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Tweaks to the new transient detector

Instead of using the average energy over the frame, we use a half-way
point between the average energy and the max energy.

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -238,6 +238,7 @@

       opus_val32 mean;

       opus_int32 unmask=0;

       opus_val32 norm;

+      opus_val16 maxE;

       mem0=0;

       mem1=0;

       /* High-pass filter: (1 - 2*z^-1 + z^-2) / (1 - z^-1 + .5*z^-2) */

@@ -276,7 +277,7 @@

       mean=0;

       mem0=0;

-      /*  Grouping by two to reduce complexity */

+      /* Grouping by two to reduce complexity */

       /* Forward pass to compute the post-echo threshold*/

       for (i=0;i<len2;i++)

       {

@@ -292,6 +293,7 @@

       }

       mem0=0;

+      maxE=0;

       /* Backward pass to compute the pre-echo threshold */

       for (i=len2-1;i>=0;i--)

       {

@@ -302,13 +304,22 @@

          tmp[i] = mem0 + MULT16_16_P15(QCONST16(0.125f,15),tmp[i]-mem0);

 #endif

          mem0 = tmp[i];

+         maxE = MAX16(maxE, mem0);

       }

-      /*for (i=0;i<len;i++)printf("%f ", tmp[i]/mean);printf("\n");*/

+      /*for (i=0;i<len2;i++)printf("%f ", tmp[i]/mean);printf("\n");*/

-      /* Compute the ratio of the mean energy over the harmonic mean of the energy.

+      /* Compute the ratio of the "frame energy" over the harmonic mean of the energy.

          This essentially corresponds to a bitrate-normalized temporal noise-to-mask

          ratio */

+      /* As a compromise with the old transient detector, frame energy is the

+         geometric mean of the energy and half the max */

+#ifdef FIXED_POINT

+      /* Costs two sqrt() to avoid overflows */

+      mean = MULT16_16(celt_sqrt(mean), celt_sqrt(MULT16_16(maxE,len2>>1)));

+#else

+      mean = sqrt(mean * maxE*.5*len2);

+#endif

       /* Inverse of the mean energy in Q15+6 */

       norm = SHL32(EXTEND32(len2),6+14)/ADD32(EPSILON,SHR32(mean,1));

       /* Compute harmonic mean discarding the unreliable boundaries

@@ -333,10 +344,10 @@

          mask_metric = unmask;

       }

    }

-   is_transient = mask_metric>141;

+   is_transient = mask_metric>200;

    /* Arbitrary metric for VBR boost */

-   tf_max = MAX16(0,celt_sqrt(64*mask_metric)-64);

+   tf_max = MAX16(0,celt_sqrt(27*mask_metric)-42);

    /* *tf_estimate = 1 + MIN16(1, sqrt(MAX16(0, tf_max-30))/20); */

    *tf_estimate = QCONST16(1.f, 14) + celt_sqrt(MAX16(0, SHL32(MULT16_16(QCONST16(0.0069,14),IMIN(163,tf_max)),14)-QCONST32(0.139,28)));

    /*printf("%d %f\n", tf_max, mask_metric);*/