My application records streaming audio on iPhone. My problem is that a small percent (~2%) of the recordings are corrupted. They appear to have some audio buffers duplicated.
For example listen to this file .
Edit: A surprising thing is that looking closely at the data using Audacity shows the repeating parts are very very similar but not identical. Since FLAC (the format I use for encoding the audio) is a loss-less compression, I guess this is not a bug in the streaming/encoding but the problem originates at the data that comes from the microphone!
Below is the code I use to setup the audio recording streaming - is there anything wrong with it?
// see functions implementation below
- (void)startRecording
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
, ^{
[self setUpRecordQueue];
[self setUpRecordQueueBuffers];
[self primeRecordQueueBuffers];
AudioQueueStart(recordQueue, NULL);
});
}
// this is called only once before any recording takes place
- (void)setUpAudioFormat
{
AudioSessionInitialize(
NULL,
NULL,
nil,
(__bridge void *)(self)
);
UInt32 sessionCategory = kAudioSessionCategory_PlayAndRecord;
AudioSessionSetProperty(
kAudioSessionProperty_AudioCategory,
sizeof(sessionCategory),
&sessionCategory
);
AudioSessionSetActive(true);
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;
bufferNumPackets = 2048; // must be power of 2 for FFT!
bufferByteSize = [self byteSizeForNumPackets:bufferNumPackets];
}
// I suspect the duplicate buffers arrive here:
static void recordCallback(
void* inUserData,
AudioQueueRef inAudioQueue,
AudioQueueBufferRef inBuffer,
const AudioTimeStamp* inStartTime,
UInt32 inNumPackets,
const AudioStreamPacketDescription* inPacketDesc)
{
Recorder* recorder = (__bridge Recorder*) inUserData;
if (inNumPackets > 0)
{
// append the buffer to FLAC encoder
[recorder recordedBuffer:inBuffer->mAudioData byteSize:inBuffer->mAudioDataByteSize packetsNum:inNumPackets];
}
AudioQueueEnqueueBuffer(inAudioQueue, inBuffer, 0, NULL);
}
- (void)setUpRecordQueue
{
OSStatus errorStatus = AudioQueueNewInput(
&audioFormat,
recordCallback,
(__bridge void *)(self), // userData
CFRunLoopGetMain(), // run loop
NULL, // run loop mode
0, // flags
&recordQueue);
UInt32 trueValue = true;
AudioQueueSetProperty(recordQueue,kAudioQueueProperty_EnableLevelMetering,&trueValue,sizeof (UInt32));
}
- (void)setUpRecordQueueBuffers
{
for (int t = 0; t < NUMBER_AUDIO_DATA_BUFFERS; ++t)
{
OSStatus errorStatus = AudioQueueAllocateBuffer(
recordQueue,
bufferByteSize,
&recordQueueBuffers[t]);
}
}
- (void)primeRecordQueueBuffers
{
for (int t = 0; t < NUMBER_AUDIO_DATA_BUFFERS; ++t)
{
OSStatus errorStatus = AudioQueueEnqueueBuffer(
recordQueue,
recordQueueBuffers[t],
0,
NULL);
}
}
事实证明,存在一个罕见的错误,几乎可以同时开始多个录音-因此两个录音并行发生,但是将音频缓冲区发送到同一回调中,从而使编码录音中的重复缓冲区失真了...
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