AAC基本结构

详细

由图可见，ADTS frame (Audio Data Transport Stream) 之间由syncword（同步字进行分离），即0xFFF。

名称 备注
syncword 总是0xFFF, 代表一个ADTS帧的开始
ID MPEG Version: 0 for MPEG-4, 1 for MPEG-2
layer always: ‘00’
protection_absent Warning, set to 1 if there is no CRC and 0 if there is CRC
profile
sampling_frequency_index
channel_configuration
aac_frame_length 一个ADTS帧的长度包括ADTS头和AAC原始流，aac_frame_length = (protection_absent == 1 ? 7 : 9) + size(AACFrame)
adts_buffer_fullness 0x7FF 说明是码率可变的码流
number_of_raw_data_blocks_in_frame 有number_of_raw_data_blocks_in_frame+1个AAC原始帧(一个AAC原始帧包含一段时间内1024个采样及相关数据)

源码解析

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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;
		}
        //找出同步码 然后找到aac_frame_length的大小 保存在size
		if((buffer[0] == 0xff) && ((buffer[1] & 0xf0) == 0xf0) ){//同步码 实际为0xF0FF?
			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){//此段不完整 buf_size为剩下的 size为应该的大小
		return 1;
	}
 
	memcpy(data, buffer, size);
	*data_size = size;
 
	return 0;
}

int aacParser(char *url){
    int data_size = 0;
	int size = 0;
	int cnt=0;
	int offset=0;
    
    unsigned char *aacframe=(unsigned char *)malloc(1024*5);
	unsigned char *aacbuffer=(unsigned char *)malloc(1024*1024);
    
    FILE *fp=fopen(url,"rb");
    
    while(!feof(fp)){
        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){//段中没有完整的ADTS了
				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;
}