[opendtv] Re: A full explanation of the PSIP time issue.
- From: "johnwillkie" <johnwillkie@xxxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 24 May 2007 17:00:48 -0700
Paul;
I've not just written a PSIP generator (EtherGuide Emissary ATSC), I've also
written a transport stream/metadata validator called EtherGuide Ferret ATSC
that ALWAYS (at least, every time so far) finds errors in transport streams
by comparing PSI to PSIP and to elementary streams and against A/65,
13818-1, 13818-2, A/69, A78 and outputs PMCP derived from the data.
Extensive user interface, presentation of data in hex, binary, decimal, and
as "EPG" data. More elegant than TS Reader, but much, much more expensive.
It's one of the reasons I say that I have mathematical proof. I can do this
closed loop: source data (pmcp or converted into PSIP) converted into PSIP
-> received, demuxed, converted into PMCP. Validated against the source
date. Tested against other implementations. Tested (at least tx) against
HP packet analyzers. Also, aside from two or three low-level routines (and
CRC-32 calculation), different code, written in different years, for encode
and decode.
Sorry if it seemed I spoke down to you. Until someone gives me their
bonafides on such a subject, I have to assume minimal PSIP knowledge.
Give me a list of the sets that include your PSIP, and I might be able to
tell you a bit as to how well they conform to the spec. There are quite a
few of PSIP implementations out there on the receive side that are
substandard or worse. I spot errors in encoder implementations, and even
other t&m gear, from time to time.
So, you believe that the UTC time value is to display the time to viewers?
Kindy kludgy to start (always) with UTC time, convert it to GPS, transmit
it, then convert it back to UTC before use, is it not?
Thankfully, my implementations aren't written into silicon.
Come to think of it, I'll send you an invitation to my PSIP list. No PR,
but more than a few PSIP experts. And, a much lower SNR than this list. I
don't think there's been a posting in more than a month.
John Willkie
_____
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Paul Freeman
Enviado el: Thursday, May 24, 2007 4:14 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: A full explanation of the PSIP time issue.
Hi John,
I realize that local time zone is set in the receiver. Just as you have
spent a number of years implementing the transmission side of PSIP, I have
spent a number of years (seven) writing different implementations of the
receiver side of PSIP. PSIP software that I have written has been licensed
by a number of different silicon customers and TV OEMs and is used in
several million TV sets. This level of experience does not mean that I am
right - as we both seem to have significant experience with PSIP and yet
only one of us is right. But I just wanted to point this out to you.
But my point had nothing to do with where the adjustment for local time zone
and daylight savings time is made. My point was to answer your question of
why the PSIP spec would require the broadcaster to transmit the
GPS_UTC_offset field if the system_time and EIT event start time were both
in GPS seconds. And my argument for requiring this is so that the receiver
can compute the correct local time for display purposes, for example in a
channel banner.
-Paul
_____
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] On
Behalf Of johnwillkie
Sent: Thursday, May 24, 2007 5:51 PM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: A full explanation of the PSIP time issue.
Paul;
I KNOW that all times expressed in STTs are UTC. Until I had worked with
the spec for a few years, I believed that the times in STTs were GPS. I was
able to mathematically prove that I was wrong and I confirmed my new
information with several of the authors. I've talked about this with
multiple vendors and they have the same knowledge as I.
I'm reminded that at one time I thought it was unlikely that the length of
the diagonal of a right triangle could be calculated accurately by getting
the square root of the sub of the square of the length of the two other
sides combined. But, I was able to prove it mathematically in eighth grade,
so I haven't had much need to check the math in the meantime to see if
something changed.
I guess you've missed the rather (to me) clear point: if the time in STTs
was GPS, there is NO USE of the GPS_UTC_offset value, because the terminal
would just be able to tell the time by comparing the start_time (32 bits)
with the system_time ()32 bits).
It would be required, if the system time was UTC, since you would only be
able to match the start time of an event by adding system_time plus
GPS_UTC_offset and compare the result with the EIT event start time.
Your comments below evidence an understanding, but you have made no argument
for ANY need of UTC time in the receiver, if the STT and EITs are denoted in
GPS. If UTC is transmitted in the STT and EITs are GPS, then the need is
clear; you add GPS_UTC_offset to the UTC integer to get the GPS integer.
I need to point something out: local time zone is set in the receiver, and
the same correction for the local time zone (minutes and seconds) is applied
to GPS or UTC to derive local time. How to handle this is left up to
implementers, but it's a value that doesn't change during the time a set is
turned on, unless the set happens to be in a moving vehicle that crosses a
time zone boundary while being turned on. Adding or subtracting static
values like this is quite simple; things get fun when you have to add or
subtract dynamic variables on the fly.
So, local time has nothing to do with this 'argument.' Daylight savings time
data is transmitted in the STT only within 30 days of a change, other than
to indicate that DST is being observed or not.
Here's something else that is commonly misread in the spec. It says to
derive UTC, you subtract GPS_UTC_offset from GPS time. It doesn't say that
you need to do this, only that when you want to do that, you follow the
procedure.
But, Bert says that PSIP is simple.
Everything one needs to know about PSIP is contained in A/65 and the
normative and informative references. That doesn't mean that one can figure
out everything on the first dozen reads. Heck, it takes more reads than
that to spot some of the subtle ambiguities. However, that doesn't apply to
this situation, as there is NO AMBIGUITY in the spec on this point, there
are just people who haven't read it enough (and played with the math and
code) to figure it out.
Let me put it another way. After 'grokking' A/65, I found that other specs
were very simple to understand, almost at a glance, because of all the
subtle issues that I've figured out with A/65. (One exception: CEA-608 is
still a bit funky to me.) But then, when I'm reading another spec for the
first time, I tend to focus on step #3 - how do I use it?
John Willkie
_____
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Paul Freeman
Enviado el: Thursday, May 24, 2007 3:05 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: A full explanation of the PSIP time issue.
Hi John,
I think the reason for sending the GPS_UTC_offset is so that the receiver
can correctly determine the time of day for purposes such as displaying a
channel banner and for labeling the columns on an EPG grid display with the
correct start times. (I believe) the STT times are expressed in GPS and (we
both believe) EIT start times are also expressed in GPS within the PSIP
data. But the user wants to see these in times displayed in their local
time. So the receiver must first convert the transmitted GPS times to UTC
time by subtracting the GPS_UTC_offset, and then adjust the UTC time for the
local time zone and local daylight savings time status. Without transmitting
the GPS_to_UTC offset, the receiver would be displaying the wrong values in
the channel banner and grid guide.
_____
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] On
Behalf Of johnwillkie
Sent: Thursday, May 24, 2007 4:55 PM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: A full explanation of the PSIP time issue.
Yes, I meant to correct that. EIT times are GPS time, and I erroneously
stated otherwise in this thread.
However, that just begs the point.
PSIP provides time of day, EPG and time for EPG events.
Since the time in EITs are GPS, and - a false notion - the STT is GPS, just
why is the GPS_UTC_value transmitted in the system time table? And, IT'S
REQUIRED TO BE ACCURATE!
It isn't needed; TV sets and receivers have no need for UTC (in this
argument). And, I can find no other circumstance in PSIP where 8 bits of
unneeded data is required to be transmitted, let alone where unneeded data
is required to have a value other than '1' in each bit position.
Now, that GPS_UTC_offset value is required in the actual PSIP time scenario,
to correct the transmitted UTC to GPS.
Once again, all this becomes obvious once you do the coding of just a PSIP
encoder.
John Willkie
_____
De: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] En
nombre de Paul Freeman
Enviado el: Thursday, May 24, 2007 1:46 PM
Para: opendtv@xxxxxxxxxxxxx
Asunto: [opendtv] Re: A full explanation of the PSIP time issue.
I am having a hard time concluding from the A/65C spec how the system_time
field in the STT is anything other than GPS time. I do not see how the
conclusion has been reached that this field represents UTC time, and so far
have been unable to understand the arguments that have been presented in
favor of the syste_time field representing UTC time.
But I have seen some of the posts claim that the EIT information is UTC
time, and nobody has refuted that yet. However, from my reading of the spec,
even though the EITs change at 12:00, 3:00, 6:00, and 9:00 UTC time (not GPS
time), the actual start times listed in the EITs are themselves expressed in
GPS time. That is, the "start_time" field in the EIT is described on page 44
of A/65C as "A 32-bit unsigned integer quantity representing the start time
of this event as the number of GPS seconds since 00:00:00 UTC, January 6,
1980".
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