[opendtv] Re: XDS and ATSC
- From: John Willkie <johnwillkie@xxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx, opendtv@xxxxxxxxxxxxx
- Date: Mon, 21 May 2007 16:30:19 -0400 (EDT)
-----Original Message-----
>From: John Limpert <john.a.limpert@xxxxxxxxx>
>Sent: May 21, 2007 6:24 AM
>To: opendtv@xxxxxxxxxxxxx
>Subject: [opendtv] XDS and ATSC
>
>I've been reading some web pages on the "autoclock" technology used by
>PBS to distribute time to consumer VCRs and television receivers. They
>use XDS packets to distribute time in their NTSC signal. Is there a
>similar capability in ATSC?
>
Actually, John, there is superior technology for transmitting time in ATSC
PSIP. It's called the system time table (a somewhat similar function is
available in DVB-SI and various SCTE specifications.)
It's superior because, unlike XDS, it nothing has higher priority access to the
bandwidth. XDS is teritiary at best with regards to priority, since caption 1
and caption 2 always have higher priority. Hence, it's not always possible to
send the XDS data to represent a particular second.
PSIP permits the STT to be sent multiple times a second, if one desires.
Your query brought up comments from Bert and John Shutt that are just plainly
wrong, and I won't refer to them except to say that
THE FIRST PERSON WHO CAN PROVE THAT ATSC A/65 PERMITS THE TRANMISSION OF GPS
TIME VALUSS WILL RECEIVE $100 FROM JOHN WILLKIE. THE FIRST PERSON WHO CAN
PROVE THAT ATSC A/65 REQUIRES THE TRANSMISSION OF GPS TIME VALUES WILL RECEIVE
$100.
This offer expires when/if the ATSC modifies the System Time Table section of
A/65. I need not worry about my bank account: this type of challenge helps
eliminate pockets of ill informed commentary.
ATSC A/65 (which I havent read in pertinent part this year, but which I know
much better than Bert and John Shutt) requires stations to transmit current UTC
time as a 32-bit unsigned integer, which I call "ATSC_seconds." There is also
a field in the STT that permits sending an 8-bit unsigned integer that is added
to ATSC_seconds to determine GPS time.
The current GPS_UTC_value is 14. One sign of a bad PSIP implementation is if
the start time of various programs has a second value of 14 (or whatever the
PSIP generator GPS_UTC_offset value is set to). This incorrect, since program
times are always in UTC.
What trips mere mortals is what the 32-bit time value represents. It is the
count of GPS seconds since January 6, 1980. You may note that is the date that
GPS became operational.
The reason the 32-bit value is the count of GPS seconds is a simple one:
ATSC_seconds always have to be continuous and increment by one for each second
elapsed. Counting UPS seconds won't work, since every couple years, the time
count becomes discontinuous for one second when the GPS_UTC_offset is
incremented by one. (This is done because the GMT/UTC time standard is not as
accurate as the one used in GPS.)
So: PSIP NEVER, NEVER, NEVER gives GPS time; it can only use UTC time and give
a value that is used to derive GPS time. (If everything is set correctly.) To
employ a consistent time base, PSIP generators must provide at least once per
second a count of the number of GPS seconds that have elapsed between UTC
January 6, 1980 and the current time in UTC.
BTW, stattions get fined for not having accurate time in their STT. ATSC A/65
requires an accuracy within one second. I should also note that time of day
from the phone company is not an accurate time reference: it can be up to 10
seconds off, in my experience. I've checked hundreds, if not thousands of
times.
John Willkie
++52 664 290 7625
>
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