如何使用 AVFoundation 以正确的音高播放不同采样率的音频文件?
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【中文标题】如何使用 AVFoundation 以正确的音高播放不同采样率的音频文件?【英文标题】:How can audio files with different sampling rates be played at the correct pitch using AVFoundation? 【发布时间】:2021-07-06 23:02:48 【问题描述】:AVAudioEngine
和 AVAudioPlayerNodes
在采样率不同时,应如何配置它们以正确的音高播放音频文件?
我读到混合器可以处理采样率转换,但我还没有实现这种行为。我正在使用扩展来播放循环音频片段,播放器应该可以使用压缩和 PCM 文件;我不知道这是否会决定解决方案。
我尝试在 AVAudioPlayerNode
的 installTap(onBus:bufferSize:format: block:)
块内使用 AVAudioConverter,但遇到了各种崩溃,并且不确定这是否是正确的解决方案。我是在正确的轨道上还是有更简单的解决方案?
import AVFoundation
import SwiftUI
@main
struct SampleRateMixerApp: App
private let audioEngine = AVAudioEngine()
private let playerA = AVAudioPlayerNode()
private let playerB = AVAudioPlayerNode()
var body: some Scene
WindowGroup
Button("Play") try? playFiles()
Button("Stop") playerA.stop(); playerB.stop()
func playFiles() throws
_ = audioEngine.mainMixerNode
try audioEngine.start()
audioEngine.prepare()
let pathA = Bundle.main.path(forResource: "32khz_sample_rate", ofType: "aif")!
try setupPlayerNode(player: playerA, withAudioEngine: audioEngine, atPath: pathA)
let pathB = Bundle.main.path(forResource: "48khz_sample_rate", ofType: "mp3")!
try setupPlayerNode(player: playerB, withAudioEngine: audioEngine, atPath: pathB)
func setupPlayerNode(player: AVAudioPlayerNode, withAudioEngine engine: AVAudioEngine, atPath path: String) throws
engine.attach(player)
engine.connect(player, to: engine.mainMixerNode, format: nil)
let url = URL(string: path)!
let file = try AVAudioFile(forReading: url)
let frameCount = AVAudioFrameCount(file.length)
let buffer = AVAudioPCMBuffer(pcmFormat: file.processingFormat, frameCapacity: frameCount)!
try file.read(into: buffer)
player.scheduleBufferSegment(buffer, range: 0.25...0.75, looping: true)
player.play()
extension AVAudioPlayerNode
func scheduleBufferSegment(_ buffer: AVAudioPCMBuffer, range: ClosedRange<Double>, looping: Bool)
let length = Double(buffer.frameLength)
let startFrame = AVAudioFramePosition(range.lowerBound * length)
let endFrame = AVAudioFramePosition(range.upperBound * length)
guard let bufferSegment = buffer.segment(from: startFrame, to: endFrame) else return
if looping
scheduleBuffer(bufferSegment, at: nil, options: [.loops, .interrupts])
else
scheduleBuffer(bufferSegment, at: nil, options: [.interrupts])
DispatchQueue.main.async [weak self] in
self?.stop()
extension AVAudioPCMBuffer
func segment(from startFrame: AVAudioFramePosition, to endFrame: AVAudioFramePosition) -> AVAudioPCMBuffer?
let framesToCopy = AVAudioFrameCount(endFrame - startFrame)
guard let segment = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: framesToCopy) else return nil
let sourcePointer = UnsafeMutableAudioBufferListPointer(mutableAudioBufferList)
let destinationPointer = UnsafeMutableAudioBufferListPointer(segment.mutableAudioBufferList)
let sampleSize = format.streamDescription.pointee.mBytesPerFrame
for (source, destination) in zip(sourcePointer, destinationPointer)
memcpy(destination.mData,
source.mData?.advanced(by: Int(startFrame) * Int(sampleSize)),
Int(framesToCopy) * Int(sampleSize))
segment.frameLength = framesToCopy
return segment
【问题讨论】:
【参考方案1】:在加载每个文件后调用AVAudioEngine
的connect(:to:format:)
传入AVAudioFiles
的processingFormat
。这可确保AVAudioPlayerNode
的输出设置为与其将播放的文件相同的采样率。
以下代码说明了这是如何完成的。该问题与播放缓冲区段无关,因此未将其包含在解决方案中。虽然音频可以在没有先断开播放器的情况下正常播放,但我怀疑它可能会泄漏内存,所以为了安全起见先断开节点。
import AVFoundation
let audioEngine = AVAudioEngine()
let player = AVAudioPlayerNode()
func playFiles() throws
_ = audioEngine.mainMixerNode // Called to ensure mainMixerNode singleton is instantiated
try audioEngine.start()
audioEngine.prepare()
audioEngine.attach(player)
let path = Bundle.main.path(forResource: "32khz", ofType: "aif")!
let url = URL(string: path)!
let file = try AVAudioFile(forReading: url)
let frameCount = AVAudioFrameCount(file.length)
let buffer = AVAudioPCMBuffer(pcmFormat: file.processingFormat, frameCapacity: frameCount)!
try file.read(into: buffer)
audioEngine.disconnectNodeOutput(player) // May be unnecessary
audioEngine.connect(player, to: audioEngine.mainMixerNode, format: file.processingFormat)
player.scheduleBuffer(buffer)
player.play()
【讨论】:
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