Extending supported sample rate from 3.0KHz to 192.0KHz for OfflineAudioContext.

Source/platform/audio/HRTFPanner.cpp

Index: Source/platform/audio/HRTFPanner.cpp
diff --git a/Source/platform/audio/HRTFPanner.cpp b/Source/platform/audio/HRTFPanner.cpp
index 63d9fc49628394fbba237fa1de46c6c77615b84d..0056652b106179d8ac338ddc71ee62bd60038c5e 100644
--- a/Source/platform/audio/HRTFPanner.cpp
+++ b/Source/platform/audio/HRTFPanner.cpp
@@ -45,6 +45,10 @@ const double MaxDelayTimeSeconds = 0.002;
 const int UninitializedAzimuth = -1;
 const unsigned RenderingQuantum = 128;
 
+const float coefficientSampleRate = 11025.f;

[Raymond Toy, 2014/07/30 16:49:38]

Where does 11025 come from? I thought the HRTF database was sampled at 44100.

[KhNo, 2014/07/31 16:45:54]

This value is just determined to calculate fftsize actually not related with
HRTF database's samplerate.
It means minimum unit to divide.
floor(sampleRate / coefficientSampleRate) * minimumFFTSize.

Actually, there might be better word and name. Please recommend if you have. 

[Raymond Toy, 2014/07/31 17:25:13]

Might be better to split this into two parts. One constant to give the  sample
rate of the data in the database (to make it explicit).  Then another constant
to produce the correct scaling.

+const size_t minimumFFTSize = 128;
+const size_t maximumFFTSize = 1024;
+
 HRTFPanner::HRTFPanner(float sampleRate, HRTFDatabaseLoader* databaseLoader)
     : Panner(PanningModelHRTF)
     , m_databaseLoader(databaseLoader)
@@ -79,8 +83,13 @@ size_t HRTFPanner::fftSizeForSampleRate(float sampleRate)
     // The HRTF impulse responses (loaded as audio resources) are 512 sample-frames @44.1KHz.
     // Currently, we truncate the impulse responses to half this size, but an FFT-size of twice impulse response size is needed (for convolution).
     // So for sample rates around 44.1KHz an FFT size of 512 is good. We double the FFT-size only for sample rates at least double this.
-    ASSERT(sampleRate >= 44100 && sampleRate <= 96000.0);
-    return (sampleRate < 88200.0) ? 512 : 1024;
+    ASSERT(sampleRate >= 8000 && sampleRate <= 96000.0);
+
+    size_t fftSize = minimumFFTSize;

[Raymond Toy, 2014/07/30 16:49:38]

This initial value seems useless since you give fftSize a new value in the next
line.

[KhNo, 2014/07/31 16:45:54]

Done.

+    fftSize = floor(sampleRate / coefficientSampleRate) * minimumFFTSize;
+    fftSize = max(minimumFFTSize, fftSize);
+    fftSize = min(maximumFFTSize, fftSize);
+    return fftSize;

[Raymond Toy, 2014/07/30 16:49:38]

If we expand the sample rate to 192 kHz, will this still work?

[KhNo, 2014/07/31 16:45:54]

Yes, it can be. kMaxFFTPow2Size is 15 for Android, for others are 24.

[Raymond Toy, 2014/07/31 17:25:13]

One other thing. Don't we need to make fftSize a power of two here? The
sampleRate could be anything in range.

 }
 
 void HRTFPanner::reset()