AM modulator

Carlos,

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RandyP

For AM you don't want 4-quadrant multiplication.  First, scale the modulation signal to lie between 0 and 1.  Then multiply it by the carrier sine (or cosine) signal, which should be balanced (+ and - values) around 0.  Of course, you need to multiply the two signals evaluated at corresponding *times*; if you have them represented in arrays but the array samples are not spaced so that they have the same time difference between adjacent elements for the two signals, you'll have to do some more work to properly align them.

Well, I finally solved the problem of implementing a simple AM modulator, basically what I did was use the support for sine functions through math.h library, and create 2 sine waves (I abandoned the tables to create them), and finally I multiplied them. I understand that this is somehow less efficient (correct me if I'm wrong), but it worked fine. The only problem is that when connecting an osciloscope to the output of the DSK I got what I expected, but when using CCS to graph the output the graph didn't correspond to the waveform observed using the osciloscope. I would be grateful if someone gave me some advice regarding the problem of graphing the output and its spectrum using CCS.

//sine_intr.c Sine generation using sin() function

#include <math.h>

#include "DSK6713_AIC23.h"                     // codec support

Uint32 fs=DSK6713_AIC23_FREQ_8KHZ;           //set sampling rate

#define DSK6713_AIC23_INPUT_MIC 0x0015

#define DSK6713_AIC23_INPUT_LINE 0x0011

Uint16 inputsource=DSK6713_AIC23_INPUT_MIC; // select input

#define SAMPLING_FREQ 8000

#define PI 3.14159265358979

float frequency = 2000.0;

float amplitude = 3000.0;

float frecuencia =400.0;

float amplitud= 1500.0;

float incremento;

float angulo=0.0;

float theta_increment;

float theta = 0.0;

float vector[100];

float salida;

short index=0;

interrupt void c_int11()

{

  theta_increment = 2*PI*frequency/SAMPLING_FREQ;

  theta += theta_increment;

  if (theta > 2*PI) theta -= 2*PI;

  incremento=2*PI*frecuencia/SAMPLING_FREQ;

  angulo +=incremento;

  if (angulo > 2*PI) angulo -= 2*PI;

  vector[index++]=(short)(amplitude*sin(theta)*(1+(amplitud/amplitude)*sin(angulo)));

  if(index>=100) index=0;

  output_left_sample((short)(amplitude*sin(theta)*(1+(amplitud/amplitude)*sin(angulo))));

  return;

}

void main()

{

  comm_intr();

  while(1);

}