1 files changed, 123 insertions, 0 deletions
diff --git a/3rdparty/portaudio/qa/loopback/src/biquad_filter.c b/3rdparty/portaudio/qa/loopback/src/biquad_filter.c
new file mode 100755
index 0000000..1715fa3
--- /dev/null
+++ b/3rdparty/portaudio/qa/loopback/src/biquad_filter.c
@@ -0,0 +1,123 @@
+#include <math.h>
+#include <string.h>
+
+#include "biquad_filter.h"
+
+/**
+ *  Unit_BiquadFilter implements a second order IIR filter.
+ *
+ *  Here is the equation that we use for this filter:
+ *      y(n) = a0*x(n) + a1*x(n-1)  + a2*x(n-2) - b1*y(n-1)  - b2*y(n-2)
+ *
+ * @author (C) 2002 Phil Burk, SoftSynth.com, All Rights Reserved
+ */
+
+#define FILTER_PI  (3.141592653589793238462643)
+/***********************************************************
+** Calculate coefficients common to many parametric biquad filters.
+*/
+static void BiquadFilter_CalculateCommon( BiquadFilter *filter, double ratio, double Q )
+{
+    double omega;
+
+    memset( filter, 0, sizeof(BiquadFilter) );
+
+/* Don't let frequency get too close to Nyquist or filter will blow up. */
+    if( ratio >= 0.499 ) ratio = 0.499;
+    omega = 2.0 * (double)FILTER_PI * ratio;
+
+    filter->cos_omega = (double) cos( omega );
+    filter->sin_omega = (double) sin( omega );
+    filter->alpha = filter->sin_omega / (2.0 * Q);
+}
+
+/*********************************************************************************
+ ** Calculate coefficients for Highpass filter.
+ */
+void BiquadFilter_SetupHighPass( BiquadFilter *filter, double ratio, double Q )
+{
+    double    scalar, opc;
+
+    if( ratio  < BIQUAD_MIN_RATIO )  ratio  = BIQUAD_MIN_RATIO;
+    if( Q < BIQUAD_MIN_Q ) Q = BIQUAD_MIN_Q;
+
+    BiquadFilter_CalculateCommon( filter, ratio, Q );
+
+    scalar = 1.0 / (1.0 + filter->alpha);
+    opc = (1.0 + filter->cos_omega);
+
+    filter->a0 = opc * 0.5 * scalar;
+    filter->a1 =  - opc * scalar;
+    filter->a2 = filter->a0;
+    filter->b1 = -2.0 * filter->cos_omega * scalar;
+    filter->b2 = (1.0 - filter->alpha) * scalar;
+}
+
+
+/*********************************************************************************
+ ** Calculate coefficients for Notch filter.
+ */
+void BiquadFilter_SetupNotch( BiquadFilter *filter, double ratio, double Q )
+{
+    double    scalar, opc;
+
+    if( ratio  < BIQUAD_MIN_RATIO )  ratio  = BIQUAD_MIN_RATIO;
+    if( Q < BIQUAD_MIN_Q ) Q = BIQUAD_MIN_Q;
+
+    BiquadFilter_CalculateCommon( filter, ratio, Q );
+
+    scalar = 1.0 / (1.0 + filter->alpha);
+    opc = (1.0 + filter->cos_omega);
+
+    filter->a0 = scalar;
+    filter->a1 =  -2.0 * filter->cos_omega * scalar;
+    filter->a2 = filter->a0;
+    filter->b1 = filter->a1;
+    filter->b2 = (1.0 - filter->alpha) * scalar;
+}
+
+/*****************************************************************
+** Perform core IIR filter calculation without permutation.
+*/
+void BiquadFilter_Filter( BiquadFilter *filter, float *inputs, float *outputs, int numSamples )
+{
+    int i;
+    double xn, yn;
+    // Pull values from structure to speed up the calculation.
+    double a0 = filter->a0;
+    double a1 = filter->a1;
+    double a2 = filter->a2;
+    double b1 = filter->b1;
+    double b2 = filter->b2;
+    double xn1 = filter->xn1;
+    double xn2 = filter->xn2;
+    double yn1 = filter->yn1;
+    double yn2 = filter->yn2;
+
+    for( i=0; i<numSamples; i++)
+    {
+        // Generate outputs by filtering inputs.
+        xn = inputs[i];
+        yn = (a0 * xn) + (a1 * xn1) + (a2 * xn2) - (b1 * yn1) - (b2 * yn2);
+        outputs[i] = yn;
+
+        // Delay input and output values.
+        xn2 = xn1;
+        xn1 = xn;
+        yn2 = yn1;
+        yn1 = yn;
+
+        if( (i & 7) == 0 )
+        {
+            // Apply a small bipolar impulse to filter to prevent arithmetic underflow.
+            // Underflows can cause the FPU to interrupt the CPU.
+            yn1 += (double) 1.0E-26;
+            yn2 -= (double) 1.0E-26;
+        }
+    }
+
+    filter->xn1 = xn1;
+    filter->xn2 = xn2;
+    filter->yn1 = yn1;
+    filter->yn2 = yn2;
+}