Re-tuning the use of LTP scaling

Making LTP scaling depend on the bitrate and whether FEC is on.
The thresholds for scaling 1 and 2 are now independent.

silk/fixed/LTP_scale_ctrl_FIX.c

@@ -43,8 +43,13 @@ void silk_LTP_scale_ctrl_FIX(
     if( condCoding == CODE_INDEPENDENTLY ) {
         /* Only scale if first frame in packet */
         round_loss = psEnc->sCmn.PacketLoss_perc + psEnc->sCmn.nFramesPerPacket;
-        psEnc->sCmn.indices.LTP_scaleIndex = (opus_int8)silk_LIMIT(
-            silk_SMULWB( silk_SMULBB( round_loss, psEncCtrl->LTPredCodGain_Q7 ), SILK_FIX_CONST( 0.1, 9 ) ), 0, 2 );
+        if ( psEnc->sCmn.LBRR_flag ) {
+            /* LBRR reduces the effective loss. In practice, it does not square the loss because
+               losses aren't independent, but that still seems to work best. We also never go below 2%. */
+            round_loss = 2 + silk_SMULBB( round_loss, round_loss ) / 100;
+        }
+        psEnc->sCmn.indices.LTP_scaleIndex = silk_SMULBB( psEncCtrl->LTPredCodGain_Q7, round_loss ) > silk_log2lin( 128*7 + 2900-psEnc->sCmn.SNR_dB_Q7 );
+        psEnc->sCmn.indices.LTP_scaleIndex += silk_SMULBB( psEncCtrl->LTPredCodGain_Q7, round_loss ) > silk_log2lin( 128*7 + 3900-psEnc->sCmn.SNR_dB_Q7 );
     } else {
         /* Default is minimum scaling */
         psEnc->sCmn.indices.LTP_scaleIndex = 0;

silk/float/LTP_scale_ctrl_FLP.c

@@ -42,7 +42,13 @@ void silk_LTP_scale_ctrl_FLP(
     if( condCoding == CODE_INDEPENDENTLY ) {
         /* Only scale if first frame in packet */
         round_loss = psEnc->sCmn.PacketLoss_perc + psEnc->sCmn.nFramesPerPacket;
-        psEnc->sCmn.indices.LTP_scaleIndex = (opus_int8)silk_LIMIT( round_loss * psEncCtrl->LTPredCodGain * 0.1f, 0.0f, 2.0f );
+        if ( psEnc->sCmn.LBRR_flag ) {
+            /* LBRR reduces the effective loss. In practice, it does not square the loss because
+               losses aren't independent, but that still seems to work best. We also never go below 2%. */
+            round_loss = 2 + silk_SMULBB( round_loss, round_loss) / 100;
+        }
+        psEnc->sCmn.indices.LTP_scaleIndex = silk_SMULBB( psEncCtrl->LTPredCodGain, round_loss ) > silk_log2lin( 2900 - psEnc->sCmn.SNR_dB_Q7 );
+        psEnc->sCmn.indices.LTP_scaleIndex += silk_SMULBB( psEncCtrl->LTPredCodGain, round_loss ) > silk_log2lin( 3900 - psEnc->sCmn.SNR_dB_Q7 );
     } else {
         /* Default is minimum scaling */
         psEnc->sCmn.indices.LTP_scaleIndex = 0;