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如何将Int16音频样本数据转换为浮点音频样本阵列

nsdata avfoundation nsarray audio swift

DEADBEEF · 技术社区 · 7 年前

我目前正在处理音频样本。我从Avassetrader那里买的,有一个 CMSampleBuffer 比如:

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)
}

正如您所见,我找到的唯一一个可以转换[Int16]->[浮动]是 samples.map { Float($0) / Float(Int16.max) 但是通过这样做,我的处理时间增加了。是否存在将Int16指针转换为浮点指针的其他方法?

4 回复 | 直到 7 年前

Martin R 7 年前

“投射”或“重新绑定”指针只会改变内存的方式解释。你想要计算整数的浮点值, 尺寸)。

不得不 迭代所有输入值 Array 创造:

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

另一种选择是使用加速框架:

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))

我不知道这是否更快,你得检查一下。 更新: 正如科尔格拉夫在回答中所证明的那样,速度更快。)

显式循环也比使用 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

在您的情况下,另一个选择可能是使用 CMBlockBufferGetDataPointer() 而不是 CMBlockBufferCopyDataBytes()

user887210 user887210 7 年前

如果使用 Accelerate Framework 对于转换:

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.

DEADBEEF 7 年前

@MartinR和@ColGraff给出了非常好的答案,感谢大家的快速回复。 AVAssetReaderAudioMixOutput 需要音频设置字典。我们可以在里面设置钥匙 AVLinearPCMIsFloatKey: true

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

swift2geek 7 年前

适用于: Xcode 8.3.3 Swift 3.1

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

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