Writing and reading an audio file with Extended Audio File

Question

I m using ExtAudioFileWriteAsync to write an audio file while using device recording, but once I get recording finished I try to read it with ExtAudioFileRead function and samples I get are not same samples I m writing... Anyone know why this could happen?

For writing:

self.audioManager.inputBlock = ^(float *data, UInt32 numFrames, UInt32 numChannels) {
  for (int i = 0; i < numFrames*numChannels; i++) {
        printf("write*%f\n", data[i]);
  }

  UInt32 numIncomingBytes = numFrames*numChannels*sizeof(float);
  UInt32 *outputBuffer =(UInt32*)malloc(numIncomingBytes);
  memcpy(outputBuffer, recordedData, numIncomingBytes);

  AudioBufferList outgoingAudio;
  outgoingAudio.mNumberBuffers = 1;
  outgoingAudio.mBuffers[0].mNumberChannels = numChannels;
  outgoingAudio.mBuffers[0].mDataByteSize = numIncomingBytes;
  outgoingAudio.mBuffers[0].mData = self.outputBuffer;

  if( 0 == pthread_mutex_trylock( &outputAudioFileLock ) ) 
  {       
      ExtAudioFileWriteAsync(outputFile, numFrames, &outgoingAudio);
  }
  pthread_mutex_unlock( &outputAudioFileLock );    
};
[self.audioManager play];

For reading:

UInt32 *outputBuffer = (UInt32 *)malloc(numFrames*numChannels*sizeof(float));

AudioBufferList convertedData;
convertedData.mNumberBuffers = 1;
convertedData.mBuffers[0].mNumberChannels = numChannels;
convertedData.mBuffers[0].mDataByteSize = numFrames*numChannels*sizeof(float);
convertedData.mBuffers[0].mData = outputBuffer;

NSMutableArray *samplesArray = [[NSMutableArray alloc]init];
while (numFrames > 0) {
    ExtAudioFileRead(inputFile, &numFrames, &convertedData);
    if (numFrames > 0)  {
        AudioBuffer audioBuffer = convertedData.mBuffers[0];
        float *samples = (float *)audioBuffer.mData;
        for (int i = 0; i < frameCount*numChannels; i++) {
            printf("read*%f\n", samples[i]);
        }
    }
}

By the way I'm using Novocaine project in order to get device audio. I can reproduce saved audio with Novocaine code or with any other player.

When writing ExtAudioFileRef output :

 ExtAudioFileCreateWithURL(audioFileRef, kAudioFileM4AType,   &outputFileDesc, NULL, kAudioFileFlags_EraseFile, &outputFile);

Where outputFileDesc is

AudioStreamBasicDescription outputFileDesc = {44100.0, kAudioFormatMPEG4AAC, 0, 0, 1024, 0, thisNumChannels, 0, 0}; outputFileDesc.mFormatFlags = kAudioFormatFlagIsBigEndian | kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;

When reading ExtAudioFileRef inputFile:

ExtAudioFileOpenURL(audioFileRef, &inputFile):

And in both cases (writing and reading) it is applied same format:

AudioStreamBasicDescription outputFormat;
_outputFormat.mSampleRate = self.samplingRate;
_outputFormat.mFormatID = kAudioFormatLinearPCM;
_outputFormat.mFormatFlags = kAudioFormatFlagIsFloat;
_outputFormat.mBytesPerPacket = 4*self.numChannels;
_outputFormat.mFramesPerPacket = 1;
_outputFormat.mBytesPerFrame = 4*self.numChannels;
_outputFormat.mChannelsPerFrame = self.numChannels;
_outputFormat.mBitsPerChannel = 32;


ExtAudioFileSetProperty(outputFile, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), &_outputFormat);


 ExtAudioFileSetProperty(inputFile, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), &_outputFormat);

And by the way, even if read samples are not equal to written samples, mean value of both signals are quite similar. But I do not fully understand why are not totally equal!

Any idea what I'm doing wrong?

Answer 1

It sounds like there is an implicit format conversion from one or both of the ExtaudioFileRefs and you are seeing different samples as a result of the conversion. You have three formats: audio_in_format, file_format, and audio_out_format. If audio_in_format is different than file_format, The writing ExtAudioFileRef will create an audio converter for you to convert the input audio to file_format before writing to disk. And the reading ExtAudioFileRef will also create a converter if file_format is different than audio_out_format.

Opinion: It's confusing that you named your writing ExtAudioFileRef "outputFile", and your reading ExtAudioFileRef "inputFile". I would use something like audioWriter and audioReader.