Simple sound wave generator with SDL in c++

Question

i am having problems understanding how the audio part of the sdl library works now, i know that when you initialize it, you have to specify the frequency and a >>callback<< function, which i think is then called automatically at the given frequency. can anyone who worked with the sdl library write a simple example that would use sdl_audio to generate a 440 hz square wave (since it is the simplest waveform) at a sampling frequency of 44000 hz?

Answer 1

This is a minimal example of how to play a sine wave in SDL2. Make sure to call SDL_Init(SDL_INIT_AUDIO) before creating an instance of Sound.

Sound.h

#include <cstdint>
#include <SDL2/SDL.h>

class Sound
{
public:
    Sound();
    ~Sound();
    void play();
    void stop();

    const double m_sineFreq;
    const double m_sampleFreq;
    const double m_samplesPerSine;
    uint32_t m_samplePos;

private:
    static void SDLAudioCallback(void *data, Uint8 *buffer, int length);

    SDL_AudioDeviceID m_device;
};

Sound.cpp

#include "Sound.h"
#include <cmath>
#include <iostream>

Sound::Sound()
    : m_sineFreq(1000),
      m_sampleFreq(44100),
      m_samplesPerSine(m_sampleFreq / m_sineFreq),
      m_samplePos(0)
{
    SDL_AudioSpec wantSpec, haveSpec;

    SDL_zero(wantSpec);
    wantSpec.freq = m_sampleFreq;
    wantSpec.format = AUDIO_U8;
    wantSpec.channels = 1;
    wantSpec.samples = 2048;
    wantSpec.callback = SDLAudioCallback;
    wantSpec.userdata = this;

    m_device = SDL_OpenAudioDevice(NULL, 0, &wantSpec, &haveSpec, SDL_AUDIO_ALLOW_FORMAT_CHANGE);
    if (m_device == 0)
    {
        std::cout << "Failed to open audio: " << SDL_GetError() << std::endl;
    }
}

Sound::~Sound()
{
    SDL_CloseAudioDevice(m_device);
}

void Sound::play()
{
    SDL_PauseAudioDevice(m_device, 0);
}

void Sound::stop()
{
    SDL_PauseAudioDevice(m_device, 1);
}

void Sound::SDLAudioCallback(void *data, Uint8 *buffer, int length)
{
    Sound *sound = reinterpret_cast<Sound*>(data);

    for(int i = 0; i < length; ++i)
    {
        buffer[i] = (std::sin(sound->m_samplePos / sound->m_samplesPerSine * M_PI * 2) + 1) * 127.5;
        ++sound->m_samplePos;
    }
}

Answer 2

The Introduction to SDL (2011 cached version: 2) has got a neat example of using SDL Sound library that should get you started: http://www.libsdl.org/intro.en/usingsound.html

EDIT: Here is a working program that does what you asked for. I modified a bit the code found here: http://www.dgames.org/beep-sound-with-sdl/

#include <SDL/SDL.h>
#include <SDL/SDL_audio.h>
#include <queue>
#include <cmath>

const int AMPLITUDE = 28000;
const int FREQUENCY = 44100;

struct BeepObject
{
    double freq;
    int samplesLeft;
};

class Beeper
{
private:
    double v;
    std::queue<BeepObject> beeps;
public:
    Beeper();
    ~Beeper();
    void beep(double freq, int duration);
    void generateSamples(Sint16 *stream, int length);
    void wait();
};

void audio_callback(void*, Uint8*, int);

Beeper::Beeper()
{
    SDL_AudioSpec desiredSpec;

    desiredSpec.freq = FREQUENCY;
    desiredSpec.format = AUDIO_S16SYS;
    desiredSpec.channels = 1;
    desiredSpec.samples = 2048;
    desiredSpec.callback = audio_callback;
    desiredSpec.userdata = this;

    SDL_AudioSpec obtainedSpec;

    // you might want to look for errors here
    SDL_OpenAudio(&desiredSpec, &obtainedSpec);

    // start play audio
    SDL_PauseAudio(0);
}

Beeper::~Beeper()
{
    SDL_CloseAudio();
}

void Beeper::generateSamples(Sint16 *stream, int length)
{
    int i = 0;
    while (i < length) {

        if (beeps.empty()) {
            while (i < length) {
                stream[i] = 0;
                i++;
            }
            return;
        }
        BeepObject& bo = beeps.front();

        int samplesToDo = std::min(i + bo.samplesLeft, length);
        bo.samplesLeft -= samplesToDo - i;

        while (i < samplesToDo) {
            stream[i] = AMPLITUDE * std::sin(v * 2 * M_PI / FREQUENCY);
            i++;
            v += bo.freq;
        }

        if (bo.samplesLeft == 0) {
            beeps.pop();
        }
    }
}

void Beeper::beep(double freq, int duration)
{
    BeepObject bo;
    bo.freq = freq;
    bo.samplesLeft = duration * FREQUENCY / 1000;

    SDL_LockAudio();
    beeps.push(bo);
    SDL_UnlockAudio();
}

void Beeper::wait()
{
    int size;
    do {
        SDL_Delay(20);
        SDL_LockAudio();
        size = beeps.size();
        SDL_UnlockAudio();
    } while (size > 0);

}

void audio_callback(void *_beeper, Uint8 *_stream, int _length)
{
    Sint16 *stream = (Sint16*) _stream;
    int length = _length / 2;
    Beeper* beeper = (Beeper*) _beeper;

    beeper->generateSamples(stream, length);
}

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

    int duration = 1000;
    double Hz = 440;

    Beeper b;
    b.beep(Hz, duration);
    b.wait();

    return 0;
}

Good luck.

Answer 3

A boiled-down variant of the beeper-example, reduced to the bare minimum (with error-handling).

#include <math.h>
#include <SDL.h>
#include <SDL_audio.h>

const int AMPLITUDE = 28000;
const int SAMPLE_RATE = 44100;

void audio_callback(void *user_data, Uint8 *raw_buffer, int bytes)
{
    Sint16 *buffer = (Sint16*)raw_buffer;
    int length = bytes / 2; // 2 bytes per sample for AUDIO_S16SYS
    int &sample_nr(*(int*)user_data);

    for(int i = 0; i < length; i++, sample_nr++)
    {
        double time = (double)sample_nr / (double)SAMPLE_RATE;
        buffer[i] = (Sint16)(AMPLITUDE * sin(2.0f * M_PI * 441.0f * time)); // render 441 HZ sine wave
    }
}

int main(int argc, char *argv[])
{
    if(SDL_Init(SDL_INIT_AUDIO) != 0) SDL_Log("Failed to initialize SDL: %s", SDL_GetError());

    int sample_nr = 0;

    SDL_AudioSpec want;
    want.freq = SAMPLE_RATE; // number of samples per second
    want.format = AUDIO_S16SYS; // sample type (here: signed short i.e. 16 bit)
    want.channels = 1; // only one channel
    want.samples = 2048; // buffer-size
    want.callback = audio_callback; // function SDL calls periodically to refill the buffer
    want.userdata = &sample_nr; // counter, keeping track of current sample number

    SDL_AudioSpec have;
    if(SDL_OpenAudio(&want, &have) != 0) SDL_LogError(SDL_LOG_CATEGORY_AUDIO, "Failed to open audio: %s", SDL_GetError());
    if(want.format != have.format) SDL_LogError(SDL_LOG_CATEGORY_AUDIO, "Failed to get the desired AudioSpec");

    SDL_PauseAudio(0); // start playing sound
    SDL_Delay(1000); // wait while sound is playing
    SDL_PauseAudio(1); // stop playing sound

    SDL_CloseAudio();

    return 0;
}