filter.c

来自「linux下的MPEG1」· C语言 代码 · 共 439 行

/*=============================================================================
 *	
 *  This software has been released under the terms of the GNU Public
 *  license. See http://www.gnu.org/copyleft/gpl.html for details.
 *
 *  Copyright 2001 Anders Johansson [e-mail]
 *
 *=============================================================================
 */

/* Design and implementation of different types of digital filters */

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif
#include <string.h>
#include <math.h>
#include "dsp.h"

/******************************************************************************
 *  FIR filter implementations
 ******************************************************************************/

/* C implementation of FIR filter y=w*x
 *
 * n number of filter taps, where mod(n,4)==0
 * w filter taps
 * x input signal must be a circular buffer which is indexed backwards 
 */
inline _ftype_t fir(register unsigned int n, _ftype_t* w, _ftype_t* x)
{
  register _ftype_t y; /* Output */
  y = 0.0; 
  do{
    n--;
    y+=w[n]*x[n];
  }while(n != 0);
  return y;
}

/* C implementation of parallel FIR filter y(k)=w(k) * x(k) (where * denotes convolution)
 *
 * n  number of filter taps, where mod(n,4)==0
 * d  number of filters
 * xi current index in xq
 * w  filter taps k by n big
 * x  input signal must be a circular buffers which are indexed backwards 
 * y  output buffer
 * s  output buffer stride
 */
inline _ftype_t* pfir(unsigned int n, unsigned int d, unsigned int xi, _ftype_t** w, _ftype_t** x, _ftype_t* y, unsigned int s)
{
  register _ftype_t* xt = *x + xi;
  register _ftype_t* wt = *w;
  register int    nt = 2*n;
  while(d-- > 0){
    *y = fir(n,wt,xt);
    wt+=n;
    xt+=nt;
    y+=s;
  }
  return y;
}

/* Add new data to circular queue designed to be used with a parallel
   FIR filter, with d filters. xq is the circular queue, in pointing
   at the new samples, xi current index in xq and n the length of the
   filter. xq must be n*2 by k big, s is the index for in.
*/
inline int updatepq(unsigned int n, unsigned int d, unsigned int xi, _ftype_t** xq, _ftype_t* in, unsigned int s)  
{
  register _ftype_t* txq = *xq + xi;
  register int nt = n*2;
  
  while(d-- >0){
    *txq= *(txq+n) = *in;
    txq+=nt;
    in+=s;
  }
  return (++xi)&(n-1);
}

/******************************************************************************
*  FIR filter design
******************************************************************************/

/* Design FIR filter using the Window method

   n     filter length must be odd for HP and BS filters
   w     buffer for the filter taps (must be n long)
   fc    cutoff frequencies (1 for LP and HP, 2 for BP and BS) 
         0 < fc < 1 where 1 <=> Fs/2
   flags window and filter type as defined in filter.h
         variables are ored together: i.e. LP|HAMMING will give a 
	 low pass filter designed using a hamming window  
   opt   beta constant used only when designing using kaiser windows
   
   returns 0 if OK, -1 if fail
*/
int design_fir(unsigned int n, _ftype_t* w, _ftype_t* fc, unsigned int flags, _ftype_t opt)
{
  unsigned int	o   = n & 1;          	/* Indicator for odd filter length */
  unsigned int	end = ((n + 1) >> 1) - o;  /* Loop end */
  unsigned int	i;			/* Loop index */

  _ftype_t k1 = 2 * M_PI;		/* 2*pi*fc1 */
  _ftype_t k2 = 0.5 * (_ftype_t)(1 - o);/* Constant used if the filter has even length */
  _ftype_t k3;				/* 2*pi*fc2 Constant used in BP and BS design */
  _ftype_t g  = 0.0;     		/* Gain */
  _ftype_t t1,t2,t3;     		/* Temporary variables */
  _ftype_t fc1,fc2;			/* Cutoff frequencies */

  /* Sanity check */
  if(!w || (n == 0)) return -1;

  /* Get window coefficients */
  switch(flags & WINDOW_MASK){
  case(BOXCAR):
    boxcar(n,w); break;
  case(TRIANG):
    triang(n,w); break;
  case(HAMMING):
    hamming(n,w); break;
  case(HANNING):
    hanning(n,w); break;
  case(BLACKMAN):
    blackman(n,w); break;
  case(FLATTOP):
    flattop(n,w); break;
  case(KAISER):
    kaiser(n,w,opt); break;
  default:
    return -1;	
  }

  if(flags & (LP | HP)){ 
    fc1=*fc;
    /* Cutoff frequency must be < 0.5 where 0.5 <=> Fs/2 */
    fc1 = ((fc1 <= 1.0) && (fc1 > 0.0)) ? fc1/2 : 0.25;
    k1 *= fc1;

    if(flags & LP){ /* Low pass filter */

      /*
       * If the filter length is odd, there is one point which is exactly
       * in the middle. The value at this point is 2*fCutoff*sin(x)/x, 
       * where x is zero. To make sure nothing strange happens, we set this
       * value separately.
       */
      if (o){
	w[end] = fc1 * w[end] * 2.0;
	g=w[end];
      }

      /* Create filter */
      for (i=0 ; i<end ; i++){
	t1 = (_ftype_t)(i+1) - k2;
	w[end-i-1] = w[n-end+i] = w[end-i-1] * sin(k1 * t1)/(M_PI * t1); /* Sinc */
	g += 2*w[end-i-1]; /* Total gain in filter */
      }
    }
    else{ /* High pass filter */
      if (!o) /* High pass filters must have odd length */
	return -1;
      w[end] = 1.0 - (fc1 * w[end] * 2.0);
      g= w[end];

      /* Create filter */
      for (i=0 ; i<end ; i++){
	t1 = (_ftype_t)(i+1);
	w[end-i-1] = w[n-end+i] = -1 * w[end-i-1] * sin(k1 * t1)/(M_PI * t1); /* Sinc */
	g += ((i&1) ? (2*w[end-i-1]) : (-2*w[end-i-1])); /* Total gain in filter */
      }
    }
  }

  if(flags & (BP | BS)){
    fc1=fc[0];
    fc2=fc[1];
    /* Cutoff frequencies must be < 1.0 where 1.0 <=> Fs/2 */
    fc1 = ((fc1 <= 1.0) && (fc1 > 0.0)) ? fc1/2 : 0.25;
    fc2 = ((fc2 <= 1.0) && (fc2 > 0.0)) ? fc2/2 : 0.25;
    k3  = k1 * fc2; /* 2*pi*fc2 */
    k1 *= fc1;      /* 2*pi*fc1 */

    if(flags & BP){ /* Band pass */
      /* Calculate center tap */
      if (o){
	g=w[end]*(fc1+fc2);
	w[end] = (fc2 - fc1) * w[end] * 2.0;
      }

      /* Create filter */
      for (i=0 ; i<end ; i++){
	t1 = (_ftype_t)(i+1) - k2;
	t2 = sin(k3 * t1)/(M_PI * t1); /* Sinc fc2 */
	t3 = sin(k1 * t1)/(M_PI * t1); /* Sinc fc1 */
	g += w[end-i-1] * (t3 + t2);   /* Total gain in filter */
	w[end-i-1] = w[n-end+i] = w[end-i-1] * (t2 - t3); 
      }
    }      
    else{ /* Band stop */
      if (!o) /* Band stop filters must have odd length */
	return -1;
      w[end] = 1.0 - (fc2 - fc1) * w[end] * 2.0;
      g= w[end];

      /* Create filter */
      for (i=0 ; i<end ; i++){
	t1 = (_ftype_t)(i+1);
	t2 = sin(k1 * t1)/(M_PI * t1); /* Sinc fc1 */
	t3 = sin(k3 * t1)/(M_PI * t1); /* Sinc fc2 */
	w[end-i-1] = w[n-end+i] = w[end-i-1] * (t2 - t3); 
	g += 2*w[end-i-1]; /* Total gain in filter */
      }
    }
  }

  /* Normalize gain */
  g=1/g;
  for (i=0; i<n; i++) 
    w[i] *= g;
  
  return 0;
}

/* Design polyphase FIR filter from prototype filter
 *
 * n     length of prototype filter
 * k     number of polyphase components
 * w     prototype filter taps
 * pw    Parallel FIR filter 
 * g     Filter gain
 * flags FWD forward indexing
 *       REW reverse indexing
 *       ODD multiply every 2nd filter tap by -1 => HP filter
 *
 * returns 0 if OK, -1 if fail
 */
int design_pfir(unsigned int n, unsigned int k, _ftype_t* w, _ftype_t** pw, _ftype_t g, unsigned int flags)
{
  int l = (int)n/k;	/* Length of individual FIR filters */
  int i;     	/* Counters */
  int j;
  _ftype_t t;	/* g * w[i] */
  
  /* Sanity check */
  if(l<1 || k<1 || !w || !pw)
    return -1;

  /* Do the stuff */
  if(flags&REW){
    for(j=l-1;j>-1;j--){ /* Columns */
      for(i=0;i<(int)k;i++){ /* Rows */
	t=g *  *w++;
	pw[i][j]=t * ((flags & ODD) ? ((j & 1) ? -1 : 1) : 1);
      }
    }
  }
  else{
    for(j=0;j<l;j++){ /* Columns */
      for(i=0;i<(int)k;i++){ /* Rows */
	t=g *  *w++;
	pw[i][j]=t * ((flags & ODD) ? ((j & 1) ? 1 : -1) : 1);
      }
    }
  }
  return -1;
}

/******************************************************************************
*  IIR filter design
******************************************************************************/

/* Helper functions for the bilinear transform */

/* Pre-warp the coefficients of a numerator or denominator.
 * Note that a0 is assumed to be 1, so there is no wrapping
 * of it.  
 */
void prewarp(_ftype_t* a, _ftype_t fc, _ftype_t fs)
{
  _ftype_t wp;
  wp = 2.0 * fs * tan(M_PI * fc / fs);
  a[2] = a[2]/(wp * wp);
  a[1] = a[1]/wp;
}

/* Transform the numerator and denominator coefficients of s-domain
 * biquad section into corresponding z-domain coefficients.
 *
 * The transfer function for z-domain is:
 *
 *        1 + alpha1 * z^(-1) + alpha2 * z^(-2)
 * H(z) = -------------------------------------
 *        1 + beta1 * z^(-1) + beta2 * z^(-2)
 *
 * Store the 4 IIR coefficients in array pointed by coef in following
 * order:
 * beta1, beta2    (denominator)
 * alpha1, alpha2  (numerator)
 *
 *  Arguments:
 * a       - s-domain numerator coefficients
 * b       - s-domain denominator coefficients
 * k 	   - filter gain factor. Initially set to 1 and modified by each
 *           biquad section in such a way, as to make it the
 *           coefficient by which to multiply the overall filter gain
 *           in order to achieve a desired overall filter gain,
 *           specified in initial value of k.  
 * fs 	   - sampling rate (Hz)
 * coef    - array of z-domain coefficients to be filled in.
 *
 * Return: On return, set coef z-domain coefficients and k to the gain
 * required to maintain overall gain = 1.0;
 */
void bilinear(_ftype_t* a, _ftype_t* b, _ftype_t* k, _ftype_t fs, _ftype_t *coef)
{
  _ftype_t ad, bd;

  /* alpha (Numerator in s-domain) */
  ad = 4. * a[2] * fs * fs + 2. * a[1] * fs + a[0];
  /* beta (Denominator in s-domain) */
  bd = 4. * b[2] * fs * fs + 2. * b[1] * fs + b[0];

  /* Update gain constant for this section */
  *k *= ad/bd;

  /* Denominator */
  *coef++ = (2. * b[0] - 8. * b[2] * fs * fs)/bd; /* beta1 */
  *coef++ = (4. * b[2] * fs * fs - 2. * b[1] * fs + b[0])/bd; /* beta2 */

  /* Numerator */
  *coef++ = (2. * a[0] - 8. * a[2] * fs * fs)/ad; /* alpha1 */
  *coef   = (4. * a[2] * fs * fs - 2. * a[1] * fs + a[0])/ad;   /* alpha2 */
}



/* IIR filter design using bilinear transform and prewarp. Transforms
 * 2nd order s domain analog filter into a digital IIR biquad link. To
 * create a filter fill in a, b, Q and fs and make space for coef and k.
 *
 *
 * Example Butterworth design: 
 *
 * Below are Butterworth polynomials, arranged as a series of 2nd
 * order sections:
 *
 * Note: n is filter order.
 *
 * n  Polynomials
 * -------------------------------------------------------------------
 * 2  s^2 + 1.4142s + 1
 * 4  (s^2 + 0.765367s + 1) * (s^2 + 1.847759s + 1)
 * 6  (s^2 + 0.5176387s + 1) * (s^2 + 1.414214 + 1) * (s^2 + 1.931852s + 1)
 *
 * For n=4 we have following equation for the filter transfer function:
 *                     1                              1
 * T(s) = --------------------------- * ----------------------------
 *        s^2 + (1/Q) * 0.765367s + 1   s^2 + (1/Q) * 1.847759s + 1
 *
 * The filter consists of two 2nd order sections since highest s power
 * is 2.  Now we can take the coefficients, or the numbers by which s
 * is multiplied and plug them into a standard formula to be used by
 * bilinear transform.
 *
 * Our standard form for each 2nd order section is:
 *
 *        a2 * s^2 + a1 * s + a0
 * H(s) = ----------------------
 *        b2 * s^2 + b1 * s + b0
 *
 * Note that Butterworth numerator is 1 for all filter sections, which
 * means s^2 = 0 and s^1 = 0
 *
 * Let's convert standard Butterworth polynomials into this form:
 *
 *           0 + 0 + 1                  0 + 0 + 1
 * --------------------------- * --------------------------
 * 1 + ((1/Q) * 0.765367) + 1   1 + ((1/Q) * 1.847759) + 1
 *
 * Section 1:
 * a2 = 0; a1 = 0; a0 = 1;
 * b2 = 1; b1 = 0.765367; b0 = 1;
 *
 * Section 2:
 * a2 = 0; a1 = 0; a0 = 1;
 * b2 = 1; b1 = 1.847759; b0 = 1;
 *
 * Q is filter quality factor or resonance, in the range of 1 to
 * 1000. The overall filter Q is a product of all 2nd order stages.
 * For example, the 6th order filter (3 stages, or biquads) with
 * individual Q of 2 will have filter Q = 2 * 2 * 2 = 8.
 *
 *
 * Arguments:
 * a       - s-domain numerator coefficients, a[1] is always assumed to be 1.0
 * b       - s-domain denominator coefficients
 * Q	   - Q value for the filter
 * k 	   - filter gain factor. Initially set to 1 and modified by each
 *           biquad section in such a way, as to make it the
 *           coefficient by which to multiply the overall filter gain
 *           in order to achieve a desired overall filter gain,
 *           specified in initial value of k.  
 * fs 	   - sampling rate (Hz)
 * coef    - array of z-domain coefficients to be filled in.
 *
 * Note: Upon return from each call, the k argument will be set to a
 * value, by which to multiply our actual signal in order for the gain
 * to be one. On second call to szxform() we provide k that was
 * changed by the previous section. During actual audio filtering
 * k can be used for gain compensation.
 *
 * return -1 if fail 0 if success.
 */
int szxform(_ftype_t* a, _ftype_t* b, _ftype_t Q, _ftype_t fc, _ftype_t fs, _ftype_t *k, _ftype_t *coef)
{
  _ftype_t at[3];
  _ftype_t bt[3];

  if(!a || !b || !k || !coef || (Q>1000.0 || Q< 1.0)) 
    return -1;

  memcpy(at,a,3*sizeof(_ftype_t));
  memcpy(bt,b,3*sizeof(_ftype_t));

  bt[1]/=Q;

  /* Calculate a and b and overwrite the original values */
  prewarp(at, fc, fs);
  prewarp(bt, fc, fs);
  /* Execute bilinear transform */
  bilinear(at, bt, k, fs, coef);

  return 0;
}