zerofilt.c

语音CELP压缩解压源代码（C语音)

/**************************************************************************
*
* ROUTINE
*               zerofilt
*
* FUNCTION
*               Direct form all-zero filter
*
* SYNOPSIS
*               subroutine zerofilt(b, n, z, xy, len)
*
*   formal 
*
*                       data    I/O
*       name            type    type    function
*       -------------------------------------------------------------------
*	b		real	i	N+1 filter coefficients
*	n		int	i	Filter order 
*	z		real	i/o	N+1 filter delay elements 
*					(maintained by the user)
*	xy		real	i/o	Input/Output data array 
*	len		int	i	Number of samples to filter
*
***************************************************************************
*       
* DESCRIPTION
*
*	Nonrecursive all-zero in-place time-domain filter.
*	The filter is implemented in a direct form realisation.
*
*               N       -i
*       H(z) = SUM b(i)z
*              i=0
*
*       x(t) ->---(z0)----- b0 >------+-----> y(t)
*                  |                  |
*                  z1------ b1 >------+
*                  |                  |
*                  z2------ b2 >------+
*                  |                  |
*                  :                  :
*                  |                  |
*                  zN------ bN >------+
*
***************************************************************************
*
* CALLED BY
*
*	celp	confg	postfilt
*
* CALLS
*
*
*
***************************************************************************
*
* REFERENCES
*
*	Oppenheim & Schafer, Digital Signal Processing, PH, 1975, p. 149.
*
**************************************************************************/
zerofilt(b, n, z, xy, len)
int n, len;
float b[], z[], xy[];
{
  register float ar;
  int t, j;

  for (t = 0; t < len; t++)
  {
    z[0] = xy[t];
    ar   = 0.0;
    for (j = n; j > 0; j--)
    {
      ar  += z[j] * b[j];
      z[j] = z[j-1];
    }
    xy[t] = ar + z[0] * b[0];
  }
}