how to read VBR audio in novacaine (as opposed to PCM) - objective-c

Question

The creator of novacaine offered example code where audio data is read from a a file and fed to a ring buffer. When the file reader is created though, the output is forced to be PCM:
- (id)initWithAudioFileURL:(NSURL *)urlToAudioFile samplingRate:(float)thisSamplingRate numChannels:(UInt32)thisNumChannels
{
...
// We're going to impose a format upon the input file
// Single-channel float does the trick.
_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;
}
I'm trying to contribute to the novacaine project by allowing it to
Read from the iPod library (which can only be accessed via AVAssetReader, rather than the audio file services library)
Read and write VBR packets rather than PCM.
So this is what my equivalent function of the above looks like (see the NOTE: parts)
- (id)initWithAudioAssetURL:(NSURL *)urlToAsset samplingRate:(float)thisSamplingRate numChannels:(UInt32)thisNumChannels
{
self = [super init];
if (self)
{
// Zero-out our timer, so we know we're not using our callback yet
self.callbackTimer = nil;
// Open a reference to the audio Asset Track and setup the reader
self.assetURL = urlToAsset;
AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:self.assetURL options:nil];
NSError * error = nil;
AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:songAsset error:&error];
AVAssetTrack* track = [songAsset.tracks objectAtIndex:0];
//NOTE: we use the track's native settings here, as opposed to forcing it to be PCM
//like the example above
_readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track
outputSettings:NULL];
_nativeTrackASBD = [self getTrackNativeSettings:track];
[reader addOutput:_readerOutput];
[reader startReading];
// Set a few defaults and presets
self.samplingRate = thisSamplingRate;
self.numChannels = thisNumChannels;
self.latency = .011609977; // 512 samples / ( 44100 samples / sec ) default
// Arbitrary buffer sizes that don't matter so much as long as they're "big enough"
self.outputBufferSize = 100000; //buffer sample sizes vary around 60-70k, we keep it # 100k to be safe
self.numSamplesReadPerPacket = 8192;
self.desiredPrebufferedSamples = self.numSamplesReadPerPacket*2;
//NOTE: these buffers are float, where as the above audio code is in SInt16
self.outputBuffer = (float *)calloc(2*self.samplingRate, sizeof(float));
self.holdingBuffer = (float *)calloc(2*self.samplingRate, sizeof(float));
// Allocate a ring buffer (this is what's going to buffer our audio)
ringBuffer = new RingBuffer(self.outputBufferSize, self.numChannels);
// Fill up the buffers, so we're ready to play immediately
[self bufferNewAudioFromAsset];
}
return self;
}
Looking at the code, it seems that everything is made in float (the audio buffers, the output format etc).. Is there a reason for this? (Keep in mind that the iOS audio canonical format is in SInt16, not float).. for example see in the Novocaine::renderCallback function:
else if ( sm.numBytesPerSample == 2 ) // then we need to convert SInt16 -> Float (and also scale)
{
float scale = (float)INT16_MAX;
vDSP_vsmul(sm.outData, 1, &scale, sm.outData, 1, inNumberFrames*sm.numOutputChannels);
for (int iBuffer=0; iBuffer < ioData->mNumberBuffers; ++iBuffer) {
int thisNumChannels = ioData->mBuffers[iBuffer].mNumberChannels;
for (int iChannel = 0; iChannel < thisNumChannels; ++iChannel) {
vDSP_vfix16(sm.outData+iChannel, sm.numOutputChannels, (SInt16 *)ioData->mBuffers[iBuffer].mData+iChannel, thisNumChannels, inNumberFrames);
}
}
}
What are the list of things that I gotta change to make this library compatible with reading and writing VBR Data?
A