Compute the tail time for an IIRFilter from its coefficients

third_party/WebKit/Source/platform/audio/IIRFilter.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IIRFilter_h
 #define IIRFilter_h
 
 #include "platform/PlatformExport.h"
 #include "platform/audio/AudioArray.h"
 #include "platform/wtf/Vector.h"
@@ skipping 12 matching lines @@
 
   void Process(const float* source_p, float* dest_p, size_t frames_to_process);
 
   void Reset();
 
   void GetFrequencyResponse(int n_frequencies,
                             const float* frequency,
                             float* mag_response,
                             float* phase_response);
 
+  // Compute the tail time of the IIR filter
+  double TailTime(double sample_rate);

[hongchan, 2017/05/12 16:24:05]

What's the frequency of this computation?

On a related note, I am aware that IIRFilter does not share its infra with any
other DSP kernel or platform components. Should we actually consider doing that?

It seems like TailTime() and ResetState() methods can be shared with other
similar components.

We can fix them later, but I think we should at least consider.

[Raymond Toy, 2017/05/12 17:33:18]

I don't quite understand this question?  The TailTime is always requested every
render quantum. For the IIR filter, the value is computed once during
construction and this just returns that value.
I don't understand what is being shared.
I don't understand this either.  The TailTime() value depends on the node.

[hongchan, 2017/05/12 17:36:45]

You answered this above. We compute just once.
I just looked at the header file. Two methods are shared with other friend
classes. Ignore my comments.

+
+  // Reset the internal state of the IIR filter to the initial state.
+  void ResetState();
+
  private:
   // Filter memory
   //
   // For simplicity, we assume |m_xBuffer| and |m_yBuffer| have the same length,
   // and the length is a power of two.  Since the number of coefficients has a
   // fixed upper length, the size of xBuffer and yBuffer is fixed. |m_xBuffer|
   // holds the old input values and |m_yBuffer| holds the old output values
   // needed to compute the new output value.
   //
   // m_yBuffer[m_bufferIndex] holds the most recent output value, say, y[n].
@@ skipping 11 matching lines @@
 
   // Coefficients of the IIR filter.  To minimize storage, these point to the
   // arrays given in the constructor.
   const AudioDoubleArray* feedback_;
   const AudioDoubleArray* feedforward_;
 };
 
 }  // namespace blink
 
 #endif  // IIRFilter_h