How do I generate a tone using SDL_audio? - c++

Question

I am trying to generate a simple, constant sine tone using SDL_audio. I have a small helper class that can be called to turn the tone on/off, change the frequency, and change the wave shape. I have followed some examples I could find on the web and got the following:
beeper.h
#pragma once
#include <SDL.h>
#include <SDL_audio.h>
#include <cmath>
#include "logger.h"
class Beeper {
private:
//Should there be sound right now
bool soundOn = true;
//Type of wave that should be generated
int waveType = 0;
//Tone that the wave will produce (may or may not be applicable based on wave type)
float waveTone = 440;
//Running index for sampling
float samplingIndex = 0;
//These are useful variables that cannot be changed outside of this file:
//Volume
const Sint16 amplitude = 32000;
//Sampling rate
const int samplingRate = 44100;
//Buffer size
const int bufferSize = 1024;
//Samples a sine wave at a given index
float sampleSine(float index);
//Samples a square wave at a given index
float sampleSquare(float index);
public:
//Initializes SDL audio, audio device, and audio specs
void initializeAudio();
//Function called by SDL audio_callback that fills stream with samples
void generateSamples(short* stream, int length);
//Turn sound on or off
void setSoundOn(bool soundOnOrOff);
//Set timbre of tone produced by beeper
void setWaveType(int waveTypeID);
//Set tone (in Hz) produced by beeper
void setWaveTone(int waveHz);
};
beeper.cpp
#include <beeper.h>
void fillBuffer(void* userdata, Uint8* _stream, int len) {
short * stream = reinterpret_cast<short*>(_stream);
int length = len;
Beeper* beeper = (Beeper*)userdata;
beeper->generateSamples(stream, length);
}
void Beeper::initializeAudio() {
SDL_AudioSpec desired, returned;
SDL_AudioDeviceID devID;
SDL_zero(desired);
desired.freq = samplingRate;
desired.format = AUDIO_S16SYS; //16-bit audio
desired.channels = 1;
desired.samples = bufferSize;
desired.callback = &fillBuffer;
desired.userdata = this;
devID = SDL_OpenAudioDevice(SDL_GetAudioDeviceName(0,0), 0, &desired, &returned, SDL_AUDIO_ALLOW_FORMAT_CHANGE);
SDL_PauseAudioDevice(devID, 0);
}
void Beeper::generateSamples(short *stream, int length) {
int samplesToWrite = length / sizeof(short);
for (int i = 0; i < samplesToWrite; i++) {
if (soundOn) {
if (waveType == 0) {
stream[i] = (short)(amplitude * sampleSine(samplingIndex));
}
else if (waveType == 1) {
stream[i] = (short)(amplitude * 0.8 * sampleSquare(samplingIndex));
}
}
else {
stream[i] = 0;
}
//INFO << "Sampling index: " << samplingIndex;
samplingIndex += (waveTone * M_PI * 2) / samplingRate;
//INFO << "Stream input: " << stream[i];
if (samplingIndex >= (M_PI*2)) {
samplingIndex -= M_PI * 2;
}
}
}
void Beeper::setSoundOn(bool soundOnOrOff) {
soundOn = soundOnOrOff;
//if (soundOnOrOff) {
// samplingIndex = 0;
//}
}
void Beeper::setWaveType(int waveTypeID) {
waveType = waveTypeID;
//samplingIndex = 0;
}
void Beeper::setWaveTone(int waveHz) {
waveTone = waveHz;
//samplingIndex = 0;
}
float Beeper::sampleSine(float index) {
double result = sin((index));
//INFO << "Sine result: " << result;
return result;
}
float Beeper::sampleSquare(float index)
{
int unSquaredSin = sin((index));
if (unSquaredSin >= 0) {
return 1;
}
else {
return -1;
}
}
The callback function is being called and the generateSamples function is loading data into the stream, but I cannot hear anything but a very slight click at irregular periods. I have had a look at the data inside the stream and it follows a pattern that I would expect for a scaled sine wave with a 440 Hz frequency. Is there something obvious that I am missing? I did notice that the size of the stream is double what I put when declaring the SDL_AudioSpec and calling SDL_OpenAudioDevice. Why is that?

Answer 1

Answered my own question! When opening the audio device I used the flag SDL_AUDIO_ALLOW_FORMAT_CHANGE which meant that SDL was actually using a float buffer instead of the short buffer that I expected. This was causing issues in a couple of places that were hard to detect (the stream being double the amount of bytes I was expecting should have tipped me off). I changed that parameter in SDL_OpenAudioDevice() to 0 and it worked as expected!