); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Just noting a few points made recently on this topic.
Do not confuse the Studio colour subcarrier frequency (4.43 or 3.58 MHz) accuracy - or the digital equivalent clock frequency - with the TV transmitters carrier frequency of transmission. They are totally independent of each other and may, or may not share a "common clock" source. As others have noted, the TV carrier frequency need only be held to kHz, but is (and has to be) held to much tighter tolerances (Hz) for co-frequency (or "offset") operation. Geostationary satellite Doppler observed on downlink signals is far from random. A vectorscope shows this very dramatically. It is entirely predictable and caused by diurnal changes in the satellite's position in it's 3 dimensional "box" resulting in the spacecraft's altitude (Z plane) varying +/- tens of kms about the mean distance of 35,786 kms. This daily oscillation is NOT corrected by the spacecraft's on-board thrusters, neither is the associated "figure 8" wobble in the X-Y plane. Gas-guzzling orbital corrections are used only when necessary, normally monthly. In the analogue PAL/NTSC world, received satellite signals that are re-broadcast at a remote Tx location have their video passed through a 'frame store' which regenerates Fsc, line & frame sync from a locally-generated crystal oscillator, so removing the received incoming Doppler shift. (This has tricked many a country T&F nut who thinks this source of 4.43 MHz is derived from a Rb or Cs source !) Similar issues exist today in a digital TV & analogue transmission chain where the incoming SMPTE multiplex from the studio (carrying an embedded master clock signal from the studio) is decoded and a composite PAL signal generated for transmission. In these instances, the incoming clock may be recovered to lock/steer an oscillator generating the required 4.43 MHz subcarrier frequency. It would be a bold system engineer who relied on the incoming digital multiplex's as his primary clock source to keep his Tx on frequency! As Murray alluded to earlier, GPS technology have made this task simpler and considerably cheaper. When the USA, Australian/NZ & UK analogue transmissions are turned off in 5-10 years time (ha !), most of these "problems" go away I guess. Cheers, Kit VK2LL _______________________________________________ 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.
