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More HRTF enhancements

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- a passive locking mechanism to enable the matrix to switch between active
and passive mode, which enhances the stereo image.

- a center front cancellation algorithm that damps the cross-talk if the
sound is coming predominantly from center (e.g. if there is dialogue).

These two new features should enhance the quality of surround downmix
noticeably.

Also a correction to the active gain control is included. The previous
implementation of Lt + Rt/Lt - Rt AGC should be fine in most cases, but the
calculation was inconsistent (gain unitarity is not guaranteed to be
preserved).

Signed off by Yue Shi Lai <ylai@users.sourceforge.net>


git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@15125 b3059339-0415-0410-9bf9-f77b7e298cf2
This commit is contained in:
henry 2005-04-11 14:01:29 +00:00
parent 7da5685bc3
commit 1fd3c733d8
2 changed files with 56 additions and 21 deletions
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72
libaf/af_hrtf.c
View File

@ -88,6 +88,16 @@ int pulse_detect(float *sx)
return 0;
}
/* Fuzzy matrix coefficient transfer function to "lock" the matrix on
a effectively passive mode if the gain is approximately 1 */
inline float passive_lock(float x)
{
const float x1 = x - 1;
const float ax1s = fabs(x - 1) * (1.0 / MATAGCLOCK);
return x1 - x1 / (1 + ax1s * ax1s) + 1;
}
/* Unified active matrix decoder for 2 channel matrix encoded surround
sources */
inline void matrix_decode(short *in, const int k, const int il,
@ -105,13 +115,22 @@ inline void matrix_decode(short *in, const int k, const int il,
(1 + l_fwr + l_fwr);
float r_gain = (l_fwr + r_fwr) /
(1 + r_fwr + r_fwr);
float lpr_gain = (lpr_fwr + lmr_fwr) /
/* The 2nd axis has strong gain fluctuations, and therefore require
limits. The factor corresponds to the 1 / amplification of (Lt
- Rt) when (Lt, Rt) is strongly correlated. (e.g. during
dialogues). It should be bigger than -12 dB to prevent
distortion. */
float lmr_lim_fwr = lmr_fwr > M9_03DB * lpr_fwr ?
lmr_fwr : M9_03DB * lpr_fwr;
float lpr_gain = (lpr_fwr + lmr_lim_fwr) /
(1 + lpr_fwr + lpr_fwr);
float lmr_gain = (lpr_fwr + lmr_fwr) /
float lmr_gain = (lpr_fwr + lmr_lim_fwr) /
(1 + lmr_lim_fwr + lmr_lim_fwr);
float lmr_unlim_gain = (lpr_fwr + lmr_fwr) /
(1 + lmr_fwr + lmr_fwr);
float lpr, lmr;
float l_agc, r_agc, lpr_agc, lmr_agc;
float f, d_gain;
float f, d_gain, c_gain, c_agc_cfk;
#if 0
static int counter = 0;
@ -119,8 +138,9 @@ inline void matrix_decode(short *in, const int k, const int il,
if(counter == 0)
fp_out = fopen("af_hrtf.log", "w");
fprintf(fp_out, "%g %g %g %g %g ", counter * (1.0 / 48000),
l_gain, r_gain, lpr_gain, lmr_gain);
if(counter % 240 == 0)
fprintf(fp_out, "%g %g %g %g %g ", counter * (1.0 / 48000),
l_gain, r_gain, lpr_gain, lmr_gain);
#endif
/*** AXIS NO. 1: (Lt, Rt) -> (C, Ls, Rs) ***/
@ -132,8 +152,8 @@ inline void matrix_decode(short *in, const int k, const int il,
*adapt_l_gain = (1 - f) * *adapt_l_gain + f * l_gain;
*adapt_r_gain = (1 - f) * *adapt_r_gain + f * r_gain;
/* Matrix */
l_agc = in[il] * *adapt_l_gain;
r_agc = in[ir] * *adapt_r_gain;
l_agc = in[il] * passive_lock(*adapt_l_gain);
r_agc = in[ir] * passive_lock(*adapt_r_gain);
cf[k] = (l_agc + r_agc) * M_SQRT1_2;
if(decode_rear) {
lr[kr] = rr[kr] = (l_agc - r_agc) * M_SQRT1_2;
@ -150,29 +170,39 @@ inline void matrix_decode(short *in, const int k, const int il,
lpr = (in[il] + in[ir]) * M_SQRT1_2;
lmr = (in[il] - in[ir]) * M_SQRT1_2;
/* AGC adaption */
d_gain = (fabs(lpr_gain - *adapt_lpr_gain) +
fabs(lmr_gain - *adapt_lmr_gain)) * 0.5;
d_gain = fabs(lmr_unlim_gain - *adapt_lmr_gain);
f = d_gain * (1.0 / MATAGCTRIG);
f = MATAGCDECAY - MATAGCDECAY / (1 + f * f);
*adapt_lpr_gain = (1 - f) * *adapt_lpr_gain + f * lpr_gain;
*adapt_lmr_gain = (1 - f) * *adapt_lmr_gain + f * lmr_gain;
/* The 2nd axis has strong gain fluctuations, and therefore require
limits. The factor is tricky. I think 2 is the reasonable
value here, which phase inverts the L, R channel if Lt, Rt is
strongly correlated (e.g. during dialogues) (1 would inhibit the
steering behavior, > 4 appears to result in distortions). */
if(*adapt_lmr_gain > 2 * *adapt_lpr_gain)
*adapt_lmr_gain = 2 * *adapt_lpr_gain;
/* Matrix */
lpr_agc = lpr * *adapt_lpr_gain;
lmr_agc = lmr * *adapt_lmr_gain;
lpr_agc = lpr * passive_lock(*adapt_lpr_gain);
lmr_agc = lmr * passive_lock(*adapt_lmr_gain);
lf[k] = (lpr_agc + lmr_agc) * M_SQRT1_2;
rf[k] = (lpr_agc - lmr_agc) * M_SQRT1_2;
/*** CENTER FRONT CANCELLATION ***/
/* A heuristic approach exploits that Lt + Rt gain contains the
information about Lt, Rt correlation. This effectively reshapes
the front and rear "cones" to concentrate Lt + Rt to C and
introduce Lt - Rt in L, R. */
/* 0.67677 is the emprical lower bound for lpr_gain. */
c_gain = 8 * (*adapt_lpr_gain - 0.67677);
c_gain = c_gain > 0 ? c_gain : 0;
/* c_gain should not be too high, not even reaching full
cancellation (~ 0.50 - 0.55 at current AGC implementation), or
the center will s0und too narrow. */
c_gain = MATCOMPGAIN / (1 + c_gain * c_gain);
c_agc_cfk = c_gain * cf[k];
lf[k] -= c_agc_cfk;
rf[k] -= c_agc_cfk;
cf[k] += c_agc_cfk + c_agc_cfk;
#if 0
fprintf(fp_out, "%g %g %g %g\n",
*adapt_l_gain, *adapt_r_gain,
*adapt_lpr_gain, *adapt_lmr_gain);
if(counter % 240 == 0)
fprintf(fp_out, "%g %g %g %g %g\n",
*adapt_l_gain, *adapt_r_gain,
*adapt_lpr_gain, *adapt_lmr_gain,
c_gain);
counter++;
#endif
}

5
libaf/af_hrtf.h
View File

@ -4,6 +4,7 @@
/* Amplitude scaling factors */
#define M17_0DB 0.1414213562
#define M9_03DB 0.3535533906
#define M6_99DB 0.4472135955
#define M4_77DB 0.5773502692
#define M3_01DB 0.7071067812
@ -25,6 +26,10 @@
#define MATAGCTRIG 8.0 /* (Fuzzy) AGC trigger */
#define MATAGCDECAY 1.0 /* AGC baseline decay rate (1/samp.) */
#define MATAGCLOCK 0.2 /* AGC range (around 1) where the
matrix behaves passively */
#define MATCOMPGAIN 0.37 /* Cross talk compensation gain,
0.50 - 0.55 is full cancellation. */
#define CFECHODELAY 360 /* Center front echo delay (samples) */
#define CFECHOAMPL M17_0DB /* Center front echo amplitude */