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Better comments in the PLC

--- a/celt/celt_decoder.c

+++ b/celt/celt_decoder.c

@@ -441,6 +441,8 @@

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

             exc[i] = ROUND16(out_mem[c][i], SIG_SHIFT);

+         /* Compute LPC coefficients for the last MAX_PERIOD samples before the loss so we can

+            work in the excitation-filter domain */

          if (st->loss_count == 0)

          {

             _celt_autocorr(exc, ac, mode->window, overlap,

@@ -468,9 +470,12 @@

          /* Samples just before the beginning of exc  */

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

             mem[i] = ROUND16(out_mem[c][-1-i], SIG_SHIFT);

+         /* Compute the excitation for MAX_PERIOD samples before the loss */

          celt_fir(exc, lpc+c*LPC_ORDER, exc, MAX_PERIOD, LPC_ORDER, mem);

-         /*for (i=0;i<MAX_PERIOD;i++)printf("%d ", exc[i]); printf("\n");*/

-         /* Check if the waveform is decaying (and if so how fast) */

+

+         /* Check if the waveform is decaying (and if so how fast)

+            We do this to avoid adding energy when concealing in a segment

+            with decaying energy */

          {

             opus_val32 E1=1, E2=1;

             int period;

@@ -489,10 +494,10 @@

             attenuation = decay;

          }

-         /* Move everything one frame to the left */

+         /* Move memory one frame to the left */

          OPUS_MOVE(decode_mem[c], decode_mem[c]+N, DECODE_BUFFER_SIZE-N+overlap);

-         /* Copy excitation, taking decay into account */

+         /* Extrapolate excitation with the right period, taking decay into account */

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

          {

             opus_val16 tmp;

@@ -505,13 +510,18 @@

             tmp = ROUND16(out_mem[c][-N+offset+i],SIG_SHIFT);

             S1 += SHR32(MULT16_16(tmp,tmp),8);

          }

-         /* Last samples correctly decoded */

+

+         /* Last samples correctly decoded so we can have a continuous signal */

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

             mem[i] = ROUND16(out_mem[c][MAX_PERIOD-N-1-i], SIG_SHIFT);

+         /* Apply the fading if not the first loss */

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

             e[i] = MULT16_32_Q15(fade, e[i]);

+         /* Synthesis filter -- back in the signal domain */

          celt_iir(e, lpc+c*LPC_ORDER, e, len, LPC_ORDER, mem);

+         /* Check if the synthesis energy is higher than expected, which can

+            happen with the signal changes during our window. If so, attenuate. */

          {

             opus_val32 S2=0;

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

@@ -542,12 +552,14 @@

             }

          }

-         /* Apply pre-filter to the MDCT overlap for the next frame (post-filter will be applied then) */

+         /* Apply pre-filter to the MDCT overlap for the next frame because the

+            post-filter will be re-applied in the decoder after the MDCT overlap */

          comb_filter(etmp, out_mem[c]+MAX_PERIOD, st->postfilter_period, st->postfilter_period, st->overlap,

                -st->postfilter_gain, -st->postfilter_gain, st->postfilter_tapset, st->postfilter_tapset,

                NULL, 0);

-         /* Simulate TDAC on the concealed audio so that it blends with the next frames */

+         /* Simulate TDAC on the concealed audio so that it blends with the

+            MDCT of next frames. */

          for (i=0;i<overlap/2;i++)

          {

             opus_val32 tmp;