[Live-devel] RTP Timestamp to UTC time

[mohita gandotra]

Hi Ross

I am not a casual hobbiest.I am using gmail id because My Official mail-id
is not accessible from home. I access Live555 mails from home also. But if
you want I will subscribe to the live555 mailing list using my official id.

I have more queries regarding timestamp. I don't understand the code
in RTPReceptionStats::noteIncomingSR
function. The function is written as:

void RTPReceptionStats::noteIncomingSR(u_int32_t ntpTimestampMSW,


u_int32_t
ntpTimestampLSW,


u_int32_t
rtpTimestamp) {

  fLastReceivedSR_NTPmsw = ntpTimestampMSW;

  fLastReceivedSR_NTPlsw = ntpTimestampLSW;



  gettimeofday(&fLastReceivedSR_time, NULL);



  // Use this SR to update time synchronization information:

  fSyncTimestamp = rtpTimestamp;

  fSyncTime.tv_sec = ntpTimestampMSW - 0x83AA7E80; // 1/1/1900 -> 1/1/1970

  double microseconds = (ntpTimestampLSW*15625.0)/0x04000000; // 10^6/2^32

  fSyncTime.tv_usec = (unsigned)(microseconds+0.5);

  fHasBeenSynchronized = True;

}
I having following confusion:

1. fSyncTime.tv_sec = ntpTimestampMSW - 0x83AA7E80; // 1/1/1900 -> 1/1/1970
      why 0x83AA7E80 is subtracted from ntpTimestampMSW?

2. double microseconds = (ntpTimestampLSW*15625.0)/0x04000000; // 10^6/2^32
Why 15625.0 is multiplied with ntpTimestampLSW and the result is divided by
0x04000000

Please reply.



On Mon, May 10, 2010 at 10:34 AM, Ross Finlayson <finlayson at live555.com>wrote:

>  We are developing a client application using Live555.
>>
>
> Do "we" not have a domain name?  (If you use a "@gmail.com" email address,
> then you are assumed to be a casual hobbiest (whose posts are moderated and
> given a very low-priority), not a "we".)
>
>
>
>  Our application have some specific requirement of converting the rtp
>> timestamp of each video frame to utc time. So I have following query
>> regarding this:
>>
>> 1.The MediaSink class of Live555 gets the presentation time as a parameter
>> in aftergettingframe function.This presentation time can be converted to RTP
>> timestamp using convertToRTPTimestamp function in RTPSink class.I just want
>> to know how this rtp timestamp can be converted to UTC time.
>>
>
> You don't need to know or care about RTP timestamps.  Our software
> automatically converts from presentation time to RTP timestamp (at the
> server end), and then back from RTP timestamp to presentation time (at the
> client end).  This is assuming, of course, that you use RTCP (which is a
> required part of the RTP/RTCP standard).
>
> So, just make sure that you give your streams proper presentation times at
> the server end - and you will get them automatically at the client end.
>  These presentation times must be aligned with 'wall clock' time - i.e., the
> time that you would get by calling "gettimeofday()".  (If "gettimeofday()"
> does not return UTC time, then you will need to apply the appropriate
> conversion at the client end.
> --
>
> Ross Finlayson
> Live Networks, Inc.
> http://www.live555.com/
> _______________________________________________
> live-devel mailing list
> live-devel at lists.live555.com
> http://lists.live555.com/mailman/listinfo/live-devel
>



-- 
Mohita Gandotra
------------------------------
Senior Software Engineer
Canon India Pvt. Ltd., Noida
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