live555时戳转换函数理解

live555时戳转换函数理解


u_int32_t RTPSink::convertToRTPTimestamp(struct timeval tv) {
  // Begin by converting from "struct timeval" units to RTP timestamp units:
  u_int32_t timestampIncrement = (fTimestampFrequency*tv.tv_sec);
  timestampIncrement += (u_int32_t)((2.0*fTimestampFrequency*tv.tv_usec + 1000000.0)/2000000);
       // 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;
  }


  u_int32_t const rtpTimestamp = fTimestampBase + timestampIncrement;
#ifdef DEBUG_TIMESTAMPS
  fprintf(stderr, "fTimestampBase: 0x%08x, tv: %lu.%06ld\n\t=> RTP timestamp: 0x%08x\n",
 fTimestampBase, tv.tv_sec, tv.tv_usec, rtpTimestamp);
  fflush(stderr);
#endif


  return rtpTimestamp;
}


u_int32_t RTPSink::presetNextTimestamp() {
  struct timeval timeNow;
  gettimeofday(&timeNow, NULL);


  u_int32_t tsNow = convertToRTPTimestamp(timeNow);
  fTimestampBase = tsNow;
  fNextTimestampHasBeenPreset = True;


  return tsNow;
}


要理解前面这个函数要结合后面这个函数及他们的调用来理解
分三部分


第一部分是转换时戳, 以频率为单位, 通常情况下该单位为90khz, 即90000
先转换秒, 再转换微妙(+1000000/2000000表示向上圆整, 5入)


第二部分
从代码看fNextTimestampHasBeenPreset如果值为真
则返回值为fTimestampBase及上一次得到的时戳
否则 则加上偏移
fNextTimestampHasBeenPreset 初始化时为真, 
fTimestampBase 初始化时为一个随机值, 可见时戳不在乎绝对值, 而只在乎差值


从整体代码看presetNextTimestamp通常只会最初调用
这样整个代码就好理解了


第三部分简单加上偏移, 并返回