Hi, Jose's correct. Co-channel TV stations are accurately offset by a fraction of the line frequency. Interestingly enough (and for this reason) independent TV stations often have very accurate transmitter frequencies, while potentially having quite poor (1e-6 or so) control of the picture timing, while larger stations (without co-channel problems), are the other way around.
In larger TV networks, the picture timing is maintained with very high precision. This allows very clever mixing desks at studios hundreds of km apart to fade between pictures from different sources, without even a change in picture hue, never mind a picture shift or roll. I understand that our biggest network, TVNZ, uses Trimble Thunderbolt GPSDO units and Neve digital desks, which can digitally advance or retard video to allow for the transmission delay between sites, which the desks negotiate between each other. In my shack, in honourable retirement, is TVNZ's first network reference - a venerable HP 5065A Rubidium, dating from about 1975. This unit has the 4.43361875MHz colour subcarrier option. The divider in this option (from the days before DDS) requires the 5MHz to be offset slightly for the colour subcarrier to be correct. It last had a new tube in 1978, and is still running fine, although the power supply required an overhaul when I first got it. Electrolytics had dried out, and the resulting hum confused the oven controller terribly, it overheated and blew the protection fuse. Regards, Murray ZL1BPU _______________________________________________ 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.
