与我们熟知的MP3格式一样,AAC是一种音频编码格式,对比MP3格式,AAC在缩小30%的前题下可以提供更好的音质。这篇博客的主要内容就是通过AudioRecorder录制PCM音频,再通过MediaCodec将PCM数据硬编码为AAC格式的音频。
通常我们使用MediaCodec的流程如下:

编解码器一个比较经典的工作原理图如下:
使用MediaCodec,进行音频AAC硬编-LMLPHP
图中的Client一般就是我们开发者,解释一下就是:我们从Codec中拿到拿到空的input buffer,然后填充上我们需要进行编码的数据,再输送给Codec,Codec对数据进行编解码,编解码完成后,Codec将处理好的数据放进output buffer,我们取出后再清空返还给Codec,形成一个环形结构。可以看作一个生产者-消费者模式。

下面我们的编码流程也基本遵守上面的这个流程。为了便于大家理解,我画个流程图讲一下我整个代码的逻辑:
使用MediaCodec,进行音频AAC硬编-LMLPHP

录音和编码分别在两个线程中进行,两个线程通过一个ArrayBlockQueue(这是一个线程安全的队列,想了解更多自己动手)队列共享数据,录音线程中的AudioRecorder通过read()将一帧数据put()到队尾,编码线程中的MediaCodec再通过take()取出队首的一帧数据进行编码。
首先看录音线程,为了使代码更加简洁易懂,我会省略掉一些代码,完整代码会在文末贴出。

    /**
     * 录音线程
     */
    public class AudioRecorder extends Thread {

        private AudioRecord mAudioRecord;
        private boolean isRecording;
        private int minBufferSize;

        public AudioRecorder() {
            isRecording = true;
            initRecorder();
        }

        @Override
        public void run() {
            super.run();
            startRecording();
        }

        /**
         * 初始化录音
         */
        public void initRecorder(){
            minBufferSize = AudioRecord.getMinBufferSize(sampleRateInHz, channelConfig, audioFormat);
            mAudioRecord = new AudioRecord(MediaRecorder.AudioSource.DEFAULT, sampleRateInHz, channelConfig, audioFormat, minBufferSize);
            if (mAudioRecord.getState() != AudioRecord.STATE_INITIALIZED) {
                isRecording = false;
                return;
            }
        }

        /**
         * 开始录音
         */
        public void startRecording(){
            if (mAudioRecord == null){
                return;
            }

            mAudioRecord.startRecording();
            while (isRecording) {
                //自定义的一个类,用来存储一帧pcm数据,即byte[],下面给出具体定义,很简单
                AudioDate audioDate = new AudioDate();
                audioDate.buffer = ByteBuffer.allocateDirect(minBufferSize);
                audioDate.size = mAudioRecord.read(audioDate.buffer, minBufferSize);
                try {
                    if (queue != null) {
                        queue.put(audioDate);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            release();
        }
    }

录音线程比较简单,主要是先初始化录音器在initRecorder()中,然后通过AudioRecorder的read方法,获取到一帧数据,通过queue.put放入队尾。
然后是编码线程。

/**
 * 音频编码线程
  */
public class AudioEncorder extends Thread {

    private MediaCodec mEncorder;
 private Boolean isEncording = false;
 private int minBufferSize;

 private OutputStream mFileStream;

 public AudioEncorder() {
        isEncording = true;
  initEncorder();
  }

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    @Override
  public void run() {
        super.run();
  startEncording();
  }

    /**
 * 初始化编码器
  */
  private void initEncorder(){
        minBufferSize = AudioRecord.getMinBufferSize(sampleRateInHz, channelConfig, audioFormat);
 try {
            mEncorder = MediaCodec.createEncoderByType(MediaFormat.MIMETYPE_AUDIO_AAC);
  } catch (IOException e) {
            e.printStackTrace();
  }
        MediaFormat format = MediaFormat.createAudioFormat(MediaFormat.MIMETYPE_AUDIO_AAC, sampleRateInHz, channelConfig);
  format.setString(MediaFormat.KEY_MIME, MediaFormat.MIMETYPE_AUDIO_AAC);
  format.setInteger(MediaFormat.KEY_AAC_PROFILE, MediaCodecInfo.CodecProfileLevel.AACObjectLC);
  format.setInteger(MediaFormat.KEY_BIT_RATE, 96000);
  format.setInteger(MediaFormat.KEY_MAX_INPUT_SIZE, minBufferSize * 2);
  mEncorder.configure(format, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);
  }

    /**
 * 开始编码
  */
  @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    public void startEncording(){
        if (mEncorder == null){
            return;
  }

        mEncorder.start();
 try {
            mFileStream = new FileOutputStream(getSDPath() + "/aac_encode.aac");
  MediaCodec.BufferInfo mBufferInfo = new MediaCodec.BufferInfo();
  AudioDate audioDate;
 while (isEncording) {
                // 从队列中取出录音的一帧音频数据
  audioDate = getAudioDate();

 if (audioDate == null) {
                    continue;
  }

                // 取出InputBuffer,填充音频数据,然后输送到编码器进行编码
  int inputBufferIndex = mEncorder.dequeueInputBuffer(0);
 if (inputBufferIndex >= 0) {
                    ByteBuffer inputBuffer = mEncorder.getInputBuffer(inputBufferIndex);
  inputBuffer.clear();
  inputBuffer.put(audioDate.buffer);
  mEncorder.queueInputBuffer(inputBufferIndex, 0, audioDate.size, System.nanoTime(), 0);
  }

                // 取出编码好的一帧音频数据,然后给这一帧添加ADTS头
  int outputBufferIndex = mEncorder.dequeueOutputBuffer(mBufferInfo, 0);
 while (outputBufferIndex >= 0) {
                    int outBitsSize = mBufferInfo.size;
 int outPacketSize = outBitsSize + 7; // ADTS头部是7个字节
  ByteBuffer outputBuffer = mEncorder.getOutputBuffer(outputBufferIndex);
  outputBuffer.position(mBufferInfo.offset);
  outputBuffer.limit(mBufferInfo.offset + outBitsSize);

 byte[] outData = new byte[outPacketSize];
  addADTStoPacket(outData, outPacketSize);

  outputBuffer.get(outData, 7, outBitsSize);
  outputBuffer.position(mBufferInfo.offset);
  mFileStream.write(outData);
  mEncorder.releaseOutputBuffer(outputBufferIndex, false);
  outputBufferIndex = mEncorder.dequeueOutputBuffer(mBufferInfo, 0);
  }
            }
            release();
  } catch (FileNotFoundException e) {
            e.printStackTrace();
  } catch (IOException e) {
            e.printStackTrace();
  }
    }

    /**
 * 添加ADTS头
  * @param packet
  * @param packetLen
  */
  private void addADTStoPacket(byte[] packet, int packetLen) {
        int profile = 2; //AAC LC
  int freqIdx = 4; //44100 根据不同的采样率修改这个值
  int chanCfg = 2; //CPE
  packet[0] = (byte) 0xFF;
  packet[1] = (byte) 0xF9;
  packet[2] = (byte) (((profile - 1) << 6) + (freqIdx << 2) + (chanCfg >> 2));
  packet[3] = (byte) (((chanCfg & 3) << 6) + (packetLen >> 11));
  packet[4] = (byte) ((packetLen & 0x7FF) >> 3);
  packet[5] = (byte) (((packetLen & 7) << 5) + 0x1F);
  packet[6] = (byte) 0xFC;
  }
}

和录音线程一样,先进行编码器的初始化在initEncorder()中,这里的参数MediaFormat比较关键,针对具体的编码格式有不同的编码格式有不同的参数,其他的编码方式自行参阅官方文档,这里的AAC编码器参数我是经过测试可行的。
然后是最关键的一步–编码。在startEncording()方法中,所有的流程完全对应上面我列出的MediaCodec的使用流程,可以对照上面的流程来阅读代码。
编码流程中非常重要的一步就是为每一帧AAC音频添加ADTS头,和WAV格式的音频不同,AAC为每一帧音频都添加了一个ADTS头,使得解码器可以从任意一帧开始解码,有时我们遇到无法编码的AAC文件无法播放,可能就是因为我们没有为其添加ADTS头。通过上面的addADTStoPacket()方法便可以未每一帧AAC音频添加ADTS头了。
至此,我就列出了我认为在AAC编码的过程中所需要注意的所有点。在完成这个功能的时候,我也参照了一些别人的做法,以及我自己的一些理解与实践,也遇到了一些令人头大的错误(例如:同样的PCM数据,放进队列中再取出来写入文件,就出现了很大的杂音)如有错误的地方,还请留言指正。
下面是完整代码:

package com.example.sisyphus.audiovideolearning;

import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.media.AudioFormat;
import android.media.AudioRecord;
import android.media.MediaCodec;
import android.media.MediaCodecInfo;
import android.media.MediaFormat;
import android.media.MediaRecorder;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.support.annotation.Nullable;
import android.support.annotation.RequiresApi;
import android.support.v4.app.ActivityCompat;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Button;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.util.concurrent.ArrayBlockingQueue;

/**
 * Created by sisyphus on 2018/8/8. * 学习 MediaCodec API,完成音频 AAC 硬编、硬解
  *
  * MediaCodec的使用流程:
  * - createEncoderByType/createDecoderByType
 * - configure * - start * - while(1) { * - dequeueInputBuffer * - queueInputBuffer * - dequeueOutputBuffer * - releaseOutputBuffer * } * - stop * - release */public class AACCodecActivity extends Activity {

    private final int sampleRateInHz = 44100;
 private final int channelConfig = 1;
 private final int audioFormat = AudioFormat.ENCODING_PCM_16BIT;

 private class AudioDate {
        private ByteBuffer buffer;
 private int size;
  }

    public static String[] MICROPHONE = {Manifest.permission.RECORD_AUDIO};
 public static String[] STORAGE = {Manifest.permission.READ_EXTERNAL_STORAGE, Manifest.permission.WRITE_EXTERNAL_STORAGE};

 private AudioRecorder mAudioRecorder;
 private AudioEncorder mAudioEncorder;

 private ArrayBlockingQueue queue;

 private Button btnStartRecording;

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_aac_codec);
  btnStartRecording = findViewById(R.id.btn_start_aac_encode);
  queue = new ArrayBlockingQueue<>(1024);
  mAudioRecorder = new AudioRecorder();
  mAudioEncorder = new AudioEncorder();
  btnStartRecording.setOnTouchListener(new View.OnTouchListener() {
            @Override
  public boolean onTouch(View view, MotionEvent motionEvent) {
                int action = motionEvent.getAction();
 if (action == MotionEvent.ACTION_DOWN) {
                    checkRecordPermission();
  btnStartRecording.setText("正在录制");
  startRecord();
  } else if (action == MotionEvent.ACTION_UP) {
                    btnStartRecording.setText("录音");
  stopRecord();
  }
                return false;
  }
        });
  }

    private void startRecord() {
        mAudioRecorder.start();
  mAudioEncorder.start();
  }

    private void stopRecord() {
        mAudioRecorder.stopRecording();
  mAudioEncorder.stopEncording();
  }

    /**
 * 录音线程
  */
  public class AudioRecorder extends Thread {

        private AudioRecord mAudioRecord;
 private boolean isRecording;
 private int minBufferSize;

 public AudioRecorder() {
            isRecording = true;
  initRecorder();
  }

        @Override
  public void run() {
            super.run();
  startRecording();
  }

        /**
 * 初始化录音
  */
  public void initRecorder(){
            minBufferSize = AudioRecord.getMinBufferSize(sampleRateInHz, channelConfig, audioFormat);
  mAudioRecord = new AudioRecord(MediaRecorder.AudioSource.DEFAULT, sampleRateInHz, channelConfig, audioFormat, minBufferSize);
 if (mAudioRecord.getState() != AudioRecord.STATE_INITIALIZED) {
                isRecording = false;
 return;  }
        }

        /**
 * 释放资源
  */
  public void release() {
            if (mAudioRecord != null && mAudioRecord.getState() == AudioRecord.STATE_INITIALIZED) {
                mAudioRecord.stop();
  }
        }

        /**
 * 开始录音
  */
  public void startRecording(){
            if (mAudioRecord == null){
                return;
  }

            mAudioRecord.startRecording();
 while (isRecording) {
                AudioDate audioDate = new AudioDate();
  audioDate.buffer = ByteBuffer.allocateDirect(minBufferSize);
  audioDate.size = mAudioRecord.read(audioDate.buffer, minBufferSize);
 try {
                    if (queue != null) {
                        queue.put(audioDate);
  }
                } catch (InterruptedException e) {
                    e.printStackTrace();
  }
            }
            release();
  }

        /**
 * 结束录音
  */
  public void stopRecording() {
            isRecording = false;
  }
    }

    /**
 * 音频编码线程
  */
  public class AudioEncorder extends Thread {

        private MediaCodec mEncorder;
 private Boolean isEncording = false;
 private int minBufferSize;

 private OutputStream mFileStream;

 public AudioEncorder() {
            isEncording = true;
  initEncorder();
  }

        @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
        @Override
  public void run() {
            super.run();
  startEncording();
  }

        /**
 * 初始化编码器
  */
  private void initEncorder(){
            minBufferSize = AudioRecord.getMinBufferSize(sampleRateInHz, channelConfig, audioFormat);
 try {
                mEncorder = MediaCodec.createEncoderByType(MediaFormat.MIMETYPE_AUDIO_AAC);
  } catch (IOException e) {
                e.printStackTrace();
  }
            MediaFormat format = MediaFormat.createAudioFormat(MediaFormat.MIMETYPE_AUDIO_AAC, sampleRateInHz, channelConfig);
  format.setString(MediaFormat.KEY_MIME, MediaFormat.MIMETYPE_AUDIO_AAC);
  format.setInteger(MediaFormat.KEY_AAC_PROFILE, MediaCodecInfo.CodecProfileLevel.AACObjectLC);
  format.setInteger(MediaFormat.KEY_BIT_RATE, 96000);
  format.setInteger(MediaFormat.KEY_MAX_INPUT_SIZE, minBufferSize * 2);
  mEncorder.configure(format, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);
  }

        /**
 * 开始编码
  */
  @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
        public void startEncording(){
            if (mEncorder == null){
                return;
  }

            mEncorder.start();
 try {
                mFileStream = new FileOutputStream(getSDPath() + "/aac_encode.aac");
  MediaCodec.BufferInfo mBufferInfo = new MediaCodec.BufferInfo();
  AudioDate audioDate;
 while (isEncording) {
                    // 从队列中取出录音的一帧音频数据
  audioDate = getAudioDate();

 if (audioDate == null) {
                        continue;
  }

                    // 取出InputBuffer,填充音频数据,然后输送到编码器进行编码
  int inputBufferIndex = mEncorder.dequeueInputBuffer(0);
 if (inputBufferIndex >= 0) {
                        ByteBuffer inputBuffer = mEncorder.getInputBuffer(inputBufferIndex);
  inputBuffer.clear();
  inputBuffer.put(audioDate.buffer);
  mEncorder.queueInputBuffer(inputBufferIndex, 0, audioDate.size, System.nanoTime(), 0);
  }

                    // 取出编码好的一帧音频数据,然后给这一帧添加ADTS头
  int outputBufferIndex = mEncorder.dequeueOutputBuffer(mBufferInfo, 0);
 while (outputBufferIndex >= 0) {
                        int outBitsSize = mBufferInfo.size;
 int outPacketSize = outBitsSize + 7; // ADTS头部是7个字节
  ByteBuffer outputBuffer = mEncorder.getOutputBuffer(outputBufferIndex);
  outputBuffer.position(mBufferInfo.offset);
  outputBuffer.limit(mBufferInfo.offset + outBitsSize);

 byte[] outData = new byte[outPacketSize];
  addADTStoPacket(outData, outPacketSize);

  outputBuffer.get(outData, 7, outBitsSize);
  outputBuffer.position(mBufferInfo.offset);
  mFileStream.write(outData);
  mEncorder.releaseOutputBuffer(outputBufferIndex, false);
  outputBufferIndex = mEncorder.dequeueOutputBuffer(mBufferInfo, 0);
  }
                }
                release();
  } catch (FileNotFoundException e) {
                e.printStackTrace();
  } catch (IOException e) {
                e.printStackTrace();
  }
        }

        /**
 * 停止编码
  */
  public void stopEncording() {
            isEncording = false;
  }

        /**
 * 从队列中取出一帧待编码的音频数据
  * @return
  */
  public AudioDate getAudioDate(){
            if (queue != null){
                try {
                    return queue.take();
  } catch (InterruptedException e) {
                    e.printStackTrace();
  }
            }
            return null;
  }

 /**
 * 添加ADTS头
  * @param packet
  * @param packetLen
  */
  private void addADTStoPacket(byte[] packet, int packetLen) {
            int profile = 2; //AAC LC
  int freqIdx = 4; //44100 根据不同的采样率修改这个值
  int chanCfg = 2; //CPE
  packet[0] = (byte) 0xFF;
  packet[1] = (byte) 0xF9;
  packet[2] = (byte) (((profile - 1) << 6) + (freqIdx << 2) + (chanCfg >> 2));
  packet[3] = (byte) (((chanCfg & 3) << 6) + (packetLen >> 11));
  packet[4] = (byte) ((packetLen & 0x7FF) >> 3);
  packet[5] = (byte) (((packetLen & 7) << 5) + 0x1F);
  packet[6] = (byte) 0xFC;
  }

 /**
 * 释放资源
  */
  public void release() {
            if (mFileStream != null) {
                try {
                    mFileStream.flush();
  mFileStream.close();
  } catch (IOException e) {
                    e.printStackTrace();
  }
            }
            if (mEncorder != null) {
                mEncorder.stop();
  }
        }
    }

    public String getSDPath() {
        // 判断是否挂载
  if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            return Environment.getExternalStorageDirectory().getAbsolutePath();
  }
        return Environment.getRootDirectory().getAbsolutePath();
  }

    public void checkRecordPermission() {
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.RECORD_AUDIO) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, MICROPHONE, 1);
 return;  }
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, STORAGE, 1);
 return;  }
    }

}
10-06 17:54