Adjust VBR mode depending on given peak bitrate and fix crash recovery usage.

Operating the FDK encoder in A2DP with variable bitrate mode
configuration resulted in unexpected encoder return value AACENC_ENCODE_ERROR.

Due to peak bitrate restriction the encoder quite often runs into requantization.
In case the bitrate constraint is not fulfilled, the so-called crash recovery is
used as final emergency step. The crash recovery reduces the overall bit
consumption considering the given number of bits to be saved. The bit difference
is extracted from audio element structures. In VBR mode the element wise bit
consumption state was not updated since there is typically no bitrate limitation
required.

The patch solves the choppy audio problems and increases audio quality for
AAC VBR encoding.

The changes in FDKaacEnc_QCMain() ensure that audio element bit info is always
updated. This is achieved by always calling FDKaacEnc_BitResRedistribution()
and FDKaacEnc_prepareBitDistribution() with maxBitsPerFrame variable as total
bits parameter.
Furthermore, VBR assumes a certain target bitrate which is used for internal
configuration and limitations. In case the peak bitrate parameter is less than
the VBR mode target bitrate the maximum of both bitrate configurations was used.

The function FDKaacEnc_AdjustVBRBitrateMode() is added to adjust the encoder
internal VBR mode to a desired target bitrate less than given peak bitrate.
It is possible that the peak bitrate is very close to the desired target bitrate.
The virtual available bitreservoir is quite low and the encoder would run quite
often into requantization with needless audio quality reduction. In such a
configuration, it is a better choice to use the CBR targeted threshold adaption
to avoid audio quality reduction.

In FDKaacEnc_Initialize(), there was already a bitResMode selection depending
on available bitreservoir for CBR. This selection will now also be used for VBR.
In case the bitResMode is AACENC_BR_MODE_FULL and VBR mode is selected, the VBR
threshold adaption is used. Otherwise, CBR threshold adaption strategy is used
and therefore, no unnecessary fill bits are written.

Bug: 161400526
Test: see bug
Change-Id: I1865f817180150da6add2623a64f1a102622784a

4 files changed, 83 insertions, 36 deletions

diff --git a/libAACenc/src/aacenc.cpp b/libAACenc/src/aacenc.cpp
index b6f733d..1af8a2e 100644
--- a/libAACenc/src/aacenc.cpp
+++ b/libAACenc/src/aacenc.cpp
@@ -1,7 +1,7 @@
 /* -----------------------------------------------------------------------------
 Software License for The Fraunhofer FDK AAC Codec Library for Android
 
-© Copyright  1995 - 2019 Fraunhofer-Gesellschaft zur Förderung der angewandten
+© Copyright  1995 - 2020 Fraunhofer-Gesellschaft zur Förderung der angewandten
 Forschung e.V. All rights reserved.
 
  1.    INTRODUCTION
@@ -245,6 +245,46 @@ INT FDKaacEnc_GetVBRBitrate(AACENC_BITRATE_MODE bitrateMode,
   return bitrate;
 }
 
+/*-----------------------------------------------------------------------------
+
+    functionname: FDKaacEnc_AdjustVBRBitrateMode
+    description:  Adjust bitrate mode to given bitrate parameter
+    input params: int vbrQuality (VBR0, VBR1, VBR2)
+                  bitrate
+                  channelMode
+    returns:      vbr bitrate mode
+
+ ------------------------------------------------------------------------------*/
+AACENC_BITRATE_MODE FDKaacEnc_AdjustVBRBitrateMode(
+    AACENC_BITRATE_MODE bitrateMode, INT bitrate, CHANNEL_MODE channelMode) {
+  AACENC_BITRATE_MODE newBitrateMode = bitrateMode;
+
+  if (bitrate != -1) {
+    const INT monoStereoMode =
+        (FDKaacEnc_GetMonoStereoMode(channelMode) == EL_MODE_STEREO) ? 1 : 0;
+    const INT nChannelsEff =
+        FDKaacEnc_GetChannelModeConfiguration(channelMode)->nChannelsEff;
+    newBitrateMode = AACENC_BR_MODE_INVALID;
+
+    for (int idx = (int)(sizeof(configTabVBR) / sizeof(*configTabVBR)) - 1;
+         idx >= 0; idx--) {
+      if (bitrate >=
+          configTabVBR[idx].chanBitrate[monoStereoMode] * nChannelsEff) {
+        if (configTabVBR[idx].chanBitrate[monoStereoMode] * nChannelsEff <
+            FDKaacEnc_GetVBRBitrate(bitrateMode, channelMode)) {
+          newBitrateMode = configTabVBR[idx].bitrateMode;
+        } else {
+          newBitrateMode = bitrateMode;
+        }
+        break;
+      }
+    }
+  }
+
+  return AACENC_BR_MODE_IS_VBR(newBitrateMode) ? newBitrateMode
+                                               : AACENC_BR_MODE_INVALID;
+}
+
 /**
  * \brief  Convert encoder bitreservoir value for transport library.
  *
@@ -397,7 +437,6 @@ AAC_ENCODER_ERROR FDKaacEnc_Initialize(
   FIXP_DBL mbfac, bw_ratio;
   QC_INIT qcInit;
   INT averageBitsPerFrame = 0;
-  int bitresMin = 0; /* the bitreservoir is always big for AAC-LC */
   const CHANNEL_MODE prevChannelMode = hAacEnc->encoderMode;
 
   if (config == NULL) return AAC_ENC_INVALID_HANDLE;
@@ -553,7 +592,6 @@ AAC_ENCODER_ERROR FDKaacEnc_Initialize(
     qcInit.minBits = fixMin(qcInit.minBits, averageBitsPerFrame & ~7);
   } else {
     INT bitreservoir = -1; /* default bitreservoir size*/
-    bitresMin = BITRES_MIN;
     if (isLowDelay(config->audioObjectType)) {
       INT brPerChannel = config->bitRate / config->nChannels;
       brPerChannel = fMin(BITRATE_MAX_LD, fMax(BITRATE_MIN_LD, brPerChannel));
@@ -567,7 +605,6 @@ AAC_ENCODER_ERROR FDKaacEnc_Initialize(
       bitreservoir = fMultI(slope, (INT)(BITRES_MAX_LD - BITRES_MIN_LD)) +
                      BITRES_MIN_LD;     /* interpolate */
       bitreservoir = bitreservoir & ~7; /* align to bytes */
-      bitresMin = BITRES_MIN_LD;
     }
 
     int maxBitres;
@@ -604,7 +641,8 @@ AAC_ENCODER_ERROR FDKaacEnc_Initialize(
   qcInit.nSubFrames = config->nSubFrames;
   qcInit.padding.paddingRest = config->sampleRate;
 
-  if (qcInit.maxBits - qcInit.averageBits >= bitresMin * config->nChannels) {
+  if (qcInit.maxBits - qcInit.averageBits >=
+      ((qcInit.isLowDelay) ? BITRES_MIN_LD : BITRES_MIN) * config->nChannels) {
     qcInit.bitResMode = AACENC_BR_MODE_FULL; /* full bitreservoir */
   } else if (qcInit.maxBits > qcInit.averageBits) {
     qcInit.bitResMode = AACENC_BR_MODE_REDUCED; /* reduced bitreservoir */
diff --git a/libAACenc/src/aacenc.h b/libAACenc/src/aacenc.h
index 0e0d8c1..b7e0ef2 100644
--- a/libAACenc/src/aacenc.h
+++ b/libAACenc/src/aacenc.h
@@ -1,7 +1,7 @@
 /* -----------------------------------------------------------------------------
 Software License for The Fraunhofer FDK AAC Codec Library for Android
 
-© Copyright  1995 - 2018 Fraunhofer-Gesellschaft zur Förderung der angewandten
+© Copyright  1995 - 2020 Fraunhofer-Gesellschaft zur Förderung der angewandten
 Forschung e.V. All rights reserved.
 
  1.    INTRODUCTION
@@ -335,6 +335,19 @@ INT FDKaacEnc_GetVBRBitrate(AACENC_BITRATE_MODE bitrateMode,
 
 /*-----------------------------------------------------------------------------
 
+    functionname: FDKaacEnc_AdjustVBRBitrateMode
+    description:  Adjust bitrate mode to given bitrate parameter
+    input params: int vbrQuality (VBR0, VBR1, VBR2)
+                  bitrate
+                  channelMode
+    returns:      vbr bitrate mode
+
+ ------------------------------------------------------------------------------*/
+AACENC_BITRATE_MODE FDKaacEnc_AdjustVBRBitrateMode(
+    AACENC_BITRATE_MODE bitrateMode, INT bitrate, CHANNEL_MODE channelMode);
+
+/*-----------------------------------------------------------------------------
+
      functionname: FDKaacEnc_AacInitDefaultConfig
      description:  gives reasonable default configuration
      returns:      ---
diff --git a/libAACenc/src/aacenc_lib.cpp b/libAACenc/src/aacenc_lib.cpp
index c3977f3..0ae329b 100644
--- a/libAACenc/src/aacenc_lib.cpp
+++ b/libAACenc/src/aacenc_lib.cpp
@@ -1028,6 +1028,13 @@ static AACENC_ERROR FDKaacEnc_AdjustEncSettings(HANDLE_AACENCODER hAacEncoder,
     case AACENC_BR_MODE_VBR_3:
     case AACENC_BR_MODE_VBR_4:
     case AACENC_BR_MODE_VBR_5:
+      /* Adjust bitrate mode in case given peak bitrate is lower than expected
+       * VBR bitrate. */
+      if ((INT)config->userPeakBitrate != -1) {
+        hAacConfig->bitrateMode = FDKaacEnc_AdjustVBRBitrateMode(
+            hAacConfig->bitrateMode, config->userPeakBitrate,
+            hAacConfig->channelMode);
+      }
       /* Get bitrate in VBR configuration */
       /* In VBR mode; SBR-modul depends on bitrate, core encoder on bitrateMode.
        */
diff --git a/libAACenc/src/qc_main.cpp b/libAACenc/src/qc_main.cpp
index bcfaa23..9a42550 100644
--- a/libAACenc/src/qc_main.cpp
+++ b/libAACenc/src/qc_main.cpp
@@ -1,7 +1,7 @@
 /* -----------------------------------------------------------------------------
 Software License for The Fraunhofer FDK AAC Codec Library for Android
 
-© Copyright  1995 - 2019 Fraunhofer-Gesellschaft zur Förderung der angewandten
+© Copyright  1995 - 2020 Fraunhofer-Gesellschaft zur Förderung der angewandten
 Forschung e.V. All rights reserved.
 
  1.    INTRODUCTION
@@ -373,13 +373,8 @@ AAC_ENCODER_ERROR FDKaacEnc_QCInit(QC_STATE* hQC, struct QC_INIT* init,
   hQC->invQuant = init->invQuant;
   hQC->maxIterations = init->maxIterations;
 
-  if (isConstantBitrateMode(hQC->bitrateMode)) {
-    /* 0: full bitreservoir, 1: reduced bitreservoir, 2: disabled bitreservoir
-     */
-    hQC->bitResMode = init->bitResMode;
-  } else {
-    hQC->bitResMode = AACENC_BR_MODE_FULL; /* full bitreservoir */
-  }
+  /* 0: full bitreservoir, 1: reduced bitreservoir, 2: disabled bitreservoir */
+  hQC->bitResMode = init->bitResMode;
 
   hQC->padding.paddingRest = init->padding.paddingRest;
 
@@ -800,10 +795,15 @@ AAC_ENCODER_ERROR FDKaacEnc_QCMain(QC_STATE* RESTRICT hQC, PSY_OUT** psyOut,
   INT avgTotalDynBits = 0; /* maximal allowed dynamic bits for all frames */
   INT totalAvailableBits = 0;
   INT nSubFrames = 1;
+  const INT isCBRAdjustment = (isConstantBitrateMode(hQC->bitrateMode) ||
+                               (hQC->bitResMode != AACENC_BR_MODE_FULL))
+                                  ? 1
+                                  : 0;
 
   /*-------------------------------------------- */
   /* redistribute total bitreservoir to elements */
-  ErrorStatus = FDKaacEnc_BitResRedistribution(hQC, cm, avgTotalBits);
+  ErrorStatus = FDKaacEnc_BitResRedistribution(
+      hQC, cm, (isCBRAdjustment == 0) ? hQC->maxBitsPerFrame : avgTotalBits);
   if (ErrorStatus != AAC_ENC_OK) {
     return ErrorStatus;
   }
@@ -831,33 +831,22 @@ AAC_ENCODER_ERROR FDKaacEnc_QCMain(QC_STATE* RESTRICT hQC, PSY_OUT** psyOut,
 
   /*-------------------------------------------- */
   /*-------------------------------------------- */
-  if (isConstantBitrateMode(hQC->bitrateMode)) {
-    /* calc granted dynamic bits for sub frame and
-       distribute it to each element */
-    ErrorStatus = FDKaacEnc_prepareBitDistribution(
-        hQC, psyOut, qcOut, cm, qcElement, avgTotalBits, &totalAvailableBits,
-        &avgTotalDynBits);
-
-    if (ErrorStatus != AAC_ENC_OK) {
-      return ErrorStatus;
-    }
-  } else {
-    qcOut[0]->grantedDynBits =
-        ((hQC->maxBitsPerFrame - (hQC->globHdrBits)) & ~7) -
-        (qcOut[0]->globalExtBits + qcOut[0]->staticBits +
-         qcOut[0]->elementExtBits);
-    qcOut[0]->maxDynBits = qcOut[0]->grantedDynBits;
-
-    totalAvailableBits = hQC->maxBitsPerFrame;
-    avgTotalDynBits = 0;
+  /* calc granted dynamic bits for sub frame and
+     distribute it to each element */
+  ErrorStatus = FDKaacEnc_prepareBitDistribution(
+      hQC, psyOut, qcOut, cm, qcElement,
+      (isCBRAdjustment == 0) ? hQC->maxBitsPerFrame : avgTotalBits,
+      &totalAvailableBits, &avgTotalDynBits);
+
+  if (ErrorStatus != AAC_ENC_OK) {
+    return ErrorStatus;
   }
 
   /* for ( all sub frames ) ... */
   for (c = 0; c < nSubFrames; c++) {
     /* for CBR and VBR mode */
     FDKaacEnc_AdjustThresholds(hQC->hAdjThr, qcElement[c], qcOut[c],
-                               psyOut[c]->psyOutElement,
-                               isConstantBitrateMode(hQC->bitrateMode), cm);
+                               psyOut[c]->psyOutElement, isCBRAdjustment, cm);
 
   } /* -end- sub frame counter */