/*

*   File:        ProcessFloatSamples.c

*

*   Contains:    A filter that implements a lowpass filter with with a 400 Hz cutoff.

*   

*   Version:    1.0

*

*   Created:    11-23-2004

*

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#include "ProcessFloatSamples.h"

/* processSamples - implements a lowpass filter with with a 400 Hz cutoff by using the coefficients in setProcessingParameters() 

   in the following difference equation:

    y[n] = b0 * x[n] + b1 * x[n-1] + b2 * x[n-2] - a1 * y[n-1] - a2 * y[n-2]

   Also saves states for subsequent processing.

*/

void processSamples (float * dataBuffer, UInt32 numSampleFrames, UInt32 numChannels, ParamStructPtr inParamStructPtr, StateStructPtr inStateStructPtr) 

{

    UInt32          index;

    const UInt32    numSamplesToProcess = numSampleFrames*numChannels;

    float           x_n;

    float           y_n;

    float           b0, b1, b2, a1, a2;

    float           xstate1l, xstate2l, ystate1l, ystate2l;

    float           xstate1r, xstate2r, ystate1r, ystate2r;

    // set up local copy of parameters

    b0 = inParamStructPtr->b0;

    b1 = inParamStructPtr->b1;

    b2 = inParamStructPtr->b2;

    a1 = inParamStructPtr->a1;

    a2 = inParamStructPtr->a2;

    // set up local copy of state variables

    xstate1l = inStateStructPtr->xstate1l;

    xstate2l = inStateStructPtr->xstate2l;

    ystate1l = inStateStructPtr->ystate1l;

    ystate2l = inStateStructPtr->ystate2l;

    xstate1r = inStateStructPtr->xstate1r;

    xstate2r = inStateStructPtr->xstate2r;

    ystate1r = inStateStructPtr->ystate1r;

    ystate2r = inStateStructPtr->ystate2r;

    // process interleaved floating point data

    for (index = 0; index < numSamplesToProcess; index++) 

    {

        x_n = *dataBuffer;

        // Example device is only stereo or mono; check to maintain correct state variables

        if (    (2 == numChannels)

             && (1 == (index % 2)))

        {

            // stereo right channel case

            y_n = b0 * x_n + b1 * xstate1r + b2 * xstate2r - a1 * ystate1r - a2 * ystate2r;

            // update state variables

            xstate2r = xstate1r;

            xstate1r = x_n;

            ystate2r = ystate1r;

            ystate1r = y_n;

            // set output data

            *dataBuffer++ = y_n;

        }

        else

        {

            // left channel or mono case

            y_n = b0 * x_n + b1 * xstate1l + b2 * xstate2l - a1 * ystate1l - a2 * ystate2l;

            // update state variables

            xstate2l = xstate1l;

            xstate1l = x_n;

            ystate2l = ystate1l;

            ystate1l = y_n;

            // set output data

            *dataBuffer++ = y_n;

        }

    }

    // save states to structure

    inStateStructPtr->xstate1l = xstate1l;

    inStateStructPtr->xstate2l = xstate2l;

    inStateStructPtr->ystate1l = ystate1l;

    inStateStructPtr->ystate2l = ystate2l;

    inStateStructPtr->xstate1r = xstate1r;

    inStateStructPtr->xstate2r = xstate2r;

    inStateStructPtr->ystate1r = ystate1r;

    inStateStructPtr->ystate2r = ystate2r;

} // processSamples

void resetProcessingState (StateStructPtr inStateStructPtr) 

{

    // reset state variables

    inStateStructPtr->xstate1l = 0.0f;

    inStateStructPtr->xstate2l = 0.0f;

    inStateStructPtr->ystate1l = 0.0f;

    inStateStructPtr->ystate2l = 0.0f;

    inStateStructPtr->xstate1r = 0.0f;

    inStateStructPtr->xstate2r = 0.0f;

    inStateStructPtr->ystate1r = 0.0f;

    inStateStructPtr->ystate2r = 0.0f;

} // resetProcessingState

void setProcessingParameters (ParamStructPtr inParamStructPtr, UInt32 inSampleRate, UInt32 inNumChannels) 

{

    // set up our parameters for 400 Hz cutoff lowpass filter

    inParamStructPtr->b0 = 0.000777753436f;

    inParamStructPtr->b1 = 0.001555506871f;

    inParamStructPtr->b2 = 0.000777753436f;

    inParamStructPtr->a1 = -1.913115889852f;

    inParamStructPtr->a2 = 0.916226903594f;

} // setProcessingParameters