计算视频文件(包含PCR)播放带宽的方法

通过对码流播放卡的分析,以及对公式的对照,对实际计算播放视频文件所用带宽的方法进行了分析(附件中,带有c语言的实现)

分析如下:

关键是最后计算带宽的公式:

PCRdelta = (uint64_t) ((uint64_t)PCR1 - (uint64_t)PCR0);

bitRate = (double) ( ((double)(40608000000ULL)*((double)mpegPackets))/(double)(PCRdelta));

PCRdelta : 是相邻N个PCR的PCR差值

mpegPackets : 是相邻N个PCR,所包含的packet的数量(MPEG packet)

我们可以理解为: mpegPackets*188表示实际的数据

而PCRdelta表示播放这段实际数据,总需要播放的总数据量.

则: (mpegPackets*188)/PCRdelta : 单位实际播放的数据中所包含的有效(MPEG packet)数据量

而我们目前所使用的VP播放卡,要求了,DVB-ASI输出带宽必须是216Mb/s

所以就有 bitRate/216000000 = (mpegPackets*188)/PCRdelta

因此得到 bitRate = (double) ( ((double)(40608000000ULL)*((double)mpegPackets))/(double)(PCRdelta));

所以,按照目前这样分析,播放同一个包含pcr的视频文件,如果播放的DVB-ASI输出总带宽不同,则播放视频文件所分配的带宽也应该不同.


////////////////////////////////////////////////////////////////////////


//


// This routine extracts the bitrate from the content file.


// The content file must contain PCRs


//


////////////////////////////////////////////////////////////////////////


double getBitRate(FILE *fp)


{


// variables used to get bit rate from content file


uint64_t PCR0 = 0, PCR1 = 0;


uint64_t PCRdelta;


double bitRate = 0;


uint32_t mpegPackets = 0;


uint32_t gotPCR = 0;


unsigned char m2tTmp[256];


int length;


unsigned int PID = 0;




//used to control "." output


int sync = 0;




printf(" Scanning content file for bit rate ");


for (;;) {




length = fread(m2tTmp, sizeof(uint8_t), 188, fp);


if(length != 188) break;


mpegPackets++;




if (sync % 1200 == 0 ) {


//show user something while we loop


printf(".");


//force it out-may not otherwise appear in real time


fflush(stdout);


}




sync++;




// look for the sync byte


if(m2tTmp[0] != 0x47) {


printf("vpSendspts: data corruption in content file ");


fclose(fp);


exit(2);


}




// we are going to scan through the first 10 PCRs in this content.


// According to the specifications that will be on the order of


// every 40 milliseconds. We don't have to scan the whole file


// to get a good sense of its validity and bit rate.




// First see if the TS header indicates the presence of an


// adaptation field.


if(GET_DVBASI_ADAPT(m2tTmp) & 0x02) {


// Get the adaptation field and look for the presence of


// a PCR in the adaptation field


if(GET_DVBASI_ADAPTLENGTH(m2tTmp) == 0) {


printf("Zero length AF ");


continue;


}




if(GET_DVBASI_ADAPTFIELD(m2tTmp) & 0x10) {


// Get the initial PCR and reset the counter


if(gotPCR == 10) { // skip the first few, some times they are bad!


PID = GET_DVBASI_PID(m2tTmp);


GET_DVBASI_PCR(m2tTmp, PCR0);


mpegPackets = 0;


}




// Look for the 10th PCR in this content and


if((gotPCR > 20) && PID == GET_DVBASI_PID(m2tTmp)) {




GET_DVBASI_PCR(m2tTmp, PCR1);


PCRdelta = (uint64_t) ((uint64_t)PCR1 - (uint64_t)PCR0);


bitRate = (double) ( ((double)(40608000000ULL)*((double)mpegPackets))/(double)(PCRdelta));




break;


}


gotPCR++;


} // GET_DVBASI_ADAPTFIELD


} //GET_DVBASI_ADAPT


} // while read




//rewind file


rewind(fp);


fflush(fp);




return(bitRate);


}

filippo.le@gmail.com