diff --git a/services/audioflinger/Threads.h b/services/audioflinger/Threads.h
index 114d43c..8eed50d 100644
--- a/services/audioflinger/Threads.h
+++ b/services/audioflinger/Threads.h
@@ -400,6 +400,8 @@
                 String16 getWakeLockTag();
 
     virtual     void        preExit() { }
+    virtual     void        setMasterMono_l(bool mono __unused) { }
+    virtual     bool        requireMonoBlend() { return false; }
 
     friend class AudioFlinger;      // for mEffectChains
 
@@ -909,6 +911,7 @@
                 //          mFastMixer->sq()    // for mutating and pushing state
                 int32_t     mFastMixerFutex;    // for cold idle
 
+                std::atomic_bool mMasterMono;
 public:
     virtual     bool        hasFastMixer() const { return mFastMixer != 0; }
     virtual     FastTrackUnderruns getFastTrackUnderruns(size_t fastIndex) const {
@@ -916,6 +919,15 @@
                               return mFastMixerDumpState.mTracks[fastIndex].mUnderruns;
                             }
 
+protected:
+    virtual     void       setMasterMono_l(bool mono) {
+                               mMasterMono.store(mono);
+                               if (mFastMixer != nullptr) { /* hasFastMixer() */
+                                   mFastMixer->setMasterMono(mMasterMono);
+                               }
+                           }
+                // the FastMixer performs mono blend if it exists.
+    virtual     bool       requireMonoBlend() { return mMasterMono.load() && !hasFastMixer(); }
 };
 
 class DirectOutputThread : public PlaybackThread {