Getting value from unSafeMutablePointer Int16 in Swift for audio data purposes

Question

I'm working to convert to Swift this code which helps get me get audio data for visualizations. The code I'm working with in Obj C, which works well, is:

    while (reader.status == AVAssetReaderStatusReading) {
           AVAssetReaderTrackOutput *trackOutput = (AVAssetReaderTrackOutput *)[reader.outputs objectAtIndex:0];
            self.sampleBufferRef = [trackOutput copyNextSampleBuffer];
            if (self.sampleBufferRef) {

                CMBlockBufferRef blockBufferRef = CMSampleBufferGetDataBuffer(self.sampleBufferRef);
                size_t bufferLength = CMBlockBufferGetDataLength(blockBufferRef);
                void *data = malloc(bufferLength);
                CMBlockBufferCopyDataBytes(blockBufferRef, 0, bufferLength, data);

                SInt16 *samples = (SInt16 *)data;
                int sampleCount = bufferLength / bytesPerInputSample;


                for (int i=0; i<sampleCount; i+=100) {
                    Float32 sample = (Float32) *samples++;

                sample = decibel(sample);
                sample = minMaxX(sample,noiseFloor,0);
                tally += sample; 

                for (int j=1; j<channelCount; j++)
                    samples++;
                tallyCount++;

                if (tallyCount == downsampleFactor) {
                    sample = tally / tallyCount;
                    maximum = maximum > sample ? maximum : sample;
                    [fullSongData appendBytes:&sample length:sizeof(sample)];//tried dividing the sample by 2
                    tally = 0;
                    tallyCount = 0;
                    outSamples++;


                }
            }

        CMSampleBufferInvalidate(self.sampleBufferRef);
        CFRelease(self.sampleBufferRef);
        free(data);
   }
}

In Swift, I'm trying to write is this part:

 while (reader.status == AVAssetReaderStatus.Reading) {
            var trackOutput = reader.outputs[0] as! AVAssetReaderTrackOutput
            self.sampleBufferRef = trackOutput.copyNextSampleBuffer()

            if (self.sampleBufferRef != nil) {

            let blockBufferRef = CMSampleBufferGetDataBuffer(self.sampleBufferRef)
            let bufferLength = CMBlockBufferGetDataLength(blockBufferRef)
            var data = NSMutableData(length: bufferLength)
            CMBlockBufferCopyDataBytes(blockBufferRef, 0, bufferLength, data!.mutableBytes)


            var samples = UnsafeMutablePointer<Int16>(data!.mutableBytes)

            var sampleCount = Int32(bufferLength)/bytesPerInputSample


            for var i = 0; i < Int(sampleCount); i++ {

                var sampleValue = CGFloat(samples[i]) etc. etc.

However, when I println() sampleValue is just comes out (Opaque Value) in the console. I can't figure out how to actually read the sampleValue.

I'm new at trying to read audio data for visualization purposes. Any help on getting a buffer of audio data to work with would be helpful. Thank you.

Tabbitha

Answer 1

Use stride?

let bytesPerInputSample = 4 // assumption ;)

var samplePtr = data.mutableBytes

for _ in stride(from: 0, to: data.length, by: bytesPerInputSample) {
    let currentSample = Data(bytes: samplePtr, count: bytesPerInputSample)
    // do whatever is needed with current sample

    //...

    // increase ptr by size of sample
    samplePtr = samplePtr + bytesPerInputSample
}

Wellread