brennanMKE/ProcessBuffer.Swift

## README.md

      
    Raw
  

              README.md
            
          
    Processing Audio with AVAssetReader and AVAssetWriter

Modifying an audio stream can be done with an AVAssetReader and AVAssetWriter. While reading a CMSampleBuffer
can be used to get CMBlockBuffer which is a pointer to a buffer within CMSampleBuffer. Those bytes can be copied
into a mutable buffer with CMBlockBufferCopyDataBytes to modify the values and then use CMBlockBufferReplaceDataBytes
to put the modified values back into the CMSampleBuffer which also modifies CMSampleBuffer before appending it
to the AVAssetWriterInput instance used by AVAssetWriter.

  
## ProcessBuffer.Swift
func processSampleBuffer(scale: Float, sampleBuffer: CMSampleBuffer, writerInput: AVAssetWriterInput) -> Bool {
    guard let blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer) else {
        return false
    }
    let length = CMBlockBufferGetDataLength(blockBuffer)
    var sampleBytes = UnsafeMutablePointer<Int16>.allocate(capacity: length)
    defer { sampleBytes.deallocate(capacity: length) }

    guard checkStatus(CMBlockBufferCopyDataBytes(blockBuffer, 0, length, sampleBytes), message: "Copying block buffer") else {
        return false
    }

    (0..<length).forEach { index in
        let ptr = sampleBytes + index
        let scaledValue = Float(ptr.pointee) * scale
        let processedValue = Int16(max(min(scaledValue, Float(Int16.max)), Float(Int16.min)))
        ptr.pointee = processedValue
    }

    guard checkStatus(CMBlockBufferReplaceDataBytes(sampleBytes, blockBuffer, 0, length), message: "Replacing data bytes in block buffer") else { return false }
    assert(CMSampleBufferIsValid(sampleBuffer))
    return writerInput.append(sampleBuffer)
}

func checkStatus(_ status: OSStatus, message: String) -> Bool {
    // See: https://www.osstatus.com/
    assert(kCMBlockBufferNoErr == noErr)
    if status != noErr {
        debugPrint("Error: \(message) [\(status)]")
    }
    return status == noErr
}
	func processSampleBuffer(scale: Float, sampleBuffer: CMSampleBuffer, writerInput: AVAssetWriterInput) -> Bool {
	guard let blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer) else {
	return false
	}
	let length = CMBlockBufferGetDataLength(blockBuffer)
	var sampleBytes = UnsafeMutablePointer<Int16>.allocate(capacity: length)
	defer { sampleBytes.deallocate(capacity: length) }

	guard checkStatus(CMBlockBufferCopyDataBytes(blockBuffer, 0, length, sampleBytes), message: "Copying block buffer") else {
	return false
	}

	(0..<length).forEach { index in
	let ptr = sampleBytes + index
	let scaledValue = Float(ptr.pointee) * scale
	let processedValue = Int16(max(min(scaledValue, Float(Int16.max)), Float(Int16.min)))
	ptr.pointee = processedValue
	}

	guard checkStatus(CMBlockBufferReplaceDataBytes(sampleBytes, blockBuffer, 0, length), message: "Replacing data bytes in block buffer") else { return false }
	assert(CMSampleBufferIsValid(sampleBuffer))
	return writerInput.append(sampleBuffer)
	}

	func checkStatus(_ status: OSStatus, message: String) -> Bool {
	// See: https://www.osstatus.com/
	assert(kCMBlockBufferNoErr == noErr)
	if status != noErr {
	debugPrint("Error: \(message) [\(status)]")
	}
	return status == noErr
	}