Sending sine wave values from array to audio output - c

Question

Im trying to get a c program to send a sine wave tone to the audio output of my raspberry pi using ALSA
Im generating the buffer of sine wave values but when i send them it doesnt sound like a sine - but like low-mid frequency noise?
I have a dumped the values from the array into a .csv and plotted in excel to verify the sine wave is good
Just wondering have a got stuff fundamentally incorrect in this program, if someone could spot something id be very grateful, thanks
edit: final working code below!!!
#include <alsa/asoundlib.h>
#include <alsa/pcm.h>
#include <math.h>
#define BUFFER_LEN 48000
static char *device = "default"; //soundcard
snd_output_t *output = NULL;
float buffer [BUFFER_LEN];
int main(void)
{
int err;
int j,k;
int f = 440; //frequency
int fs = 48000; //sampling frequency
snd_pcm_t *handle;
snd_pcm_sframes_t frames;
// ERROR HANDLING
if ((err = snd_pcm_open(&handle, device, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
printf("Playback open error: %s\n", snd_strerror(err));
exit(EXIT_FAILURE);
}
if ((err = snd_pcm_set_params(handle,
SND_PCM_FORMAT_FLOAT,
SND_PCM_ACCESS_RW_INTERLEAVED,
1,
48000,
1,
500000)) < 0) {
printf("Playback open error: %s\n", snd_strerror(err));
exit(EXIT_FAILURE);
}
// SINE WAVE
printf("Sine tone at %dHz ",f);
for (k=0; k<BUFFER_LEN; k++){
buffer[k] = (sin(2*M_PI*f/fs*k)); //sine wave value generation
}
for (j=0; j<5; j++){
frames = snd_pcm_writei(handle, buffer, BUFFER_LEN); //sending values to sound driver
}
snd_pcm_close(handle);
return 0;
}

Answer 1

You have configured the sample format SND_PCM_FORMAT_U8, but the actuall buffer contains 32-bit floating-point samples.
Use SND_PCM_FORMAT_FLOAT, or define the buffer as an array of unsigned char.
Furthermore, you have confused the loop to initialize the buffer and the loop to play the data, and many bytes/frames numbers, and fs is wrong; you need to use something like this:
for (i = 0; i < BUFFER_LEN; i++)
buffer [i] = sin(2*M_PI*f/48000*i); // sine wave value generation
for (i = 0; i < 10 * 48000 / BUFFER_LEN; i++) { // 10 seconds
frames = snd_pcm_writei(handle, buffer, BUFFER_LEN);
if (frames < 0)
frames = snd_pcm_recover(handle, frames, 0);
if (frames < 0) {
printf("snd_pcm_writei failed: %s\n", snd_strerror(frames));
break;
}
if (frames > 0 && frames < BUFFER_LEN)
printf("Short write (expected %li, wrote %li)\n", BUFFER_LEN, frames);
}