改善音訊播放品質

當您嘗試透過 USB API 直接存取 USB 音訊週邊裝置時，可能會發生問題。這些問題可能包括安全性問題、限制其他應用程式的媒體播放，以及透過 USB 裝置播放鬧鐘、通知和鈴聲的遺失情況。

為改善音訊播放品質，請改為設定混音器屬性。

設定混音器屬性

使用 AudioMixerAttributes API 時，您可以透過 USB 設定應用程式使用偏好的混合器屬性。

當應用程式播放與偏好混音器屬性的編碼格式、頻道遮罩和取樣率相符時，系統會將該播放內容附加至音訊輸出串流，其已設定偏好混合器屬性的混音器屬性。

只要 USB 裝置支援設定，應用程式就能在任何設定下串流至硬體抽象層 (HAL) 和裝置。

AudioMixerAttributes 中允許的兩個混合行為是 DEFAULT 和 BIT_PERFECT。如果混音器行為為 DEFAULT，表示混用不同來源的音訊資料。

當混音器行為為 BIT_PERFECT 時，系統不會為播放套用任何音訊混音、音量調整或音訊處理效果。資料會原封不動傳送至 HAL，然後再向下傳送到 USB 裝置。

只要使用 BIT_PERFECT，即可在 Android 裝置上直接串流數位 (DSD) 訊息，藉此減少脈衝程式碼調節 (PCM)。下列程式碼範例說明如何完成這項操作：

val EXPECTED_FORMAT: AudioFormat = AudioFormat.Builder()
  .setEncoding(AudioFormat.ENCODING_PCM_24BIT_PACKED)
  .setChannelMask(AudioFormat.CHANNEL_OUT_STEREO)
  .setSampleRate(44100)
  .build()

fun startPlayback() {
  // Query all supported mixer attributes
  val mixerAttributesList: List<AudioMixerAttributes?> =
    mAudioManager.getSupportedMixerAttributes(usbDevice)

  // Find the wanted mixer attributes
  val mixerAttributes = mixerAttributesList.stream()
    .filter { mixerAttr: AudioMixerAttributes? ->
      EXPECTED_FORMAT.equals(
        mixerAttr!!.format
      )
    }
    .findAny()
    .orElse(null)

  // Register a listener to mixer attributes changed
  val listener = MyPreferredMixerAttributesChangedListener()
  mAudioManager.addOnPreferredMixerAttributesChangedListener(
    Executors.newSingleThreadExecutor(), listener
  )

  // Currently, only media usage over USB devices will be allowed
  val attr: AudioAttributes = AudioAttributes.Builder()
    .setUsage(AudioAttributes.USAGE_MEDIA).build()
  // Set preferred mixer attributes
  mAudioManager.setPreferredMixerAttributes(
    attr, usbDevice, mixerAttributes
  )

  // Start playback, note the playback and the audio format must
  // match what is set when calling `setPreferredMixerAttriutes`
  // API.
  val audioTrack = AudioTrack.Builder()
    .setAudioAttributes(attr)
    .setAudioFormat(mixerAttributes!!.format)
    .build()

  // Clear all preferred mixer attributes related stuff when
  // playback task is completed
  mAudioManager.clearPreferredMixerAttributes(attr, usbDevice)
  mAudioManager.removeOnPreferredMixerAttributesChangedListener(listener)
}

private class MyPreferredMixerAttributesChangedListener :
  AudioManager.OnPreferredMixerAttributesChangedListener {
  override fun onPreferredMixerAttributesChanged(
    attributes: AudioAttributes,
    device: AudioDeviceInfo,
    mixerAttributes: AudioMixerAttributes?,
  ) {
    // Do something when preferred mixer attributes changed
  }
}

final AudioFormat EXPECTED_FORMAT = new AudioFormat.Builder()
        .setEncoding(AudioFormat.ENCODING_PCM_24BIT_PACKED)
        .setChannelMask(AudioFormat.CHANNEL_OUT_STEREO)
        .setSampleRate(44100)
        .build();

void startPlayback() {
    // Query all supported mixer attributes
    List<AudioMixerAttributes> mixerAttributesList =
    mAudioManager.getSupportedMixerAttributes(usbDevice);

    // Find the wanted mixer attributes
    AudioMixerAttributes mixerAttributes =
        mixerAttributesList.stream()
            .filter(mixerAttr -> EXPECTED_FORMAT.equals(mixerAttr.getFormat()))
            .findAny()
            .orElse(null);

    // Register a listener to mixer attributes changed
    MyPreferredMixerAttributesChangedListener listener =
        new MyPreferredMixerAttributesChangedListener();
    mAudioManager.addOnPreferredMixerAttributesChangedListener(
            Executors.newSingleThreadExecutor(), listener);

    // Currently, only media usage over USB devices will be allowed
    AudioAttributes attr = new AudioAttributes.Builder()
           .setUsage(AudioAttributes.USAGE_MEDIA).build();
    // Set preferred mixer attributes
    mAudioManager.setPreferredMixerAttributes(
            attr, usbDevice, mixerAttributes);

    // Start playback, note the playback and the audio format must
    // match what is set when calling `setPreferredMixerAttriutes`
    // API.
    AudioTrack audioTrack = new AudioTrack.Builder()
            .setAudioAttributes(attr)
            .setAudioFormat(mixerAttributes.getFormat())
            .build();

    // Clear all preferred mixer attributes related stuff when
    // playback task is completed
    mAudioManager.clearPreferredMixerAttributes(attr, usbDevice);
    mAudioManager.removeOnPreferredMixerAttributesChangedListener(
            listener);
}

private class MyPreferredMixerAttributesChangedListener
        implements AudioManager.OnPreferredMixerAttributesChangedListener {
    @Override
    public void onPreferredMixerAttributesChanged(
        AudioAttributes attributes,
        AudioDeviceInfo device,
        AudioMixerAttributes mixerAttributes) {
        // Do something when preferred mixer attributes changed
    }
}