AAC encoder bitrate limitation

AAC encoder: Make the bit rate limiting functionality more stable for multichannel
     configurations where element bit rate for SBR can differ significantly.

Bug 9428126

Change-Id: I35b134c5b3c160a8f5a16b4314782b731fe49cd8

libAACenc/src/aacenc_lib.cpp

@@ -98,7 +98,7 @@ amm-info@iis.fraunhofer.de
 /* Encoder library info */
 #define AACENCODER_LIB_VL0 3
 #define AACENCODER_LIB_VL1 4
-#define AACENCODER_LIB_VL2 4
+#define AACENCODER_LIB_VL2 5
 #define AACENCODER_LIB_TITLE "AAC Encoder"
 #define AACENCODER_LIB_BUILD_DATE __DATE__
 #define AACENCODER_LIB_BUILD_TIME __TIME__
@@ -525,14 +525,20 @@ INT aacEncoder_LimitBitrate(
   /* Limit bit rate in respect to available SBR modes if active */
   if (sbrActive)
   {
-    SBR_ELEMENT_INFO sbrElInfo[6];
-    INT sbrBitRate = 0;
-    int e, tooBig=-1;
+    int numIterations = 0;
+    INT initialBitrate, adjustedBitrate;
+    initialBitrate = adjustedBitrate = bitRate;
 
+    /* Find total bitrate which provides valid configuration for each SBR element. */
+    do {
+      int e;
+      SBR_ELEMENT_INFO sbrElInfo[(6)];
       FDK_ASSERT(cm.nElements <= (6));
 
+      initialBitrate = adjustedBitrate;
+
       /* Get bit rate for each SBR element */
-    aacEncDistributeSbrBits(&cm, sbrElInfo, bitRate);
+      aacEncDistributeSbrBits(&cm, sbrElInfo, initialBitrate);
 
       for (e=0; e<cm.nElements; e++)
       {
@@ -546,35 +552,31 @@ INT aacEncoder_LimitBitrate(
         if (sbrBitRateOut == 0) {
           return 0;
         }
+
+        /* If bitrates don't match, distribution and limiting needs to be determined again.
+           Abort element loop and restart with adapted bitrate. */
+        if (sbrElementBitRateIn != sbrBitRateOut) {
+
           if (sbrElementBitRateIn < sbrBitRateOut) {
-        FDK_ASSERT(tooBig != 1);
-        tooBig = 0;
-        if (e == 0) {
-          sbrBitRate = 0;
-        }
+            adjustedBitrate = fMax(initialBitrate, (INT)fDivNorm((FIXP_DBL)(sbrBitRateOut+8), cm.elInfo[e].relativeBits));
+            break;
           }
+
           if (sbrElementBitRateIn > sbrBitRateOut) {
-        FDK_ASSERT(tooBig != 0);
-        tooBig = 1;
-        if (e == 0) {
-          sbrBitRate = 5000000;
-        }
-      }
-      if (tooBig != -1)
-      {
-        INT sbrBitRateLimit = (INT)fDivNorm((FIXP_DBL)sbrBitRateOut, cm.elInfo[e].relativeBits);
-        if (tooBig) {
-          sbrBitRate = fMin(sbrBitRate, sbrBitRateLimit-16);
-          FDK_ASSERT( (INT)fMultNorm(cm.elInfo[e].relativeBits, (FIXP_DBL)sbrBitRate) < sbrBitRateOut);
-        } else {
-          sbrBitRate = fMax(sbrBitRate, sbrBitRateLimit+16);
-          FDK_ASSERT( (INT)fMultNorm(cm.elInfo[e].relativeBits, (FIXP_DBL)sbrBitRate) >= sbrBitRateOut);
-        }
-      }
-    }
-    if (tooBig != -1) {
-      bitRate = sbrBitRate;
+            adjustedBitrate = fMin(initialBitrate, (INT)fDivNorm((FIXP_DBL)(sbrBitRateOut-8), cm.elInfo[e].relativeBits));
+            break;
           }
+
+        } /* sbrElementBitRateIn != sbrBitRateOut */
+
+      } /* elements */
+
+      numIterations++; /* restrict iteration to worst case of num elements */
+
+    } while ( (initialBitrate!=adjustedBitrate) && (numIterations<=cm.nElements) );
+
+    /* Unequal bitrates mean that no reasonable bitrate configuration found. */
+    bitRate = (initialBitrate==adjustedBitrate) ? adjustedBitrate : 0;
   }
 
   FDK_ASSERT(bitRate > 0);