DCfilter.h

/*
 * DCfilter.h
 *
 * Copyright 2012 Tim Barrass.
 *
 * This file is part of Mozzi.
 *
 * Mozzi is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
 *
 */

#ifndef DCFILTER_H
#define DCFILTER_H

/*
tb2010 adapted from:
robert bristow-johnson, DSP Trick: Fixed-Point DC Blocking Filter with Noise-Shaping
http://www.dspguru.com/book/export/html/126

y[n] = x[n] - x[n-1] + a * y[n-1]

Where y[n] is the output at the current time n, and x[n] is the input at the current time n.

also, see DC Blocker Algorithms, http://www.ingelec.uns.edu.ar/pds2803/materiales/articulos/04472252.pdf
 */

/**
A DC-blocking filter useful for highlighting changes in control signals.
The output of the filter settles to 0 if the incoming signal stays constant.  If the input changes, the
filter output swings to track the change and eventually settles back to 0.
*/
class DCfilter
{
public:
/**
Instantiate a DC-blocking filter.
@param pole sets the responsiveness of the filter,
how long it takes to settle to 0 if the input signal levels out at a constant value.
*/
    DCfilter(float pole):acc(0),prev_x(0),prev_y(0)
    {
        A = (int)(32768.0*(1.0 - pole));
    }

/*  almost original
    // timing: 20us
    int next(int x)
    {
        setPin13High();
        acc   -= prev_x;
        prev_x = (long)x<<15;
        acc   += prev_x;
        acc   -= A*prev_y;
        prev_y = acc>>15;               // quantization happens here
        int filtered = (int)prev_y;
        // acc has y[n] in upper 17 bits and -e[n] in lower 15 bits
        setPin13Low();
        return filtered;
    }
    */

    /**
    Filter the incoming value and return the result.
    @param x the value to filter
    @return filtered signal
    */
    // timing :8us
    inline
    int next(int x)
    {
        acc += ((long)(x-prev_x)<<16)>>1;
        prev_x = x;
        acc   -= (long)A*prev_y;    // acc has y[n] in upper 17 bits and -e[n] in lower 15 bits
        prev_y = (acc>>16)<<1; // faster than >>15 but loses bit 0
        if (acc & 32784) prev_y += 1; // adds 1 if it was in the 0 bit position lost in the shifts above
        return prev_y;
    }

private:
    long acc;
    int prev_x, prev_y,A;
};

/**
@example 05.Control_Filters/DCFilter/DCFilter.ino
This example demonstrates the DCFilter class.
*/

#endif        //  #ifndef DCFILTER_H