Dealing with Audio Output Hardware

本文介绍如何在Android设备上管理音频输出硬件,包括检查当前使用的输出设备(如扬声器、有线耳机或蓝牙设备),以及如何处理音频输出硬件变化的情况,例如耳机拔出或蓝牙设备断开时自动调整音频输出。

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分任务链接地址:http://docs.eoeandroid.com/training/managing-audio/audio-output.html

译者:长剑耿介

完成时间:2012年9月5日


处理音频输出硬件

当用户谈到享受他们的Android设备中的音频时,有很多处理方式。大多数设备有一个内置扬声器,有线耳机的耳机插孔,不少还配备了蓝牙连接,支持A2DP的音频。


检查正在使用什么硬件

硬件的输出如何被发送将会影响到您的应用程序的效果。

您可以通过以下代码片段来查询AudioManager来确定的音频路由是否被发送到扬声器,有线耳机或连接的蓝牙设备:

if (isBluetoothA2dpOn()) {
    // Adjust output for Bluetooth.
} else if (isSpeakerphoneOn()) {
    // Adjust output for Speakerphone.
} else if (isWiredHeadsetOn()) {
    // Adjust output for headsets
} else { 
    // If audio plays and noone can hear it, is it still playing?
}

变动音频输出硬件时的处理

当耳机被拔掉,或蓝牙设备断开连接,音频流将会自动重新路由到内置的扬声器。如果你正用很高的音量听歌,那将会是一个嘈杂的“惊喜”。

幸运的是,当这一切发生的时候,系统会广播ACTION_AUDIO_BECOMING_NOISY意图。当你正播放音频时,注册BroadcastReceiver,并监听这个意图是很好的做法。在音乐播放器案例中,用户通常期望要暂停播放,而在游戏中,你可以选择大大地降低音量。

private class NoisyAudioStreamReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        if (AudioManager.ACTION_AUDIO_BECOMING_NOISY.equals(intent.getAction())) {
            // Pause the playback
        }
    }
}
 
private IntentFilter intentFilter = new IntentFilter(AudioManager.ACTION_AUDIO_BECOMING_NOISY);
 
private void startPlayback() {
    registerReceiver(myNoisyAudioStreamReceiver(), intentFilter);
}
 
private void stopPlayback() {
    unregisterReceiver(myNoisyAudioStreamReceiver);
}


### AAC Profile Information and Configuration In the context of audio encoding, particularly with Advanced Audio Coding (AAC), profiles define specific configurations that determine how an audio stream is encoded to ensure compatibility across different devices and platforms. When dealing with AAC streams, it is crucial to understand these profiles as they dictate various parameters such as bit rate, sampling frequency, and channel configuration. The absence of ADTS headers in each frame of an AAC file can lead to playback issues on both PC and mobile devices[^1]. This indicates that proper encapsulation of the AAC data into a container format like MP4 or inclusion of ADTS headers is necessary for successful decoding by media players. Regarding the `audio_policy` macro definition found within the Android framework's header files at `/hardware/libhardware/include/hardware/audio_policy.h`, its primary purpose relates more directly towards managing hardware-level policies rather than detailing specifics about AAC profiles themselves: ```c #define AUDIO_POLICY_HARDWARE_MODULE_ID "audio_policy" ``` This identifier refers specifically to modules responsible for handling sound policy management but does not delve deeply enough into actual codec settings required during real-time scenarios involving noise reduction techniques mentioned elsewhere regarding similar problems encountered while using libav tools[^2]. For addressing potential quality concerns including unwanted noises when employing certain encoders under particular conditions—such discussions have been raised previously among users working closely alongside FFmpeg libraries where adjustments might be needed based upon input source characteristics along side chosen output formats.[^3] Here’s an example demonstrating basic usage incorporating relevant aspects discussed above through Python scripting utilizing pydub library which internally leverages ffmpeg/ffprobe binaries installed separately beforehand ensuring appropriate flags set according preferences specified earlier : ```python from pydub import AudioSegment import os def convert_to_aac(input_file_path,output_dir=".",output_filename=None,audio_profile='aac_he_v2',bitrate='128k'): if not output_filename : base=os.path.basename(input_file_path).split('.')[0]+'.m4a' else : base=output_filename+'.m4a' full_output_path=os.path.join(output_dir,base) sound=AudioSegment.from_file(input_file_path) export_params={ 'format':'ipod', 'codec': ['libfdk_aac'], 'parameters':['-vbr','4','-profile:a',audio_profile,'-b:a',bitrate] } sound.export(full_output_path,**export_params) convert_to_aac('example.wav') ```
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