*AVAudioEngine - Multichannel output does not play on channel 1 and 2*
We have an existing iOS application that uses the Core Audio C API and
achieves multichannel output using the Matrix Mixer. I’m currently
rewriting the audio engine using AVAudioEngine but I am having problems
getting multichannel output working correctly.
In terms of hardware, I am running * iPad Air 2 (iOS 10.2) ——>
iConnectAudio 4+* (which receives 8 channels of audio from the iPad via
USB). In my audio graph, I have a single custom AVAudioUnit which is
outputting 8 channels of audio to the outputNode. I am setting up my audio
format as follows:
// Muti channel format
let numChannels = UInt32(8)
let multiChannelLayout = AVAudioChannelLayout(layoutTag:
kAudioChannelLayoutTag_DiscreteInOrder | numChannels)
let multiChannelFormat = AVAudioFormat(commonFormat:
stereoFormat.commonFormat, sampleRate: stereoFormat.sampleRate,
interleaved: stereoFormat.isInterleaved, channelLayout: multiChannelLayout)
I have confirmed that my custom AVAudioUnit is constantly sending all 8
channels of audio to the outputNode. However, on the iConnectAudio 4+
control panel, I can see that the device is only receiving audio on
channels 3 - 8. For some reason, it is not receiving any audio from the app
on the first 2 channels. I have confirmed this in listening tests too.
However, the audio device *IS* receiving all of the system sounds on
channels 1 and 2. For example, the click sounds when editing the text in a
UITextField can be heard on channels 1 and 2. It seems as though either -
* iOS is blocking all audio on channels 1 and 2
* The stream for audio channels 1 and 2 and getting zeroed inside the
AVAudioOutputNode
There seems to be no issue with the audio hardware itself because I am able
to output audio on all 8 channels using our existing iOS application and
other apps, such as Cubasis. So, it would suggest that the issue perhaps
lies in the AVAudioOutputNode. If I set the AVAudioFormat to a 2 channel
configuration, the audio will output on channels 1 and 2 (mixed with the
system sounds) but any configuration with more than 2 channels results in
no audio on channels 1 and 2, other than the system sounds.
It seems to me that '*kAudioChannelLayoutTag_DiscreteInOrder |
numChannels'* would be the correct layout to use in this case - but please
do correct me if I am wrong - there's very little information on how
AVAudioEngine handles this information. In essence - I want to output each
channel independently to the audio interface, I want the outputNode to just
output the audio channels as it receives them - this is a studio
application, not a surround sound/positional audio scenario. I have also
tried setting the AVAudioChannelLayout to some value other than
kAudioChannelLayoutTag_DiscreteInOrder
but to no avail - resulting in either silence or 2-channel audio.
Has anyone had this or a similar issue? Or could anybody suggest what’s
happening here and how I might fix the issue.
Many thanks,
Dave
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