dan@rna.UUCP (Dan Ts'o) (10/01/84)
Hi, The discussion of using the Heath WWV receiver as a clock brings a question to my mind. Does such a receiver used as a computer clock really win over using the 60Hz line frequency ? Although for any short period of time, if the receiver picks up WWV, its time keeping should be more accurate than using the 60Hz, it is my understanding that the power companies strive to make the long-range accuracy of the 60Hz signal very good. Whereas with the WWV receiver, there would be no mechanism to insure that time losses would be balanced by time gains. My current experience is that digital clocks operating off the 60Hz are more accurate on a month by month or year by year basis than most crystal-controlled watches. Cheers, Dan Ts'o ...cmcl2!rna!dan
henry@utzoo.UUCP (Henry Spencer) (10/02/84)
> The discussion of using the Heath WWV receiver as a clock brings > a question to my mind. Does such a receiver used as a computer clock really > win over using the 60Hz line frequency ? Although for any short period of > time, if the receiver picks up WWV, its time keeping should be more accurate > than using the 60Hz, it is my understanding that the power companies strive > to make the long-range accuracy of the 60Hz signal very good. Whereas with > the WWV receiver, there would be no mechanism to insure that time losses > would be balanced by time gains. My current experience is that digital clocks > operating off the 60Hz are more accurate on a month by month or year by year > basis than most crystal-controlled watches. This says more about the cruddiness of crystal-controlled watches than about the accuracy of 60 Hz. The 60 Hz signal normally is controlled by reference to a precision crystal oscillator, better than the one in watches but still just a crystal oscillator. The WWV signal is derived directly from the atomic clocks that are *the* time standard for the US and are part of the collective time standard for the world. You just do not get timekeeping that is better than that. The National Bureau of Standards most assuredly puts more effort into accurate clocks than your local power company does. *Provided* that the Heathkit clock reliably receives and decodes the WWV signal -- not entirely a safe assumption, as DMR has related -- it's definitely a better time source. -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry
darrelj@sdcrdcf.UUCP (Darrel VanBuer) (10/02/84)
Power line frequencies are generally pretty accurate (long term), in part
because they use the National Bureau of Standards as a reference (e.g. WWV).
It's essential to have consistent frequencies when you run a national
power sharing grid. Then can notably run slow during high load conditions
(e.g. hot summer afternoons, but will (usually) eventually catch up.
WWV provides standards for both power and frequencies (their carrier frequencies
are all exact multiples of 2.5 MHz.
The long term stability of WWV comes from a battery of atomic clocks with
accuracies approaching 1 in 10^12 (and short term stability better by a couple
orders of magnitude.
The big advantage of WWV over any power line clock is with the proper
equipment (Heath clock or voice recognition -- your head) it's an absolute
standard, rather than relative.
We had a serious problem with a powerline clock on our PDP 11/70 after
our motor-generator power conditioner was installed; it ran about 58 Hertz
output, so our processor ran 40 minutes off each day!
If you ever want a really accurate, cheap frequency standard, try your
color TV. For the color to come out right, it has to stay quite close to
the color burst frequency at the transmitter, so is resyncronized every
60 microseconds to keep it within a tenth of a cycle, or so. The major networks
maintain theirs against NBS. (it's 315/88 MHz).
--
Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
!sdcrdcf!darrelj
VANBUER@USC-ECL.ARPAjbn@wdl1.UUCP (jbn ) (10/04/84)
The Heath WWV receiver is not just counting cycles of a waveform;
WWV is actually sending the time and date in a suitable format. So
this is an absolute reference, not a relative one.
For those of you who do not know what WWV is, a brief explaination is
in order. WWV is a radio station operated by the US Government National
Bureau of Standards. WWV broadcasts various time signals on different
frequencies. The time broadcast is controlled directly by multiple
cross-checked cesium-beam atomic clocks at the WWV site in Boulder, Colorado.
These are in turn checked against the United States Master Clock at the
U.S. Naval Observatory in Washington (composed of over a dozen cesium-beam
atomic clocks cross-checking each other). Cumulative error in WWV signals
is under 300 microseconds per century.
WWV time is defined relative to the Earth's rotation, and the rotation
of the earth is slowing down; the Naval Observatory measures this and
every few years WWV inserts a ``leap second'' to compensate. When this
occurs, the time sent by WWV jumps one second exactly. These adjustments are
made only at the beginning of a year. (Exactly at the beginning of a year,
0000Z UT, 1 January).
Receivers which obtain WWV signals on bands reflected off the ionosphere
(these are the short wave bands) are subject to some drift due to ionospheric
height variations; this drift is measured in tens of milliseconds but the
long-term drift is zero. There are also satellite signals which can be used
for timekeeping and navigation; these are also very tightly controlled.
Unless you have some very exotic timekeeping application requiring
millisecond-level syncronization at distant points, the Heath clock is
fine.rpw3@redwood.UUCP (Rob Warnock) (10/04/84)
Get a voice-recognition card for your computer, and let it call the local dial-a-time number... ;-} No, seriously folks,... +--------------- | If you ever want a really accurate, cheap frequency standard, try your | color TV. For the color to come out right, it has to stay quite close to | the color burst frequency at the transmitter, so is resyncronized every | 60 microseconds to keep it within a tenth of a cycle, or so. The major | networks maintain theirs against NBS. (it's 315/88 MHz). +--------------- True, but there are some things you have to watch out for. (This comes from memory of an old Scientific American article in the Amateur Experimenter column, something on frequency standards.) First and foremost, you must be watching a DIRECT network telecast (that is LIVE from the network, whether or not the program material itself is live or recorded). "Network" programs which are taped by local stations and re-broadcast later use the crystal oscillator in the local station, not the secondary rubidium standard used to sync up the network. The accuracy required (and maintained) by the local station is MUCH less. (Instead of 315/88 Mhz = 3579545.5 Hz, the usual standards, RS-170A and NTSC, call for 3579545 Hz +/- 10 Hz.) Next, 3579545 Hz is not a convenient multiple or submultiple of anything you might want to use for time (such as a 1 or 10 Mhz crystal), so what you have to measure is the DRIFT of phase of the color burst w.r.t. some local oscillator. (The Scientific American article went into some extensive explanations of how to do this.) (I believe the networks do not "correct" their rubidium clocks, but only check periodically what the offsets and drifts are from NBS. In fact, the nets might just use a WWV ==> Heath clock to get the daytime... ;-} ) Then, as a result of the preceding points, you will actually want to use an oven-controlled crystal oscillator as your local tertiary (NBS = primary, network = secondary) reference, the sort of free-standing 1 or 10 Mhz units amateur radio operators use for frequency calibration. (This is good, because such crystals can hold fairly good short-term stability across "Network Trouble On The Cable".) Lastly, this still only gives you a frequency (delta-time) reference. You must obtain an "absolute" reference somehow. ("I know what a second is, but what is TODAY?") So it's back to the Heath clock, or some other radio, or, you can carry an atomic (cesium or rubidium) clock to Boulder and get NBS to calibrate it for you. ;-} Misc. points: On black-and-white sets, the color sub-carrier frequency is in the passband of the normal video, and strongly colored area would therefore have strong stripes in them, since the color hue/saturation is sent with double-sideband modulation. But the horizontal frequency DURING COLOR TRANSMISSIONS is not the usual 15750 Hz, but EXACTLY color-burst * 2 / 455 or ~15734.264... This means that alternate horizontal lines will have the stripes in alternate dot positions AND the dots/stripes will not wander over the screen. This causes strongly colored areas to show up as a sort of halftone stipple (45-degree "stripes") on a black-and-white set, which is not supposed to be noticable. Since there are still 525 / 2 lines in an interlaced frame, the vertical rate is not 60 Hz but ~59.94... Hz. This can cause slowly rolling dark bars on black-and-white sets with poor power supplies (the power grid IS exactly 60.00... Hz ). (Hmmm... let's move this to net.tv.hardware...) Rob Warnock UUCP: {ihnp4,ucbvax!amd}!fortune!redwood!rpw3 DDD: (415)572-2607 [ until 10/5/84: (415)369-7437 ] Envoy: rob.warnock/kingfisher USPS: 510 Trinidad Ln, Foster City, CA 94404
jbn@wdl1.UUCP (jbn ) (10/06/84)
For those of you with Internet access, the site DCN1.ARPA (Comsat Corporation) has a clock synchronized to WWV via satellite, and offers an Internet time server. The 4.2BSD command "rdate DCN1.ARPA" will synchronize one's local clock to WWV within a few seconds (network transit time) of accuracy.