本文记录了AAC音频码流解析，音频码流在视频播放器中的位置如下所示

原理

AAC原始码流是由一个一个的ADTS frame组成的。其结构如下图所示。

其中每个ADTS frame之间通过syncword（同步字）进行分隔。同步字为0xFFF（二进制“111111111111”）。AAC码流解析的步骤就是首先从码流中搜索0xFFF，分离出ADTS frame；然后再分析ADTS frame的首部各个字段。本程序可以从AAC码流中分析得到它的基本单元ADTS frame，并且可以简单解析ADTS frame首部的字段。

代码

#include <jni.h>
#include <string>
#include <math.h>

#ifdef ANDROID
#include <jni.h>
#include <android/log.h>
#define LOGE(format, ...)  __android_log_print(ANDROID_LOG_ERROR, "xohnffmpeg", format, ##__VA_ARGS__)
#define LOGI(format, ...)  __android_log_print(ANDROID_LOG_INFO,  "xohnffmpeg", format, ##__VA_ARGS__)
#else
#define LOGE(format, ...)  printf("xohnffmpeg" format "\n", ##__VA_ARGS__)
#define LOGI(format, ...)  printf("xohnffmpeg " format "\n", ##__VA_ARGS__)
#endif

int getADTSframe(unsigned char* buffer, int buf_size, unsigned char* data ,int* data_size){
    int size = 0;

    if(!buffer || !data || !data_size ){
        return -1;
    }

    while(1){
        if(buf_size  < 7 ){
            //return -1;
            return 1; //长度小于7，也做缓存处理
        }
        //Sync words
        if((buffer[0] == 0xff) && ((buffer[1] & 0xf0) == 0xf0) ){
            size |= ((buffer[3] & 0x03) <<11);     //high 2 bit
            size |= buffer[4]<<3;                //middle 8 bit
            size |= ((buffer[5] & 0xe0)>>5);        //low 3bit
            break;
        }
        --buf_size;
        ++buffer;
    }

    if(buf_size < size){
        return 1;
    }

    memcpy(data, buffer, size);
    *data_size = size;

    return 0;
}

/**
 * Analysis AAC Bitstream
 * @param jstr_url    Location of input AAC bitstream file.
 * AAC原始码流（又称为“裸流”）是由一个一个的ADTS frame组成的
 * ADTS 可参考：https://wiki.multimedia.cx/index.php/ADTS
 * 每个ADTS frame之间通过syncword（同步字）进行分隔。同步字为0xFFF（二进制“111111111111”）
 * AAC码流解析的步骤就是首先从码流中搜索0xFFF，分离出ADTS frame；然后再分析ADTS frame的首部各个字段
 */
extern "C" JNIEXPORT jint JNICALL
Java_com_xohn_ffmpeg_AVUtils_aacParser(
        JNIEnv *env,jobject /* this */,
        jstring jstr_url) {
    int data_size = 0;
    int size = 0;
    int cnt = 0;
    int offset = 0;

    const char *url = env->GetStringUTFChars(jstr_url, NULL);
    FILE *ifile = fopen(url, "rb");
    env->ReleaseStringUTFChars(jstr_url,url);
    if (ifile == NULL){
        return -1;
    }

    //FILE *myout = fopen("output_log.txt","wb+");
    //FILE *myout = stdout;

    unsigned char *aacframe = (unsigned char *)malloc(1024*8); //frame len 占位13bit，最大长度8k
    unsigned char *aacbuffer = (unsigned char *)malloc(1024*1024);

    LOGI("-----+- ADTS Frame Table -+------+\n");
    LOGI(" NUM | Profile | Frequency| Size |\n");
    LOGI("-----+---------+----------+------+\n");

    while(!feof(ifile)){
        data_size = fread(aacbuffer+offset, 1, 1024*1024-offset, ifile);
        data_size += offset; //实际读出来的长度 + 前面缓存的偏移量的长度
        unsigned char* input_data = aacbuffer;

        while(1){
            int ret = getADTSframe(input_data, data_size, aacframe, &size);
            if(ret == -1){
                break;
            }else if(ret == 1){
                memcpy(aacbuffer,input_data,data_size);
                offset = data_size;
                break;
            }

            char profile_str[10] = {0};
            char frequence_str[10] = {0};

            unsigned char profile = aacframe[2]&0xC0;
            profile = profile>>6;
            switch(profile){
                case 0: sprintf(profile_str,"Main");break;
                case 1: sprintf(profile_str,"LC");break;
                case 2: sprintf(profile_str,"SSR");break;
                default:sprintf(profile_str,"unknown");break;
            }

            unsigned char sampling_frequency_index = aacframe[2]&0x3C;
            sampling_frequency_index = sampling_frequency_index>>2;
            switch(sampling_frequency_index){
                case 0: sprintf(frequence_str,"96000Hz");break;
                case 1: sprintf(frequence_str,"88200Hz");break;
                case 2: sprintf(frequence_str,"64000Hz");break;
                case 3: sprintf(frequence_str,"48000Hz");break;
                case 4: sprintf(frequence_str,"44100Hz");break;
                case 5: sprintf(frequence_str,"32000Hz");break;
                case 6: sprintf(frequence_str,"24000Hz");break;
                case 7: sprintf(frequence_str,"22050Hz");break;
                case 8: sprintf(frequence_str,"16000Hz");break;
                case 9: sprintf(frequence_str,"12000Hz");break;
                case 10: sprintf(frequence_str,"11025Hz");break;
                case 11: sprintf(frequence_str,"8000Hz");break;
                default:sprintf(frequence_str,"unknown");break;
            }


            //fprintf(myout,"%5d| %8s|  %8s| %5d|\n",cnt,profile_str ,frequence_str,size);
            LOGI("%5d| %8s|  %8s| %5d|\n",cnt,profile_str ,frequence_str,size);
            data_size -= size;
            input_data += size;
            cnt++;
        }

    }
    fclose(ifile);
    free(aacbuffer);
    free(aacframe);

    return 0;
}

函数调用

AVUtils.getInstance().aacParser(ROOT+"nocturne.aac");

输出为该码流中ADTS frame的统计数据，如下图

参考文章：

雷神文章 https://blog.csdn.net/leixiaohua1020/article/details/50535042

ADTS frame详细说明 https://wiki.multimedia.cx/index.php/ADTS

项目git地址：

https://gitee.com/xohn/FFmpeg.git 中的FFMpeg1 - xohn_avutils_aac.cpp