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frequency standard from color TV signal?

Discussion in 'Electronic Design' started by Ancient_Hacker, Apr 17, 2007.

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  1. IIRC there are very accurate signals in your basic color TV off the
    air signal. Now the question is it the 3.58 MHz color subcarrier or
    is it the 3.545... color burst? What about the 4.5MHz sound
    subcarrier? And how accurate are they?

    Any info appreciated.
     
  2. http://msl.irl.cri.nz/services/time/
    Seems pretty good, I assume NTSC is similar, but if your video comes
    via digital and gets converted to composite video in the STB, all bets
    may be off. And the way analogue Txs are being shut down in favour of
    digital.....

    Have a browse at SMPTE, they may even have the specs for free


    martin
     
  3. Guest

    in the old days, when transcontinental microwave was the feed from NY
    or LA to the station, things were excellent, timing being generated
    by a network's master cesium clock. Now each station gets a digitized
    fiber feed and its ran through a buffer, so unless for some reason
    that station decides to have a rubidium master clock for its timing
    chain, your probably screwed.

    some smaller countries still use microwave and things are good, but in
    the us and mainland europe, its on a station by station basis. Big
    stations (NY,LA,chicago) probably have a decent clock.

    Steve Roberts
     
  4. Don Bowey

    Don Bowey Guest

    Most multiplexing today is SONET. Network sync traceable to Stratum 1 is
    available from the multiplexer.
    Don
     
  5. Any time standard that depends on transmission through the atmosphere
    must deal with the variabilities of the medium. Unless your receiver
    is in direct line of sight from the transmitter, the effective length
    of the transmission channel is not constant.
     
  6. Guest


    The color sub-carrier and the colorburst refer to the same frequency
    in NTSC. 3.579545 MHz.
     
  7. Tom Bruhns

    Tom Bruhns Guest


    The nominal colour subcarrier frequency for NTSC is 315/88 MHz; you
    can do some multiplication and division and get to one of the
    "standard" reference frequencies like 10MHz = colour subcarrier *
    176/63 = c.s.c.*2^4*11/(3*3*7). The FCC tolerance is +/-10Hz, last I
    knew; heaven only knows under deregulation. Though stations MAY
    broadcast it with very high accuracy, they may not, too. And if you
    get your signal via cable, be aware that cable companies have a habit
    of moving channels around. Even if the signal is in the same channel
    it was broadcast in, there is no guarantee it will be synchronous with
    the broadcast signal.

    If you want an accurate frequency standard, why not use an ovenized
    10MHz crystal oscillator? If that's not good enough, why not
    discipline it with the 1 pulse per second from a GPS receiver?

    Cheers,
    Tom
     
  8. Tom Bruhns

    Tom Bruhns Guest

    This is true, and technically even if you are receiving your signal
    line-of-sight, there's variation due to changes in atmospheric
    pressure and humidity, for example. I believe you can find some good
    information on the accuracy to expect for off-the-air reception of WWV/
    WWVB/WWVH on various frequencies, on the NIST website. A few years
    ago I was playing with how accurately I could calibrate a good oven
    standard from WWV's sky wave from Colorado to the Seattle area, and
    found that using FFT techniques I was able to consistently do better
    than they suggested, but not reliably better than about one part in
    10^8, as I recall. That took some understanding of how and when the
    propagation path experiences the most change.

    Cheers,
    Tom
     
  9. GregS

    GregS Guest

    I think orginally, the live TV shows had direst feeds and the carriers were more accurate.
    I used to run NASA timming at the Goldstone Apollo site, back when they
    actually had ground stations before the TDRS satillite. The sky waves were pretty
    radical. We used loran to monitor drifting. Were pretty close to Colorado, but still
    a lot of drift. I usd to cal my frequency counter with the stations Cesium clock
    using the basic 5Mhz sent around the station.

    greg
     
  10. one of my favorites is
    http://www.rt66.com/~shera/index_fs.htm


    martin
     
  11. GregS

    GregS Guest

    I think the WWV signals were only good enough to tell what second you were on.
    The Collins digital equipment had battery backup, and there was Rubidium and
    crystal backups. Of, course the Cesium was a crystal oscillator.
    There seemed an unwritten backup, anyone with a good wristwatch.

    greg
     
  12. My first experience with this was about 1980 while testing some oven-
    controlled oscillators by comparing to a bench frequency standard that
    received a reference signal from WWVB. One night I was testing late
    in the day, and as the sun wnet down my numbers wandered all over the
    place. The next day I showed the results to my boss and he had a good
    laugh at my ignorance.
     
  13. Guest

    Shera is the granddaddy of all the reflock boards,

    but if you want something simple and fast, start with a jupiter gps
    board (~20$ US) from ebay


    and go here:

    http://mysite.verizon.net/n1jez/

    Jupiter boards output a 1 pulse per second signal and a 10Khz signal

    Steve Roberts
     
  14. On 17 Apr 2007 13:14:05 -0700, in sci.electronics.design
    Not Found
    The requested URL /n1jez/ was not found on this server.

    Any about theShera Grandad, I've never quite followed
    P2 on the pdf

    "Interestingly, it is desirable to have the
    frequency of U7 drift slightly rather than
    being synchronized with the VCXO. A
    slight random drift averages out the ±1
    count ambiguity that is inherent in any
    pulse-counting device.

    My measurements indicate that the simple phase-measuring
    circuit I use is consistently accurate to 2 or
    3 ns (for a 30-second measurement), while
    without drift, the resolution would be lim-
    ited to 42 ns. "

    Do you have a simple explanation?




    martin
     
  15. Mike Monett

    Mike Monett Guest

    Yes, a very good one.

    That is one of many places where the Binary Sampling technique can
    dramatically improve the accuracy and speed of sampling noisy data.

    For a description, see "Noise-Rejecting Wideband Sampler" at:

    http://www3.sympatico.ca/add.automation/sampler/intro.htm

    The same technique works in software as well as hardware. A delta
    value (dv) is selected appropriate for the signal. The system is
    initialized by averaging, then storing the result in the current
    value (cv).

    At each sample time, the raw input signal is measured and compared
    to the current value (cv).

    If the raw signal is greater than the current value, the current
    value is incremented by the delta value, otherwise it is
    decremented:

    if (raw > cv) then
    cv := cv + dv
    else
    cv := cv - dv

    This allows the software to track the signal much better than
    conventional averaging techniques allow. The reason is the magnitude
    of the large amplitude noise spikes are not included in the result,
    only the direction.

    Since random noise has zero mean, the system will track the mean of
    the signal much more accurately than averaging techniques allow.

    The same software can also perform a moving average on the acquired
    output signal. A simple moving average filter is a very fast method
    since each sample requires one addition, one subtraction and one
    division. This can easily be done in integer math, so inexpensive
    microprocessors and PIC's can be used.

    A single stage acts as a low-pass filter with a cutoff floor of
    about -13 dB. Cascading 4 filters provides a gaussian response with
    a floor of about -60dB. This is in addition to the filtering
    provided by the Binary Sampling technique.

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://members.spsdialup.com//index.htm
    SPICE Analysis of Crystal Oscillators:
    http://members.spsdialup.com//spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  16. MassiveProng

    MassiveProng Guest

    http://www.smpte.org/standards/
     
  17. MassiveProng

    MassiveProng Guest

    http://www.novia.net/~ereitan/NTSC_overview.html
     
  18. MassiveProng

    MassiveProng Guest


    Or DWDM (Dense Wave Division Multiplexing)

    Sun is coming out with a 10Gb/s link set, and some nebulous startup
    is claiming 100GB/s capability.
     
  19. MassiveProng

    MassiveProng Guest

    Instead of your gear, YOU were affected by sunspots. :-]
     
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