); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY From: "Kit Scally" <[EMAIL PROTECTED]> Subject: [time-nuts] TV Frequency control - slightly O/T. Date: Tue, 9 Oct 2007 09:28:55 +1000 Message-ID: <[EMAIL PROTECTED]>
> ); 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. 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 _______________________________________________ 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.
