Simple sound wave generator with SDL in c++ - 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

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 2

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;
}

Answer 3

SDL 2 C example
The following code produces a sinusoidal sound, it is adapted from: https://codereview.stackexchange.com/questions/41086/play-some-sine-waves-with-sdl2
main.c
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <SDL2/SDL.h>
const double ChromaticRatio = 1.059463094359295264562;
const double Tao = 6.283185307179586476925;
Uint32 sampleRate = 48000;
Uint32 frameRate = 60;
Uint32 floatStreamLength = 1024;
Uint32 samplesPerFrame;
Uint32 msPerFrame;
double practicallySilent = 0.001;
Uint32 audioBufferLength = 48000;
float *audioBuffer;
SDL_atomic_t audioCallbackLeftOff;
Sint32 audioMainLeftOff;
Uint8 audioMainAccumulator;
SDL_AudioDeviceID AudioDevice;
SDL_AudioSpec audioSpec;
SDL_Event event;
SDL_bool running = SDL_TRUE;
typedef struct {
float *waveform;
Uint32 waveformLength;
double volume;
double pan;
double frequency;
double phase;
} voice;
void speak(voice *v) {
float sample;
Uint32 sourceIndex;
double phaseIncrement = v->frequency/sampleRate;
Uint32 i;
if (v->volume > practicallySilent) {
for (i = 0; (i + 1) < samplesPerFrame; i += 2) {
v->phase += phaseIncrement;
if (v->phase > 1)
v->phase -= 1;
sourceIndex = v->phase*v->waveformLength;
sample = v->waveform[sourceIndex]*v->volume;
audioBuffer[audioMainLeftOff+i] += sample*(1-v->pan);
audioBuffer[audioMainLeftOff+i+1] += sample*v->pan;
}
}
else {
for (i=0; i<samplesPerFrame; i+=1)
audioBuffer[audioMainLeftOff+i] = 0;
}
audioMainAccumulator++;
}
double getFrequency(double pitch) {
return pow(ChromaticRatio, pitch-57)*440;
}
int getWaveformLength(double pitch) {
return sampleRate / getFrequency(pitch)+0.5f;
}
void buildSineWave(float *data, Uint32 length) {
Uint32 i;
for (i=0; i < length; i++)
data[i] = sin(i*(Tao/length));
}
void logSpec(SDL_AudioSpec *as) {
printf(
" freq______%5d\n"
" format____%5d\n"
" channels__%5d\n"
" silence___%5d\n"
" samples___%5d\n"
" size______%5d\n\n",
(int) as->freq,
(int) as->format,
(int) as->channels,
(int) as->silence,
(int) as->samples,
(int) as->size
);
}
void logVoice(voice *v) {
printf(
" waveformLength__%d\n"
" volume__________%f\n"
" pan_____________%f\n"
" frequency_______%f\n"
" phase___________%f\n",
v->waveformLength,
v->volume,
v->pan,
v->frequency,
v->phase
);
}
void logWavedata(float *floatStream, Uint32 floatStreamLength, Uint32 increment) {
printf("\n\nwaveform data:\n\n");
Uint32 i=0;
for (i = 0; i < floatStreamLength; i += increment)
printf("%4d:%2.16f\n", i, floatStream[i]);
printf("\n\n");
}
void audioCallback(void *unused, Uint8 *byteStream, int byteStreamLength) {
float* floatStream = (float*) byteStream;
Sint32 localAudioCallbackLeftOff = SDL_AtomicGet(&audioCallbackLeftOff);
Uint32 i;
for (i = 0; i < floatStreamLength; i++) {
floatStream[i] = audioBuffer[localAudioCallbackLeftOff];
localAudioCallbackLeftOff++;
if (localAudioCallbackLeftOff == audioBufferLength)
localAudioCallbackLeftOff = 0;
}
SDL_AtomicSet(&audioCallbackLeftOff, localAudioCallbackLeftOff);
}
int init(void) {
SDL_Init(SDL_INIT_AUDIO | SDL_INIT_TIMER);
SDL_AudioSpec want;
SDL_zero(want);
want.freq = sampleRate;
want.format = AUDIO_F32;
want.channels = 2;
want.samples = floatStreamLength;
want.callback = audioCallback;
AudioDevice = SDL_OpenAudioDevice(NULL, 0, &want, &audioSpec, SDL_AUDIO_ALLOW_FORMAT_CHANGE);
if (AudioDevice == 0) {
printf("\nFailed to open audio: %s\n", SDL_GetError());
return 1;
}
printf("want:\n");
logSpec(&want);
printf("audioSpec:\n");
logSpec(&audioSpec);
if (audioSpec.format != want.format) {
printf("\nCouldn't get Float32 audio format.\n");
return 2;
}
sampleRate = audioSpec.freq;
floatStreamLength = audioSpec.size / 4;
samplesPerFrame = sampleRate / frameRate;
msPerFrame = 1000 / frameRate;
audioMainLeftOff = samplesPerFrame * 8;
SDL_AtomicSet(&audioCallbackLeftOff, 0);
if (audioBufferLength % samplesPerFrame)
audioBufferLength += samplesPerFrame - (audioBufferLength % samplesPerFrame);
audioBuffer = malloc(sizeof(float) * audioBufferLength);
return 0;
}
int onExit(void) {
SDL_CloseAudioDevice(AudioDevice);
SDL_Quit();
return 0;
}
int main(int argc, char *argv[]) {
float syncCompensationFactor = 0.0016;
Sint32 mainAudioLead;
Uint32 i;
voice testVoiceA;
voice testVoiceB;
voice testVoiceC;
testVoiceA.volume = 1;
testVoiceB.volume = 1;
testVoiceC.volume = 1;
testVoiceA.pan = 0.5;
testVoiceB.pan = 0;
testVoiceC.pan = 1;
testVoiceA.phase = 0;
testVoiceB.phase = 0;
testVoiceC.phase = 0;
testVoiceA.frequency = getFrequency(45);
testVoiceB.frequency = getFrequency(49);
testVoiceC.frequency = getFrequency(52);
Uint16 C0waveformLength = getWaveformLength(0);
testVoiceA.waveformLength = C0waveformLength;
testVoiceB.waveformLength = C0waveformLength;
testVoiceC.waveformLength = C0waveformLength;
float sineWave[C0waveformLength];
buildSineWave(sineWave, C0waveformLength);
testVoiceA.waveform = sineWave;
testVoiceB.waveform = sineWave;
testVoiceC.waveform = sineWave;
if (init())
return 1;
SDL_Delay(42);
SDL_PauseAudioDevice(AudioDevice, 0);
while (running) {
while (SDL_PollEvent(&event) != 0) {
if (event.type == SDL_QUIT) {
running = SDL_FALSE;
}
}
for (i = 0; i < samplesPerFrame; i++)
audioBuffer[audioMainLeftOff+i] = 0;
speak(&testVoiceA);
speak(&testVoiceB);
speak(&testVoiceC);
if (audioMainAccumulator > 1) {
for (i=0; i<samplesPerFrame; i++) {
audioBuffer[audioMainLeftOff+i] /= audioMainAccumulator;
}
}
audioMainAccumulator = 0;
audioMainLeftOff += samplesPerFrame;
if (audioMainLeftOff == audioBufferLength)
audioMainLeftOff = 0;
mainAudioLead = audioMainLeftOff - SDL_AtomicGet(&audioCallbackLeftOff);
if (mainAudioLead < 0)
mainAudioLead += audioBufferLength;
if (mainAudioLead < floatStreamLength)
printf("An audio collision may have occured!\n");
SDL_Delay(mainAudioLead * syncCompensationFactor);
}
onExit();
return 0;
}
Compile and run:
gcc -ggdb3 -O3 -std=c99 -Wall -Wextra -pedantic -o main.out main.c -lSDL2 -lm
./main.out
Should be easy to turn this into a simple piano with: https://github.com/cirosantilli/cpp-cheat/blob/f734a2e76fbcfc67f707ae06be7a2a2ef5db47d1/c/interactive/audio_gen.c#L44
For wav manipulation, also check the official examples:
http://hg.libsdl.org/SDL/file/e12c38730512/test/testresample.c
http://hg.libsdl.org/SDL/file/e12c38730512/test/loopwave.c
Tested on Ubuntu 19.10, SDL 2.0.10.

Answer 4

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;
}
}