Playing .wav file through Bonsai distorts its spectral content to lower frequencies

[m-lockwood]

Hi,

I'm trying to play an audio .wav file of white noise filtered between 16-22kHz (generated using MATLAB's audiowrite function). The sampling frequency I specified in MATLAB is 48kHz, which matches the sampling rate of the sound card in the PC from which I am playing the sound. When I play the sound through windows media player the spectral content is as 16-22kHz expected, however when I play the sound through Bonsai using the following test code, the sound is distorted to lower frequencies:

I initially tried setting the sampling rate of the audio reader and audio playback nodes to 48kHz, and the buffer length to 10, and the sound was audibly distorted to lower frequencies. Since then I've tried playing with the parameters: Changing the buffer length made no difference unless I set it to 0 (then no sound comes out). Changing the sampling frequencies of the audio reader and audio playback nodes didn't make any difference unless I decrease them below 40kHz, then they are audibly lower as you would expect.

Does anyone know what could be causing this issue?

I will attach the test workflow and the audio file which I'm trying to play here in case it's helpful:

Test sound.zip

[banchan86]

Hi @m-lockwood I don't have much experience with audio applications, but I was able to reproduce this issue with another set of 48kHz and higher frequency sound files from https://www.audiocheck.net/ and I think this might be the problem you are having https://www.audiocheck.net/audiotests_aliasing.php. The sounds played fine in the browser but not in Bonsai.

Unfortunately I do not know how to solve it, but I did notice that when I converted your file to 44.1kHz they played fine in Bonsai (I wasn't able to accurately measure the spectral response beyond 20kHz though as that is what my iPhone microphone is capped at). Since it is within the Nyquist frequency for your white noise, if you try regenerating your file in Matlab with that sampling frequency, will that work for you in the meantime? I will raise an issue separately on the Bonsai core repo for further investigation (or to see if someone knows what is going on or how to resolve it).

I only tested with these nodes:

In AudioReader you should not need to set the SampleRate, I believe that is only needed to overwrite the sampling rate if it the wav file header has the wrong SampleRate (I checked your file and it seems to be fine, so I left it at 0). I also don't think the `BufferLength is an issue. I did sometimes detect a click in playback if I had it at 10ms, increasing it to 1000ms made it go away but I don't think it affects the playback frequency.

In AudioPlayback I set the SampleRate to the SampleRate of the wav file (44100 in the wav file I converted).

[m-lockwood]

Hello, thanks for these suggestions!

I also tried creating an audio file with a sampling frequency of 44.1kHz and setting the audio playback node to 44.1kHz in Bonsai, but I had the same issue. How did you convert the audio file?

I generated the file from this code in MATLAB, in which you can set the sampling rate and bandpass frequencies:
generate_audio_file.zip

[banchan86]

Hi @m-lockwood. I converted it in audacity, unfortunately I don't have Matlab. I attached both the bonsai file and wav file I converted.
What soundcard are you using?

test_sound_44.1kz_bonsai_and_wav.zip

[adam-hockley]

I was having a similar issue and found that bonsai cannot handle 24-bit audio. Outputting the matlab audio at 16-bit fixed the problem for me and worked at any sampling rate my dacs could handle
audiowrite('filename',var,fs,'BitsPerSample',16)

[glopesdev]

@adam-hockley indeed the original OpenAL implementation on Windows did not support 24-bit formats.

This has since changed in modern APIs. To use these you would need to go through either the new bonsai-rx/mixer package or the upcoming upgrade of the Audio package to use OpenAL Soft which from release notes it seems might support 24-bit audio.

Might make sense to open an issue with this so we can track developments.