Generating sine wave array in C with noise

Question

Total noob here. Can someone give me an example on how i can generate a 2kHz sine wave array with white noise of variance 0.01 in C? This is what I have so far:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define PI 3.141592653589793

int main() {
    int i;
    double buffer[10000];

    for(i = 0; i < 10000; i++)
    {
        buffer[i] = sin(2000 * (2 * PI) * i / 6000) + sqrt(0.01)rand;
    }

    return 0;
}

not sure how to add the white noise with the rand function :S — rwar, Sep 19 '15 at 17:29
(Except for the missing random seed and the typos at sqrt(0.01)rand) — Peter Goldsborough, Sep 19 '15 at 17:29
Well you would certainly have to seed your random number generator with srand((unsigned)time(0)), and the sqrt(0.01)rand should probably be sqrt(0.01) * rand()? — Peter Goldsborough, Sep 19 '15 at 17:30
what do you mean by "seed your random number generator with srand((unsigned)time(0))"? — rwar, Sep 19 '15 at 17:32
How about reading the man pages for `rand` and `srand`? Too complicated? — too honest for this site, Sep 19 '15 at 17:35
A random-number-generator will not be random unless you give it a different start/seed value each time you call it (rand isn't magic, it's just an algorithm that does fancy things given an input to output almost-random values). Before you use rand() in your program, you have to call srand() once with a unique value. Because time doesn't stop that often it's a good idea to pass it the number of seconds since the epoch. But that information really is out there, so see here for example: http://stackoverflow.com/questions/822323/how-to-generate-a-random-number-in-c — Peter Goldsborough, Sep 19 '15 at 17:36
one thing though, when i print out the results from "buffer array", i get huge values. what I'm trying to do is replicate this matlab code into c: x = sin(2*pi*(2000)*(0:1/6000:100)); signal = x + sqrt(0.01)*(randn(size(x)); — rwar, Sep 19 '15 at 22:14

score 0 · Accepted Answer · answered Sep 19 '15 at 17:49

(For reference)

You first have to seed the random-number generator using srand(), to which you should pass a unique value at every program-run.

Your code, modified for correctness:

#include <math.h>
#include <stdlib.h>
#include <time.h>

int main() {
    srand((unsigned)time(NULL));

    int i;
    double buffer[10000];

    for(i = 0; i < 10000; i++)
    {
        buffer[i] = sin(2000 * (2 * M_PI) * i / 6000) + sqrt(0.01) * rand();
    }

    /* ... */

    return 0;
}

Generating sine wave array in C with noise

1 Answers1