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.
Last active
November 2, 2023 09:25
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Process a CMSampleBuffer to modify the audio using an AVAssetReader and AVAssetWriter in Swift
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| 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 | |
| } |
what is the value of 'scale', I can't find it
what is the value of 'scale', I can't find it
@daobao777 scale could be specified by yourself. it could be 0.0-1.0
what is the value of 'scale', I can't find it
@daobao777 scale could be specified by yourself. it could be 0.0-1.0
line 15: let scaledValue = Float(ptr.pointee) * scale
what is the value of 'scale', I can't find it
@daobao777 scale could be specified by yourself. it could be 0.0-1.0
I would get the wrong answer when scale is 1.0
So, how can i get accurate data
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this code helped me, but it spent about 5-7ms