当您尝试使用 USB API 直接访问 USB 音频外设时,会出现问题。这些问题可能包括:安全问题、限制其他应用的媒体播放,以及无法通过 USB 设备丢失闹钟、通知和铃声。
如需改善音频播放效果,请改为配置混音器属性。
配置混音器属性
通过使用 AudioMixerAttributes
API,您可以通过 USB 为应用配置首选混音器属性。
如果您的应用播放符合首选混音器属性的编码格式、声道掩码和采样率,系统会将播放附加到音频输出流,该流的混音器配置了首选混音器属性。
只要 USB 设备支持相应的配置,您的应用就可以在任何配置下流式传输到硬件抽象层 (HAL) 和设备。
AudioMixerAttributes
中允许的两种混音器行为是 DEFAULT
和 BIT_PERFECT
。当混音器行为为 DEFAULT
时,表示混合了来自不同来源的音频数据。
当混音器行为为 BIT_PERFECT
时,不会向播放应用任何混音、音量调整或音频处理效果。数据按原样发送到 HAL,并最终发送到 USB 设备。
借助 BIT_PERFECT
,您可以在 Android 设备上通过脉冲编码调制 (PCM) 定向流式传输数字 (DSD)。以下代码示例展示了如何实现这一点:
Kotlin
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 } }
Java
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 } }