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

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.
tags/n0.8
Justin Justin Ruggles 15 years ago
parent
6e7cf13b6b
commit
323e6fead0
7 changed files with 65 additions and 39 deletions
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  1. +12
    -13
      libavcodec/ac3enc.c
  2. +36
    -13
      libavcodec/ac3enc_fixed.c
  3. +2
    -0
      libavcodec/ac3enc_fixed.h
  4. +2
    -0
      libavcodec/ac3enc_float.h
  5. +1
    -1
      tests/ref/acodec/ac3_fixed
  6. +1
    -1
      tests/ref/lavf/rm
  7. +11
    -11
      tests/ref/seek/ac3_rm

+ 12
- 13
libavcodec/ac3enc.c View File

@@ -78,7 +78,7 @@ typedef struct AC3Block {
int16_t **band_psd; ///< psd per critical band
int16_t **mask; ///< masking curve
uint16_t **qmant; ///< quantized mantissas
int8_t exp_shift[AC3_MAX_CHANNELS]; ///< exponent shift values
uint8_t coeff_shift[AC3_MAX_CHANNELS]; ///< fixed-point coefficient shift values
uint8_t new_rematrixing_strategy; ///< send new rematrixing flags in this block
uint8_t rematrixing_flags[4]; ///< rematrixing flags
} AC3Block;
@@ -269,7 +269,7 @@ static void apply_mdct(AC3EncodeContext *s)

apply_window(&s->dsp, s->windowed_samples, input_samples, s->mdct.window, AC3_WINDOW_SIZE);

block->exp_shift[ch] = normalize_samples(s);
block->coeff_shift[ch] = normalize_samples(s);

mdct512(&s->mdct, block->mdct_coef[ch], s->windowed_samples);
}
@@ -328,10 +328,10 @@ static void compute_rematrixing_strategy(AC3EncodeContext *s)
CoefType rt = block->mdct_coef[1][i];
CoefType md = lt + rt;
CoefType sd = lt - rt;
sum[0] += lt * lt;
sum[1] += rt * rt;
sum[2] += md * md;
sum[3] += sd * sd;
MAC_COEF(sum[0], lt, lt);
MAC_COEF(sum[1], rt, rt);
MAC_COEF(sum[2], md, md);
MAC_COEF(sum[3], sd, sd);
}

/* compare sums to determine if rematrixing will be used for this band */
@@ -416,14 +416,13 @@ static void extract_exponents(AC3EncodeContext *s)
AC3Block *block = &s->blocks[blk];
uint8_t *exp = block->exp[ch];
int32_t *coef = block->fixed_coef[ch];
int exp_shift = block->exp_shift[ch];
for (i = 0; i < AC3_MAX_COEFS; i++) {
int e;
int v = abs(coef[i]);
if (v == 0)
e = 24;
else {
e = 23 - av_log2(v) + exp_shift;
e = 23 - av_log2(v);
if (e >= 24) {
e = 24;
coef[i] = 0;
@@ -1139,7 +1138,7 @@ static inline int asym_quant(int c, int e, int qbits)
* Quantize a set of mantissas for a single channel in a single block.
*/
static void quantize_mantissas_blk_ch(AC3EncodeContext *s, int32_t *fixed_coef,
int8_t exp_shift, uint8_t *exp,
uint8_t *exp,
uint8_t *bap, uint16_t *qmant, int n)
{
int i;
@@ -1147,7 +1146,7 @@ static void quantize_mantissas_blk_ch(AC3EncodeContext *s, int32_t *fixed_coef,
for (i = 0; i < n; i++) {
int v;
int c = fixed_coef[i];
int e = exp[i] - exp_shift;
int e = exp[i];
int b = bap[i];
switch (b) {
case 0:
@@ -1243,7 +1242,7 @@ static void quantize_mantissas(AC3EncodeContext *s)
s->qmant1_ptr = s->qmant2_ptr = s->qmant4_ptr = NULL;

for (ch = 0; ch < s->channels; ch++) {
quantize_mantissas_blk_ch(s, block->fixed_coef[ch], block->exp_shift[ch],
quantize_mantissas_blk_ch(s, block->fixed_coef[ch],
block->exp[ch], block->bap[ch],
block->qmant[ch], s->nb_coefs[ch]);
}
@@ -1507,10 +1506,10 @@ static int ac3_encode_frame(AVCodecContext *avctx, unsigned char *frame,

apply_mdct(s);

compute_rematrixing_strategy(s);

scale_coefficients(s);

compute_rematrixing_strategy(s);

apply_rematrixing(s);

process_exponents(s);


+ 36
- 13
libavcodec/ac3enc_fixed.c View File

@@ -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]);
}
}
}




+ 2
- 0
libavcodec/ac3enc_fixed.h View File

@@ -36,6 +36,8 @@ typedef int16_t SampleType;
typedef int32_t CoefType;
typedef int64_t CoefSumType;

#define MAC_COEF(d,a,b) MAC64(d,a,b)


/**
* Compex number.


+ 2
- 0
libavcodec/ac3enc_float.h View File

@@ -36,6 +36,8 @@ typedef float SampleType;
typedef float CoefType;
typedef float CoefSumType;

#define MAC_COEF(d,a,b) ((d)+=(a)*(b))


typedef struct AC3MDCTContext {
const float *window; ///< MDCT window function


+ 1
- 1
tests/ref/acodec/ac3_fixed View File

@@ -1,2 +1,2 @@
07bd593823ebd721b3a32ef298bdfc20 *./tests/data/acodec/ac3.rm
b3a8f0a8809a58b2ece90744f06fff96 *./tests/data/acodec/ac3.rm
98751 ./tests/data/acodec/ac3.rm

+ 1
- 1
tests/ref/lavf/rm View File

@@ -1,2 +1,2 @@
d149fc272dfd21fb8908ee21d7b1651b *./tests/data/lavf/lavf.rm
7da378131db880bcf2e58305d54418ec *./tests/data/lavf/lavf.rm
346706 ./tests/data/lavf/lavf.rm

+ 11
- 11
tests/ref/seek/ac3_rm View File

@@ -4,28 +4,29 @@ ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret: 0 st:-1 flags:1 ts: 1.894167
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret: 0 st: 0 flags:0 ts: 0.788000
ret: 0 st: 0 flags:1 dts:4160806.587000 pts:4160806.587000 pos: 3883 size: 116
ret: 0 st: 0 flags:1 dts:12581.487000 pts:12581.487000 pos: 5822 size: 916
ret: 0 st: 0 flags:1 ts:-0.317000
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret: 0 st:-1 flags:0 ts: 2.576668
ret: 0 st: 0 flags:1 dts:4160806.587000 pts:4160806.587000 pos: 3883 size: 116
ret: 0 st: 0 flags:1 dts:524.800000 pts:524.800000 pos: 6155 size: 244
ret:-1 st:-1 flags:1 ts: 1.470835
ret: 0 st: 0 flags:0 ts: 0.365000
ret: 0 st: 0 flags:1 dts:4160806.587000 pts:4160806.587000 pos: 3883 size: 116
ret: 0 st: 0 flags:1 dts:12581.487000 pts:12581.487000 pos: 5822 size: 916
ret: 0 st: 0 flags:1 ts:-0.741000
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret:-1 st:-1 flags:0 ts: 2.153336
ret:-1 st:-1 flags:1 ts: 1.047503
ret: 0 st:-1 flags:1 ts: 1.047503
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret: 0 st: 0 flags:0 ts:-0.058000
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret:-1 st: 0 flags:1 ts: 2.836000
ret: 0 st: 0 flags:1 ts: 2.836000
ret: 0 st: 0 flags:1 dts: 2.681000 pts: 2.681000 pos: 44105 size: 558
ret:-1 st:-1 flags:0 ts: 1.730004
ret: 0 st:-1 flags:1 ts: 0.624171
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret: 0 st: 0 flags:0 ts:-0.482000
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret: 0 st: 0 flags:1 ts: 2.413000
ret: 0 st: 0 flags:1 dts: 2.229000 pts: 2.229000 pos: 36705 size: 556
ret:-1 st: 0 flags:1 ts: 2.413000
ret:-1 st:-1 flags:0 ts: 1.306672
ret: 0 st:-1 flags:1 ts: 0.200839
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
@@ -33,13 +34,12 @@ ret: 0 st: 0 flags:0 ts:-0.905000
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret:-1 st: 0 flags:1 ts: 1.989000
ret: 0 st:-1 flags:0 ts: 0.883340
ret: 0 st: 0 flags:1 dts:4160806.587000 pts:4160806.587000 pos: 3883 size: 116
ret: 0 st: 0 flags:1 dts:12581.487000 pts:12581.487000 pos: 5822 size: 916
ret: 0 st:-1 flags:1 ts:-0.222493
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556
ret: 0 st: 0 flags:0 ts: 2.672000
ret: 0 st: 0 flags:1 dts:6354.691000 pts:6354.691000 pos: 10783 size: 304
ret:-1 st: 0 flags:0 ts: 2.672000
ret:-1 st: 0 flags:1 ts: 1.566000
ret: 0 st:-1 flags:0 ts: 0.460008
ret: 0 st: 0 flags:1 dts:4160806.587000 pts:4160806.587000 pos: 3883 size: 116
ret: 0 st: 0 flags:1 dts:12581.487000 pts:12581.487000 pos: 5822 size: 916
ret: 0 st:-1 flags:1 ts:-0.645825
ret: 0 st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos: 271 size: 556

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