[opendtv] Re: A full explanation of the PSIP time issue.

So, I was off by 2 seconds on a 27 year old time value.  Franky, I couldn't
remember if it was 7, 8 or 9, and I didn't care to look it up, because GPS
representations of time are irrelevant to STT time, although the count of
GPS seconds is important.

Less worse than being off 14 seconds now. 

So, starting on July 1, 1991, you couldn't code STTs using GPS time
displays, and you can't now.  This is before the grand alliance was even
formed, so you are tending now to support my view?

John Willkie


-----Mensaje original-----
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de John Shutt
Enviado el: Thursday, May 24, 2007 6:02 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: A full explanation of the PSIP time issue.

John posted this in his "Challenge" email earlier today:

"Lesson: the trick is getting the right time value: either UTC, or another 
time value plus an integer that corrects that time value to its UTC 
equivalent.  UTC requires only UTC; GPS or other time bases require the time

and a time offset.  Here's another help:  At Jan 6, 1980 00:00:00 UTC, GPS 
displays said Jan 6, 1980 00:00:07."

From all I can find, At January 6th, 1980, 00:00:00 UTC, GPS time was also 
January 6th, 1980, 00:00:00 GPS.  The two did not diverge until 1 July 1981 
00:00:00 UTC, when the first leap second since the inception of GPS time was

inserted into UTC.  Making the equivalent GPS time 1 July 1981, 00:00:01 
GPS.

I'm not sure where he got his "GPS displays said Jan 6, 1980 00:00:07", 
because even in 1980 there had been 9 prior leap seconds inserted into UTC, 
not 7.  Regardless, GPS doesn't care what happened to UTC prior to it's 
epoch date of 6 January 1980 00:00:00 UTC.  That is time zero for GPS and 
only the leap seconds after that date are calculated when converting between

UTC and GPS time.

John Shutt


----- Original Message ----- 
From: "Paul Freeman" <paul@xxxxxxxxxxxxx>


> Hi John,
>
>
>
> I would like to respond to this, but I have to admit that I am having a 
> hard
> time understanding your post. As I see it, Mark Eyer's example STT
> transmitted a system_time field in GPS. At the receiver side I would
> subtract the transmitted GPS_UTC_offset to arrive at a time expressed as
> UTC. This time would then correspond to exactly 8029 days and 13 hours 
> (and
> no remaining seconds). I would then add this amount of time to my start 
> time
> of 00:00:00 Jan 6, 1980 UTC to arrive at the current UTC of 1:00 PM 
> December
> 30, 2001.
>
>
>
> -Paul


 
 
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