Timestamp of AAC, h264, etc

aac Time stamp

    aac： Advanced audio coding , The sampling rate is generally 44100, How to calculate the timestamp ？ Generally speaking , Everyone will calculate like this :1024*1000/44100 = 23 millisecond , So this one 1024 To come from ？

    1024 It means nb_sample, Is to choose from 1024 Bytes for encoding , And according to the sampling rate , One second produces 44100 byte , that 1024 How much , Namely 1024/44100, multiply 1000, Into milliseconds . Got to know this , That is, the correct timestamp can be generated .

     about RTP Agreement for , Direct transmission 1024 Interval as timestamp .

h264 h265 Time stamp

RTP The agreement is different from other agreements , If using RTP agreement , Both the audio timestamp and the video timestamp need to be adjusted . For video , We give a calculation formula code

static uint32_t convertToRTPTimestamp(/*struct timeval tv*/)
{
    
	timeval tv;
	gettimeofday(&tv, NULL);
	UINT32 timestampIncrement = (90000 * tv.tv_sec);
	timestampIncrement += (UINT32)((2.0 * 90000 * tv.tv_usec + 1000000.0) / 2000000);
	//UINT32 const rtpTimestamp = timestampIncrement; 
	return timestampIncrement;
}

This is a h264 h265 Function of calculating timestamp by encoding , It is obvious that you have multiplied by one 90000, Why? , because 90000 When multiplied, it can become an integer , that 0, 4500,9000,13500 How to calculate the frame rate of this integer ,x * 90000 = 4500 , that x = 4500/90000 = 1/20, Obviously , Apart, One twentieth of a second , At this moment, you can see that it is one second 20 frame .

RTMP Protocol timestamp

For things like RTMP Wait for the protocol timestamp , It uses relative and absolute timestamps , The first is the absolute timestamp , Then use the relative timestamp , That is, the relative timestamp relative to the first absolute time . Timestamps for protocols like this , It uses the normal timestamp .

ffmpeg

ffmpeg Whether the current relative timestamp can be used directly ？ It is also related to the agreement . about RTMP Push flow and RTSP To push the flow , Follow the agreement ,RTSP The agreement is obviously to be observed RTP Protocol timestamp ,RTMP Use the normal timestamp , have access to 0, Then follow the passing time , That is, absolute timestamp use 0, Relative timestamp uses the time difference relative to zero .