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

From: "Kit Scally" <kscally@bytecan.com.au> Subject: [time-nuts] TV Frequency control - slightly O/T. Date: Tue, 9 Oct 2007 09:28:55 +1000 Message-ID: <001a01c80a02$feb3d5b0$3601a8c0@BYTECAN.com.au> > ); SAEximRunCond expanded to false > Errors-To: time-nuts-bounces+magnus=rubidium.dyndns.org@febo.com 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. Good point. I ment to comment on that. > 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. For analogue TV systems, that may be true... yes. > 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. It should be mentioned that when the 8 meter parabols is not doing anything "usefull" then train themselfs on the figure of 8 patter of various satelites so that they have a recent number. > 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 !) Most frame-stores I've seen either have a PLL or is externally timed. > 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. It has also made the TV broadcasting considerably more vunreble. The DVB-ASI interface (which is used to bring the MPEG-2 Transport Stream to the transmitter) is a mess timingwise, and no-one in their right mind would use THAT for their carrier timing. It could have been good, but reality speaks against it. With the use of Single-Frequency Network techniques, there is need for PPS and 10 MHz at all transmitters. This is used both for SFN timing and carrier frequency. Neither the DVB-ASI or SDI would allow the required timing and stability. The SFN advantage makes system engineering of a national broadcasting network easier, but it also raises new problems. There are countries in which GPS can not be expected to work on a continous basis. This brings "interesting" challanges for those doing modern broadcasting networks. There are naturally solutions to be applied. GPS have started to become a real system-hazzard for some. It has been too easy to just apply. The backside of the coin have now started to show. Oh, some of the new DVB-T transmitters even transmitt on channels so they may locally kill the GPS signal on their overtone. :) > 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. Well, in the meanwhile we are doing it here. We are learning from the mistakes as we go along. Cheers, Magnus