FFmpeg编码aac音频，编码文件无法播放

Question

我正在尝试对 aac 音频进行编码。 MP2 的例子在这里。 我遵循了这个文档。 我通过调用startEncoding使用下面的代码对音频进行编码，2 秒后我调用stopEncoding 。 一切似乎都很好我得到了一个大小不一的文件，但问题是我无法打开或播放它。 我不知道为什么。 我一定在代码中做错了什么。

header：

class MediaEncoder {
public:
    MediaEncoder(char *filePath);

    void startEncoding();
    void stopEncoding();

    void encode(AVFrame *frame);
    bool isEncoding() const;

    void startEncoderWorker();

    int32_t check_sample_fmt(enum AVSampleFormat sample_fmt);
    
    bool signalExitFuture = false;
    int32_t ret;

private:
    std::future<void> encodingFuture;
    AVCodecContext *avCodecContext;
    AVFrame *avFrame;
    AVPacket *avPacket;
    AVCodec *codec;
    FILE* file;
};

cp:

MediaEncoder::MediaEncoder(char *filePath){
    buffer = std::make_unique<LockFreeQueue<float>>(recorderBufferSize);

    /* find the encoder */
    codec = avcodec_find_encoder(AV_CODEC_ID_AAC);
    avCodecContext = avcodec_alloc_context3(codec);

    avCodecContext->bit_rate = 64000;

    /* check that the encoder supports given sample format */
    avCodecContext->sample_fmt = AVSampleFormat::AV_SAMPLE_FMT_FLTP;
    
    /* select other audio parameters supported by the encoder */
    avCodecContext->sample_rate = defaultSampleRate;
    avCodecContext->channel_layout = AV_CH_LAYOUT_STEREO;
    avCodecContext->channels = av_get_channel_layout_nb_channels(avCodecContext->channel_layout);

    /* open it */
    avcodec_open2(avCodecContext, codec, nullptr)
    
    file = fopen(filePath, "wb");
    
    /* packet for holding encoded output */
    avPacket = av_packet_alloc();
    
    /* frame containing input raw audio */
    avFrame = av_frame_alloc();
    
    avFrame->nb_samples = avCodecContext->frame_size;
    avFrame->format = avCodecContext->sample_fmt;
    avFrame->channel_layout = avCodecContext->channel_layout;

    /* allocate the data buffers */
    av_frame_get_buffer(avFrame, 0);
}


void MediaEncoder::startEncoding() {
    // set flags for decoding thread
    signalExitFuture = false;

    encodingFuture = std::async(std::launch::async, &MediaEncoder::startEncoderWorker, this);
}

void MediaEncoder::stopEncoding() {
    signalExitFuture = true;
}

bool MediaEncoder::isEncoding() const {
    return !signalExitFuture;
}

void MediaEncoder::encode(AVFrame *frame) {
    /* send the frame for encoding */
    ret = avcodec_send_frame(avCodecContext, frame);
    if (ret < 0) {
        LOGE("Error sending the frame to the encoder %s",
             av_err2str(ret));
        *recorderStatePointer = MediaEncoderState::RUNTIME_FAIL;
        return;
    }

    /* read all the available output packets (in general there may be any
     * number of them */
    while (ret >= 0) {
        ret = avcodec_receive_packet(avCodecContext, avPacket);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
            return;
        } else if (ret < 0) {
            LOGE("Error encoding audio frame %s",
                 av_err2str(ret));
            *recorderStatePointer = MediaEncoderState::RUNTIME_FAIL;
            return;
        }
       
        /* Solution begins here
        int aac_profile = 2;           // AAC LC
        int frequencey_index = 4;      // 44,100Hz
        int channel_configuration = 2; // stereo (left, right)

        int frame_length = avPacket->size + 7;              // your frame length
     
        unsigned char adts_header[7];

        // fill in ADTS data
        adts_header[0] = (unsigned char) 0xFF;
        adts_header[1] = (unsigned char) 0xF9;
        adts_header[2] = (unsigned char) (((aac_profile -1) << 6 ) + (frequencey_index << 2) + (channel_configuration >> 2));
        adts_header[3] = (unsigned char) (((channel_configuration & 3) << 6) + (frame_length >> 11));
        adts_header[4] = (unsigned char) ((frame_length & 0x7FF) >> 3);
        adts_header[5] = (unsigned char) (((frame_length & 7) << 5) + 0x1F);
        adts_header[6] = (unsigned char) 0xFC;

        fwrite(adts_header, 1, 7, file); 
        Solution ends here */ 
        fwrite(avPacket->data, 1, avPacket->size, file);
        av_packet_unref(avPacket);
    }
}

void MediaEncoder::startEncoderWorker() {
    try {
        float *leftChannel;
        float *rightChannel;
        float val;

        while (!signalExitFuture) {
            ret = av_frame_make_writable(avFrame);
            if (ret < 0) {
                LOGE("av_frame_make_writable Can not Ensure that the frame data is writable. %s",
                     av_err2str(ret));
                *recorderStatePointer = MediaEncoderState::RUNTIME_FAIL;
                return;
            }

            leftChannel = (float *) avFrame->data[0];
            rightChannel = (float *) avFrame->data[1];

            for (int32_t i = 0; i < avCodecContext->frame_size; ++i) {
               
                leftChannel[i] = 0.4;
                rightChannel[i] = 0.4;
            }

            encode(avFrame);
        }

        /* flush the encoder */
        encode(nullptr);

        fclose(file);
        LOGE("Encoding finished!");

        av_frame_free(&avFrame);
        av_packet_free(&avPacket);
        avcodec_free_context(&avCodecContext);
    } catch (std::exception &e) {
        LOGE("startEncoderWorker uncaught exception %s", e.what());
    }

    LOGE("Deleting Media Encoder!");

}

这是用真实浮点 pcm 数据记录的 11 秒 aac 文件，而不是我发布的代码中的 0.4 秒。 谷歌云端硬盘链接

上面的代码有效。 感谢传奇@Markus-Schumann

Answer 1

我假设 AAC 编码器输出原始 AAC 帧。 将原始 AAC 帧写入文件不会创建可播放的 output。 因此，您必须添加帧头或将 output 混合到 MP4 文件中。 寻找 ADIF、ADTS 或 MP4 作为文件格式。

假设您想做 ADTS：

int aac_profile = 2;           // AAC LC
int frequencey_index = 4;      // 44,100Hz
int channel_configuration = 2; // stereo (left, right)

// https://wiki.multimedia.cx/index.php/MPEG-4_Audio#Channel_Configurations

int frame_length;              // your frame length + 7 bytes for the header


unsigned char adts_header[7];
// Take look here: https://wiki.multimedia.cx/index.php/ADTS

// fill in ADTS data
adts_header[0] = (unsigned char) 0xFF;
adts_header[1] = (unsigned char) 0xF9;
adts_header[2] = (unsigned char) (((aac_profile -1) << 6 ) + (frequencey_index << 2) + (channel_configuration >> 2));
adts_header[3] = (unsigned char) (((channel_configuration & 3) << 6) + (frame_length >> 11));
adts_header[4] = (unsigned char) ((frame_length & 0x7FF) >> 3);
adts_header[5] = (unsigned char) (((frame_length & 7) << 5) + 0x1F);
adts_header[6] = (unsigned char) 0xFC;

因此，您必须使用上述 header 为来自编码器的每个帧添加前缀，确保将 frame_length 设置为来自编码器的 AAC 缓冲区长度。 在磁盘上它应该看起来像：|ADTS 头|AAC 帧|ADTS 头|AAC 帧|...|ADTS 头|AAC 帧|