On Sunday, June 03, 2012 12:38:19 AM Charles Steinkuehler did opine: > On 6/2/2012 8:02 PM, gene heskett wrote: > > We used to be proud of saying our color subcarrier frequency was > > 3.57954545454545454545454545 megahertz until you ran out of wind to > > say the 45. Actually there was about 12 digits that were good but > > it started to get a little fuzzy after that. ;-) I think you can > > buy a serviceable one for about $1000 now. > > Very impressive (even today!). > > ...but that must have been some time ago. I design video equipment > for my day job, and the subcarrier frequency has been rounded off to > 3,579,545.0 Hz +/- 10 Hz for as long as I can remember, at least in > the international standards. The (old) US documents show a reference > 5 MHz signal x 63/88 which gets your .5454 repeating, but the newer > standards base everything off the defined subcarrier frequency > (nothing after the decimal for the US television standard). > > But none of that really matters any more. Now that over the air TV > transmission has gone full digital, you can no longer use your local > network affiliates as a free timing reference good to a couple parts > per million (or better...most of our local stations were easily within > 1 Hz of the reference ~3.58MHz spec). :( > > Then again, these days I guess that's what GPS satellites are for. :)
But for precise timing, the satellite signal sucks, big big time. Problem? The GPS satellite has an assigned orbit, but they are LEO, not Clark Belt. In any event, the doppler motion caused by the LEO is considerable since it can be moving at a relative to you, velocity of something over 17 thousand mph. The answers you get from a quality GPS are actually derived from the signals of several birds who may or may not be equidistant from your location, and you get the average, which is at least an order of magnitude better than you would get from just one bird. Even the geosync birds can move fast enough in their orbital box that despite being fed with one of those standards, the color subcarrier frequency coming back down was well outside the commishes rules, 15 hz or worse just from the doppler as it moves. You can watch that very easily with a vectorscope. Synched to the station generator, you can watch the birds move, and they'll wind the pattern back and forth as the day progresses. So if you wanted to calibrate the station generator against the network Rhubidium standard, you would have to rig an updown counter, zero the diff at noon, and freeze the diff a fraction of a second before noon the next day. A long, tedious process & I had better things to do, so I bought an HP counter with the super stable timebase & used that. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> <Knghtbrd> I SNEAK TO BUN <Knghtbrd> HELP ME FOR TO QUACK <Venom> kb: what the hell are you talking about? <Knghtbrd> bwahahaha.. It's a long story. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
