); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Jeffrey Pawlan said the following on 10/07/2007 12:03 PM:
> All larger TV stations use Cs standards. What you don't know is that the FCC > assigns SLIGHTLY different scan frequencies to each station on the same > channel in a close area so when you are in a fringe area between two stations > on > the same channel, you will intentionally see both pictures superimposed with a > rolling of the scan lines. Otherwise they would be locked and you would see > only black and white bars. (Sorry this is replying to an old post; I'm just catching up after being out of town.) This is a little different than my understanding. I thought that the carrier frequencies, not the scan frequencies, were offset. The options were either on frequency, +10khz, or -10kHz. In fact, one of my friends does TV DXing and he identifies stations by their carrier offset. When I was a kid, I lived in an area where co-channel interference was very common in the summer. We had channel 2, 5, and 11 in Green Bay, Wisconsin, and all three of those channels were also used in Chicago, about 200 miles down Lake Michigan. We lived about 70 miles north of Green Bay, and both sets of stations were within the beamwidth of our TV antenna. Due to ducting effects along the edge of the lake, in the summer channel 2 was unwatchable much of the time, channel 5 sometimes, and channel 11 a couple of times. Back in the '60s, an attempt was made to reduce this problem by controlling the carrier frequency much more tightly than usual. This may have been before the -10/0/+10 offset scheme. I believe the goal was to keep the two carriers so close in frequency that the beat frequency would be a small fraction of a Hertz. In any event, the Green Bay and Chicago stations bought very stable oscillators and WWVB tracking receivers. The experiment lasted a few years and didn't make much difference. Ultimately, all the high-stability stuff was scrapped and they went back to the standard oscilltors. As for us, when the interference got bad we turned the antenna to watch stations in the Upper Peninsula of Michigan. Regarding scan frequencies, back in the old days, the scan and colorburst were controlled by the networks, and Rb oscillators were used for stability. The FCC monitored the network stations and reported on their offset (using the colorburst). However, with digital techniques things like frame synchronizers came along and the ability to trace sync or colorburst back to the networks disappeared. Today, there is a master clock in each station, but its quality is an unknown factor; almost certainly, it's not calibrated against anything except whether it's close enough to make everything sync up (this last bit was passed on to me by a local TV station chief engineer). John _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
