text: /*
   * FILE: fresample.h
   *
   * The configuration constants below govern
   * the number of bits in the input sample and filter coefficients, the 
   * number of bits to the right of the binary-point for fixed-point math, etc.
   *
   */
  
  /* #include "stdefs.h" */
  
  typedef double SAMPLE_TYPE;
  #define SCALE_FACTOR_TO_SHORT 32767
  
  /* Conversion constants */
  #define Nhc       8
  /* #define Na        7 */
  /* #define Np       (Nhc+Na)*/
  #define Npc      (1<<Nhc)
  /* #define Amask    ((1<<Na)-1) */
  /* #define Pmask    ((1<<Np)-1) */
  /* #define Nh       16 */
  /* #define Nb       16 */
  /* #define Nhxn     14 */
  /* #define Nhg      (Nh-Nhxn) */
  /* #define NLpScl   13 */
  
  /* Description of constants:
   *
   * Npc - is the number of look-up values available for the lowpass filter
   *    between the beginning of its impulse response and the "cutoff time"
   *    of the filter.  The cutoff time is defined as the reciprocal of the
   *    lowpass-filter cut off frequence in Hz.  For example, if the
   *    lowpass filter were a sinc function, Npc would be the index of the
   *    impulse-response lookup-table corresponding to the first zero-
   *    crossing of the sinc function.  (The inverse first zero-crossing
   *    time of a sinc function equals its nominal cutoff frequency in Hz.)
   *    Npc must be a power of 2 due to the details of the current
   *    implementation. The default value of 512 is sufficiently high that
   *    using linear interpolation to fill in between the table entries
   *    gives approximately 16-bit accuracy in filter coefficients.
   *
   * Nhc - is log base 2 of Npc.
   *
   * Na - is the number of bits devoted to linear interpolation of the
   *    filter coefficients.
   *
   * Np - is Na + Nhc, the number of bits to the right of the binary point
   *    in the integer "time" variable. To the left of the point, it indexes
   *    the input array (X), and to the right, it is interpreted as a number
   *    between 0 and 1 sample of the input X.  Np must be less than 16 in
   *    this implementation.
   *
   * Nh - is the number of bits in the filter coefficients. The sum of Nh and
   *    the number of bits in the input data (typically 16) cannot exceed 32.
   *    Thus Nh should be 16.  The largest filter coefficient should nearly
   *    fill 16 bits (32767).
   *
   * Nb - is the number of bits in the input data. The sum of Nb and Nh cannot
   *    exceed 32.
   *
   * Nhxn - is the number of bits to right shift after multiplying each input
   *    sample times a filter coefficient. It can be as great as Nh and as
   *    small as 0. Nhxn = Nh-2 gives 2 guard bits in the multiply-add
   *    accumulation.  If Nhxn=0, the accumulation will soon overflow 32 bits.
   *
   * Nhg - is the number of guard bits in mpy-add accumulation (equal to Nh-Nhxn)
   *
   * NLpScl - is the number of bits allocated to the unity-gain normalization
   *    factor.  The output of the lowpass filter is multiplied by LpScl and
   *    then right-shifted NLpScl bits. To avoid overflow, we must have 
   *    Nb+Nhg+NLpScl < 32.
   */
  
  
  