如何将Int16音频样本数据转换为浮点音频样本数组

Question

我目前正在处理音频样本。我从AVAssetReader获得它们，并有一个cmsampleBuffer，其内容如下：

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)
}

如您所见，我找到的唯一用于转换[Int16]->[Float]的方法是samples.map{Float($0)/Float(Int16.max)，但这样做会增加我的处理时间。是否存在将Int16指针转换为Float指针的其他方法？

Answer 1

“强制转换”或“重新绑定”指针只会改变内存的解释方式。如果要从整数计算浮点值，则新值具有不同的内存表示形式（以及不同的大小）。

因此，您必须以某种方式迭代所有输入值并计算新值。您可以做的是省略数组创建：

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中的vDSP函数：

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))

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，另一个选项可能是在分配的内存中使用CMBlockBufferGetDataPointer()而不是CMBlockBufferCopyDataBytes()。