Document the discrete time constant formula

Add comments to show how to get the discrete time constant value for
the formula for setTargetAtTime.

No code changes at all.

Change-Id: I6af196475671e030c665e5a968c18579bc8f7e83
Reviewed-on: https://chromium-review.googlesource.com/940455Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539922}

third_party/WebKit/Source/platform/audio/AudioUtilities.cpp

@@ -43,6 +43,26 @@ float LinearToDecibels(float linear) {
 
 double DiscreteTimeConstantForSampleRate(double time_constant,
                                          double sample_rate) {
+  // From the WebAudio spec, the formula for setTargetAtTime is
+  //
+  //   v(t) = V1 + (V0 - V1)*exp(-t/tau)
+  //
+  // where tau is the time constant, V1 is the target value and V0 is
+  // the starting value.
+  //
+  // Rewrite this as
+  //
+  //   v(t) = V0 + (V1 - V0)*(1-exp(-t/tau))
+  //
+  // The implementation of setTargetAtTime uses this form.  So at the
+  // sample points, we have
+  //
+  //   v(n/Fs) = V0 + (V1 - V0)*(1-exp(-n/(Fs*tau)))
+  //
+  // where Fs is the sample rate of the sampled systme.  Thus, the
+  // discrete time constant is
+  //
+  //   1 - exp(-1/(Fs*tau)
   return 1 - exp(-1 / (sample_rate * time_constant));
 }