Distorted (robotic) audio recording on iOS using AudioQueue - ios

Question

I'm using AudioQueue in iOS to implement streaming of recording to the web.
The problem is that it usually works quite well, but sometimes (~20% of my attempts) the sound is horribly distorted - it sounds robotic.
Edit:
I am able to reproduce it quite easily on the ios6 and ios6.1 simulator - but I wasn't able to reproduce it on a real phone (ios6.1.3).
Attempting to debug it I save the PCM data to a file. The same distortion appears in the PCM file, so this is not a problem in the encoding or upload code. I've also tried to play with the number of buffers and the size of the buffers - nothing helped.
The problem is I don't know how to debug it farther - it appears the buffer is distorted as input to the callback - before my code is activated (except for the audio queue config).
do you have ideas what could be the problem?
or how to debug it further?
Queue setup code:
audioFormat.mFormatID = kAudioFormatLinearPCM;
audioFormat.mSampleRate = SAMPLE_RATE; //16000.0;
audioFormat.mChannelsPerFrame = CHANNELS; //1;
audioFormat.mBitsPerChannel = 16;
audioFormat.mFramesPerPacket = 1;
audioFormat.mBytesPerFrame = audioFormat.mChannelsPerFrame * sizeof(SInt16);
audioFormat.mBytesPerPacket = audioFormat.mBytesPerFrame * audioFormat.mFramesPerPacket;
audioFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger
| kLinearPCMFormatFlagIsPacked;
AudioQueueNewInput(
&audioFormat,
recordCallback,
self, // userData
CFRunLoopGetMain(), // run loop
NULL, // run loop mode
0, // flags
&recordQueue);
UInt32 trueValue = true;
AudioQueueSetProperty(recordQueue,kAudioQueueProperty_EnableLevelMetering,&trueValue,sizeof (UInt32));
for (int t = 0; t < NUMBER_AUDIO_DATA_BUFFERS; ++t)
{
AudioQueueAllocateBuffer(
recordQueue,
bufferByteSize,
&recordQueueBuffers[t]);
}
for (int t = 0; t < NUMBER_AUDIO_DATA_BUFFERS; ++t)
{
AudioQueueEnqueueBuffer(
recordQueue,
recordQueueBuffers[t],
0,
NULL);
}
Start recording function:
pcmFile = [[NSOutputStream alloc] initToFileAtPath:pcmFilePath append:YES];
[pcmFile scheduleInRunLoop:[NSRunLoop currentRunLoop]
forMode:NSDefaultRunLoopMode];
[pcmFile open];
setupQueue(); // see above
AudioQueueStart(recordQueue, NULL);
Callback code:
static void recordCallback(
void* inUserData,
AudioQueueRef inAudioQueue,
AudioQueueBufferRef inBuffer,
const AudioTimeStamp* inStartTime,
UInt32 inNumPackets,
const AudioStreamPacketDescription* inPacketDesc)
Recorder* recorder = (Recorder*) inUserData;
if (!recorder.recording)
return;
[recorder.pcmFile write:inBuffer->mAudioData maxLength:inBuffer->mAudioDataByteSize];
AudioQueueEnqueueBuffer(inAudioQueue, inBuffer, 0, NULL);
}

Answer 1

we had the exact same issue in SoundJS Web Audio on iOS whenever a video element was present and the cache was empty. Once cached, the problem went away. We were unable to find a fix or a work around for the issue.
You can read the details in our community forum:
http://community.createjs.com/discussions/soundjs/162-ios-audio-distortion-when-video-element-exists
Hope that helps.