具体 Box 分析

Avcc Box

介绍

该 Box 则包含了音频的编码信息和音频码率信息，所以解码音频时非常关键。Esds中可以分为三层，每层为包含关系，分别为 MP4ESDescr，MP4DecConfigDescr，MP4DecSpecificDescr。

工具分析

直接分析字段值，这个 Box 是 Full Box，这里 length， type 和 version flag 就不分析了，具体可以看上面，下面我们就直接分析 Data 数据。这里要注意上面红框标注的 80 80 80，这个相当于分隔符，对应 Esds 每一层的数据。当然有一些 MP4文件下的 Esds Box' 是没有这个80 80 80，那就直接解析就行了

上面分析基本是和程序是一致的，下面是对上面各个字段不同值的补充解释。

ed flag：

一般默认00:0x00:00000000

其中每个bit还代表是否后面有相应的字段。

第一bit为1，则有16bit的dependOn_ES_IS字段；

第二bit为1，则有8bit的URL ing字段；

第三bit为1，则有16bit的OCR_ES_ID字段；

最后5bit，代表streamPriority

asc object type**（5bit）**:

0: Null

1: AAC Main

2: AAC LC (Low Complexity)

3: AAC SSR (Scalable Sample Rate)

asc frequency index**（4bit）**

0: 96000 Hz

1: 88200 Hz

2: 64000 Hz

3: 48000 Hz

4: 44100 Hz

5: 32000 Hz

6: 24000 Hz

7: 22050 Hz

8: 16000 Hz

9: 12000 Hz

10: 11025 Hz

11: 8000 Hz

12: 7350 Hz

13: Reserved

14: Reserved

15: frequency is written explictly

asc channel configuration**（4bit）**

0: Defined in AOT Specifc Config

1: 1channel: front-center 单声道

2: 2channels:front-left, front-right 双声道

3: 3channels:front-center,front-left, front-right 3声道

asc frame length flag**（1bit）**：

0: Each packet contains 1024 samples

1: Each packet contains 960 samples

参照： https://wiki.multimedia.cx/index.php?title=MPEG-4_Audio

代码定义

// BaseBox.h  

// ...
// 其他 Box 的定义

class TimeEsdsBox :public BaseBox {
public:
    Timebyte version = 0;
    Timebyte flags = 0;

    unsigned char es_description_tag = 0; // 基本流描述标记：默认0x03

    unsigned char ed_tag_szie = 0; // 长度
    unsigned short ed_track_id = 0; // es id 原始音频流的id
    unsigned char ed_flag = 0; // 一般默认00


    unsigned char decoder_config_descriptor_tag = 0; // 解码配置参数描述标记：默认0x04
    unsigned char dcd_tag_size = 0; // Length Field长度
    unsigned char dcd_mepg_audio = 0; // 如果是mp4则计算得到：0x40
    unsigned char dcd_audio_stream = 0; // 按照标准或计算得到：此处一般默认0x15
    unsigned int dcd_buffersize_db = 0; // 3byte 建议的解码器缓存大小
    unsigned int dcd_max_bitrate = 0; // 音频数据最大码率
    unsigned int dcd_avg_bitrate = 0; // 音频数据平均码率


    unsigned char decoder_specific_info_description_tag = 0; // 解码规格标记 默认值：0x05
    unsigned char dsid_tag_szie = 0 ;// 解码规格标记及其后面值大小
    // 音频规格数据(2byte) 16bit代表的含义
    unsigned char asc_object_type = 0 ;// 5bit 表示采用的音频编码规格
    unsigned char asc_frequency_index = 0 ;// 4bit 表示采样率
    unsigned char asc_channel_configuration = 0 ;// 4bit 表示通道数
    unsigned char asc_frame_length_flag = 0 ;// 1bit 表示一帧音频的大小
    // ... 还有2bite 暂时不关心



    TimeEsdsBox(BoxHeader h, Timebyte * d): BaseBox(h, d){};
    void PrintDataInfo() override;
};

// TimeEsdsBox.cpp

void TimeEsdsBox::PrintDataInfo() {
    TimeBufferStream bufferStream(data, h.GetDataSize());
    version = bufferStream.GetUChar();
    bufferStream.GetLenData(&flags, 3);
    {
        es_description_tag = bufferStream.GetUChar();
        // 0x80 0x80 0x80
        unsigned char catNumb[3] = {0};
        bufferStream.GetLenData(catNumb, 3);
        if (catNumb[0] == 0x80 && catNumb[1] == 0x80 && catNumb[2] == 0x80) {
            ed_tag_szie = bufferStream.GetUChar();
            ed_track_id = bufferStream.GetUShort();
        } else {
            ed_tag_szie = catNumb[0];
            ed_track_id = catNumb[1] << 8 | catNumb[2];
        }
        ed_flag = bufferStream.GetUChar();
    }


    {
        decoder_config_descriptor_tag = bufferStream.GetUChar();
        // 0x80 0x80 0x80
        unsigned char catNumb[3] = {0};
        bufferStream.GetLenData(catNumb, 3);
        if (catNumb[0] == 0x80 && catNumb[1] == 0x80 && catNumb[2] == 0x80) {

            dcd_tag_size = bufferStream.GetUChar();
            dcd_mepg_audio = bufferStream.GetUChar();
            dcd_audio_stream = bufferStream.GetUChar();
        } else {
            ed_tag_szie = catNumb[0];
            dcd_mepg_audio = catNumb[1];
            dcd_audio_stream = catNumb[2];
        }
        bufferStream.GetLenData(&dcd_buffersize_db, 3);

        dcd_max_bitrate = bufferStream.GetUInt();
        dcd_avg_bitrate = bufferStream.GetUInt();
    }

    {
        decoder_specific_info_description_tag = bufferStream.GetUChar();
        // 0x80 0x80 0x80
        unsigned char catNumb[3] = {0};
        unsigned short temp = 0;
        bufferStream.GetLenData(catNumb, 3);
        if (catNumb[0] == 0x80 && catNumb[1] == 0x80 && catNumb[2] == 0x80) {
            dsid_tag_szie = bufferStream.GetUChar();
            temp = bufferStream.GetUShort();

        } else {
            dsid_tag_szie = catNumb[0];
            temp = catNumb[1] << 8 | catNumb[2];
        }
        asc_object_type = (temp & 0xf800) >> 11;
        asc_frequency_index = (temp & 0x780) >> 7;
        asc_channel_configuration = (temp & 0x78) >> 3;
        asc_channel_configuration = (temp & 0b100) >> 2;
    }

    printf("===========================\n");
    h.to_string();
    printf("es_description_tag: %d\n", es_description_tag);
    printf("ed_tag_szie: %d\n", ed_tag_szie);
    printf("ed_track_id: %d\n", ed_track_id);
    printf("ed_flag: %d\n", ed_flag);

    printf("decoder_config_descriptor_tag: %d\n", decoder_config_descriptor_tag);
    printf("dcd_tag_size: %d\n", dcd_tag_size);
    printf("dcd_mepg_audio: %d\n", dcd_mepg_audio);
    printf("dcd_audio_stream: %d\n", dcd_audio_stream);
    printf("dcd_buffersize_db: %d\n", dcd_buffersize_db);
    printf("dcd_max_bitrate: %ud => max: %.2f Kb/s\n", dcd_max_bitrate, (float) dcd_max_bitrate / 1000.0);
    printf("dcd_avg_bitrate: %ud => avg: %.2f Kb/s\n", dcd_avg_bitrate, (float) dcd_avg_bitrate / 1000.0);

    printf("decoder_specific_info_description_tag: %d\n", decoder_specific_info_description_tag);
    printf("dsid_tag_szie: %d\n", dsid_tag_szie);
    switch (asc_object_type) { // 5bit
        case 0:
            printf("asc_object_type: Null\n");
            break;
        case 1:
            printf("asc_object_type: AAC Main\n");
            break;
        case 2:
            printf("asc_object_type:  AAC LC (Low Complexity)\n");
            break;
        case 3:
            printf("asc_object_type:  AAC SSR (Scalable Sample Rate)\n");
            break;
            // ... 后面的不常见
    }
    switch (asc_frequency_index) { // 4bit
        case 0:
            printf("samples: 96000 Hz\n");
            break;
        case 1:
            printf("samples: 88200 Hz\n");
            break;
        case 2:
            printf("samples: 64000 Hz\n");
            break;
        case 3:
            printf("samples: 48000 Hz\n");
            break;
        case 4:
            printf("samples: 44100 Hz\n");
            break;
        case 5:
            printf("samples: 32000 Hz\n");
            break;
        case 6:
            printf("samples: 24000 Hz\n");
            break;
        case 7:
            printf("samples: 22050 Hz\n");
            break;
        case 8:
            printf("samples: 16000 Hz\n");
            break;
        case 9:
            printf("samples: 12000 Hz\n");
            break;
        case 10:
            printf("samples: 11025 Hz\n");
            break;
        case 11:
            printf("samples: 8000 Hz\n");
            break;
        case 12:
            printf("samples: 7350 Hz\n");
            break;
            // ... 后面的不常见
    }
    switch (asc_channel_configuration) { // 4bit
        case 0:
            printf("channels: Defined in AOT Specifc Config\n");
            break;
        case 1:
            printf("channels: 1 channel: front-center\n");
            break;
        case 2:
            printf("channels: 2 channels:front-left, front-right\n");
            break;
        case 3:
            printf("channels: 3 channels:front-center,front-left, front-right \n");
            break;
            // ... 不明
    }
    switch (asc_frame_length_flag) { // 1bit
        case 0:
            printf("packet: Each packet contains 1024 samples\n");
            break;
        case 1:
            printf("packet: Each packet contains 960 samples\n");
            break;
    }
}