iOS 相机:`AVCaptureAudioDataOutput` 即时激活音频会话，避免背景音乐卡顿 [英] iOS Camera: `AVCaptureAudioDataOutput` activate audio session on the fly, avoid background music stutter

问题描述

我使用 AVFoundation 创建了一个相机，它能够使用 AVCaptureVideoDataOutput 和 AVCaptureAudioDataOutput 录制视频和音频.我创建我的捕获会话，连接所有输入以及视频和音频数据输出，然后相机处于空闲状态.用户现在可以开始视频录制了.

I have created a Camera using AVFoundation which is able to record video and audio using AVCaptureVideoDataOutput and AVCaptureAudioDataOutput. I create my capture session, attach all inputs and the video- and audio-data outputs, and the Camera then sits idle. The user is now able to start a video recording.

问题在于，在我开始捕获会话 (captureSession.startRunning()) 后，背景音乐立即断断续续.我认为这是因为一旦捕获会话开始运行，AVCaptureAudioDataOutput 在内部激活 AVAudioSession (AVAudioSession.setActive(...))，我不希望它这样做.我希望它处于空闲状态(并且不提供任何音频输出缓冲区)，直到我明确激活音频会话(一旦用户开始录制).

The problem with this is that immediately after I start the capture session (captureSession.startRunning()), the background music stutters. I assume this is because once the capture session starts running, the AVCaptureAudioDataOutput internally activates the AVAudioSession (AVAudioSession.setActive(...)), which I don't want it doing. I want it sitting idle (and not providing any audio output buffers) until I explicitly activate the Audio Session (once the user starts recording).

这真的很烦人，因为相机是我们应用程序的开始屏幕，每次用户打开或关闭应用程序时，他的音乐都会断断续续.

This is really annoying, since the Camera is the start-screen in our app and everytime the user opens or closes the app his music stutters.

我知道这在某种程度上是可能的，因为 Snapchat 就是这样工作的——你打开应用程序，背景音频继续流畅地播放.开始录制后，背景音乐会出现轻微的卡顿，但相机会顺利运行并在短暂的卡顿结束后开始录制.

I know that this is somehow possible because Snapchat works that way - you open the App and background audio smoothly continues to play. Once you start recording, there is a small stutter on the background music, but the Camera smoothly operates and starts recording once the short stutter is over.

我的代码:

func configureSession() {

    captureSession.beginConfiguration()

    // Video, Photo and Audio Inputs
    ...

    // Video Output
    ...

    // Audio Output
    audioOutput = AVCaptureAudioDataOutput()
    guard captureSession.canAddOutput(audioOutput!) else {
      throw CameraError.parameter(.unsupportedOutput(outputDescriptor: "audio-output"))
    }
    audioOutput!.setSampleBufferDelegate(self, queue: audioQueue)
    captureSession.addOutput(audioOutput!)

    try AVAudioSession.sharedInstance().setCategory(AVAudioSession.Category.playAndRecord,
                                                    options: [.mixWithOthers,
                                                              .allowBluetoothA2DP,
                                                              .defaultToSpeaker,
                                                              .allowAirPlay])

    captureSession.commitConfiguration()
}

我尝试了什么

延迟配置AVAudioSession.sharedInstance()

我尝试首先使用类别AVAudioSession.Category.playback 配置AVAudioSession.sharedInstance，并在需要时切换到.playAndRecord开始录制音频.

What I tried

Delay configuring the AVAudioSession.sharedInstance()

I tried to first configure the AVAudioSession.sharedInstance with the category AVAudioSession.Category.playback, and switch to .playAndRecord once I want to start recording audio.

这不起作用，AVCaptureSessionRuntimeError 事件在启动相机后立即被调用，错误代码为 -10851，这意味着 kAudioUnitErr_InvalidPropertyValue.我认为这意味着 AVCaptureAudioDataOutput 不允许从音频会话中录制，但我现在不想这样做 - 它应该只是空闲.

This didn't work and the AVCaptureSessionRuntimeError event gets invoked immediately after starting the Camera with the Error code -10851, which means kAudioUnitErr_InvalidPropertyValue. I think this means that the AVCaptureAudioDataOutput is not allowed to record from the Audio Session, but I don't event want to do that right now - it should just be idle.

我尝试在开始时不添加音频输出 (AVCaptureAudioDataOutput)，而仅按需"添加它一旦用户开始录制，虽然这对背景音乐效果很好(开始时没有口吃，用户开始录制后只有短暂的口吃，正是我想要的)，它使预览冻结了很短的时间(因为捕获会话正在通过 beginConfiguration + 添加音频输出 + commitConfiguration)

I tried to not add the audio output (AVCaptureAudioDataOutput) in the beginning, and only add it "on-demand" once the user starts recording, and while that worked fine for the background music (no stutter when starting, only short stutter once the user starts recording, exactly how I want it), it made the Preview freeze for a short amount of time (because the Capture Session is being reconfigured via beginConfiguration + audio output adding + commitConfiguration)

有谁知道如何实现我在这里想要做的事情 - 或者 Snapchat 是如何做到的?感谢任何帮助，谢谢！

Does anyone know how it's possible to achieve what I'm trying to do here - or how Snapchat does it? Any help appreciated, thanks!

推荐答案

终于想通了.我只是创建了一个单独的 AVCaptureSession，专门用于与主捕获会话的 masterClock 同步的音频输入/输出.然后我可以即时启动/停止辅助捕获会话(在开始录制前不久)

Finally figured it out. I simply created a separate AVCaptureSession specifically for the audio input/output which I synchronize with the main capture session's masterClock. I can then start/stop the secondary capture session on the fly (shortly before start recording)

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