Alan Larson wrote: > Hi Dave, > > Since the application for ATSC broadcasting seems to be mostly setting > the clocks in the receiver's equipment, the 232 picoseconds of NTP seems to > rather far beyond what is needed. After all, 232 picoseconds is just under > 7 centimeters at light speed. Thus, broadcasting the time of day (to set > the clock in your TV recorder) to that resolution seems a bit excessive. > > 0.01 second seems adequate to me to not miss the program. > > The problem as I see it is that the sources are way bad (as you note). > > Clearly, NTP would be a useful and available way of keeping a machine > transmitting time on a broadcast signal in good sync. > > With ATSC, there seems to be some difficulty with daylight saving time -- > when the time advanced this spring, I noticed that the electronic program > guides apparently did not advance. Attempts to set the clock on my ATSC > receiver from over the air signals got various times which did not match > local time. I eventually gave up and went back to the recommendation in > the addendum to the manual to manually set it to local non-DST time, with > the timezone set. That gets the program guide to match, but the clock of > course reads an hour slow. > > It is unclear whether the Asians who programmed the thing didn't > understand time, or whether the standard is that completely flawed that it > doesn't account for properly dealing with transferred times in UTC and > converting for display. (Though, transmitting daylight saving time info in > the WWVB style seems a good idea, given the government's willingness to > fiddle with it and almost certainly break lots of things with built in > advance/fallback calculations.) > > > Precision is not a solution for lack of accuracy. > > Neither precision nor accuracy will make up for implementation errors. > > > Alan > > > p.s. Doesn't CBS news still do the ping at the top of the hour? I think > they are still coming out from KCBS here. (I don't know what they do about > satellite delay of the feed (signal travel time, and any digital coding/decode > times). I have no idea how they will deal with the variable time delays of > IBOC digital processing. > Alan
Be aware that, at least in LA, some stations put the 'ping' ahead of an 8-10 second delay. Realistically, do you even need to be more accurate than a frame of video? I think 0.01 seconds is 10-100 times better than it needs to be. We're not trying to pinpoint the position of the news helicopter, just start your DVR without missing the head of the show. The phone company time is sufficient for that. I suspect the times are 'way bad' because it isn't automatically loaded yet and the stations aren't getting any complaints and those guys have plenty to do withouit setting a clock almost nobody checks. Personally, I set the times and record times manually with no problems. GG _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
