RTP timestamp(时间戳)算法 (live555)

live555中进行RTP打包时，生成RTP header时，需要计算该RTP的时间戳.

RTP header

RTP header

可以看出 RTP的时间戳在 RTP header的第 5字节到第 8字节，共 4个字节，即 32bit.

timeval

/*
 * Structure used in select() call, taken from the BSD file sys/time.h.
 */
struct timeval {
        long    tv_sec;         /* seconds */
        long    tv_usec;        /* and microseconds */
};

• tv_sec
  秒.
• tv_usec
  u秒, 即1000000分之一秒.

调用堆栈

以视频为H.264为例:

RTPSink::convertToRTPTimestamp**(timeval tv)  行 97  C++
MultiFramedRTPSink::setTimestamp(timeval framePresentationTime)  行 113 + 0x10 字节    C++
H264or5VideoRTPSink::doSpecialFrameHandling(unsigned int __formal, unsigned int __formal, unsigned int __formal, timeval framePresentationTime, unsigned int __formal)  行 152   C++
MultiFramedRTPSink::afterGettingFrame1(unsigned int frameSize, unsigned int numTruncatedBytes, timeval presentationTime, unsigned int durationInMicroseconds)  行 319 + 0x27 字节  C++
MultiFramedRTPSink::afterGettingFrame(void * clientData, unsigned int numBytesRead, unsigned int numTruncatedBytes, timeval presentationTime, unsigned int durationInMicroseconds)  行 235   C++
FramedSource::afterGetting(FramedSource * source)  行 91 + 0x31 字节   C++

核心函数

• setTimestamp

void MultiFramedRTPSink::setTimestamp(struct timeval framePresentationTime) {
  // First, convert the presentation time to a 32-bit RTP timestamp:
  fCurrentTimestamp = convertToRTPTimestamp(framePresentationTime);

  // Then, insert it into the RTP packet:
  fOutBuf->insertWord(fCurrentTimestamp, fTimestampPosition);
}

• convertToRTPTimestamp
  生成32bit的RTP时间戳.

u_int32_t RTPSink::convertToRTPTimestamp(struct timeval tv) {
  // 将 "struct timeval" 单元转换为RTP 时间戳单元:
  u_int32_t timestampIncrement = (fTimestampFrequency*tv.tv_sec);
// 加0.5是为了实现四舍五入
  timestampIncrement += (u_int32_t)(fTimestampFrequency*(tv.tv_usec/1000000.0) + 0.5); // note: rounding

  // Then add this to our 'timestamp base':
  if (fNextTimestampHasBeenPreset) {
    // Make the returned timestamp the same as the current "fTimestampBase",
    // so that timestamps begin with the value that was previously preset:
    fTimestampBase -= timestampIncrement;
    fNextTimestampHasBeenPreset = False;
  }
// 生成32位的时间戳
// 时间戳 = 基准时间戳 + 时间戳增量
  u_int32_t const rtpTimestamp = fTimestampBase + timestampIncrement;

  return rtpTimestamp;
}

fTimestampFrequency对H.264来说一般为90000.

References

http://www.siptutorial.net/RTP/header.html