Re: [time-nuts] TV sync
); 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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] TV sync
Hi Tom, a related very interesting experiment is to take the Horizontal Sync signal from an old Tube-type TV, and feed it to a frequency counter with GPSDO time base (preferrably). Got to be careful about the high voltages inside the TV though! BTW: it's not 15374KHz, there are some significant decimal digits (I don't recall the formula to calculate the frequency exactly, but it was the total number of frame lines multiplied by 59.94Hz or so). Then you can test how accurate the broadcasters' 27MHz reference clock is. One would be surprised how inaccurate(!) some broadcasters are, and how much drift some Satellite providers have. A lot of the stability is dependent on the receiver of course, and how well the receiver's VCXO locks to the Broadcaster. bye, Said wrong 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. Satellite broadcasts are completely different and I do not know the FCC specification for those. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] TV sync
); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY - Original Message - From: Jeffrey Pawlan [EMAIL PROTECTED] To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Sunday, October 07, 2007 6:03 PM Subject: Re: [time-nuts] TV sync Hi Tom, a related very interesting experiment is to take the Horizontal Sync signal from an old Tube-type TV, and feed it to a frequency counter with GPSDO time base (preferrably). Got to be careful about the high voltages inside the TV though! BTW: it's not 15374KHz, there are some significant decimal digits (I don't recall the formula to calculate the frequency exactly, but it was the total number of frame lines multiplied by 59.94Hz or so). Then you can test how accurate the broadcasters' 27MHz reference clock is. One would be surprised how inaccurate(!) some broadcasters are, and how much drift some Satellite providers have. A lot of the stability is dependent on the receiver of course, and how well the receiver's VCXO locks to the Broadcaster. bye, Said wrong 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. Satellite broadcasts are completely different and I do not know the FCC specification for those. Actually, according to ITU Recommendations, requirement for horizontal scanning frequency is only 2 parts in 10^7, and +- 1 Hz for chroma frequency, at least for 625 lines PAL standard. Regarding co-channel interference, an offset to the vision carriers frequency can be applied between two transmitters; the offset steps are in 12th fractions of the horizontal scanning frequency, i.e. every 1302 Hz, for the 15625 Hz standards. If we choose, for example, a 3906 Hz offset between the two carriers, we have a visual perception threshold of the interference, on the TV screen, of 36dB between carrier levels, but if no control of the frequencies exists we can need as much as 60dB between carriers for no visible interference appreciation on the screen. This level of protection needs a +-1 Hz frequency stability requirement for the relative offset. Regards, José, EA1PX ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
