[time-nuts] TV sync
jpradoes at telefonica.net
Sun Oct 7 17:59:22 EDT 2007
----- Original Message -----
From: "Jeffrey Pawlan" <jpawlan at pawlan.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
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
>> from an old Tube-type TV, and feed it to a frequency counter with GPSDO
>> 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
>> recall the formula to calculate the frequency exactly, but it was the
>> number of frame lines multiplied by 59.94Hz or so).
>> Then you can test how accurate the broadcasters' 27MHz reference clock
>> One would be surprised how inaccurate(!) some broadcasters are, and how
>> drift some Satellite providers have.
>> A lot of the stability is dependent on the receiver of course, and how
>> the receiver's VCXO locks to the Broadcaster.
> All larger TV stations use Cs standards. What you don't know is that the
> 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
> 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, José, EA1PX
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