At 12:40 AM 1/27/96 -0500, Dave Emery wrote:
A peripheral I've long wanted to see, commonly available: ACCURATE time, broadcast to the millisecond/microsecond/nanosecond, available from sources as varied as TV VIR's, FM subcarriers, and other sources, available as an easy input (via a peripheral card) to a computer.
The only technology that I'd trust to be useful much below 10-100 ms is GPS. The others are unlikely to be controlled well enough at the source to be trusted.
What about Loran? WWV(B)? Receptor-type signals? Now, I agree that CURRENTLY few people "depend" on those other (non-GPS, non-Loran, non-WWVB) systems, but to some extent that's a "chicken and egg problem"
Current TV broadcasting, for example, usually involves multiple passes though digital frame stores and time base correctors - most homes get the signal via cable which itself involves significant uncontrolled delays (juat the thermal changes in propagation delay in a long CATV cable and amplifier chain due to weather changes run into the many microseconds).
Perhaps, but I'm assuming broadcast tv. Single source. Limited variability in path length. I admit there are limitations; my argument is that the signals SHOULD contain accurately-defined points, even if it is only one per frame.
And humans beings being as imperfect as they are, it is hard to beleive that making sure that the time being broadcast is really kept accurate is going to be a priority when most people use it for purposes that require plus or minus a few seconds timing.
I think that if there WERE some reliably-available timecode system, plus a cheap single-chip system to drive it, it WOULD be kept reliable enough because of demand.
I have a 12-year-old Heathkit "Most Accurate Clock" that I assembled myself, and had the foresight to install it with its computer interface option. (receives 5, 10, or 15 MHz signals broadcast from Boulder, Colorado, containing "exact" time.)
While I've never taken the time to connect it to my PC, it provides (through an RS232 jack) correct time with a rated accuracy of about 5 milliseconds, as I vaguely recall. (Even has a dipswitch setup on the bottom to tell it how many 500 mile increments you are away from WWVB... corrects for delay to a first order of magnitude.)
WWVB is the 60 khz broadcast (which is more accurate due to more stable propagation)
Not much of a difference, given the context. For example, I'm probably 1000 miles away from Boulder; it is highly unlikely that the path length differences for the HF bands could exceed about 100 miles, or about 0.5 millisecond. Given the context, it's accurate enough for anti-spoofing work in networks. . the HF ones are WWV. Commercial time receivers
are available that work off the 60 khz time code (very narrow bandwidth ASK), but the 60 khz is most used as a standard frequency for long term tracking of error in local standards.
Any more? I don't think so. GPS has probably pretty much taken over as the "gold standard" for clock synchronization, I suspect. Path length is known, by definition, and the resolution must (as a consequence of the distance accuracy requirements) be in the low-nanosecond level.
(BTW, if anybody knows how to easily connect it to the pc, or has the appropriate software, please tell me The task isn't difficult from a hardware standpoint; it's just RS-232 serial ASCII timecode at about 9600 bps which either continuously retransmits or on request. The problem is the software:
If you run unix
Nope.
there are some quite sophisticated programs that can use this specific clock (connected to a serial port) that allow sync to the full accuracy possible at good times of day (around 1 ms). The programs also allow time distribution to other computers on a network - thus their name - ntp - which stands for network time protocol (and the network time program that implements it). This protocol and the various unix programs that implement it are quite widely used on commercial LANs and the Internet to sychronize time amoung unix workstations, servers, and bridges and routers. Current implementations are capable of tracking clock oscillator error on a system and adjusting the time periodically to compensate for the frequency error of the clock and even to predict (polynomial approximation) the change in frequency error with time.
The man behind much of this (at least the early research) is Dave Mills who used to be at louie.udel.edu which hosted a ftp site for the programs. An archie search will reveal where they are kept now, and there is a newsgroup (comp.protocols.time.ntp) for this which no doubt has a substantial faq file about this.
Thanks for the reference.
(Then again, there are those "Receptor" watches which have (at least) similar accuracy, which as I understand it work on FM subcarrier principles.)
Yes they use the RDS broadcast on the 57 khz subcarrier for this. Of course there is no certainty the station has the clock set accurately.
Chicken and egg, again. I assume that any radio station can afford $300 for a GPS receiver that can put out time accurate to 1 microsecond. If enough people start USING such broadcasts, they will be considered NECESSARY and will be maintained. The Receptor watch is an excellent interest-developing product to assist in this problem; the only problem might be that errors of greater than the 5 msec spec'd are not necessarily immediately apparent to the common watch-on-wrist user.
TV stations could be made to maintain a local clock sync'd to GPS and use that to do the final level of clocking out before feeding the transmitter and could thus ensure that some reference point in some frame happened at an exact time, but given that a user who can see a TV signal can probably see GPS signals and can do the same timekeeping himself for a couple hundred bucks it hardly seems worth it any more. I do expect that time codes with modest accuracy (few tens of ms at best) will become common as part of the Starsite (or whatever they call it now) program guide distribution on PBS, simply because this has defined a format that can conveniantly contain time messages multiplexed with other data and the box displays the time. DSS and VC-II both also have this capability, but of course the uncertainty of the satellite delay limits accuracy and neither has provisions for providing time to other devices.
This is possible, but I bet the variations in phase in the local distribution system due to power factor, choice of phase to use, propagation time through transmission lines and substations and so forth would mean that phase as observed at two distant sites was rather random and maybe even subject to shifts over time as load conditions varied.
I'm hoping some HV engineer will make a comment as to this factor.