An amplitude shift keying (ASK) demodulator.

ask-demod.c

// An ASK Demodulator
// Simplicity first. Get it working, then improve!
// gcc ask-demod.c -o askdemod -lm -Wall
// Usage: ./ask-demod ask-msg.raw ask-msg.txt

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

int main(int argc, char* argv[]){

  int sampleRate = 8000;
  int baud = 50;
  int sampPerSym = sampleRate/baud;

  // this is half the value of RMS for a sine wave 
  int threshold = ((sqrt(2)/2)*pow(2,15))/2;
  int sign = -1;

  short int sample = 0;
  short int output = 0;
  short int rsample = 0;
  int numBits = 0;
  int sampsLeft = 0;
  int counter = 0;

  short int movAvg[sampPerSym+1];
  int movAvgI = 0;
  int sumMovAvg = 0;
  memset(&movAvg, 0, sizeof(movAvg));

  char bitBuffer = 0;
  char bitCounter = 0;
  int i = 0;

  // open file
  FILE *filein;
  FILE *fileout;

  // opens the file as read only
  filein = fopen(argv[1], "rb");
  if(!filein) {
    printf("Unable to open input file\n\n");
    return 1;
  }


  // opens the file as overwrite, binary
  fileout = fopen(argv[2], "wb");
  if(!fileout) {
    printf("Unable to open output file\n\n");
    return 1;
  }

  while(!feof(filein)){
    // read another sample in
    fread(&sample, sizeof(short), 1, filein);

    // rectify by taking abs
    rsample = abs(sample);

    // moving average
    movAvg[movAvgI] = rsample/sampPerSym;
    sumMovAvg = sumMovAvg + (movAvg[movAvgI]) - (movAvg[(movAvgI+1)%sampPerSym]);
    movAvgI++;
    if(movAvgI >= sampPerSym) movAvgI = 0;

    // hard limit the samples
    if (sumMovAvg >= threshold) {
      output = 1;
    } else {
      output = -1;
    }

    // make bits out of the data
    if (output == sign) {
      counter++;
    } else {
      // count number of bits we have adding a partal if its good enough
      numBits = (int)counter/sampPerSym;
      sampsLeft = counter%sampPerSym;
      if (sampsLeft > (0.7*sampPerSym)){
        numBits++;
      }

      // add the bits to our buffer
      for (i = 0; i < numBits; i++) {
        bitBuffer <<= 1;

        // put a 1 or zero in the lsb after shifting the buffer to the left
        if (sign == 1) {
          bitBuffer ^= (0x01);
        } else {
          bitBuffer &= (0xfe);
        }
        bitCounter++;

        // when we have enough to write a char out do so
        if(bitCounter > 7){
          bitCounter = 0;
          fwrite(&bitBuffer, sizeof(char), 1, fileout);
        }
      }

      counter = 0;
      sign *= -1;
    }

    // audio stuff
    // fwrite(&rsample, sizeof(short), 1, fileout);
    // fwrite(&sumMovAvg, sizeof(short), 1, fileout);
    // fwrite(&output, sizeof(short), 1, fileout);
  }

return 0;
}

/*
BSD 2-clause license

Copyright 2013 Jimmy Carter, KG4SGP. All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are
permitted provided that the following conditions are met:

   1. Redistributions of source code must retain the above copyright notice, this list of
      conditions and the following disclaimer.

   2. Redistributions in binary form must reproduce the above copyright notice, this list
      of conditions and the following disclaimer in the documentation and/or other materials
      provided with the distribution.

THIS SOFTWARE IS PROVIDED BY Jimmy Carter ''AS IS'' AND ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL Jimmy Carter OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/