问题描述
我想使用具有以下功能的 AVAudioEngine 构建一个简单的节拍器应用程序:
- 可靠的时间安排(我知道,我知道,我应该使用音频单元,但我仍在为 Core Audio 内容/Obj-C 包装器等而苦苦挣扎)
- 小节的“1”和节拍“2”/“3”/“4”有两种不同的声音。
- 某种需要与音频同步的视觉反馈(至少是当前节拍的显示)。
所以我创建了两个短点击声音(26ms / 1150 个样本 @ 16 位 / 44,1 kHz / 立体声 wav 文件)并将它们加载到 2 个缓冲区中。它们的长度将被设置为代表一个时期。
我的 UI 设置很简单:一个用于切换开始/暂停的按钮和一个显示当前节拍的标签(我的“计数器”变量)。
当使用 scheduleBuffer 的循环属性时,时间是可以的,但是因为我需要有 2 种不同的声音和一种在循环点击时同步/更新我的 UI 的方法,所以我不能使用它。我想出使用 completionHandler 代替它重新启动我的 playClickLoop() 函数 - 请参阅下面附上的代码。
不幸的是,在执行此操作时,我并没有真正测量计时的准确性。现在事实证明,当将 bpm 设置为 120 时,它仅以大约 117.5 bpm 的速度播放循环 - 相当稳定,但仍然太慢。当 bpm 设置为 180 时,我的应用以大约 172.3 bpm 播放。
这是怎么回事?这种延迟是通过使用 completionHandler 引入的吗?有什么办法可以改善时间?还是我的整个方法都错了?
提前致谢! 亚历克斯
import UIKit
import AVFoundation
class ViewController: UIViewController {
private let engine = AVAudioEngine()
private let player = AVAudioPlayerNode()
private let fileName1 = "sound1.wav"
private let fileName2 = "sound2.wav"
private var file1: AVAudioFile! = nil
private var file2: AVAudioFile! = nil
private var buffer1: AVAudioPCMBuffer! = nil
private var buffer2: AVAudioPCMBuffer! = nil
private let sampleRate: Double = 44100
private var bpm: Double = 180.0
private var periodLengthInSamples: Double { 60.0 / bpm * sampleRate }
private var counter: Int = 0
private enum metronomeState {case run; case stop}
private var state: metronomeState = .stop
@IBOutlet weak var label: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
//
// MARK: Loading buffer1
//
let path1 = Bundle.main.path(forResource: fileName1,ofType: nil)!
let url1 = URL(fileURLWithPath: path1)
do {file1 = try AVAudioFile(forReading: url1)
buffer1 = AVAudioPCMBuffer(
pcmFormat: file1.processingFormat,frameCapacity: AVAudioFrameCount(periodLengthInSamples))
try file1.read(into: buffer1!)
buffer1.frameLength = AVAudioFrameCount(periodLengthInSamples)
} catch { print("Error loading buffer1 \(error)") }
//
// MARK: Loading buffer2
//
let path2 = Bundle.main.path(forResource: fileName2,ofType: nil)!
let url2 = URL(fileURLWithPath: path2)
do {file2 = try AVAudioFile(forReading: url2)
buffer2 = AVAudioPCMBuffer(
pcmFormat: file2.processingFormat,frameCapacity: AVAudioFrameCount(periodLengthInSamples))
try file2.read(into: buffer2!)
buffer2.frameLength = AVAudioFrameCount(periodLengthInSamples)
} catch { print("Error loading buffer2 \(error)") }
//
// MARK: Configure + start engine
//
engine.attach(player)
engine.connect(player,to: engine.mainmixerNode,format: file1.processingFormat)
engine.prepare()
do { try engine.start() } catch { print(error) }
}
//
// MARK: Play / Pause toggle action
//
@IBAction func buttonPresed(_ sender: UIButton) {
sender.isSelected = !sender.isSelected
if player.isPlaying {
state = .stop
} else {
state = .run
try! engine.start()
player.play()
playClickLoop()
}
}
private func playClickLoop() {
//
// MARK: Completion handler
//
let scheduleBufferCompletionHandler = { [uNowned self] /*(_: AVAudioPlayerNodeCompletionCallbackType)*/ in
dispatchQueue.main.async {
switch state {
case .run:
self.playClickLoop()
case .stop:
engine.stop()
player.stop()
counter = 0
}
}
}
//
// MARK: Schedule buffer + play
//
if engine.isRunning {
counter += 1; if counter > 4 {counter = 1} // Counting from 1 to 4 only
if counter == 1 {
//
// MARK: Playing sound1 on beat 1
//
player.scheduleBuffer(buffer1,at: nil,options: [.interruptsAtLoop],//completionCallbackType: .dataPlayedBack,completionHandler: scheduleBufferCompletionHandler)
} else {
//
// MARK: Playing sound2 on beats 2,3 & 4
//
player.scheduleBuffer(buffer2,//completionCallbackType: .dataRendered,completionHandler: scheduleBufferCompletionHandler)
}
//
// MARK: display current beat on UILabel + to console
//
dispatchQueue.main.async {
self.label.text = String(self.counter)
print(self.counter)
}
}
}
}
解决方法
正如 Phil Freihofner 上面所建议的,这是我自己的问题的解决方案:
我学到的最重要的一课:scheduleBuffer 命令提供的completionHandler 回调没有足够早地被调用以在第一个缓冲区仍在播放时触发另一个缓冲区的重新调度。这将导致声音之间的(听不清)间隙并弄乱时间。必须已经有另一个缓冲区“保留”,即在当前缓冲区被调度之前已经被调度。
考虑到完成回调的时间,使用 scheduleBuffer 的 completionCallbackType 参数并没有太大变化:当将其设置为 .dataRendered 或 .dataConsumed 时,回调已经来不及重新安排另一个缓冲区。使用 .dataPlayedback 只会让事情变得更糟 :-)
因此,为了实现无缝播放(具有正确的计时!)我只是激活了一个每周期触发两次的计时器。所有奇数定时器事件将重新安排另一个缓冲区。
有时解决方案是如此简单以至于令人尴尬...但有时您必须首先尝试几乎所有错误的方法才能找到它;-)
我的完整工作解决方案(包括两个声音文件和 UI)可以在 GitHub 上找到:
https://github.com/Alexander-Nagel/Metronome-using-AVAudioEngine
import UIKit
import AVFoundation
private let DEBUGGING_OUTPUT = true
class ViewController: UIViewController{
private var engine = AVAudioEngine()
private var player = AVAudioPlayerNode()
private var mixer = AVAudioMixerNode()
private let fileName1 = "sound1.wav"
private let fileName2 = "sound2.wav"
private var file1: AVAudioFile! = nil
private var file2: AVAudioFile! = nil
private var buffer1: AVAudioPCMBuffer! = nil
private var buffer2: AVAudioPCMBuffer! = nil
private let sampleRate: Double = 44100
private var bpm: Double = 133.33
private var periodLengthInSamples: Double {
60.0 / bpm * sampleRate
}
private var timerEventCounter: Int = 1
private var currentBeat: Int = 1
private var timer: Timer! = nil
private enum MetronomeState {case running; case stopped}
private var state: MetronomeState = .stopped
@IBOutlet weak var beatLabel: UILabel!
@IBOutlet weak var bpmLabel: UILabel!
@IBOutlet weak var playPauseButton: UIButton!
override func viewDidLoad() {
super.viewDidLoad()
bpmLabel.text = "\(bpm) BPM"
setupAudio()
}
private func setupAudio() {
//
// MARK: Loading buffer1
//
let path1 = Bundle.main.path(forResource: fileName1,ofType: nil)!
let url1 = URL(fileURLWithPath: path1)
do {file1 = try AVAudioFile(forReading: url1)
buffer1 = AVAudioPCMBuffer(
pcmFormat: file1.processingFormat,frameCapacity: AVAudioFrameCount(periodLengthInSamples))
try file1.read(into: buffer1!)
buffer1.frameLength = AVAudioFrameCount(periodLengthInSamples)
} catch { print("Error loading buffer1 \(error)") }
//
// MARK: Loading buffer2
//
let path2 = Bundle.main.path(forResource: fileName2,ofType: nil)!
let url2 = URL(fileURLWithPath: path2)
do {file2 = try AVAudioFile(forReading: url2)
buffer2 = AVAudioPCMBuffer(
pcmFormat: file2.processingFormat,frameCapacity: AVAudioFrameCount(periodLengthInSamples))
try file2.read(into: buffer2!)
buffer2.frameLength = AVAudioFrameCount(periodLengthInSamples)
} catch { print("Error loading buffer2 \(error)") }
//
// MARK: Configure + start engine
//
engine.attach(player)
engine.connect(player,to: engine.mainMixerNode,format: file1.processingFormat)
engine.prepare()
do { try engine.start() } catch { print(error) }
}
//
// MARK: Play / Pause toggle action
//
@IBAction func buttonPresed(_ sender: UIButton) {
sender.isSelected = !sender.isSelected
if state == .running {
//
// PAUSE: Stop timer and reset counters
//
state = .stopped
timer.invalidate()
timerEventCounter = 1
currentBeat = 1
} else {
//
// START: Pre-load first sound and start timer
//
state = .running
scheduleFirstBuffer()
startTimer()
}
}
private func startTimer() {
if DEBUGGING_OUTPUT {
print("# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # ")
print()
}
//
// Compute interval for 2 events per period and set up timer
//
let timerIntervallInSamples = 0.5 * self.periodLengthInSamples / sampleRate
timer = Timer.scheduledTimer(withTimeInterval: timerIntervallInSamples,repeats: true) { timer in
//
// Only for debugging: Print counter values at start of timer event
//
// Values at begin of timer event
if DEBUGGING_OUTPUT {
print("timerEvent #\(self.timerEventCounter) at \(self.bpm) BPM")
print("Entering \ttimerEventCounter: \(self.timerEventCounter) \tcurrentBeat: \(self.currentBeat) ")
}
//
// Schedule next buffer at 1st,3rd,5th & 7th timerEvent
//
var bufferScheduled: String = "" // only needed for debugging / console output
switch self.timerEventCounter {
case 7:
//
// Schedule main sound
//
self.player.scheduleBuffer(self.buffer1,at:nil,options: [],completionHandler: nil)
bufferScheduled = "buffer1"
case 1,3,5:
//
// Schedule subdivision sound
//
self.player.scheduleBuffer(self.buffer2,completionHandler: nil)
bufferScheduled = "buffer2"
default:
bufferScheduled = ""
}
//
// Display current beat & increase currentBeat (1...4) at 2nd,4th,6th & 8th timerEvent
//
if self.timerEventCounter % 2 == 0 {
DispatchQueue.main.async {
self.beatLabel.text = String(self.currentBeat)
}
self.currentBeat += 1; if self.currentBeat > 4 {self.currentBeat = 1}
}
//
// Increase timerEventCounter,two events per beat.
//
self.timerEventCounter += 1; if self.timerEventCounter > 8 {self.timerEventCounter = 1}
//
// Only for debugging: Print counter values at end of timer event
//
if DEBUGGING_OUTPUT {
print("Exiting \ttimerEventCounter: \(self.timerEventCounter) \tcurrentBeat: \(self.currentBeat) \tscheduling: \(bufferScheduled)")
print()
}
}
}
private func scheduleFirstBuffer() {
player.stop()
//
// pre-load accented main sound (for beat "1") before trigger starts
//
player.scheduleBuffer(buffer1,at: nil,completionHandler: nil)
player.play()
beatLabel.text = String(currentBeat)
}
}
非常感谢大家的帮助!这是一个很棒的社区。p>
亚历克斯
,您用来衡量的工具或过程的准确度如何?
由于我不是 C 程序员,因此我无法确定您的文件具有正确的 PCM 帧数。加载文件时,似乎包含来自 wav 标头的数据。这让我想知道在每次播放或循环开始时重复处理标头信息时,播放是否会产生一些延迟。
我很幸运通过使用从读取 PCM 帧中获得的连续输出无限流的计划,在 Java 中构建了一个节拍器。根据所选节拍器设置的周期和 PCM 帧中的咔嗒声长度,通过计算 PCM 帧并在静音(PCM 数据点 = 0)或咔嗒声的 PCM 数据中路由来实现计时。