[Development] QMediaPlayer changes hardware sample rate unexpectedly on MacOS

[Tim Blechmann]

can you possibly file a bug report and assign it to me? there are a few 
aspects that need to be refined:
* ideally we want to avoid software resampling due to the DSP involved
* iirc coreaudio only resamples on audio output, but not on audio input
* in a perfect world we had an interface to allow customization of the 
resampling, but it's not trivial to do this in a cross-platform manner

cheers,
tim

On 4/13/26 20:31, Эдвард Даньковский wrote:
> Hello,
> 
> I'm working on a messaging application that uses QMediaPlayer to play 
> audio messages.
> These audio messages typically use a 22 kHz sample rate. I've noticed 
> that playing an
> audio message on MacOS causes the output device's sample rate to change. 
> In some cases,
> this significantly degrades the user experience:
> 
> 1. When using Bose NC700 wireless headphones, the sample rate changes to 
> 16 kHz, which
>     causes the headphones to disable active noise cancellation.
> 
> 2. When background music is playing at 48 kHz, its playback quality 
> significantly
>     decreases after the sample rate change. Notably, the sample rate 
> change persists
>     even after the application exits, leaving the device in an altered 
> state.
> 
> The sample rate change occurs in QCoreAudioSinkStream::open() [1]:
> 
>      bool QCoreAudioSinkStream::open()
>      {
>          // ...
>      #ifdef Q_OS_MACOS
>          std::optional<int> bestNominalSamplingRate =
>              audioObjectFindBestNominalSampleRate(nativeDeviceId, 
> QAudioDevice::Output, m_format.sampleRate());
> 
>          if (bestNominalSamplingRate) {
>              if (!audioObjectSetSamplingRate(nativeDeviceId, 
> *bestNominalSamplingRate))
>                  return false;
>          }
>          // ...
>      #endif
>          // ...
>      }
> 
> 
> Here, audioObjectFindBestNominalSampleRate [2] finds the sample rate 
> closest to
> m_format.sampleRate() among the device's supported sample rates, and
> audioObjectSetSamplingRate [2] sets the 
> kAudioDevicePropertyNominalSampleRate
> property of the output device. In my case, the output device supports 16 
> kHz and
> 44.1 kHz rates, so the 22 kHz audio message is closer to 16 kHz, and Qt 
> switches
> the device to that rate.
> 
> As I understand it, this logic serves an optimization purpose — to avoid 
> resampling
> when the output device supports the required sample rate. However, as 
> described above,
> this can lead to unexpected behavior.
> 
> On Windows (Shared Mode AudioClient) and with PulseAudio/PipeWire, this 
> is not a problem
> because resampling is handled internally by the audio subsystem. In the 
> ALSA backend I'm
> not sure if it is possible NOT to update the sample rate.
> 
> I see two possible solutions:
> 
> 1. Remove the sample rate change entirely from the MacOS code. This is 
> the simplest approach.
>     The documentation does not mention sample rate changes as part of 
> QAudioSink behavior,
>     so this shouldn't break documented contracts.
> 
> 2. Add a new property (e.g., allowHardwareSampleRateChange) to QAudioSink
>     and QAudioOutput, allowing the sample rate change to be disabled 
> conditionally
>     while preserving the current default behavior.
> 
> The first option seems cleanest, but it may affect users who rely on the 
> current
> behavior for low-latency playback without resampling.
> 
> The second option preserves backward compatibility but introduces 
> complexity:
> - With ALSA, I'm not sure if it's possible to avoid setting the sample rate
>    for a hardware device.
> - With PipeWire, it may be impossible to guarantee the sample rate won't 
> change,
>    since PipeWire can update it when only one stream is active.
> - Every media player backend would need updates to support this property.
> 
> My questions:
> 
> 1. Is this considered a bug that needs fixing?
> 2. If so, can we simply remove the sample rate change from 
> QCoreAudioSinkStream::open?
> 3. If removal is not acceptable, what are your thoughts on adding an
>     allowHardwareSampleRateChange property?
> 
> [1] qtmultimedia/src/multimedia/darwin/qdarwinaudiosink.mm <https:// 
> eur01.safelinks.protection.outlook.com/? 
> url=http%3A%2F%2Fqdarwinaudiosink.mm%2F&data=05%7C02%7C%7Ccc107b41a43e48fad19408de9959210f%7C20d0b167794d448a9d01aaeccc1124ac%7C0%7C0%7C639116805446889327%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C80000%7C%7C%7C&sdata=cQTbKLNDL6m6WPQQ1nYEZN%2FOhtG2COFqoSiuS1RrB0g%3D&reserved=0>
> [2] qtmultimedia/src/multimedia/darwin/qcoreaudioutils.cpp
>