c++ beginner

why do we use header in c++ and how can we understand which class call from main program?

can u explain an example of code here an code:

Appendix A. The C++ Source Code

/***************** A.1 APPENDIX aec.h *****************/

/* aec.h
 *
 * Copyright (C) DFS Deutsche Flugsicherung (2004). All Rights Reserved.
 *
 * Acoustic Echo Cancellation NLMS-pw algorithm
 *
 * Version 1.3 filter created with www.dsptutor.freeuk.com
 */

#ifndef _AEC_H                  /* include only once */

// use double if your CPU does software-emulation of float
typedef float REAL;       

/* dB Values */
const REAL M0dB = 1.0f;
const REAL M3dB = 0.71f;
const REAL M6dB = 0.50f;
const REAL M9dB = 0.35f;
const REAL M12dB = 0.25f;
const REAL M18dB = 0.125f;
const REAL M24dB = 0.063f;

/* dB values for 16bit PCM */
/* MxdB_PCM = 32767 * 10 ^(x / 20) */
const REAL M10dB_PCM = 10362.0f;
const REAL M20dB_PCM = 3277.0f;
const REAL M25dB_PCM = 1843.0f;
const REAL M30dB_PCM = 1026.0f;
const REAL M35dB_PCM = 583.0f;
const REAL M40dB_PCM = 328.0f;
const REAL M45dB_PCM = 184.0f;
const REAL M50dB_PCM = 104.0f;
const REAL M55dB_PCM = 58.0f;
const REAL M60dB_PCM = 33.0f;
const REAL M65dB_PCM = 18.0f;
const REAL M70dB_PCM = 10.0f;
const REAL M75dB_PCM = 6.0f;
const REAL M80dB_PCM = 3.0f;
const REAL M85dB_PCM = 2.0f;
const REAL M90dB_PCM = 1.0f;

const REAL MAXPCM = 32767.0f;

/* Design constants (Change to fine tune the algorithms */

/* The following values are for hardware AEC and studio quality
 * microphone */

/* maximum NLMS filter length in taps. A longer filter length gives
 * better Echo Cancellation, but slower convergence speed and
 * needs more CPU power (Order of NLMS is linear) */
#define NLMS_LEN  (80*8)

/* convergence speed. Range: >0 to <1 (0.2 to 0.7). Larger values give
 * more AEC in lower frequencies, but less AEC in higher frequencies. */
const REAL Stepsize = 0.7f;

/* minimum energy in xf. Range: M70dB_PCM to M50dB_PCM. Should be equal
 * to microphone ambient Noise level */
const REAL Min_xf = M75dB_PCM;

/* Double Talk Detector Speaker/Microphone Threshold. Range <=1
 * Large value (M0dB) is good for Single-Talk Echo cancellation,
 * small value (M12dB) is good for Doulbe-Talk AEC */
const REAL GeigelThreshold = M6dB;

/* Double Talk Detector hangover in taps. Not relevant for Single-Talk
 * AEC */
const int Thold = 30 * 8;

/* for Non Linear Processor. Range >0 to 1. Large value (M0dB) is good
 * for Double-Talk, small value (M12dB) is good for Single-Talk */
const REAL NLPAttenuation = M12dB;

/* Below this line there are no more design constants */


/* Exponential Smoothing or IIR Infinite Impulse Response Filter */
class IIR_HP {
  REAL x;

public:
   IIR_HP() { x = 0.0f; };
  REAL highpass(REAL in) {
    const REAL a0 = 0.01f;   /* controls Transfer Frequency */
    /* Highpass = Signal - Lowpass. Lowpass = Exponential Smoothing */
    x += a0 * (in - x);
    return in - x;
  };
};


/* 13 taps FIR Finite Impulse Response filter
 * Coefficients calculated with
 * www.dsptutor.freeuk.com/KaiserFilterDesign/KaiserFilterDesign.html
 */
class FIR_HP13 {
  REAL z[14];
 
public:
   FIR_HP13() { memset(this, 0, sizeof(FIR_HP13)); };
  REAL highpass(REAL in) {
    const REAL a[14] = {
      // Kaiser Window FIR Filter, Filter type: High pass
      // Passband: 300.0 - 4000.0 Hz, Order: 12
      // Transition band: 100.0 Hz, Stopband attenuation: 10.0 dB
      -0.043183226f, -0.046636667f, -0.049576525f, -0.051936015f,
      -0.053661242f, -0.054712527f, 0.82598513f, -0.054712527f,
      -0.053661242f, -0.051936015f, -0.049576525f, -0.046636667f,
      -0.043183226f, 0.0f
    };
    memmove(z+1, z, 13*sizeof(REAL));
    z[0] = in;
    REAL sum0 = 0.0, sum1 = 0.0;
    int j;

    for (j = 0; j < 14; j+= 2) {
      // optimize: partial loop unrolling
      sum0 += a[j] * z[j];
      sum1 += a[j+1] * z[j+1];
    }
    return sum0+sum1;
  }
};


/* Recursive single pole IIR Infinite Impulse response filter
 * Coefficients calculated with
 * http://www.dsptutor.freeuk.com/IIRFilterDesign/IIRFiltDes102.html
 */
class IIR1 {
  REAL x, y;

public:
   IIR1() { memset(this, 0, sizeof(IIR1)); };
  REAL highpass(REAL in) {
    // Chebyshev IIR filter, Filter type: HP
    // Passband: 3700 - 4000.0 Hz
    // Passband ripple: 1.5 dB, Order: 1
    const REAL a0 = 0.105831884f;
    const REAL a1 = -0.105831884;
    const REAL b1 = 0.78833646f;
    REAL out = a0 * in + a1 * x + b1 * y;
    x = in;
    y = out;
    return out;
  }
};

/* Recursive two pole IIR Infinite Impulse Response filter
 * Coefficients calculated with
 * http://www.dsptutor.freeuk.com/IIRFilterDesign/IIRFiltDes102.html
 */
class IIR2 {
  REAL x[2], y[2];

public:
   IIR2() { memset(this, 0, sizeof(IIR2)); };
  REAL highpass(REAL in) {
    // Butterworth IIR filter, Filter type: HP
    // Passband: 2000 - 4000.0 Hz, Order: 2
    const REAL a[] = { 0.29289323f, -0.58578646f, 0.29289323f };
    const REAL b[] = { 1.3007072E-16f, 0.17157288f };
    REAL out =
      a[0] * in +
      a[1] * x[0] +
      a[2] * x[1] -
      b[0] * y[0] -
      b[1] * y[1];

    x[1] = x[0];
    x[0] = in;
    y[1] = y[0];
    y[0] = out;
    return out;
  }
};

// Extention in taps to reduce mem copies
#define NLMS_EXT  (10*8)  

// block size in taps to optimize DTD calculation
#define DTD_LEN   16      


class AEC {
  // Time domain Filters
  IIR_HP hp00, hp1;             // DC-level remove Highpass)
  FIR_HP13 hp0;                 // 300Hz cut-off Highpass
  IIR1 Fx, Fe;                  // pre-whitening Highpass for x, e

  // Geigel DTD (Double Talk Detector)
  REAL max_max_x;               // max(|x[0]|, .. |x[L-1]|)
  int hangover;
  // optimize: less calculations for max()
  REAL max_x[NLMS_LEN / DTD_LEN];  
  int dtdCnt;
  int dtdNdx;

  // NLMS-pw
  REAL x[NLMS_LEN + NLMS_EXT];  // tap delayed loudspeaker signal
  REAL xf[NLMS_LEN + NLMS_EXT]; // pre-whitening tap delayed signal
  REAL w[NLMS_LEN];             // tap weights
  int j;                        // optimize: less memory copies
  int lastupdate;               // optimize: iterative dotp(x,x)
  double dotp_xf_xf;            // double to avoid loss of precision
  double Min_dotp_xf_xf;
  REAL s0avg;

public:
   AEC();

/* Geigel Double-Talk Detector
 *
 * in d: microphone sample (PCM as REALing point value)
 * in x: loudspeaker sample (PCM as REALing point value)
 * return: 0 for no talking, 1 for talking
 */
  int dtd(REAL d, REAL x);

/* Normalized Least Mean Square Algorithm pre-whitening (NLMS-pw)
 * The LMS algorithm was developed by Bernard Widrow
 * book: Widrow/Stearns, Adaptive Signal Processing, Prentice-Hall, 1985
 *
 * in mic: microphone sample (PCM as REALing point value)
 * in spk: loudspeaker sample (PCM as REALing point value)
 * in update: 0 for convolve only, 1 for convolve and update
 * return: echo cancelled microphone sample
 */
  REAL nlms_pw(REAL mic, REAL spk, int update);

/* Acoustic Echo Cancellation and Suppression of one sample
 * in   d:  microphone signal with echo
 * in   x:  loudspeaker signal
 * return:  echo cancelled microphone signal
 */
  int AEC::doAEC(int d, int x);

  float AEC::getambient() {
    return s0avg;
  };
  void AEC::setambient(float Min_xf) {
    dotp_xf_xf = Min_dotp_xf_xf = NLMS_LEN * Min_xf * Min_xf;
  };
};

#define _AEC_H
#endif


/***************** A.2 APPENDIX aec.cpp *****************/

/* aec.cpp
 *
 * Copyright (C) DFS Deutsche Flugsicherung (2004). All Rights Reserved.
 *
 * Acoustic Echo Cancellation NLMS-pw algorithm
 *
 * Version 1.3 filter created with www.dsptutor.freeuk.com
 */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include "aec.h"


/* Vector Dot Product */
REAL dotp(REAL a[], REAL b[]) {
  REAL sum0 = 0.0, sum1 = 0.0;
  int j;
 
  for (j = 0; j < NLMS_LEN; j+= 2) {
    // optimize: partial loop unrolling
    sum0 += a[j] * b[j];
    sum1 += a[j+1] * b[j+1];
  }
  return sum0+sum1;
}


AEC::AEC()
{
  max_max_x = 0.0f;
  hangover = 0;
  memset(max_x, 0, sizeof(max_x));
  dtdCnt = dtdNdx = 0;
 
  memset(x, 0, sizeof(x));
  memset(xf, 0, sizeof(xf));
  memset(w, 0, sizeof(w));
  j = NLMS_EXT;
  lastupdate = 0;
  s0avg = M80dB_PCM;
  setambient(Min_xf);
}

REAL AEC::nlms_pw(REAL mic, REAL spk, int update)
{
  REAL d = mic;                    // desired signal
  x[j] = spk;
  xf[j] = Fx.highpass(spk);     // pre-whitening of x
 
  // calculate error value
  // (mic signal - estimated mic signal from spk signal)
  REAL e = d - dotp(w, x + j);
  REAL ef = Fe.highpass(e);    // pre-whitening of e
  // optimize: iterative dotp(xf, xf)
  dotp_xf_xf += (xf[j]*xf[j] - xf[j+NLMS_LEN-1]*xf[j+NLMS_LEN-1]);
  if (update) {
    // calculate variable step size
    REAL mikro_ef = Stepsize * ef / dotp_xf_xf;

    // update tap weights (filter learning)
    int i;
    for (i = 0; i < NLMS_LEN; i += 2) {
      // optimize: partial loop unrolling
      w[i] += mikro_ef*xf[i+j];
      w[i+1] += mikro_ef*xf[i+j+1];
    }
  }

  if (--j < 0) {
    // optimize: decrease number of memory copies
    j = NLMS_EXT;
    memmove(x+j+1, x, (NLMS_LEN-1)*sizeof(REAL));    
    memmove(xf+j+1, xf, (NLMS_LEN-1)*sizeof(REAL));    
  }

  return e;
}


int AEC::dtd(REAL d, REAL x)
{
  // optimized implementation of max(|x[0]|, |x[1]|, .., |x[L-1]|):
  // calculate max of block (DTD_LEN values)
  x = fabsf(x);
  if (x > max_x[dtdNdx]) {
    max_x[dtdNdx] = x;
    if (x > max_max_x) {
      max_max_x = x;
    }
  }
  if (++dtdCnt >= DTD_LEN) {
    dtdCnt = 0;
    // calculate max of max
    max_max_x = 0.0f;
    for (int i = 0; i < NLMS_LEN/DTD_LEN; ++i) {
      if (max_x[i] > max_max_x) {
        max_max_x = max_x[i];
      }
    }
    // rotate Ndx
    if (++dtdNdx >= NLMS_LEN/DTD_LEN) dtdNdx = 0;
    max_x[dtdNdx] = 0.0f;
  }

  // The Geigel DTD algorithm with Hangover timer Thold
  if (fabsf(d) >= GeigelThreshold * max_max_x) {
    hangover = Thold;
  }
   
  if (hangover) --hangover;
 
  return (hangover > 0);
}


int AEC::doAEC(int d, int x)
{
  REAL s0 = (REAL)d;
  REAL s1 = (REAL)x;
 
  // Mic Highpass Filter - to remove DC
  s0 = hp00.highpass(s0);
 
  // Mic Highpass Filter - telephone users are used to 300Hz cut-off
  s0 = hp0.highpass(s0);

  // ambient mic level estimation
  s0avg += 1e-4f*(fabsf(s0) - s0avg);
 
  // Spk Highpass Filter - to remove DC
  s1 = hp1.highpass(s1);

  // Double Talk Detector
  int update = !dtd(s0, s1);

  // Acoustic Echo Cancellation
  s0 = nlms_pw(s0, s1, update);

  // Acoustic Echo Suppression
  if (update) {
    // Non Linear Processor (NLP): attenuate low volumes
    s0 *= NLPAttenuation;
  }
 
  // Saturation
  if (s0 > MAXPCM) {
    return (int)MAXPCM;
  } else if (s0 < -MAXPCM) {
    return (int)-MAXPCM;
  } else {
    return (int)roundf(s0);
  }
}


/***************** A.3 APPENDIX aec_test.cpp *****************/

/* aec_test.cpp
 *
 * Copyright (C) DFS Deutsche Flugsicherung (2004). All Rights Reserved.
 *
 * Test stub for Acoustic Echo Cancellation NLMS-pw algorithm
 * Author: Andre Adrian, DFS Deutsche Flugsicherung
 * <Andre.Adrian@dfs.de>
 *
 * compile
c++ -O2 -o aec_test aec_test.cpp aec.cpp -lm
 *
 * Version 1.3 set/get ambient in dB
 */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

#include "aec.h"

#define TAPS        (80*8)

typedef signed short MONO;

typedef struct {
  signed short l;
  signed short r;
} STEREO;

float dB2q(float dB)
{
  /* Dezibel to Ratio */
  return powf(10.0f, dB / 20.0f);
}

float q2dB(float q)
{
  /* Ratio to Dezibel */
  return 20.0f * log10f(q);
}

/* Read a raw audio file (8KHz sample frequency, 16bit PCM, stereo)
 * from stdin, echo cancel it and write it to stdout
 */
int main(int argc, char *argv[])
{
  STEREO inbuf[TAPS], outbuf[TAPS];

  fprintf(stderr, "usage: aec_test [ambient in dB] <in.raw >out.raw\n");

  AEC aec;

  if (argc >= 2) {
    aec.setambient(MAXPCM*dB2q(atof(argv[1])));    
  }

  int taps;
  while (taps = fread(inbuf, sizeof(STEREO), TAPS, stdin)) {
    int i;
    for (i = 0; i < taps; ++i) {
      int s0 = inbuf[i].l;      /* left channel microphone */
      int s1 = inbuf[i].r;      /* right channel speaker */

      /* and do NLMS */
      s0 = aec.doAEC(s0, s1);

      /* copy back */
      outbuf[i].l = 0;          /* left channel silence */
      outbuf[i].r = s0;         /* right channel echo cancelled mic */
    }

    fwrite(outbuf, sizeof(STEREO), taps, stdout);
  }

  float ambient = aec.getambient();
  float ambientdB = q2dB(ambient / 32767.0f);
  fprintf(stderr, "Ambient = %2.0f dB\n", ambientdB);
  fflush(NULL);
  return 0;
}