音頻碼流在視頻播放器中的位置如下所示。

本文中的程序是一個AAC碼流解析程序。該程序可以從AAC碼流中分析得到它的基本單元ADTS frame，并且可以簡單解析ADTS frame首部的字段。通過修改該程序可以實現不同的AAC碼流處理功能。

原理

AAC原始碼流（又稱為“裸流”）是由一個一個的ADTS frame組成的。他們的結構如下圖所示。

其中每個ADTS frame之間通過syncword（同步字）進行分隔。同步字為0xFFF（二進制“111111111111”）。AAC碼流解析的步驟就是首先從碼流中搜索0x0FFF，分離出ADTS frame；然后再分析ADTS frame的首部各個字段。本文的程序即實現了上述的兩個步驟。

代碼

整個程序位于simplest_aac_parser()函數中，如下所示。

/** 
 * 最簡單的視音頻數據處理示例 
 * Simplest MediaData Test 
 * 
 * 雷霄驊 Lei Xiaohua 
 * leixiaohua1020@126.com 
 * 中國傳媒大學/數字電視技術 
 * Communication University of China / Digital TV Technology 
 * http://blog.csdn.net/leixiaohua1020 
 * 
 * 本項目包含如下幾種視音頻測試示例： 
 *  (1)像素數據處理程序。包含RGB和YUV像素格式處理的函數。 
 *  (2)音頻采樣數據處理程序。包含PCM音頻采樣格式處理的函數。 
 *  (3)H.264碼流分析程序。可以分離并解析NALU。 
 *  (4)AAC碼流分析程序。可以分離并解析ADTS幀。 
 *  (5)FLV封裝格式分析程序。可以將FLV中的MP3音頻碼流分離出來。 
 *  (6)UDP-RTP協議分析程序。可以將分析UDP/RTP/MPEG-TS數據包。 
 * 
 * This project contains following samples to handling multimedia data: 
 *  (1) Video pixel data handling program. It contains several examples to handle RGB and YUV data. 
 *  (2) Audio sample data handling program. It contains several examples to handle PCM data. 
 *  (3) H.264 stream analysis program. It can parse H.264 bitstream and analysis NALU of stream. 
 *  (4) AAC stream analysis program. It can parse AAC bitstream and analysis ADTS frame of stream. 
 *  (5) FLV format analysis program. It can analysis FLV file and extract MP3 audio stream. 
 *  (6) UDP-RTP protocol analysis program. It can analysis UDP/RTP/MPEG-TS Packet. 
 * 
 */  
#include <stdio.h>  
#include <stdlib.h>  
#include <string.h>  
  
  
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;  
        }  
        //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;  
}  
  
int simplest_aac_parser(char *url)  
{  
    int data_size = 0;  
    int size = 0;  
    int cnt=0;  
    int offset=0;  
  
    //FILE *myout=fopen("output_log.txt","wb+");  
    FILE *myout=stdout;  
  
    unsigned char *aacframe=(unsigned char *)malloc(1024*5);  
    unsigned char *aacbuffer=(unsigned char *)malloc(1024*1024);  
  
    FILE *ifile = fopen(url, "rb");  
    if(!ifile){  
        printf("Open file error");  
        return -1;  
    }  
  
    printf("-----+- ADTS Frame Table -+------+\n");  
    printf(" NUM | Profile | Frequency| Size |\n");  
    printf("-----+---------+----------+------+\n");  
  
    while(!feof(ifile)){  
        data_size = fread(aacbuffer+offset, 1, 1024*1024-offset, ifile);  
        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);  
            data_size -= size;  
            input_data += size;  
            cnt++;  
        }     
  
    }  
    fclose(ifile);  
    free(aacbuffer);  
    free(aacframe);  
  
    return 0;  
} 

結果

本程序的輸入為一個AAC原始碼流（裸流）的文件路徑，輸出為該碼流中ADTS frame的統計數據，如下圖所示。

Ps:非原創,原作者雷霄驊