@ static void mdct512(AC3MDCTContext *mdct, int32_t *out, int16_t *in)
for (i = 0; i < n4; i++) {
re = x[i].re;
im = x[i].im;
CMUL(out[n2-1-2*i], out[2*i], re, im, mdct->xsin1[i], mdct->xcos1[i]);
CMUL(out[n2-1-2*i], out[2*i], re, im, mdct->xsin1[i], mdct->xcos1[i], 0);
}
}
@ -294,10 +294,26 @@ static void lshift_tab(int16_t *tab, int n, unsigned int lshift)
}
/**
* Right-shift each value in an array of int32_t by a specified amount.
* @param src input array
* @param len number of values in the array
* @param shift right shift amount
*/
static void ac3_rshift_int32_c(int32_t *src, unsigned int len, unsigned int shift)
{
int i;
if (shift > 0) {
for (i = 0; i < len; i++)
src[i] >>= shift;
}
}
/**
* Normalize the input samples to use the maximum available precision.
* This assumes signed 16-bit input samples. Exponents are reduced by 9 to
* match the 24-bit internal precision for MDCT coefficients.
* This assumes signed 16-bit input samples.
*
* @return exponent shift
*/
@ -305,18 +321,25 @@ static int normalize_samples(AC3EncodeContext *s)
{
int v = 14 - log2_tab(s, s->windowed_samples, AC3_WINDOW_SIZE);
lshift_tab(s->windowed_samples, AC3_WINDOW_SIZE, v);
return v - 9;
/* +6 to right-shift from 31-bit to 25-bit */
return v + 6;
}
/**
* Scale MDCT coefficients from float to fixed-point.
* Scale MDCT coefficients to 25-bit signed fixed-point.
*/
static void scale_coefficients(AC3EncodeContext *s)
{
/* scaling/conversion is obviously not needed for the fixed-point encoder
since the coefficients are already fixed-point. */
return;
int blk, ch;
for (blk = 0; blk < AC3_MAX_BLOCKS; blk++) {
AC3Block *block = &s->blocks[blk];
for (ch = 0; ch < s->channels; ch++) {
ac3_rshift_int32_c(block->mdct_coef[ch], AC3_MAX_COEFS,
block->coeff_shift[ch]);
}
}
}