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: Xcode 8.3.3 • Swift 3.1

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    

来源：https://stackoverflow.com/questions/45863240/how-to-convert-data-of-int16-audio-samples-to-array-of-float-audio-samples