Sound playback not working when using QueueAudio on C++ - c++

Question

I am trying to play a sin wave sound with SDL2 by using the audio queue on C++. In order to do that, I have created a class "Speaker", which has a pushBeep function that is called every time a beep needs to be generated. I have created an AudioDevice successfully, and it is also successful when I do the QueueAudio to the device (I have checked on the debugger) but I can't seem to get any sound out of it.
I have tried changing the way I generate the samples in numerous ways, also, as I said previously, I have checked that the device is properly opened and the QueueAudio returns 0 for success.
This is the class
Speaker::Speaker()
{
SDL_AudioSpec ds;
ds.freq = Speaker::SPEAKER_FREQUENCY;
ds.format = AUDIO_F32;
ds.channels = 1;
ds.samples = 4096;
ds.callback = NULL;
ds.userdata = this;
SDL_AudioSpec os;
this->dev = SDL_OpenAudioDevice(NULL, 0, &ds, &os, NULL);
std::cout << "DEVICE: " << this->dev << std::endl;
SDL_PauseAudioDevice(this->dev, 0);
}
Speaker::~Speaker()
{
SDL_CloseAudioDevice(this->dev);
}
void Speaker::pushBeep(double freq, int duration) {
int nSamples = duration * Speaker::SPEAKER_FREQUENCY / 1000;
float* samples = new float[nSamples];
double v = 0.0;
for (int idx = 0; idx < nSamples; idx++) {
//float value = (float)Speaker::SPEAKER_AMPLITUDE * std::sin(v * 2 * M_PI / Speaker::SPEAKER_FREQUENCY);
float value = 440.0;
samples[idx] = value;
v += freq;
}
int a = SDL_QueueAudio(this->dev, (void*)samples, nSamples * sizeof(float));
std::cout << a << std::endl;
delete[] samples;
samples = NULL;
}
And this is how I call it
Speaker s;
s.pushBeep(440.0, 1000);
When I try with the sin wave generation code (commented) it gives me a "double to float loss of precision" error. When I use the fixed value (not commented) it does not give the error, but it still does not work.
I expect the program to output the sound.

Answer 1

Couple of things you are missing, or maybe you didn't add to your code snippet. You didn't specify an audio callback so when you call SDL_QueueAudio(); it didn't know what to do with the data I'm pretty sure. And you weren't calling SDL_PauseAudioDevice() in your example with the delay.
#include <math.h>
#include <SDL2/SDL.h>
#include <SDL2/SDL_audio.h>
#include <iostream>
namespace AudioGen
{
const int AMPLITUDE = 1;
const int SAMPLE_RATE = 44000;
// Globals
float *in_buffer;
SDL_atomic_t callback_sample_pos;
SDL_Event event;
SDL_bool running = SDL_TRUE;
/**
* Structure for holding audio metadata such as frequency
*/
struct AudioData
{
int sampleNum;
float frequency;
};
void audio_callback(void *user_data, Uint8 *raw_buffer, int bytes)
{
float *buffer = (float*)raw_buffer;
AudioData &audio_data(*static_cast<AudioData*>(user_data));
int nSamples = bytes / 4; // For F32
std::cout << nSamples << std::endl;
for(int i = 0; i < nSamples; i++, audio_data.sampleNum++)
{
double time = (double)audio_data.sampleNum / (double)SAMPLE_RATE;
buffer[i] = (float)(AMPLITUDE * sin(2.0f * M_PI * audio_data.frequency * time));
}
}
int buffer_length;
void callback(void *user_data, Uint8 *raw_buffer, int bytes)
{
float *buffer = (float*)raw_buffer;
int nSamples = bytes/4;
auto local_sample_pos = SDL_AtomicGet(&callback_sample_pos);
for(int i = 0; i < nSamples; ++i)
{
// Stop running audio if all samples are finished playing
if(buffer_length == local_sample_pos)
{
running = SDL_FALSE;
break;
}
buffer[i] = in_buffer[local_sample_pos];
++local_sample_pos;
}
SDL_AtomicSet(&callback_sample_pos, local_sample_pos);
}
class Speaker
{
public:
Speaker()
{
SDL_Init(SDL_INIT_AUDIO);
SDL_AudioSpec ds;
ds.freq = SAMPLE_RATE;
ds.format = AUDIO_F32;
ds.channels = 1;
ds.samples = 4096;
ds.callback = callback;
ds.userdata = &ad; // metadata for frequency
SDL_AudioSpec os;
dev = SDL_OpenAudioDevice(NULL, 0, &ds, &os, SDL_AUDIO_ALLOW_FORMAT_CHANGE);
}
~Speaker()
{
SDL_CloseAudioDevice(dev);
SDL_Quit();
}
void pushBeep(float frequency, int duration)
{
ad.frequency = frequency; // set the frequency for the beep
SDL_PauseAudioDevice(dev, 0);
SDL_Delay(duration); // wait while sound is playing
SDL_PauseAudioDevice(dev, 1);
}
void pushBeep2(float frequency, int duration )
{
int nSamples = duration * SAMPLE_RATE / 1000;
in_buffer = new float[nSamples];
buffer_length = nSamples;
for (int idx = 0; idx < nSamples; idx++) {
double time = (double)idx / (double)SAMPLE_RATE;
in_buffer[idx] = (float)(AMPLITUDE * std::sin(2.0f * M_PI * frequency * time));
}
SDL_QueueAudio(dev, in_buffer, nSamples * sizeof(float));
SDL_PauseAudioDevice(dev, 0);
while(running){
while(SDL_PollEvent(&event)!=0);
}
delete[] in_buffer;
}
private:
SDL_AudioDeviceID dev;
AudioData ad;
int sampleNum = 0;
};
} // End of namespace AudioGen
int main(int argc, char *argv[])
{
AudioGen::Speaker speaker;
//speaker.pushBeep(440, 1000);
speaker.pushBeep2(440.0f, 1000);
return 0;
}