Audio Decoding

You can call the native APIs provided by the audio codec module to decode audio, that is, to decode media data into PCM streams.

Currently, the following decoding capabilities are supported:

Usage Scenario

• Audio playback

  Decode audio and transmit the data to the speaker for playing.

• Audio rendering

  Decode audio and transmit the data to the audio processing module for audio rendering.

• Audio editing

  Decode audio and transmit the data for audio editing (for example, adjusting the playback speed of a channel). Audio editing is performed based on PCM streams.

How to Develop

Read Audio Codec for the API reference.

Refer to the code snippet below to complete the entire audio decoding process, including creating a decoder, setting decoding parameters (such as the sampling rate, bit rate, and number of audio channels), and starting, refreshing, resetting, and destroying the decoder.

During application development, you must call the APIs in the defined sequence. Otherwise, an exception or undefined behavior may occur.

The figure below shows the call relationship of audio decoding.

Linking the Dynamic Library in the CMake Script

target_link_libraries(sample PUBLIC libnative_media_codecbase.so)
target_link_libraries(sample PUBLIC libnative_media_core.so)
target_link_libraries(sample PUBLIC libnative_media_acodec.so)

How to Develop

1. Add the header files.

   #include <multimedia/player_framework/native_avcodec_audiocodec.h>
   #include <multimedia/native_audio_channel_layout.h>
   #include <multimedia/player_framework/native_avcapability.h>
   #include <multimedia/player_framework/native_avcodec_base.h>
   #include <multimedia/player_framework/native_avformat.h>
   #include <multimedia/player_framework/native_avbuffer.h>
   

2. Create a decoder instance. In the code snippet below, **OH_AVCodec *** is the pointer to the decoder instance created.

   // Namespace of the C++ standard library.
   using namespace std;
   // Create a decoder by name.
   OH_AVCapability *capability = OH_AVCodec_GetCapability(OH_AVCODEC_MIMETYPE_AUDIO_MPEG, false);
   const char *name = OH_AVCapability_GetName(capability);
   OH_AVCodec *audioDec_ = OH_AudioCodec_CreateByName(name);
   

   // Specify whether encoding is used. The value **false** means decoding.
   bool isEncoder = false;
   // Create a decoder by MIME type.
   OH_AVCodec *audioDec_ = OH_AudioCodec_CreateByMime(OH_AVCODEC_MIMETYPE_AUDIO_MPEG, isEncoder);
   

   // Initialize the queues.
   class ADecBufferSignal {
   public:
       std::mutex inMutex_;
       std::mutex outMutex_;
       std::mutex startMutex_;
       std::condition_variable inCond_;
       std::condition_variable outCond_;
       std::condition_variable startCond_;
       std::queue<uint32_t> inQueue_;
       std::queue<uint32_t> outQueue_;
       std::queue<OH_AVBuffer *> inBufferQueue_;
       std::queue<OH_AVBuffer *> outBufferQueue_;
   };
   ADecBufferSignal *signal_;
   

3. Call OH_AudioCodec_RegisterCallback() to register callback functions.

   Register the OH_AVCodecCallback struct that defines the following callback function pointers:

   • OH_AVCodecOnError, a callback used to report a codec operation error
   • OH_AVCodecOnStreamChanged, a callback used to report a codec stream change, for example, audio channel change
   • OH_AVCodecOnNeedInputBuffer, a callback used to report input data required, which means that the decoder is ready for receiving data
   • OH_AVCodecOnNewOutputBuffer, a callback used to report output data generated, which means that decoding is complete

   You need to process the callback functions to ensure that the decoder runs properly.

   // Implement the OH_AVCodecOnError callback function.
   static void OnError(OH_AVCodec *codec, int32_t errorCode, void *userData)
   {
       (void)codec;
       (void)errorCode;
       (void)userData;
   }
   // Implement the OH_AVCodecOnStreamChanged callback function.
   static void OnOutputFormatChanged(OH_AVCodec *codec, OH_AVFormat *format, void *userData)
   {
       (void)codec;
       (void)format;
       (void)userData;
   }
   // Implement the OH_AVCodecOnNeedInputBuffer callback function.
   static void OnInputBufferAvailable(OH_AVCodec *codec, uint32_t index, OH_AVBuffer *data, void *userData)
   {
       (void)codec;
       ADecBufferSignal *signal = static_cast<ADecBufferSignal *>(userData);
       unique_lock<mutex> lock(signal->inMutex_);
       signal->inQueue_.push(index);
       signal->inBufferQueue_.push(data);
       signal->inCond_.notify_all();
       // The input stream is sent to inBufferQueue_.
   }
   // Implement the OH_AVCodecOnNewOutputBuffer callback function.
   static void OnOutputBufferAvailable(OH_AVCodec *codec, uint32_t index, OH_AVBuffer *data, void *userData)
   {
       (void)codec;
       ADecBufferSignal *signal = static_cast<ADecBufferSignal *>(userData);
       unique_lock<mutex> lock(signal->outMutex_);
       signal->outQueue_.push(index);
       signal->outBufferQueue_.push(data);
       signal->outCond_.notify_all();
       // The index of the output buffer is sent to outQueue_.
       // The decoded data is sent to outBufferQueue_.
   }
   signal_ = new ADecSignal();
   OH_AVCodecCallback cb_ = {&OnError, &OnOutputFormatChanged, &OnInputBufferAvailable, &OnOutputBufferAvailable};
   int32_t ret = OH_AudioCodec_RegisterCallback(audioDec_, cb_, signal_);
   if (ret != AVCS_ERR_OK) {
       // Exception handling.
   }
   

4. Call OH_AudioCodec_Configure() to configure the decoder.

   Key values of configuration options are described as follows:

// Set the decoding resolution.
int32_t ret;
// (Mandatory) Configure the audio sampling rate.
constexpr uint32_t DEFAULT_SMAPLERATE = 44100; 
// (Mandatory) Configure the audio bit rate.
constexpr uint32_t DEFAULT_BITRATE = 32000;
// (Mandatory) Configure the number of audio channels.
constexpr uint32_t DEFAULT_CHANNEL_COUNT = 2;
// (Optional) Configure the maximum input length.
constexpr uint32_t DEFAULT_MAX_INPUT_SIZE = 1152;
// Configure whether to use ADTS decoding (ACC).
constexpr uint32_t DEFAULT_AAC_TYPE = 1;
OH_AVFormat *format = OH_AVFormat_Create();
// Set the format.
OH_AVFormat_SetIntValue(format, OH_MD_KEY_AUD_SAMPLE_RATE, DEFAULT_SMAPLERATE);
OH_AVFormat_SetIntValue(format, OH_MD_KEY_BITRATE, DEFAULT_BITRATE);
OH_AVFormat_SetIntValue(format, OH_MD_KEY_AUD_CHANNEL_COUNT, DEFAULT_CHANNEL_COUNT);
OH_AVFormat_SetIntValue(format, OH_MD_KEY_MAX_INPUT_SIZE, DEFAULT_MAX_INPUT_SIZE);
OH_AVFormat_SetIntValue(format, OH_MD_KEY_AAC_IS_ADTS, DEFAULT_AAC_TYPE);
// Configure the decoder.
ret = OH_AudioCodec_Configure(audioDec_, format);
if (ret != AV_ERR_OK) {
    // Exception handling.
}

5. Call OH_AudioCodec_Prepare() to prepare internal resources for the decoder.

   ret = OH_AudioCodec_Prepare(audioDec_);
   if (ret != AV_ERR_OK) {
       // Exception handling.
   }
   

6. Call OH_AudioCodec_Start() to start the decoder.

   unique_ptr<ifstream> inputFile_ = make_unique<ifstream>();
   unique_ptr<ofstream> outFile_ = make_unique<ofstream>();
   // Open the path of the binary file to be decoded.
   inputFile_->open(inputFilePath.data(), ios::in | ios::binary); 
   // Configure the path of the output file.
   outFile_->open(outputFilePath.data(), ios::out | ios::binary);
   // Start decoding.
   ret = OH_AudioCodec_Start(audioDec_);
   if (ret != AV_ERR_OK) {
       // Exception handling.
   }
   

7. Call OH_AudioCodec_PushInputBuffer() to write the data to decode.

   To indicate the End of Stream (EOS), pass in the AVCODEC_BUFFER_FLAGS_EOS flag.

   uint32_t index = signal_->inQueue_.front();
   auto buffer = signal_->inBufferQueue_.front();
   int64_t size;
   int64_t pts;
   inputFile_.read(reinterpret_cast<char *>(&size), sizeof(size));
   inputFile_.read(reinterpret_cast<char *>(&pts), sizeof(pts));
   inputFile_.read((char *)OH_AVMemory_GetAddr(buffer), size);
   OH_AVCodecBufferAttr attr = {0};
   if (inputFile_->eof()) {
       attr.size = 0;
       attr.flags = AVCODEC_BUFFER_FLAGS_EOS;
   } else {
       attr.size = size;
       attr.flags = AVCODEC_BUFFER_FLAGS_NONE;
   }
   OH_AVBuffer_SetBufferAttr(buffer, &attr);
   int32_t ret = OH_AudioCodec_PushInputBuffer(audioDec_, index);
   if (ret != AV_ERR_OK) {
       // Exception handling.
   }
   

8. Call OH_AudioCodec_FreeOutputBuffer() to output decoded PCM streams.

   uint32_t index = signal_->outQueue_.front();
   OH_AVBuffer *data = signal_->outBufferQueue_.front();
   // Obtain the buffer attributes.
   OH_AVCodecBufferAttr attr = {0};
   ret = OH_AVBuffer_GetBufferAttr(data, &attr);
   if (ret != AV_ERR_OK) {
       // Exception handling.
   }
   // Write the decoded data (specified by data) to the output file.
   pcmOutputFile_.write(reinterpret_cast<char *>(OH_AVBuffer_GetAddr(data)), attr.size);
   ret = OH_AudioCodec_FreeOutputBuffer(audioDec_, index);
   if (ret != AV_ERR_OK) {
       // Exception handling.
   }
   if (attr.flags == AVCODEC_BUFFER_FLAGS_EOS) {
       // End
   }
   

9. (Optional) Call OH_AudioCodec_Flush() to refresh the decoder.

   After OH_AudioCodec_Flush() is called, the decoder remains in the running state, but the current queue is cleared and the buffer storing the decoded data is freed. To continue decoding, you must call OH_AudioCodec_Start() again.

   You need to call OH_AudioCodec_Start() in the following cases:

   • The EOS of the file is reached.
   • An error with OH_AudioCodec_IsValid set to true (indicating that the execution can continue) occurs.

   // Refresh the decoder.
   ret = OH_AudioCodec_Flush(audioDec_);
   if (ret != AV_ERR_OK) {
       // Exception handling.
   }
   // Start decoding again.
   ret = OH_AudioCodec_Start(audioDec_);
   if (ret != AV_ERR_OK) {
       // Exception handling.
   }
   

10. (Optional) Call OH_AudioCodec_Reset() to reset the decoder.

    After OH_AudioCodec_Reset() is called, the decoder returns to the initialized state. To continue decoding, you must call OH_AudioCodec_Configure() and then OH_AudioCodec_Start().

    // Reset the decoder.
    ret = OH_AudioCodec_Reset(audioDec_);
    if (ret != AV_ERR_OK) {
        // Exception handling.
    }
    // Reconfigure the decoder.
    ret = OH_AudioCodec_Configure(audioDec_, format);
    if (ret != AV_ERR_OK) {
    // Exception handling.
    }
    

11. Call OH_AudioCodec_Stop() to stop the decoder.

    // Stop the decoder.
    ret = OH_AudioCodec_Stop(audioDec_);
    if (ret != AV_ERR_OK) {
        // Exception handling.
    }
    

12. Call OH_AudioCodec_Destroy() to destroy the decoder instance and release resources.

    NOTE

    You only need to call this API once.

    // Call OH_AudioCodec_Destroy to destroy the decoder.
    ret = OH_AudioCodec_Destroy(audioDec_);
    if (ret != AV_ERR_OK) {
        // Exception handling.
    } else {
        audioDec_ = NULL; // The decoder cannot be destroyed repeatedly.
    }