lpc10/preemp.c

   1 /*
   2
   3  * Revision 1.1  1996/08/19  22:30:58  jaf
   4  * Initial revision
   5  *
   6
   7 */
   8
   9 /*  -- translated by f2c (version 19951025).
  10    You must link the resulting object file with the libraries:
  11         -lf2c -lm   (in that order)
  12 */
  13
  14 #include "f2c.h"
  15
  16 extern int preemp_(real *inbuf, real *pebuf, integer *nsamp, real *coef, real *z__);
  17
  18 /* ******************************************************************* */
  19
  20 /*      PREEMP Version 55 */
  21
  22 /*
  23  * Revision 1.1  1996/08/19  22:30:58  jaf
  24  * Initial revision
  25  * */
  26 /* Revision 1.3  1996/03/14  23:16:29  jaf */
  27 /* Just added a few comments about which array indices of the arguments */
  28 /* are used, and mentioning that this subroutine has no local state. */
  29
  30 /* Revision 1.2  1996/03/11  23:23:34  jaf */
  31 /* Added a bunch of comments to an otherwise simple subroutine. */
  32
  33 /* Revision 1.1  1996/02/07 14:48:48  jaf */
  34 /* Initial revision */
  35
  36
  37 /* ******************************************************************* */
  38
  39 /*   Preemphasize speech with a single-zero filter. */
  40 /*  (When coef = .9375, preemphasis is as in LPC43.) */
  41
  42 /* Inputs: */
  43 /*  NSAMP  - Number of samples to filter */
  44 /*  INBUF  - Input speech buffer */
  45 /*           Indices 1 through NSAMP are read. */
  46 /*  COEF   - Preemphasis coefficient */
  47 /* Input/Output: */
  48 /*  Z      - Filter state */
  49 /* Output: */
  50 /*  PEBUF  - Preemphasized speech buffer (can be equal to INBUF) */
  51 /*           Indices 1 through NSAMP are modified. */
  52
  53 /* This subroutine has no local state. */
  54
  55 /* Subroutine */ int preemp_(real *inbuf, real *pebuf, integer *nsamp, real *
  56         coef, real *z__)
  57 {
  58     /* System generated locals */
  59     integer i__1;
  60
  61     /* Local variables */
  62     real temp;
  63     integer i__;
  64
  65 /*       Arguments */
  66 /*       Local variables */
  67
  68 /*       None of these need to have their values saved from one */
  69 /*       invocation to the next. */
  70
  71 /*       Logically, this subroutine computes the output sequence */
  72 /*       pebuf(1:nsamp) defined by: */
  73
  74 /*       pebuf(i) = inbuf(i) - coef * inbuf(i-1) */
  75
  76 /*       where inbuf(0) is defined by the value of z given as input to */
  77 /*       this subroutine. */
  78
  79 /*       What is this filter's frequency response and phase response? */
  80
  81 /*       Why is this filter applied to the speech? */
  82
  83 /*       Could it be more efficient to apply multiple filters */
  84 /*       simultaneously, by combining them into one equivalent filter? */
  85
  86 /*       Are there ever cases when "factoring" one high-order filter into
  87 */
  88 /*       multiple smaller-order filter actually reduces the number of */
  89 /*       arithmetic operations needed to perform them? */
  90 /*       When I first read this subroutine, I didn't understand why the */
  91 /*       variable temp was used.  It seemed that the statements in the do
  92 */
  93 /*       loop could be replaced with the following: */
  94
  95 /*           pebuf(i) = inbuf(i) - coef * z */
  96 /*           z = inbuf(i) */
  97
  98 /*       The reason for temp is so that even if pebuf and inbuf are the */
  99 /*       same arrays in memory (i.e., they are aliased), then this */
 100 /*       subroutine will still work correctly.  I didn't realize this */
 101 /*       until seeing the comment after PEBUF above that says "(can be */
 102 /*       equal to INBUF)". */
 103     /* Parameter adjustments */
 104     --pebuf;
 105     --inbuf;
 106
 107     /* Function Body */
 108     i__1 = *nsamp;
 109     for (i__ = 1; i__ <= i__1; ++i__) {
 110         temp = inbuf[i__] - *coef * *z__;
 111         *z__ = inbuf[i__];
 112         pebuf[i__] = temp;
 113 /* L10: */
 114     }
 115     return 0;
 116 } /* preemp_ */
 117