dataCallback void* pOutput format

[duane34]

This seems like an simple question, but I can not find the answer ...

For an audio output call back .. (from the examples) ..
deviceConfig = ma_device_config_init(ma_device_type_playback);
deviceConfig.dataCallback = data_callback;

void data_callback(ma_device* pDevice, void* pOutput, const void* pInput, ma_uint32 frameCount)
{
}

I want to append directly to "void *pOutput".

Do I always need to append a full "frameCount"?
How do I indicated to the data_callback() how much I have appended?
Is "void *pOutput" really a uint8 *?
How do I know what type of pointer it is?

thanks!

[duane34]

I found the following in the miniaudio.h ...
Seems like the buffer is initialized to silence, so one can write as much as they want ...
I assume the "void *pOutput" is a byte pointer to the beginning of the buffer??

Is the ma_device_config documented somewhere in https://miniaud.io/docs/manual/index.html??

Thanks!

17. Miscellaneous Notes
    =======================

• The contents of the output buffer passed into the data callback will always be pre-initialized to
  silence unless the noPreSilencedOutputBuffer config variable in ma_device_config is set to
  true, in which case it'll be undefined which will require you to write something to the entire
  buffer.

[duane34]

From line 6411 in miniaudi.h

I assume the period has some affect on the amount of buffer allocated??

frameCount (in)
The number of PCM frames to process. Note that this will not necessarily be equal to what you requested when you initialized the device. The
periodSizeInFrames and periodSizeInMilliseconds members of the device config are just hints, and are not necessarily exactly what you'll get. You must
not assume this will always be the same value each time the callback is fired.

from line 6629 in miniaudio.h

"struct ma_device_config" ... shows the fields of this structure.

[mackron]

You don't append to pOutput, you just copy into it. It's been preallocated internally by miniaudio. You need to copy as much data into pOutput as you can, but not more than frameCount. The only time you would ever copy over less is if you run out of data.

The data type is the format you configured your device with:

deviceConfig.playback.format = ma_format_f32;

If you used f32, it's a buffer or floats. s16 is short, etc.

[duane34]

I assume the correct approach is to:

1. set "periodSizeInFrames" or "periodSizeInMilliseconds"
   to the amount of buffering desired.
2. do not copy anything in the callback until can fill the full call back request (ie frameCount).
   (this will result in silence at the beginning of the stream)
3. after the first copying of data should always have enough data ready to fill the full call back request
   (otherwise the source is falling behind the player .. and will inject silence)

Is this correct??

Thanks!

[mackron]

The periodSizeInFrames/Milliseconds configuration is used for controlling the size of frameCount in the data callback, not necessarily the amount of buffering you'd want to do (you may want to buffer a bit more so you've got a bigger margin to work with).

The data callback runs continuously. miniaudio fires it when the backend needs more data for playback. Whatever is contained in the output buffer when the callback returns is what will be played out of the speakers. The output buffer will be pre-filled with silence. If you don't have any data to play back, you have no other option than to just not copy anything into the output buffer which is the only time it's acceptable to partially fill the output buffer. If you have the data available, copy as much of it as you can, up to frameCount frames.

[duane34]

Many thanks for the quick reply. Very helpful!!!