计时问题：节拍器使用 AVAudioEngine scheduleBuffer 的完成处理程序
Timing issues: Metronome using AVAudioEngine scheduleBuffer's completion handler
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：
- 稳定的时间安排（我知道，我知道，我应该使用音频单元，但我仍在努力处理核心音频的东西/Obj-C 包装器等）
- 小节的“1”和节拍“2”/“3”/“4”有两种不同的声音。
- 需要与音频同步的某种视觉反馈（至少显示当前节拍）。
所以我创建了两个短的点击声音（26 毫秒/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周期。
我的 UI 设置很简单：一个切换开始/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 时序没问题，但是因为我需要有 2 种不同的声音和一种方法来 sync/update 我的 UI 在循环点击时我无法使用这个。我想出使用 completionHandler 而不是重新启动我的 playClickLoop() 函数 - 请参阅下面的代码附件。
不幸的是，在实现这个时我并没有真正测量时间的准确性。正如现在将 bpm 设置为 120 时所证明的那样，它仅以大约 117.5 bpm 的速度播放循环 - 相当稳定但仍然太慢了。当 bpm 设置为 180 时，我的应用程序以大约 172.3 bpm 的速度播放。
这是怎么回事？这种延迟是使用 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,
at: nil,
options: [.interruptsAtLoop],
//completionCallbackType: .dataRendered,
completionHandler: scheduleBufferCompletionHandler)
}
//
// MARK: Display current beat on UILabel + to console
//
DispatchQueue.main.async {
self.label.text = String(self.counter)
print(self.counter)
}
}
}
}
您用来测量的工具或过程的准确性如何？
由于我不是 C 程序员，我无法确定您的文件具有正确数量的 PCM 帧。加载文件时，似乎包含来自 wav header 的数据。这让我想知道在每次播放或循环开始时重复处理 header 信息时，播放是否会产生一些延迟。
我很幸运地在 Java 中构建了一个节拍器，方法是使用连续输出从读取 PCM 帧中派生的无尽流的计划。根据所选节拍器设置的周期和 PCM 帧中点击的长度，通过计算 PCM 帧并在静音（PCM 数据点 = 0）或点击的 PCM 数据中路由来实现计时。
正如 Phil Freihofner 在上面所建议的，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 回调调用不够早，无法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。这将导致声音之间出现（听不见的）间隙并弄乱时间。必须已经有另一个缓冲区“保留”，即在当前缓冲区被安排之前已经被安排。
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，回调已经太晚了，无法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（正确计时！）我只是激活了一个定时器，每个周期触发两次。所有奇数计时器事件将重新安排另一个缓冲区。
有时解决方案太简单了，令人尴尬......但有时你必须先尝试几乎所有错误的方法才能找到它;-)
我完整的工作解决方案（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, at:nil, options: [], 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, options: [], 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。
亚历克斯
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：
- 稳定的时间安排（我知道，我知道，我应该使用音频单元，但我仍在努力处理核心音频的东西/Obj-C 包装器等）
- 小节的“1”和节拍“2”/“3”/“4”有两种不同的声音。
- 需要与音频同步的某种视觉反馈（至少显示当前节拍）。
所以我创建了两个短的点击声音（26 毫秒/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周期。
我的 UI 设置很简单：一个切换开始/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 时序没问题，但是因为我需要有 2 种不同的声音和一种方法来 sync/update 我的 UI 在循环点击时我无法使用这个。我想出使用 completionHandler 而不是重新启动我的 playClickLoop() 函数 - 请参阅下面的代码附件。
不幸的是，在实现这个时我并没有真正测量时间的准确性。正如现在将 bpm 设置为 120 时所证明的那样，它仅以大约 117.5 bpm 的速度播放循环 - 相当稳定但仍然太慢了。当 bpm 设置为 180 时，我的应用程序以大约 172.3 bpm 的速度播放。
这是怎么回事？这种延迟是使用 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,
at: nil,
options: [.interruptsAtLoop],
//completionCallbackType: .dataRendered,
completionHandler: scheduleBufferCompletionHandler)
}
//
// MARK: Display current beat on UILabel + to console
//
DispatchQueue.main.async {
self.label.text = String(self.counter)
print(self.counter)
}
}
}
}
您用来测量的工具或过程的准确性如何？
由于我不是 C 程序员，我无法确定您的文件具有正确数量的 PCM 帧。加载文件时，似乎包含来自 wav header 的数据。这让我想知道在每次播放或循环开始时重复处理 header 信息时，播放是否会产生一些延迟。
我很幸运地在 Java 中构建了一个节拍器，方法是使用连续输出从读取 PCM 帧中派生的无尽流的计划。根据所选节拍器设置的周期和 PCM 帧中点击的长度，通过计算 PCM 帧并在静音（PCM 数据点 = 0）或点击的 PCM 数据中路由来实现计时。
正如 Phil Freihofner 在上面所建议的，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 回调调用不够早，无法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。这将导致声音之间出现（听不见的）间隙并弄乱时间。必须已经有另一个缓冲区“保留”，即在当前缓冲区被安排之前已经被安排。
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，回调已经太晚了，无法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（正确计时！）我只是激活了一个定时器，每个周期触发两次。所有奇数计时器事件将重新安排另一个缓冲区。
有时解决方案太简单了，令人尴尬......但有时你必须先尝试几乎所有错误的方法才能找到它;-)
我完整的工作解决方案（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, at:nil, options: [], 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, options: [], 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。
亚历克斯