This one keeps me awake:
I have an OS X audio application which has to react if the user changes the current sample rate of the device.
To do this I register a callback for both in- and output devices on ‘kAudioDevicePropertyNominalSampleRate’.
So if one of the devices sample rates get changed I get the callback and set the new sample rate on the devices with ‘AudioObjectSetPropertyData’ and ‘kAudioDevicePropertyNominalSampleRate’ as the selector.
The next steps were mentioned on the apple mailing list and i followed them:
- stop the input AudioUnit and the AUGraph which consists of a mixer and the output AudioUnit
- uninitalize them both.
- check for the node count, step over them and use AUGraphDisconnectNodeInput to disconnect the mixer from the output
- now set the new sample rate on the output scope of the input unit
- and on the in- and output scope on the mixer unit
- reconnect the mixer node to the output unit
- update the graph
- init input and graph
- start input and graph
Render and Output callbacks start again but now the audio is distorted. I believe it’s the input render callback which is responsible for the signal but I’m not sure.
What did I forget?
The sample rate doesn’t affect the buffer size as far as i know.
If I start my application with the other sample rate everything is OK, it’s the change that leads to the distorted signal.
I look at the stream format (kAudioUnitProperty_StreamFormat) before and after. Everything stays the same except the sample rate which of course changes to the new value.
As I said I think it’s the input render callback which needs to be changed. Do I have to notify the callback that more samples are needed? I checked the callbacks and buffer sizes with 44k and 48k and nothing was different.
I wrote a small test application so if you want me to provide code, I can show you.
Edit: I recorded the distorted audio(a sine) and looked at it in Audacity.
What I found was that after every 495 samples the audio drops for another 17 samples.
I think you see where this is going: 495 samples + 17 samples = 512 samples. Which is the buffer size of my devices.
But I still don’t know what I can do with this finding.
I checked my Input and Output render procs and their access of the RingBuffer(I’m using the fixed Version of CARingBuffer)
Both store and fetch 512 frames so nothing is missing here…
Got it!
After disconnecting the Graph it seems to be necessary to tell both devices the new sample rate.
I already did this before the callback but it seems this has to be done at a later time.