); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY
Ah-ha! Satellite broadcasters have poor stability - not necessarily so. Many years ago (about 1978, actually) we were sending a signal from Johannesburg to UK by satellite. In our control room, we could see both the signal as sent, and as returned from satellite. Comparing the phase of the two (vectorscope locked to local, looking at satellite return) a constant rotational phase change was seen. One of our brighter guys did some quick maths, rang the satellite control people in the US, and said "Did you know that your satellite is moving out of position at x meters/second?". the answer was "Yes, that's within normal parameters, it gets corrected, but how the hell did you know?!" So the "fixed" geostationary satellites aren't that fixed, and it turns out that one of the major factors in deciding the life of a satellite is the quantity of fuel left in the tanks for the positioning jets. And yes, the subcarrier frequency will suffer from random Doppler effects. Paul Galpin ZS2PG -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED] Sent: 08 October 2007 18:00 To: [email protected] Subject: time-nuts Digest, Vol 39, Issue 12 Send time-nuts mailing list submissions to [email protected] To subscribe or unsubscribe via the World Wide Web, visit https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts or, via email, send a message with subject or body 'help' to [EMAIL PROTECTED] You can reach the person managing the list at [EMAIL PROTECTED] When replying, please edit your Subject line so it is more specific than "Re: Contents of time-nuts digest..." Today's Topics: 1. Re: TV sync (Jeffrey Pawlan) 2. Re: TV sync (Jose Manuel) ---------------------------------------------------------------------- Message: 1 Date: Sun, 7 Oct 2007 09:03:45 -0700 (PDT) From: Jeffrey Pawlan <[EMAIL PROTECTED]> Subject: Re: [time-nuts] TV sync To: Discussion of precise time and frequency measurement <[email protected]> Message-ID: <[EMAIL PROTECTED]> Content-Type: TEXT/PLAIN; charset=US-ASCII > 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. ------------------------------ Message: 2 Date: Sun, 7 Oct 2007 23:59:22 +0200 From: "Jose Manuel" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] TV sync To: "Discussion of precise time and frequency measurement" <[email protected]> Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original ----- Original Message ----- From: "Jeffrey Pawlan" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <[email protected]> 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, Jos?, EA1PX ------------------------------ _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts End of time-nuts Digest, Vol 39, Issue 12 ***************************************** _______________________________________________ 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.
