iOS上的低延迟音频输出问题（也就是如何击败AUAudioUnit sampleRate，maximumFramesToRender和ioBufferDuration提交）

Question

好吧，我显然在这里缺少了一些重要的内容。我正在尝试通过网络进行低延迟音频，而我的基本帧为10ms。我希望这没问题。我的目标手机是iPhone X扬声器，因此我的硬件采样率应锁定为48000Hz。我要求10毫秒，这是一个很好的偶数除数，应为480、960、1920或3840，具体取决于您要对帧/样本/字节进行切片的方式。

然而，对于我的一生，我绝对无法让iOS做我认为理智的事情。我得到10.667ms的缓冲区持续时间，这很荒谬-iOS淘汰了这种方法，使我的缓冲区大小不是sampleRate的整数倍。更糟糕的是，该帧明显是LONG，这意味着我必须吸收一个而不是two个延迟数据包，才能填充该缓冲区。我完全无法更改maximumFrameToRender，并且系统返回的是[[0作为我的采样率，即使它很明显是在48000Hz处呈现。

我显然缺少重要的东西-这是什么？我是否忘了断开/连接某些东西以获得直接的硬件映射？ （我的格式是1，即pcmFormatFloat32 －我希望pcmFormatInt16或pcmFormatInt32直接映射到硬件，所以OS中的某些内容可能会妨碍您使用）指针很受赞赏，我很乐意阅读更多内容。还是AUAudioUnit只是半生半熟，我需要返回到更旧，更有用的API？还是我完全错过了情节，低延迟音频人员使用了一套完全不同的音频管理功能？

感谢您的帮助-非常感谢。

代码输出：

2019-11-07 23:28:29.782786-0800 latencytest[3770:50382] Ready to receive user events
2019-11-07 23:28:34.727478-0800 latencytest[3770:50382] Start button pressed
2019-11-07 23:28:34.727745-0800 latencytest[3770:50382] Launching auxiliary thread
2019-11-07 23:28:34.729278-0800 latencytest[3770:50445] Thread main started
2019-11-07 23:28:35.006005-0800 latencytest[3770:50445] Sample rate: 0
2019-11-07 23:28:35.016935-0800 latencytest[3770:50445] Buffer duration: 0.010667
2019-11-07 23:28:35.016970-0800 latencytest[3770:50445] Number of output busses: 2
2019-11-07 23:28:35.016989-0800 latencytest[3770:50445] Max frames: 4096
2019-11-07 23:28:35.017010-0800 latencytest[3770:50445] Can perform output: 1
2019-11-07 23:28:35.017023-0800 latencytest[3770:50445] Output Enabled: 1
2019-11-07 23:28:35.017743-0800 latencytest[3770:50445] Bus channels: 2
2019-11-07 23:28:35.017864-0800 latencytest[3770:50445] Bus format: 1
2019-11-07 23:28:35.017962-0800 latencytest[3770:50445] Bus rate: 0
2019-11-07 23:28:35.018039-0800 latencytest[3770:50445] Sleeping 0
2019-11-07 23:28:35.018056-0800 latencytest[3770:50445] Buffer count: 2 4096
2019-11-07 23:28:36.023220-0800 latencytest[3770:50445] Sleeping 1
2019-11-07 23:28:36.023400-0800 latencytest[3770:50445] Buffer count: 190 389120
2019-11-07 23:28:37.028610-0800 latencytest[3770:50445] Sleeping 2
2019-11-07 23:28:37.028790-0800 latencytest[3770:50445] Buffer count: 378 774144
2019-11-07 23:28:38.033983-0800 latencytest[3770:50445] Sleeping 3
2019-11-07 23:28:38.034142-0800 latencytest[3770:50445] Buffer count: 566 1159168
2019-11-07 23:28:39.039333-0800 latencytest[3770:50445] Sleeping 4
2019-11-07 23:28:39.039534-0800 latencytest[3770:50445] Buffer count: 756 1548288
2019-11-07 23:28:40.041787-0800 latencytest[3770:50445] Sleeping 5
2019-11-07 23:28:40.041943-0800 latencytest[3770:50445] Buffer count: 944 1933312
2019-11-07 23:28:41.042878-0800 latencytest[3770:50445] Sleeping 6
2019-11-07 23:28:41.043037-0800 latencytest[3770:50445] Buffer count: 1132 2318336
2019-11-07 23:28:42.048219-0800 latencytest[3770:50445] Sleeping 7
2019-11-07 23:28:42.048375-0800 latencytest[3770:50445] Buffer count: 1320 2703360
2019-11-07 23:28:43.053613-0800 latencytest[3770:50445] Sleeping 8
2019-11-07 23:28:43.053771-0800 latencytest[3770:50445] Buffer count: 1508 3088384
2019-11-07 23:28:44.058961-0800 latencytest[3770:50445] Sleeping 9
2019-11-07 23:28:44.059119-0800 latencytest[3770:50445] Buffer count: 1696 3473408

实际代码：
import UIKit

import os.log

import Foundation
import AudioToolbox
import AVFoundation

class AuxiliaryWork: Thread {
    let II_SAMPLE_RATE = 48000

    var iiStopRequested: Int32 = 0;  // Int32 is normally guaranteed to be atomic on most architectures

    var iiBufferFillCount: Int32 = 0;
    var iiBufferByteCount: Int32 = 0;

    func requestStop() {
        iiStopRequested = 1;
    }

    func myAVAudioSessionInterruptionNotificationHandler(notification: Notification ) -> Void {
        os_log(OSLogType.info, "AVAudioSession Interrupted: %s", notification.debugDescription)
    }

    func myAudioUnitProvider(actionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>, timestamp: UnsafePointer<AudioTimeStamp>,
                             frameCount: AUAudioFrameCount, inputBusNumber: Int, inputData: UnsafeMutablePointer<AudioBufferList>) -> AUAudioUnitStatus {
        let ppInputData = UnsafeMutableAudioBufferListPointer(inputData)
        let iiNumBuffers = ppInputData.count

        if (iiNumBuffers > 0) {
            assert(iiNumBuffers == 2)

            for bbBuffer in ppInputData {
                assert(Int(bbBuffer.mDataByteSize) == 2048)  // FIXME: This should be 960 or 1920 ...

                iiBufferFillCount += 1
                iiBufferByteCount += Int32(bbBuffer.mDataByteSize)

                memset(bbBuffer.mData, 0, Int(bbBuffer.mDataByteSize))  // Just send silence

            }
        } else {
            os_log(OSLogType.error, "Zero buffers from system")
            assert(iiNumBuffers != 0)  // Force crash since os_log would cause an audio hiccup due to locks anyway
        }

        return noErr
    }

    override func main() {
        os_log(OSLogType.info, "Thread main started")

#if os(iOS)
        let kOutputUnitSubType = kAudioUnitSubType_RemoteIO
#else
        let kOutputUnitSubType = kAudioUnitSubtype_HALOutput
#endif

        let audioSession = AVAudioSession.sharedInstance()  // FIXME: Causes the following message No Factory registered for id
        try! audioSession.setCategory(AVAudioSession.Category.playback, options: [])
        try! audioSession.setMode(AVAudioSession.Mode.measurement)

        try! audioSession.setPreferredSampleRate(48000.0)
        try! audioSession.setPreferredIOBufferDuration(0.010)

        NotificationCenter.default.addObserver(
            forName: AVAudioSession.interruptionNotification,
            object: nil,
            queue: nil,
            using: myAVAudioSessionInterruptionNotificationHandler
        )

        let ioUnitDesc = AudioComponentDescription(
            componentType: kAudioUnitType_Output,
            componentSubType: kOutputUnitSubType,
            componentManufacturer: kAudioUnitManufacturer_Apple,
            componentFlags: 0,
            componentFlagsMask: 0)

        let auUnit = try! AUAudioUnit(componentDescription: ioUnitDesc,
                                      options: AudioComponentInstantiationOptions())

        auUnit.outputProvider = myAudioUnitProvider;
        auUnit.maximumFramesToRender = 256


        try! audioSession.setActive(true)

        try! auUnit.allocateRenderResources()  // Make sure audio unit has hardware resources--we could provide the buffers from the circular buffer if we want
        try! auUnit.startHardware()


        os_log(OSLogType.info, "Sample rate: %d", audioSession.sampleRate);
        os_log(OSLogType.info, "Buffer duration: %f", audioSession.ioBufferDuration);

        os_log(OSLogType.info, "Number of output busses: %d", auUnit.outputBusses.count);
        os_log(OSLogType.info, "Max frames: %d", auUnit.maximumFramesToRender);


        os_log(OSLogType.info, "Can perform output: %d", auUnit.canPerformOutput)
        os_log(OSLogType.info, "Output Enabled: %d", auUnit.isOutputEnabled)
        //os_log(OSLogType.info, "Audio Format: %p", audioFormat)

        var bus0 = auUnit.outputBusses[0]
        os_log(OSLogType.info, "Bus channels: %d", bus0.format.channelCount)
        os_log(OSLogType.info, "Bus format: %d", bus0.format.commonFormat.rawValue)
        os_log(OSLogType.info, "Bus rate: %d", bus0.format.sampleRate)

        for ii in 0..<10 {
            if (iiStopRequested != 0) {
                os_log(OSLogType.info, "Manual stop requested");
                break;
            }

            os_log(OSLogType.info, "Sleeping %d", ii);
            os_log(OSLogType.info, "Buffer count: %d %d", iiBufferFillCount, iiBufferByteCount)
            Thread.sleep(forTimeInterval: 1.0);
        }

        auUnit.stopHardware()
    }
}

class FirstViewController: UIViewController {
    var thrAuxiliaryWork: AuxiliaryWork? = nil;

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    @IBAction func startButtonPressed(_ sender: Any) {
        os_log(OSLogType.error, "Start button pressed");
        os_log(OSLogType.error, "Launching auxiliary thread");

        thrAuxiliaryWork = AuxiliaryWork();
        thrAuxiliaryWork?.start();
    }

    @IBAction func stopButtonPressed(_ sender: Any) {
        os_log(OSLogType.error, "Stop button pressed");
        os_log(OSLogType.error, "Manually stopping auxiliary thread");
        thrAuxiliaryWork?.requestStop();
    }

    @IBAction func muteButtonPressed(_ sender: Any) {
        os_log(OSLogType.error, "Mute button pressed");
    }

    @IBAction func unmuteButtonPressed(_ sender: Any) {
        os_log(OSLogType.error, "Unmute button pressed");
    }
}

Answer 1

假设API可以帮您完成，您就无法击败iOS芯片硬件。如果要抽象硬件，则必须自己做缓冲。

为了获得最佳（最低）延迟，您的软件将（可能会动态地）适应实际的硬件功能，实际的硬件功能可能因设备而异，并且因模式而异。

硬件采样率似乎是44.1ksps（较旧的iOS设备），48ksps（较新的arm64 iOS设备）或其整数倍（以及插入非AirPod蓝牙耳机时的其他采样率）。实际的硬件DMA（或等效的）缓冲区似乎总是2的幂，在最新的设备上可能减少到64个样本。但是，各种iOS省电模式都会将缓冲区大小（以2的幂为单位）增加到4k个样本，尤其是在较旧的iOS设备上。如果您请求的采样率不是硬件速率，则操作系统可能会将缓冲区重新采样为与2的幂不同的大小，并且如果重新采样率不是精确的整数，则此大小可以从Audio Unit回调更改为后续回调。

音频单元是可通过iOS设备上的公共API访问的最低级别。其他所有内容都建立在顶部，因此可能会变慢。例如，如果您将Audio Queue API与非硬件缓冲区大小配合使用，则操作系统将使用2次幂的音频缓冲区，并将其切碎或连接起来以返回或获取请求的Audio Queue缓冲区数据量。较慢和抖动。