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Fixing bandwidth selection hysteresis

Previously, the bandwidth detection could (e.g.) change the bandwidth
from fullband to superwideband, and the hysteresis would then cause
bandwidth to be stuck in superwideband.

--- a/src/opus_encoder.c

+++ b/src/opus_encoder.c

@@ -102,6 +102,8 @@

     int          prev_channels;

     int          prev_framesize;

     int          bandwidth;

+    /* Bandwidth determined automatically from the rate (before any other adjustment) */

+    int          auto_bandwidth;

     int          silk_bw_switch;

     /* Sampling rate (at the API level) */

     int          first;

@@ -1477,7 +1479,7 @@

             hysteresis = bandwidth_thresholds[2*(bandwidth-OPUS_BANDWIDTH_MEDIUMBAND)+1];

             if (!st->first)

             {

-                if (st->bandwidth >= bandwidth)

+                if (st->auto_bandwidth >= bandwidth)

                     threshold -= hysteresis;

                 else

                     threshold += hysteresis;

@@ -1485,7 +1487,7 @@

             if (equiv_rate >= threshold)

                 break;

         } while (--bandwidth>OPUS_BANDWIDTH_NARROWBAND);

-        st->bandwidth = bandwidth;

+        st->bandwidth = st->auto_bandwidth = bandwidth;

         /* Prevents any transition to SWB/FB until the SILK layer has fully

            switched to WB mode and turned the variable LP filter off */

         if (!st->first && st->mode != MODE_CELT_ONLY && !st->silk_mode.inWBmodeWithoutVariableLP && st->bandwidth > OPUS_BANDWIDTH_WIDEBAND)