How can I get sample from AudioBufferList

Question

I get the AudioBufferList from a wav file(Which Sample Rate is 44100HZ and the long time is 2second). But I can't get 44100*2=88200 samples. In actually I got an AudiobufferList which contain 512 nNumberBuffers. How can I get the sample from the AudioBufferList?

score 2 · Answer 1 · answered Nov 06 '14 at 07:13

here is my method to get the samples from a wav file

-(float *) GetSoundFile:(NSString *)fileName
{
NSBundle *bundle = [NSBundle mainBundle];
NSString *path = [bundle pathForResource:[fileName stringByDeletingPathExtension]
                                  ofType:[fileName pathExtension]];

NSURL *audioURL = [NSURL fileURLWithPath:path];
if (!audioURL)
{
    NSLog(@"file: %@ not found.", fileName);
}

OSStatus                        err = 0;
                                theFileLengthInFrames = 0;  //this is global
AudioStreamBasicDescription     theFileFormat;
UInt32                          thePropertySize = sizeof(theFileFormat);
ExtAudioFileRef                 extRef = NULL;
void*                           theData = NULL;
AudioStreamBasicDescription     theOutputFormat;

// Open a file with ExtAudioFileOpen()
err = ExtAudioFileOpenURL((__bridge CFURLRef)(audioURL), &extRef);

// Get the audio data format
err = ExtAudioFileGetProperty(extRef, kExtAudioFileProperty_FileDataFormat, &thePropertySize, &theFileFormat);

theOutputFormat.mSampleRate = samplingRate = theFileFormat.mSampleRate;
theOutputFormat.mChannelsPerFrame = numChannels = 1;

theOutputFormat.mFormatID = kAudioFormatLinearPCM;
theOutputFormat.mBytesPerFrame = sizeof(Float32) * theOutputFormat.mChannelsPerFrame ;
theOutputFormat.mFramesPerPacket = theFileFormat.mFramesPerPacket;
theOutputFormat.mBytesPerPacket = theOutputFormat.mFramesPerPacket * theOutputFormat.mBytesPerFrame;
theOutputFormat.mBitsPerChannel = sizeof(Float32) * 8 ;
theOutputFormat.mFormatFlags = 9 | 12;

//set the output property
err = ExtAudioFileSetProperty(extRef, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), &theOutputFormat);

// Here I Get the total frame count and write it into gloabal variable
thePropertySize = sizeof(theFileLengthInFrames);
err = ExtAudioFileGetProperty(extRef, kExtAudioFileProperty_FileLengthFrames, &thePropertySize, &theFileLengthInFrames);

UInt32 dataSize = (UInt32)(theFileLengthInFrames * theOutputFormat.mBytesPerFrame);
theData = malloc(dataSize);

AudioBufferList theDataBuffer;
if (theData)
{
    theDataBuffer.mNumberBuffers = 1;
    theDataBuffer.mBuffers[0].mDataByteSize = dataSize;
    theDataBuffer.mBuffers[0].mNumberChannels = theOutputFormat.mChannelsPerFrame;
    theDataBuffer.mBuffers[0].mData = theData;

    // Read the data into an AudioBufferList
    err = ExtAudioFileRead(extRef, (UInt32*)&theFileLengthInFrames, &theDataBuffer);
}

return (float*)theDataBuffer.mBuffers[0].mData;  
//here is the data that has thefilelengthInframes amount frames 

}

score 1 · Answer 2 · answered Feb 11 '13 at 10:29

AudioBufferList should not be set to a fixed size because is a temporary buffer, hardware dependent. The size is unpredictable and you can only set a preferable size, moreover is not granted to be the same size between 2 reading calls. To get 88200 samples (or what you like) you must incrementally fill a new buffer using, time by time, the samples in AudioBufferList. I suggest to use this circular buffer https://github.com/michaeltyson/TPCircularBuffer made for this purpose. Hope this help

score 0 · Answer 3 · answered Sep 17 '12 at 16:18

As I know, AudioBufferList must be configured by you to receive data to it and then it must be filled by some reading function (e.g. ExtAudioFileRead()). So you should prepare it by allocating buffers you need (usually 1 or 2), set nNumberBuffers to that number and read audio data to them. AudioBufferList just stores that buffers and they are will contain frames values.

How can I get sample from AudioBufferList

3 Answers3