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PLC fixes

Fixed an off-by-one in the handling of the IIR filter memory and
disabled "TDAC blending" at the beginning of a lost packet until it
can be made to work properly.

--- a/libcelt/celt.c

+++ b/libcelt/celt.c

@@ -1433,6 +1433,8 @@

          _celt_lpc(lpc+c*LPC_ORDER, ac, LPC_ORDER);

       }

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

+         mem[i] = out_mem[c][MAX_PERIOD-1-i];

       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) */

@@ -1465,7 +1467,7 @@

          S1 += SHR32(MULT16_16(out_mem[c][offset+i],out_mem[c][offset+i]),8);

       }

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

-         mem[i] = out_mem[c][MAX_PERIOD-i];

+         mem[i] = out_mem[c][MAX_PERIOD-1-i];

       for (i=0;i<len+st->mode->overlap;i++)

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

       iir(e, lpc+c*LPC_ORDER, e, len+st->mode->overlap, LPC_ORDER, mem);

@@ -1515,8 +1517,13 @@

          out_mem[c][MAX_PERIOD-N+i] += MULT16_32_Q15(st->mode->window[i], tmp1);

          out_mem[c][MAX_PERIOD-N+overlap-i-1] -= MULT16_32_Q15(st->mode->window[overlap-i-1], tmp1);

       }

+#if 0

       for (i=0;i<N-overlap;i++)

          out_mem[c][MAX_PERIOD-N+overlap+i] = e[overlap+i];

+#else

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

+         out_mem[c][MAX_PERIOD-N+i] = e[i];

+#endif

 #ifdef ENABLE_POSTFILTER

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