How to convert Data of Int16 audio samples to array of float audio samples

问题:

I'm currently working with audio samples. I get them from AVAssetReader and have a CMSampleBuffer with something like this:

guard let sampleBuffer = readerOutput.copyNextSampleBuffer() else { guard reader.status == .completed else { return nil } // Completed // samples is an array of Int16 let samples = sampleData.withUnsafeBytes {   Array(UnsafeBufferPointer<Int16>(   start: $0, count: sampleData.count / MemoryLayout<Int16>.size))  }   // The only way I found to convert [Int16] -> [Float]...  return samples.map { Float($0) / Float(Int16.max)} }  guard let blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer) else { return nil }  let length = CMBlockBufferGetDataLength(blockBuffer) let sampleBytes = UnsafeMutablePointer<UInt8>.allocate(capacity: length)       CMBlockBufferCopyDataBytes(blockBuffer, 0, length, sampleBytes)        sampleData.append(sampleBytes, count: length) } 

As you can see the only I found to convert [Int16] -> [Float] issamples.map { Float($0) / Float(Int16.max) but by doing this my processing time is increasing. Does it exist an other way to cast a pointer of Int16 to a pointer of Float?

回答1:

"Casting" or "rebinding" a pointer only changes the way how memory is interpreted. You want to compute floating point values from integers, the new values have a different memory representation (and also a different size).

Therefore you somehow have to iterate over all input values and compute the new values. What you can do is to omit the Array creation:

let samples = sampleData.withUnsafeBytes {     UnsafeBufferPointer<Int16>(start: $0, count: sampleData.count / MemoryLayout<Int16>.size) } return samples.map { Float($0) / Float(Int16.max) } 

Another option would be to use the vDSP functions from the Accelerate framework:

import Accelerate // ...  let numSamples = sampleData.count / MemoryLayout<Int16>.size var factor = Float(Int16.max) var floats: [Float] = Array(repeating: 0.0, count: numSamples)  // Int16 array to Float array: sampleData.withUnsafeBytes {     vDSP_vflt16($0, 1, &floats, 1, vDSP_Length(numSamples)) } // Scaling: vDSP_vsdiv(&floats, 1, &factor, &floats, 1, vDSP_Length(numSamples)) 

I don't know if that is faster, you'll have to check. (Update: It is faster, as ColGraff demonstrated in his answer.)

An explicit loop is also much faster than using map:

let factor = Float(Int16.max) let samples = sampleData.withUnsafeBytes {     UnsafeBufferPointer<Int16>(start: $0, count: sampleData.count / MemoryLayout<Int16>.size) } var floats: [Float] = Array(repeating: 0.0, count: samples.count) for i in 0..<samples.count {     floats[i] = Float(samples[i]) / factor } return floats 

---

An additional option in your case might be to use CMBlockBufferGetDataPointer() instead of CMBlockBufferCopyDataBytes() into allocated memory.

回答2:

You can do considerably better if you use the Accelerate Framework for the conversion:

import Accelerate  // Set up random [Int] var randomInt = [Int16]()  randomInt.reserveCapacity(10000) for _ in 0..<randomInt.capacity {   let value = Int16(Int32(arc4random_uniform(UInt32(UInt16.max))) - Int32(UInt16.max / 2))   randomInt.append(value) }  // Time elapsed helper: https://stackoverflow.com/a/25022722/887210 func printTimeElapsedWhenRunningCode(title:String, operation:()->()) {   let startTime = CFAbsoluteTimeGetCurrent()   operation()   let timeElapsed = CFAbsoluteTimeGetCurrent() - startTime   print("Time elapsed for \(title): \(timeElapsed) s.") }  // Testing  printTimeElapsedWhenRunningCode(title: "vDSP") {   var randomFloat = [Float](repeating: 0, count: randomInt.capacity)   vDSP_vflt16(randomInt, 1, &randomFloat, 1, vDSP_Length(randomInt.capacity)) }  printTimeElapsedWhenRunningCode(title: "map") {   randomInt.map { Float($0) } }  // Results // // Time elapsed for vDSP   : 0.000429034233093262 s. // Time elapsed for flatMap: 0.00233501195907593 s. 

It's an improvement of about 5 times faster.

(Edit: Added some changes suggested by Martin R)

回答3:

@MartinR and @ColGraff gave really good answers, and thank you for everybody and the fast replies. however I found an easier way to do that without any computation. AVAssetReaderAudioMixOutput requires an audio settings dictionary. Inside we can set the key AVLinearPCMIsFloatKey: true. This way I will read my data like this

let samples = sampleData.withUnsafeBytes {     UnsafeBufferPointer<Float>(start: $0,                                 count: sampleData.count / MemoryLayout<Float>.size) } 

回答4:

for:

extension Collection where Iterator.Element == Int16 {     var floatArray: [Float] {         return flatMap{ Float($0) }     } } 

usage:

let int16Array: [Int16] = [1, 2, 3 ,4]     let floatArray = int16Array.floatArray     

文章来源: How to convert Data of Int16 audio samples to array of float audio samples