[time-nuts] Re TV sync

Mon Oct 8 13:36:07 EDT 2007

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-----
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Sent: 08 October 2007 18:00
To: time-nuts at febo.com
Subject: time-nuts Digest, Vol 39, Issue 12

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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 <jpawlan at pawlan.com>
Subject: Re: [time-nuts] TV sync
To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Message-ID:
	<Pine.GSO.4.31.0710070855490.4611-100000 at jeffrey150.pawlan.com>
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" <jpradoes at telefonica.net>
Subject: Re: [time-nuts] TV sync
To: "Discussion of precise time and frequency measurement"
	<time-nuts at febo.com>
Message-ID: <002701c8092d$51ee5010$2201a8c0 at PC2>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
	reply-type=original

----- 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
>> 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

------------------------------

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