No sound generated by SDL Audio Callback - c++

Question

I am trying to get a simple sinewave sound generation example working using SDL 2.0.12 on Windows 10, but no sound is being output.
I have no idea if it is a problem with the code or with the output device or the audio drivers.
I'd really appreciate suggestions of how I can debug the problem further.
#include <iostream>
#include "SDL.h"
float sine_freq = 200.0f;
float audio_volume = 4000.0f;
float audio_frequency;
void SineAudioCallback(void* userdata, Uint8* stream, int len) {
float* buf = (float*)stream;
for (int i = 0; i < len / 4; ++i) {
buf[i] = (float)(audio_volume * sin(2 * M_PI * i * audio_frequency));
}
return;
}
int main(int argc, char* argv[])
{
if (SDL_Init(SDL_INIT_AUDIO)) {
return 1;
}
std::cout << "[SDL] Audio driver: " << SDL_GetCurrentAudioDriver() << std::endl;
SDL_AudioSpec want, have;
SDL_zero(want);
want.freq = 5000;
want.format = AUDIO_F32;
want.channels = 2;
want.samples = 4096;
want.callback = SineAudioCallback;
std::cout <<"[SDL] Desired - frequency: " << want.freq
<< ", format: f " << SDL_AUDIO_ISFLOAT(want.format) << " s " << SDL_AUDIO_ISSIGNED(want.format) << " be " << SDL_AUDIO_ISBIGENDIAN(want.format) << " sz " << SDL_AUDIO_BITSIZE(want.format)
<< ", channels: " << (int)want.channels << ", samples: " << want.samples << std::endl;
SDL_AudioDeviceID dev = SDL_OpenAudioDevice(NULL, 0, &want, &have, SDL_AUDIO_ALLOW_ANY_CHANGE);
if (!dev) {
SDL_Quit();
return 1;
}
std::cout << "[SDL] Desired - frequency: " << have.freq
<< ", format: f " << SDL_AUDIO_ISFLOAT(have.format) << " s " << SDL_AUDIO_ISSIGNED(have.format) << " be " << SDL_AUDIO_ISBIGENDIAN(have.format) << " sz " << SDL_AUDIO_BITSIZE(have.format)
<< ", channels: " << (int)have.channels << ", samples: " << have.samples << std::endl;
audio_frequency = sine_freq / have.freq;
SDL_PauseAudioDevice(dev, 0);
SDL_Delay(10000);
SDL_CloseAudioDevice(dev);
SDL_Quit();
return 0;
}
The output I get is
[SDL] Audio driver: wasapi
[SDL] Desired - frequency: 5000, format: f 256 s 32768 be 0 sz 32, channels: 2, samples: 4096
[SDL] Desired - frequency: 5000, format: f 256 s 32768 be 0 sz 32, channels: 2, samples: 118
So there is a difference between SDL_AudioSpec I want and have, in the number of samples being reduced.
I also get an error message relating to the dlls although I'm not sure if it is important.
avcore\audiocore\client\audioclient\audioclientcore.cpp(1839)\AUDIOSES.DLL!00007FFC48E00F8E: (caller: 0000000070D6BE39) ReturnHr(1) tid(46dc) 80070057 The parameter is incorrect.

Answer 1

It turns out there were two things I needed to do to solve this problem.
For a float type the sound wave can only take values from -1 to 1, whilst I was using a much greater volume boost. I needed to change
float audio_volume = 1.0f;
For Windows, the direct sound and winmm drivers give better sample lengths than wasapi. For SDL2 on Windows, you need SDL_AUDIODRIVER=directsound or SDL_AUDIODRIVER=winmm set as an environment variable. More details here