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:
* 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
ADTS frame详细说明
项目git地址:
中的FFMpeg1 – xohn_avutils_aac.cpp