Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site redwood.UUCP Path: utzoo!linus!decvax!decwrl!amd!fortune!foros1!redwood!rpw3 From: rpw3@redwood.UUCP (Rob Warnock) Newsgroups: net.unix-wizards Subject: Re: Almost Accurrate Clock Message-ID: <56@redwood.UUCP> Date: Thu, 4-Oct-84 05:35:03 EDT Article-I.D.: redwood.56 Posted: Thu Oct 4 05:35:03 1984 Date-Received: Fri, 5-Oct-84 20:43:39 EDT References: <297@rna.UUCP> <1356@sdcrdcf.UUCP> Organization: Rob Warnock, Redwood City, CA Lines: 69 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