从 AVAudioEngine 总线抽头生成的文件中的音频与设备输出上播放的速度不同

Question

我正在使用 AVAudioEngine 生成音频。然后我在引擎的 mainMixerNode 输出上安装一个分路器，它提供 AVAudioPCMBuffer 然后写入 MPEG4ACC AVAudioFile。我遇到的问题是，在生成的 .m4a 文件上播放的音频速度比您在设备输出本身（扬声器、耳机）上听到的速度慢。这是我正在实施的代码：


// Audio Format
let audioFormat = AVAudioFormat(standardFormatWithSampleRate: 44100, channels: 2)

// Engine
var engine = AVAudioEngine()

// Two AVAudioNodes are hooked up to the AVAudioEngine
engine.connect(myAVAudioNode0, to: engine.mainMixerNode, format: audioFormat)
engine.connect(myAVAudioNode1, to: engine.mainMixerNode, format: audioFormat)

// Function to Write Audio to a File
func writeAudioToFile() {
        // File to write
        let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
        let audioURL = documentsDirectory.appendingPathComponent("share.m4a")

        // Format parameters
        let sampleRate = Int(audioFormat!.sampleRate)
        let channels = Int(audioFormat!.channelCount)

        // Audio File settings
        let settings = [
            AVFormatIDKey: Int(kAudioFormatMPEG4AAC),
            AVSampleRateKey: Int(audioFormat!.sampleRate),
            AVNumberOfChannelsKey: Int(audioFormat!.channelCount),
            AVEncoderAudioQualityKey: AVAudioQuality.max.rawValue
        ]

        // Audio File
        var audioFile = AVAudioFile()
        do {
            audioFile = try AVAudioFile(forWriting: audioURL, settings: settings, commonFormat: .pcmFormatFloat32, interleaved: false)
        }
        catch {
            print ("Failed to open Audio File For Writing: \(error.localizedDescription)")
        }

        // Install Tap on mainMixer
        // Write into buffer and then write buffer into AAC file
        engine.mainMixerNode.installTap(onBus: 0, bufferSize: 8192, format: nil, block: { (pcmBuffer, when) in
            do {
                try audioFile.write(from: pcmBuffer)
            }
            catch {
                print("Failed to write Audio File: \(error.localizedDescription)")
            }
        })
}

Answer 1

我打印了AVAudioPCMBuffer格式，发现它默认的采样率为48000Hz。但情况并非总是如此。首先，确定音频会话采样率并将该采样率用于音频格式：

let audioSession = AVAudioSession.sharedInstance()
let sampleRate = audioSession.sampleRate
let audioFormat = AVAudioFormat(standardFormatWithSampleRate: sampleRate, channels: 2)

从 AVAudioEngine 总线抽头生成的文件中的音频与设备输出上播放的速度不同

Audio from a file generated from an AVAudioEngine bus tap has different speed than the one played on device output

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avaudioengine

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