ios - 播放 AVAudioPlayerNode 时 AVAudioEngine inputNode 的格式发生变化

Question

我将从我创建的一个简单的“ Playground ” View Controller 类开始，该类演示了我的问题:

class AudioEnginePlaygroundViewController: UIViewController {
    private var audioEngine: AVAudioEngine!
    private var micTapped = false
    override func viewDidLoad() {
        super.viewDidLoad()
        configureAudioSession()
        audioEngine = AVAudioEngine()
    }

    @IBAction func toggleMicTap(_ sender: Any) {
        guard let mic = audioEngine.inputNode else {
            return
        }
        if micTapped {
            mic.removeTap(onBus: 0)
            micTapped = false
            return
        }
        stopAudioPlayback()

        let micFormat = mic.inputFormat(forBus: 0)
        print("installing tap: \(micFormat.sampleRate) -- \(micFormat.channelCount)")
        mic.installTap(onBus: 0, bufferSize: 2048, format: micFormat) { (buffer, when) in
            print("in tap completion")
            let sampleData = UnsafeBufferPointer(start: buffer.floatChannelData![0], count: Int(buffer.frameLength))
        }
        micTapped = true
        startEngine()
    }

    @IBAction func playAudioFile(_ sender: Any) {
        stopAudioPlayback()
        let playerNode = AVAudioPlayerNode()

        let audioUrl = Bundle.main.url(forResource: "test_audio", withExtension: "wav")!
        let audioFile = readableAudioFileFrom(url: audioUrl)
        audioEngine.attach(playerNode)
        audioEngine.connect(playerNode, to: audioEngine.outputNode, format: audioFile.processingFormat)
        startEngine()
        playerNode.scheduleFile(audioFile, at: nil, completionHandler: nil)
        playerNode.play()
    }

    // MARK: Internal Methods

    private func configureAudioSession() {
        do {
            try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord, with: [.mixWithOthers, .defaultToSpeaker])
            try AVAudioSession.sharedInstance().setActive(true)
        } catch { }
    }

    private func readableAudioFileFrom(url: URL) -> AVAudioFile {
        var audioFile: AVAudioFile!
        do {
            try audioFile = AVAudioFile(forReading: url)
        } catch { }
        return audioFile
    }

    private func startEngine() {
        guard !audioEngine.isRunning else {
            return
        }

        do {
            try audioEngine.start()
        } catch { }
    }

    private func stopAudioPlayback() {
        audioEngine.stop()
        audioEngine.reset()
    }
}

上面的 VC 有一个 AVAudioEngine 实例和两个 UIButton Action :一个播放在硬编码 url 中找到的音频文件，另一个在引擎的 inputNode。

我的目标是让实时麦克风敲击和音频文件播放同时工作，但彼此完全互斥。也就是说，无论麦克风的当前状态如何，我都希望能够触发播放，反之亦然。如果我在触发音频文件播放之前安装水龙头，一切都会完全按预期工作。但是，如果我先播放音频文件，然后尝试安装水龙头，我会遇到以下崩溃:

[avae] AVAEInternal.h:70:_AVAE_Check: required condition is false: [AVAEGraphNode.mm:810:CreateRecordingTap: (IsFormatSampleRateAndChannelCountValid(format))]

这促使我通过 installTap 调用上方的日志语句检查麦克风格式的数据。果然，当我在播放前安装 tap 时，我得到了 44100.0 的预期采样率和 1 的 channel 数。但是当我先播放音频文件并然后安装 mic tap 时，我的日志显示0 的采样率和 2 的 channel 数给我上面显示的错误。

我已经尝试修改 AVAudioEngine 的启动/重置流程，尝试了我的 AVAudioSession 的不同类别/模式组合(请参阅我的configureAudioSession 方法)，并尝试手动创建点击格式，如下所示:

let micFormat = mic.inputFormat(forBus: 0)
var trueFormat: AVAudioFormat!
if micFormat.sampleRate == 0 {
    trueFormat = AVAudioFormat(standardFormatWithSampleRate: 44100, channels: 1)
} else {
    trueFormat = micFormat
}
print("installing tap: \(micFormat.sampleRate) -- \(micFormat.channelCount)")
mic.installTap(onBus: 0, bufferSize: 2048, format: trueFormat) { (buffer, when) in
    print("in tap completion")
    let sampleData = UnsafeBufferPointer(start: buffer.floatChannelData![0], count: Int(buffer.frameLength))
}

这给了我一个类似但不同的错误:

[avae] AVAEInternal.h:70:_AVAE_Check: required condition is false: [AVAudioIONodeImpl.mm:896:SetOutputFormat: (IsFormatSampleRateAndChannelCountValid(hwFormat))]

我看不出麦克风的格式数据会根据是否播放 AVAudioPlayerNode 而有所不同的任何原因。

Answer 1

经过一番搜索，我发现了问题所在。问题在于音频引擎的 inputNode 单例。来自文档:

The audio engine creates a singleton on demand when inputNode is first accessed. To receive input, connect another audio node from the output of the input audio node, or create a recording tap on it.

加上对我遇到的格式问题的引用:

Check the input format of input node (specifically, the hardware format) for a non-zero sample rate and channel count to see if input is enabled.

在我的 playground 类中，触发音频文件播放的流程在创建“事件链”之前从不访问引擎的 inputNode:

audioEngine.connect(playerNode, to: audioEngine.outputNode, format: audioFile.processingFormat)

如果您希望引擎在内部为输入配置自身，您似乎必须在启动之前访问 AVAudioEngine 的 inputNode。即使停止()和重置()引擎也不会导致访问 inputNode 以重新配置引擎。 (我怀疑通过 disconnectNode 调用手动中断事件链将允许内部重新配置，但我还不确定)。

因此在代码方面修复很简单:只需在实例化后立即访问引擎的输入节点，以便为音频输入配置引擎。这是整个类(class)的文件播放和麦克风窃听一起工作:

import UIKit

class AudioEnginePlaygroundViewController: UIViewController {
    private var audioEngine: AVAudioEngine!
    private var mic: AVAudioInputNode!
    private var micTapped = false

    override func viewDidLoad() {
        super.viewDidLoad()
        configureAudioSession()
        audioEngine = AVAudioEngine()
        mic = audioEngine.inputNode!
    }

    @IBAction func toggleMicTap(_ sender: Any) {
        if micTapped {
            mic.removeTap(onBus: 0)
            micTapped = false
            return
        }

        let micFormat = mic.inputFormat(forBus: 0)
        mic.installTap(onBus: 0, bufferSize: 2048, format: micFormat) { (buffer, when) in
            let sampleData = UnsafeBufferPointer(start: buffer.floatChannelData![0], count: Int(buffer.frameLength))
        }
        micTapped = true
        startEngine()
    }

    @IBAction func playAudioFile(_ sender: Any) {
        stopAudioPlayback()
        let playerNode = AVAudioPlayerNode()

        let audioUrl = Bundle.main.url(forResource: "test_audio", withExtension: "wav")!
        let audioFile = readableAudioFileFrom(url: audioUrl)
        audioEngine.attach(playerNode)
        audioEngine.connect(playerNode, to: audioEngine.outputNode, format: audioFile.processingFormat)
        startEngine()
        playerNode.scheduleFile(audioFile, at: nil, completionHandler: nil)
        playerNode.play()
    }

    // MARK: Internal Methods

    private func configureAudioSession() {
        do {
            try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord, with: [.mixWithOthers, .defaultToSpeaker])
            try AVAudioSession.sharedInstance().setActive(true)
        } catch { }
    }

    private func readableAudioFileFrom(url: URL) -> AVAudioFile {
        var audioFile: AVAudioFile!
        do {
            try audioFile = AVAudioFile(forReading: url)
        } catch { }
        return audioFile
    }

    private func startEngine() {
        guard !audioEngine.isRunning else {
            return
        }

        do {
            try audioEngine.start()
        } catch { }
    }

    private func stopAudioPlayback() {
        audioEngine.stop()
        audioEngine.reset()
    }
}