What determines a new call to the callback function

[yanostulens]

Hi,

The documentation of miniaudio tells us: 'The idea is that you choose a physical device to emit or capture audio from, and then move data to/from the device when miniaudio tells you to.' I was wondering how this works internally (especially for playback), how does miniaudio decide when it needs to request more audio frames?

Thanks in advance!

[mackron]

It's all ultimately controlled by the backend (WASAPI, PulseAudio, etc.). miniaudio will supply a hint to the backend that specifies what we would like, but the backend is ultimately in control of when the callback is fired. The specific way in which each backend notifies miniaudio that it needs more data is different for each backend. Some use a callback whereas others use blocking reads and writes.

In the device config there's two variables that control the size of the callback: periodSizeInFrames and periodSizeInMilliseconds (you would typically only set one of these). By default, miniaudio will always fire the callback at an interval directly related to the values, and the frame count will always be constant. In this case there is an internal buffer that is allocated by miniaudio. When the backend requests more data, miniaudio will first deliver any data that's already available in that buffer. When no more data is available in the buffer, miniaudio will fire the callback to get it refilled with fresh data which is then passed on the backend.

There is also a config option called noFixedSizedCallback. If that's set to true, all of that internal buffering stuff is skipped, and the callback is fired whenever the backend asks for more data, the frequency of which could be anything.

If you want to get into the real details, do a search for ma_device__on_data() in miniaudio.h. If you want details on how each specific backend requests more data you would need to look at the code for each one as they're all different and there's too much to go through here.

[yanostulens]

Great answer, thank you for the enlightenment!