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ecdis.h

/*
 * SpanDSP - a series of DSP components for telephony
 *
 * ec_disable_detector.h - A detector which should eventually meet the
 *                         G.164/G.165 requirements for detecting the
 *                         2100Hz echo cancellor disable tone.
 *
 * Written by Steve Underwood <steveu@coppice.org>
 *
 * Copyright (C) 2001 Steve Underwood
 *
 * All rights reserved.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 */

#include "biquad.h"

typedef struct
{
    biquad2_state_t notch;
    int notch_level;
    int channel_level;
    int tone_present;
    int tone_cycle_duration;
    int good_cycles;
    int hit;
} echo_can_disable_detector_state_t;


#define FALSE 0
#define TRUE (!FALSE)

static inline void echo_can_disable_detector_init (echo_can_disable_detector_state_t *det)
{
    /* Elliptic notch */
    /* This is actually centred at 2095Hz, but gets the balance we want, due
       to the asymmetric walls of the notch */
    biquad2_init (&det->notch,
              (int32_t) (-0.7600000*32768.0),
              (int32_t) (-0.1183852*32768.0),
              (int32_t) (-0.5104039*32768.0),
              (int32_t) ( 0.1567596*32768.0),
              (int32_t) ( 1.0000000*32768.0));

    det->channel_level = 0;
    det->notch_level = 0;    
    det->tone_present = FALSE;
    det->tone_cycle_duration = 0;
    det->good_cycles = 0;
    det->hit = 0;
}
/*- End of function --------------------------------------------------------*/

static inline int echo_can_disable_detector_update (echo_can_disable_detector_state_t *det,
                                      int16_t amp)
{
    int16_t notched;
    
      notched = biquad2 (&det->notch, amp);
      /* Estimate the overall energy in the channel, and the energy in
         the notch (i.e. overall channel energy - tone energy => noise).
         Use abs instead of multiply for speed (is it really faster?).
         Damp the overall energy a little more for a stable result.
         Damp the notch energy a little less, so we don't damp out the
         blip every time the phase reverses */
        det->channel_level += ((abs(amp) - det->channel_level) >> 5);
      det->notch_level += ((abs(notched) - det->notch_level) >> 4);
      if (det->channel_level > 280)
      {
          /* There is adequate energy in the channel. Is it mostly at 2100Hz? */
          if (det->notch_level*6 < det->channel_level)
          {
                /* The notch says yes, so we have the tone. */
            if (!det->tone_present)
            {
                    /* Do we get a kick every 450+-25ms? */
                if (det->tone_cycle_duration >= 425*8
                  &&
                  det->tone_cycle_duration <= 475*8)
                  {
                  det->good_cycles++;
                  if (det->good_cycles > 2)
                      det->hit = TRUE;
                }
                det->tone_cycle_duration = 0;
            }
            det->tone_present = TRUE;
          }
          else
          {
            det->tone_present = FALSE;
          }
          det->tone_cycle_duration++;
      }
      else
      {
          det->tone_present = FALSE;
          det->tone_cycle_duration = 0;
          det->good_cycles = 0;
      }
    return  det->hit;
}
/*- End of function --------------------------------------------------------*/
/*- End of file ------------------------------------------------------------*/

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