Fixes a SILK bandwidth switching regression

The bug was triggered because f982b84d started using prefill for
SILK bandwidth changes, which reinitialized the encoder state and
prevented the variable lowpass from working properly. To fix the problem,
we preserve the sampling rate and variable low-pass when prefilling.

3 files changed, 22 insertions, 5 deletions

diff --git a/silk/control_audio_bandwidth.c b/silk/control_audio_bandwidth.c
index 4f9bc5cb..f6d22d83 100644
--- a/silk/control_audio_bandwidth.c
+++ b/silk/control_audio_bandwidth.c
@@ -39,9 +39,15 @@ opus_int silk_control_audio_bandwidth(
 )
 {
     opus_int   fs_kHz;
+    opus_int   orig_kHz;
     opus_int32 fs_Hz;
 
-    fs_kHz = psEncC->fs_kHz;
+    orig_kHz = psEncC->fs_kHz;
+    /* Handle a bandwidth-switching reset where we need to be aware what the last sampling rate was. */
+    if( orig_kHz == 0 ) {
+        orig_kHz = psEncC->sLP.saved_fs_kHz;
+    }
+    fs_kHz = orig_kHz;
     fs_Hz = silk_SMULBB( fs_kHz, 1000 );
     if( fs_Hz == 0 ) {
         /* Encoder has just been initialized */
@@ -61,7 +67,7 @@ opus_int silk_control_audio_bandwidth(
         }
         if( psEncC->allow_bandwidth_switch || encControl->opusCanSwitch ) {
             /* Check if we should switch down */
-            if( silk_SMULBB( psEncC->fs_kHz, 1000 ) > psEncC->desiredInternal_fs_Hz )
+            if( silk_SMULBB( orig_kHz, 1000 ) > psEncC->desiredInternal_fs_Hz )
             {
                 /* Switch down */
                 if( psEncC->sLP.mode == 0 ) {
@@ -76,7 +82,7 @@ opus_int silk_control_audio_bandwidth(
                     psEncC->sLP.mode = 0;
 
                     /* Switch to a lower sample frequency */
-                    fs_kHz = psEncC->fs_kHz == 16 ? 12 : 8;
+                    fs_kHz = orig_kHz == 16 ? 12 : 8;
                 } else {
                    if( psEncC->sLP.transition_frame_no <= 0 ) {
                        encControl->switchReady = 1;
@@ -90,12 +96,12 @@ opus_int silk_control_audio_bandwidth(
             }
             else
             /* Check if we should switch up */
-            if( silk_SMULBB( psEncC->fs_kHz, 1000 ) < psEncC->desiredInternal_fs_Hz )
+            if( silk_SMULBB( orig_kHz, 1000 ) < psEncC->desiredInternal_fs_Hz )
             {
                 /* Switch up */
                 if( encControl->opusCanSwitch ) {
                     /* Switch to a higher sample frequency */
-                    fs_kHz = psEncC->fs_kHz == 8 ? 12 : 16;
+                    fs_kHz = orig_kHz == 8 ? 12 : 16;
 
                     /* New transition */
                     psEncC->sLP.transition_frame_no = 0;
diff --git a/silk/enc_API.c b/silk/enc_API.c
index 7ae31a9e..55a33f37 100644
--- a/silk/enc_API.c
+++ b/silk/enc_API.c
@@ -200,15 +200,25 @@ opus_int silk_Encode(                                   /* O    Returns error co
     tot_blocks = ( nBlocksOf10ms > 1 ) ? nBlocksOf10ms >> 1 : 1;
     curr_block = 0;
     if( prefillFlag ) {
+        silk_LP_state save_LP;
         /* Only accept input length of 10 ms */
         if( nBlocksOf10ms != 1 ) {
             celt_assert( 0 );
             RESTORE_STACK;
             return SILK_ENC_INPUT_INVALID_NO_OF_SAMPLES;
         }
+        if ( prefillFlag == 2 ) {
+            save_LP = psEnc->state_Fxx[ 0 ].sCmn.sLP;
+            /* Save the sampling rate so the bandwidth switching code can keep handling transitions. */
+            save_LP.saved_fs_kHz = psEnc->state_Fxx[ 0 ].sCmn.fs_kHz;
+        }
         /* Reset Encoder */
         for( n = 0; n < encControl->nChannelsInternal; n++ ) {
             ret = silk_init_encoder( &psEnc->state_Fxx[ n ], psEnc->state_Fxx[ n ].sCmn.arch );
+            /* Restore the variable LP state. */
+            if ( prefillFlag == 2 ) {
+                psEnc->state_Fxx[ n ].sCmn.sLP = save_LP;
+            }
             celt_assert( !ret );
         }
         tmp_payloadSize_ms = encControl->payloadSize_ms;
diff --git a/silk/structs.h b/silk/structs.h
index 4ff590b0..3380c757 100644
--- a/silk/structs.h
+++ b/silk/structs.h
@@ -78,6 +78,7 @@ typedef struct {
     opus_int32                   In_LP_State[ 2 ];           /* Low pass filter state */
     opus_int32                   transition_frame_no;        /* Counter which is mapped to a cut-off frequency */
     opus_int                     mode;                       /* Operating mode, <0: switch down, >0: switch up; 0: do nothing           */
+    opus_int32                   saved_fs_kHz;               /* If non-zero, holds the last sampling rate before a bandwidth switching reset. */
 } silk_LP_state;
 
 /* Structure containing NLSF codebook */