We have a long-form audio recording app built on AVAudioEngine. We install a tap on inputNode, accumulate the PCM buffers, and encode them to AAC in ~60-second chunks. Setup is essentially:
let session = AVAudioSession.sharedInstance()
try session.setCategory(.playAndRecord, mode: .default,
options: [.defaultToSpeaker, .allowBluetooth, .allowBluetoothA2DP])
try session.setActive(true)
let engine = AVAudioEngine()
let input = engine.inputNode
let format = input.inputFormat(forBus: 0) // valid, e.g. 48 kHz, 1 ch
input.installTap(onBus: 0, bufferSize: 1024, format: format) { buffer, _ in
// In the failure case, buffer.floatChannelData is entirely 0.0
// (accumulate + encode to AAC)
}
engine.prepare()
try engine.start()
Intermittently — and so far only reported from the field, never reproduced in normal testing — a recording comes out completely silent. When we decode the resulting AAC and inspect the raw PCM, every sample is exactly 0.0. The signature is very specific:
The engine is running and the tap keeps firing for the full duration (normal number of buffers / full-length chunks).
inputFormat is valid (sampleRate ≠ 0, e.g. 48 kHz).
No error is thrown anywhere — setCategory, setActive, start(), and the tap callback all succeed.
The PCM is literally all zeros (not low-level noise / room tone — exact 0.0).
Two separate silent recordings decode to byte-identical AAC, confirming pure digital silence rather than corruption.
So as far as our error handling, format checks, and tap-liveness are concerned, everything looks healthy — yet the microphone is delivering pure silence.
One way we can reproduce it: recording while the iPhone is being driven via macOS iPhone Mirroring (the iPhone stays locked, the mic is effectively unavailable from the device, but our session still activates with a valid format and the tap fires zero-filled buffers for the whole recording — with no error at any point).
What we've ruled out: microphone permission is granted; it's not truncation or short capture (full-length, full frame count); it's not our encoding step (the input buffers themselves are zero); it's not a quiet/obstructed mic (that would be low noise, not exact 0.0).
We also found two existing threads describing what looks like the same symptom:
https://developer.apple.com/forums/thread/834950
https://developer.apple.com/forums/thread/808072
Both of those are PushToTalk apps where the system activates the audio session, and an Apple
engineer notes it may be related to a CallKit issue (r.157725305). For context on our side: we do
use CallKit, but only CXCallObserver — purely to detect whether a phone call comes in while a
recording is in progress, so we can pause and resume around it. We do not use CXProvider or
PushToTalk, and we activate our own AVAudioSession ourselves with setActive(true).
I'm trying to understand whether there are other scenarios or device states that could leave a
running AVAudioEngine tap returning all-zero buffers like this, and whether this is the same
underlying CallKit issue (r.157725305) from those threads . And since nothing throws an error, any guidance on how
to detect this at runtime and recover from it would be really helpful.