Hello,
In an iOS app I am trying to capture audio from my graph in AVAudioEngine as it
is sent to the output route. The purpose is a record-sum function.
There is a method AVAudioNode.installTap() which works great — if I invoke it
on engine.mainMixerNode.
The tap is not called when I install it on engine.outputNode.
Has anyone successfully managed to install a tap on engine.outputNode?
I’d like to avoid the overhead of a mixer node just so I can install a tap on
it when the output node should be able to handle it.
Note that the API documentation explicitly mentions that for an
AVAudioOutputNode the format must be nil. This implies that AVAudioOutputNode
is supported.
Here is some code from my test project. Each time the tap is called a text
message should be seen in the console. It repeatedly appears when I use
engine.mainMixerNode, but it never appears with engine.outputNode. Assume
AVAudioEngine has already been started. An instance of AVAudioUnitSampler is
connected to engine.outputNode.
func startRecording() -> Bool {
guard let url =
ViewController.documentsFolderURL()?.appendingPathComponent("Audio.aiff") else
{ return false }
do {
// settings = Output file format
// commonFormat = what comes from PCM buffer
// Write 16-bit AIFF file
let settings: [String: Any] = [
AVLinearPCMBitDepthKey: 16,
AVLinearPCMIsFloatKey: false,
AVLinearPCMIsNonInterleaved: false,
AVFormatIDKey: kAudioFormatLinearPCM,
AVNumberOfChannelsKey: stereoFormat.channelCount,
AVSampleRateKey: stereoFormat.sampleRate
]
currentAudioFile = try AVAudioFile(forWriting: url, settings:
settings, commonFormat: .pcmFormatFloat32, interleaved: false)
engine.outputNode.installTap(onBus: 0, bufferSize: 8192,
format: nil) { (pcmBuffer, when) in
// Use serial queue to prevent overlaps
self.audioFileQueue.addOperation {
guard let file = self.currentAudioFile else {
return }
do {
print("Writing
length=\(pcmBuffer.frameLength), pos=\(file.framePosition),
when=\(when.sampleTime)")
try file.write(from: pcmBuffer)
} catch {
print("Could not write to audio file")
}
}
}
print("Started new audio file")
} catch {
print("Could not create audio file")
}
return true
}
Thanks for any ideas.
Sven
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