Getting level values from PCM raw data using Core Audio
- by John
I am trying to extract level data from a PCM audio file using core audio. I have gotten as far as (I believe) getting the raw data into a byte array (UInt8) but it is 16 bit PCM data and I am having trouble reading the data out. The input is from the iPhone microphone, which I have set as:
[recordSetting setValue:[NSNumber numberWithInt:kAudioFormatLinearPCM] forKey:AVFormatIDKey];
[recordSetting setValue:[NSNumber numberWithFloat:44100.0] forKey:AVSampleRateKey];
[recordSetting setValue:[NSNumber numberWithInt:1] forKey:AVNumberOfChannelsKey];
[recordSetting setValue:[NSNumber numberWithInt:16] forKey:AVLinearPCMBitDepthKey];
[recordSetting setValue:[NSNumber numberWithBool:NO] forKey:AVLinearPCMIsBigEndianKey];
[recordSetting setValue:[NSNumber numberWithBool:NO] forKey:AVLinearPCMIsFloatKey];
which is obviously 16 bits. I am then trying to just print out a few values to see if they look reasonable for debug purposes below, and they do not look reasonable (many 0's).
ExtAudioFileRef inputFile = NULL;
ExtAudioFileOpenURL(track.location, &inputFile);
AudioStreamBasicDescription inputFileFormat;
UInt32 dataSize = (UInt32)sizeof(inputFileFormat);
ExtAudioFileGetProperty(inputFile, kExtAudioFileProperty_FileDataFormat, &dataSize, &inputFileFormat);
UInt8 *buffer = malloc(BUFFER_SIZE);
AudioBufferList bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0].mNumberChannels = 1;
bufferList.mBuffers[0].mData = buffer; //pointer to buffer of audio data
bufferList.mBuffers[0].mDataByteSize = BUFFER_SIZE; //number of bytes in the buffer
while(true) {
UInt32 frameCount = (bufferList.mBuffers[0].mDataByteSize / inputFileFormat.mBytesPerFrame);
// Read a chunk of input
OSStatus status = ExtAudioFileRead(inputFile, &frameCount, &bufferList);
// If no frames were returned, conversion is finished
if(0 == frameCount)
break;
NSLog(@"---");
int16_t *bufferl = &buffer;
for(int i=0;i<100;i++){
//const int16_t *bufferl = bufferl[i];
NSLog(@"%d",bufferl[i]);
}
}
Not sure what I am doing wrong, I think it has to do with reading the byte array. Sorry for the long code post...
