Question

I am trying to get the samples out of a CMSampleBufferRef and into an array of floats in order to change the pitch. I have found a method CMSampleBufferCallForEachSample in the documentation but don’t know how to use it and there are no examples or explanations. Please I have been running around in circles with this thing for a while, can anyone help?

Answer 1

I was solving similar problem. I needed to split AAC samples from one SampleBuffer.

    ...
    CMSampleBufferCallForEachSample(sampleBuffer, &sampler, NULL);
    ...

static OSStatus sampler(CMSampleBufferRef sampleBuffer, CMItemCount index, void *refcon) {
    CMTime presentationTimeStamp = CMSampleBufferGetOutputPresentationTimeStamp(sampleBuffer);
    CMBlockBufferRef buff = CMSampleBufferGetDataBuffer(sampleBuffer);
    UInt32 timeStamp = (1000*presentationTimeStamp.value) / presentationTimeStamp.timescale;
    size_t size = CMBlockBufferGetDataLength(buff);

    void *sampleData = malloc(size);
    CMBlockBufferCopyDataBytes(buff, 0, size, sampleData);
    NSData * data = [NSData dataWithBytes:sampleData length:size];
    NSLog(@"Audio %ldx samples of size %lu has been transfered at time %ld", CMSampleBufferGetNumSamples(sampleBuffer), size, timeStamp); 
}

but callback is called only once for first sample in each sampleBuffer, even there is the array of 76 samples and CMSampleBufferGetNumSamples returning this number

i found excellent solution here (method fillMovieSampleBuffer row 283)

for documentation of CoreMedia framework check this URL

@function   CMSampleBufferCallForEachSample
@abstract   Calls a function for every individual sample in a sample buffer.
@discussion Temporary sample buffers will be created for individual samples,
referring to the sample data and containing its timing, size and attachments.
The callback function may retain these sample buffers if desired.
If the callback function returns an error, iteration will stop immediately
and the error will be returned.

If there are no sample sizes in the provided sample buffer, kCMSampleBufferError_CannotSubdivide will be returned.
This will happen, for example, if the samples in the buffer are non-contiguous (eg. non-interleaved audio, where
the channel values for a single sample are scattered through the buffer).