Apply an all-pass on the existing output when upsampling ambisonics

Alc/alu.cpp

@@ -128,8 +128,8 @@ inline HrtfDirectMixerFunc SelectHrtfMixer(void)
 void ProcessHrtf(ALCdevice *device, ALsizei SamplesToDo)
 {
     if(AmbiUpsampler *ambiup{device->AmbiUp.get()})
-        ambiup->process(device->Dry.Buffer, device->FOAOut.Buffer, device->FOAOut.NumChannels,
-            SamplesToDo);
+        ambiup->process(device->Dry.Buffer, device->Dry.NumChannels, device->FOAOut.Buffer,
+            device->FOAOut.NumChannels, SamplesToDo);
 
     /* HRTF is stereo output only. */
     const int lidx{(device->RealOut.ChannelName[0]==FrontLeft) ? 0 : 1};
@@ -147,16 +147,16 @@ void ProcessAmbiDec(ALCdevice *device, ALsizei SamplesToDo)
 {
     BFormatDec *ambidec{device->AmbiDecoder.get()};
     if(device->Dry.Buffer != device->FOAOut.Buffer)
-        ambidec->upSample(device->Dry.Buffer, device->FOAOut.Buffer, device->FOAOut.NumChannels,
-            SamplesToDo);
+        ambidec->upSample(device->Dry.Buffer, device->Dry.NumChannels, device->FOAOut.Buffer,
+            device->FOAOut.NumChannels, SamplesToDo);
     ambidec->process(device->RealOut.Buffer, device->RealOut.NumChannels, device->Dry.Buffer,
         SamplesToDo);
 }
 
 void ProcessAmbiUp(ALCdevice *device, ALsizei SamplesToDo)
 {
-    device->AmbiUp->process(device->RealOut.Buffer, device->FOAOut.Buffer,
-        device->FOAOut.NumChannels, SamplesToDo);
+    device->AmbiUp->process(device->RealOut.Buffer, device->RealOut.NumChannels,
+        device->FOAOut.Buffer, device->FOAOut.NumChannels, SamplesToDo);
 }
 
 void ProcessUhj(ALCdevice *device, ALsizei SamplesToDo)

Alc/bformatdec.cpp

@@ -171,6 +171,9 @@ void BFormatDec::reset(const ALsizei inchans, const ALfloat xover_norm, const AL
         mUpsampler[1].Gains[HF_BAND] = Ambi3DDecoderHFScale[1] / hfscales[1];
         mUpsampler[1].Gains[LF_BAND] = 1.0f;
         std::fill(std::begin(mUpsampler)+2, std::end(mUpsampler), mUpsampler[1]);
+
+        mUpAllpass[0].init(xover_norm);
+        std::fill(std::begin(mUpAllpass)+1, std::end(mUpAllpass), mUpAllpass[0]);
     }
 
     for(ALsizei i{0};i < chancount;i++)
@@ -220,9 +223,10 @@ void BFormatDec::process(ALfloat (*OutBuffer)[BUFFERSIZE], const ALsizei OutChan
     }
 }
 
-void BFormatDec::upSample(ALfloat (*OutBuffer)[BUFFERSIZE], const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo)
+void BFormatDec::upSample(ALfloat (*OutBuffer)[BUFFERSIZE], const ALsizei OutChannels, const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo)
 {
     ASSUME(InChannels > 0);
+    ASSUME(OutChannels > InChannels);
 
     /* This up-sampler leverages the differences observed in dual-band higher-
      * order decoder matrices compared to first-order. For the same output
@@ -236,11 +240,21 @@ void BFormatDec::upSample(ALfloat (*OutBuffer)[BUFFERSIZE], const ALfloat (*InSa
      */
     for(ALsizei i{0};i < InChannels;i++)
     {
+        /* NOTE: Because we can't treat the first-order signal as completely
+         * decorrelated from the existing output (it may contain the reverb,
+         * echo, etc, portion) phase interference is a possibility if not kept
+         * coherent. As such, we need to apply an all-pass on the existing
+         * output so that it stays aligned with the upsampled signal.
+         */
+        mUpAllpass[i].process(OutBuffer[i], SamplesToDo);
+
         mUpsampler[i].Splitter.process(mSamples[HF_BAND].data(), mSamples[LF_BAND].data(),
             InSamples[i], SamplesToDo);
         MixRowSamples(OutBuffer[i], mUpsampler[i].Gains,
             &reinterpret_cast<ALfloat(&)[BUFFERSIZE]>(mSamples[0]), sNumBands, 0, SamplesToDo);
     }
+    for(ALsizei i{InChannels};i < OutChannels;i++)
+        mUpAllpass[i].process(OutBuffer[i], SamplesToDo);
 }
 
 
@@ -255,16 +269,23 @@ void AmbiUpsampler::reset(const ALsizei out_order, const ALfloat xover_norm)
     mInput[1].Gains[HF_BAND] = Ambi3DDecoderHFScale[1] / hfscales[1];
     mInput[1].Gains[LF_BAND] = 1.0f;
     std::fill(std::begin(mInput)+2, std::end(mInput), mInput[1]);
+
+    mAllpass[0].init(xover_norm);
+    std::fill(std::begin(mAllpass)+1, std::end(mAllpass), mAllpass[0]);
 }
 
-void AmbiUpsampler::process(ALfloat (*OutBuffer)[BUFFERSIZE], const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo)
+void AmbiUpsampler::process(ALfloat (*OutBuffer)[BUFFERSIZE], const ALsizei OutChannels, const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo)
 {
     ASSUME(InChannels > 0);
+    ASSUME(OutChannels > InChannels);
 
     for(ALsizei i{0};i < InChannels;i++)
     {
+        mAllpass[i].process(OutBuffer[i], SamplesToDo);
         mInput[i].Splitter.process(mSamples[HF_BAND], mSamples[LF_BAND], InSamples[i],
             SamplesToDo);
         MixRowSamples(OutBuffer[i], mInput[i].Gains, mSamples, sNumBands, 0, SamplesToDo);
     }
+    for(ALsizei i{InChannels};i < OutChannels;i++)
+        mAllpass[i].process(OutBuffer[i], SamplesToDo);
 }

Alc/bformatdec.h

@@ -36,6 +36,7 @@ private:
         BandSplitter Splitter;
         ALfloat Gains[sNumBands];
     } mUpsampler[4];
+    SplitterAllpass mUpAllpass[MAX_OUTPUT_CHANNELS];
 
     ALsizei mNumChannels;
     ALboolean mDualBand;
@@ -49,7 +50,7 @@ public:
     void process(ALfloat (*OutBuffer)[BUFFERSIZE], const ALsizei OutChannels, const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei SamplesToDo);
 
     /* Up-samples a first-order input to the decoder's configuration. */
-    void upSample(ALfloat (*OutBuffer)[BUFFERSIZE], const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo);
+    void upSample(ALfloat (*OutBuffer)[BUFFERSIZE], const ALsizei OutChannels, const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo);
 
     DEF_NEWDEL(BFormatDec)
 };
@@ -60,15 +61,17 @@ public:
  */
 class AmbiUpsampler {
     static constexpr ALsizei sNumBands{2};
+
+    alignas(16) ALfloat mSamples[sNumBands][BUFFERSIZE];
     struct {
         BandSplitter Splitter;
         ALfloat Gains[sNumBands];
     } mInput[4];
-    alignas(16) ALfloat mSamples[sNumBands][BUFFERSIZE];
+    SplitterAllpass mAllpass[MAX_OUTPUT_CHANNELS];
 
 public:
     void reset(const ALsizei out_order, const ALfloat xover_norm);
-    void process(ALfloat (*OutBuffer)[BUFFERSIZE], const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo);
+    void process(ALfloat (*OutBuffer)[BUFFERSIZE], const ALsizei OutChannels, const ALfloat (*InSamples)[BUFFERSIZE], const ALsizei InChannels, const ALsizei SamplesToDo);
 
     DEF_NEWDEL(AmbiUpsampler)
 };