如何在Android上解码M4A音频

Question

我正在尝试在Android上解码音频，并获取原始数据应用过滤器。

我正在使用MediaExtractor从文件中提取编码的数据，这似乎有效。然后，我尝试将MediaExtractor Docs和MediaCodec的代码混合使用，以使用缓冲区来提取数据并将它们解码为块。

。

所以我首先配置了从extractor.getTrackFormat(0);

获得的格式配置的解码器

MediaExtractor extractor = new MediaExtractor();
String path = "...";
extractor.setDataSource(path);
MediaFormat format = extractor.getTrackFormat(0);
mAudioKeyMine = format.getString(MediaFormat.KEY_MIME);
extractor.selectTrack(0);
MediaCodec decoder;
decoder = MediaCodec.createDecoderByType(mAudioKeyMine);
decoder.configure(format, null, null, 0);

然后尝试获取数据：

public void getData(MediaExtractor extractor)
{
    int offset = 0;
    ByteBuffer inputBuffer = ByteBuffer.allocate(2048);
    MediaFormat outputFormat = decoder.getOutputFormat();
    Log.v(TAG, "outputFormat: " + outputFormat.toString());
    decoder.start();
    int index = decoder.dequeueInputBuffer(1000);
    boolean sawInputEOS = false;
    int sample = 0;
    while (sample >= 0)
    {
        int inputBufferId = decoder.dequeueInputBuffer(1000);
        if (inputBufferId >= 0)
        {
            inputBuffer = decoder.getInputBuffer(index);
            sample = extractor.readSampleData(inputBuffer, 0);
            long presentationTimeUs = 0;
            if (sample < 0)
            {
                sawInputEOS = true;
                sample = 0;
            }
            else
            {
                int trackIndex = extractor.getSampleTrackIndex();
                presentationTimeUs = extractor.getSampleTime();
                Log.v(TAG, "trackIndex: " + trackIndex + ", presentationTimeUs: " + presentationTimeUs);
                Log.v(TAG, "sample: " + sample + ", offset: " + offset);
                Log.v(TAG, "inputBuffer: " + inputBuffer.toString());
            }
            decoder.queueInputBuffer(inputBufferId, 0, sample, presentationTimeUs, sawInputEOS ? MediaCodec.BUFFER_FLAG_END_OF_STREAM : 0);
            if (!sawInputEOS)
            {
                extractor.advance();
            }
        }
        MediaCodec.BufferInfo info = new MediaCodec.BufferInfo();
        int outputBufferId = decoder.dequeueOutputBuffer(info, 1000);
        Log.v(TAG, "info: " + info.toString());
        if (outputBufferId >= 0)
        {
            ByteBuffer outputBuffer = decoder.getOutputBuffer(outputBufferId);
            MediaFormat bufferFormat = decoder.getOutputFormat(outputBufferId);
            Log.v(TAG, "option A");
            Log.v(TAG, "outputBufferId: " + outputBufferId);
            if (outputBuffer != null)
            {
                Log.v(TAG, "outputBuffer: " + outputBuffer.toString());
            }
            else
            {
                Log.v(TAG, "outputBuffer: null");
            }
            Log.v(TAG, "bufferFormat: " + bufferFormat.toString());
            if (outputBuffer != null)
            {
                int cont = 0;
                while (outputBuffer.hasRemaining())
                {
                    int pos = outputBuffer.position();
                    byte data = outputBuffer.get();
                    // do something with the data
                    if (cont < 10)
                    {
                        Log.v(TAG, "outputBuffer: " + pos + " -> " + data);
                    }
                    cont++;
                }
            }
            else
            {
                Log.v(TAG, "outputBuffer: null");
            }
            decoder.releaseOutputBuffer(outputBufferId, 0);
        }
        else if (outputBufferId == MediaCodec.INFO_OUTPUT_FORMAT_CHANGED)
        {
            Log.v(TAG, "option B");
            outputFormat = decoder.getOutputFormat(); 
            Log.v(TAG, "outputFormat: " + outputFormat.toString());
        }
        Log.v(TAG, "extractor.advance()");
        offset += sample;
    }
    Log.v(TAG, "end of track");
    extractor.release();
    extractor = null;
    decoder.stop();
    decoder.release();
}

但是我在线路int outputBufferId = decoder.dequeueOutputBuffer(info, 1000);上遇到了一个错误 IllegalStateException。

我搜索了该错误以及如何正确解码M4A，但大多数解决方案在API 21上进行了弃用，现在我陷入了此错误。

因此，有一个API 26/28的音频解码的示例，或者有人可以解释如何正确执行操作？

整个项目托管在GitHub上。

Answer 1

i使用回调以异步模式解决了这个问题。

基本工作流程是：

• 使用MediaExtractor从文件中提取编码数据
• 将其传递给MediaCodec进行解码
• 将解码的数据传递给Audiotrack以重现它（或对数据做任何您想做的事情）

首先，我们需要一些初始化，我将其放在我用来解码和复制文件的类的构造函数中：

// inizialize the mediaExtractor and set the source file
mediaExtractor = new MediaExtractor();
mediaExtractor.setDataSource(fileName);
// select the first audio track in the file and return it's format
mediaFormat = null;
int i;
int numTracks = mediaExtractor.getTrackCount();
for (i = 0; i < numTracks; i++)
{
    mediaFormat = mediaExtractor.getTrackFormat(i);
    if (mediaFormat.getString(MediaFormat.KEY_MIME).startsWith("audio/"))
    {
        mediaExtractor.selectTrack(i);
        break;
    }
}
// we get the parameter from the mediaFormat
channelCount = mediaFormat.getInteger(MediaFormat.KEY_CHANNEL_COUNT);
sampleRate = mediaFormat.getInteger(MediaFormat.KEY_SAMPLE_RATE);
duration = mediaFormat.getLong(MediaFormat.KEY_DURATION);
mimeType = mediaFormat.getString(MediaFormat.KEY_MIME);
// we can get the minimum buffer size from audioTrack passing the parameter of the audio
// to keep it safe it's good practice to create a buffer that is 8 times bigger
int minBuffSize = AudioTrack.getMinBufferSize(sampleRate,
                                              AudioFormat.CHANNEL_OUT_STEREO,
                                              AudioFormat.ENCODING_PCM_16BIT);
// to reproduce the data we need to initialize the audioTrack, by passing the audio parameter
// we use the MODE_STREAM so we can put more data dynamically with audioTrack.write()
audioTrack = new AudioTrack(AudioManager.STREAM_MUSIC,
                            sampleRate,
                            AudioFormat.CHANNEL_OUT_STEREO,
                            AudioFormat.ENCODING_PCM_16BIT,
                            minBuffSize * 8,
                            AudioTrack.MODE_STREAM);

对于开发人员指南，我用来初始化 audioTrack的方法已弃用，但它对我有用，而新方法不适用，因此出于本示例的目的，我保留了这种类型的初始化。

在初始化阶段之后，我们需要创建解码器，为其设置回调，然后启动解码器和Audiotrack。MediaCodec的回调是：

• onutputbufferavailable：当输入缓冲区可用时调用。
• onoutputbufferavailable：当输出缓冲区可用时调用。
• onerror：当媒体代码遇到错误
时调用
• OnOutputFormatchAnged：输出格式已更改时调用

所以我们需要：

• 使用提取器从文件中提取编码的数据，并使用它填充我们从编解码器中获得的输入袋。
• ，在解码完成编解码器后，我们可以从输出缓冲程序中获取解码数据并将其传递给IDIOTRACK。

我的代码是：

// we get the mediaCodec by creating it using the mime_type extracted form the track
MediaCodec decoder = MediaCodec.createDecoderByType(mimeType);
// to decode the file in asynchronous mode we set the callbacks
decoder.setCallback(new MediaCodec.Callback()
{
    private boolean mOutputEOS = false;
    private boolean mInputEOS = false;
    @Override
    public void onInputBufferAvailable (@NonNull MediaCodec codec,
                                        int index)
    {
        // if i reached the EOS i either the input or the output file i just skip
        if (mOutputEOS | mInputEOS) return;
        // i must use the index to get the right ByteBuffer from the codec
        ByteBuffer inputBuffer = codec.getInputBuffer(index);
        // if the codec is null i just skip and wait for another buffer
        if (inputBuffer == null) return;
        long sampleTime = 0;
        int result;
        // with this method i fill the inputBuffer with the data read from the mediaExtractor
        result = mediaExtractor.readSampleData(inputBuffer, 0);
        // the return parameter of readSampleData is the number of byte read from the file
        // and if it's -1 it means that i reached EOS
        if (result >= 0)
        {
            // if i read some bytes i can pass the index of the buffer, the number of bytes
            // that are in the buffer and the sampleTime to the codec, so that it can decode
            // that data
            sampleTime = mediaExtractor.getSampleTime();
            codec.queueInputBuffer(index, 0, result, sampleTime, 0);
            mediaExtractor.advance();
        }
        else
        {
            // if i reached EOS i need to tell the codec
            codec.queueInputBuffer(index, 0, 0, -1, MediaCodec.BUFFER_FLAG_END_OF_STREAM);
            mInputEOS = true;
        }
    }
    @Override
    public void onOutputBufferAvailable (@NonNull MediaCodec codec,
                                         int index,
                                         @NonNull MediaCodec.BufferInfo info)
    {
        // i can get the outputBuffer from the codec using the relative index
        ByteBuffer outputBuffer = codec.getOutputBuffer(index);
        // if i got a non null buffer
        if (outputBuffer != null)
        {
            outputBuffer.rewind();
            outputBuffer.order(ByteOrder.LITTLE_ENDIAN);
            // i just need to write the outputBuffer into the audioTrack passing the number of
            // bytes it contain and using the WRITE_BLOCKING so that this call will block
            // until it doesn't finish to write the data
            int ret = audioTrack.write(outputBuffer,
                                       outputBuffer.remaining(),
                                       AudioTrack.WRITE_BLOCKING);
        }
        // if the flags in the MediaCodec.BufferInfo contains the BUFFER_FLAG_END_OF_STREAM
        // it mean that i reached EOS so i set mOutputEOS to true, and to assure
        // that it remain true even if this callback is called again i use the logical or
        mOutputEOS |= ((info.flags & MediaCodec.BUFFER_FLAG_END_OF_STREAM) != 0);
        // i always need to release the buffer i use so the system can recycle them and use
        // it again
        codec.releaseOutputBuffer(index, false);
        // if i reached the end of the output stream i need to stop and release the codec
        // and the extractor
        if (mOutputEOS)
        {
            codec.stop();
            codec.release();
            mediaExtractor.release();
            audioTrack.release();
        }
    }
    @Override
    public void onError (@NonNull MediaCodec codec,
                         @NonNull MediaCodec.CodecException e)
    {
        Timber.e(e, "mediacodec collback onError: %s", e.getMessage());
    }
    @Override
    public void onOutputFormatChanged (@NonNull MediaCodec codec,
                                       @NonNull MediaFormat format)
    {
        Timber.d("onOutputFormatChanged: %s", format.toString());
    }
});
// now we can configure the codec by passing the mediaFormat and start it
decoder.configure(mediaFormat, null, null, 0);
decoder.start();
// also we need to start the audioTrack.
audioTrack.play();