It used to be that they had to be super accurate so they could switch
feeds with their networks without causing a rolling picture.  I imagine
now that they fix such problems with time base correctors, but I'll bet
they use a GPS timebase anyway, so they know /when/ their parent is
sending the next commercial and they can catch and juggle and send
things without missing frames.

-Adam

Jack Smith wrote:

>        Another source of accurate frequency in most peoples houses is the TV set.
>        Just run a wire across the top of the TV set and you can pick up the scan
>        line frequency (depends what country you are in as to the actually
>        frequency), this frequency is ok for a calibration check.
>
>        If you know your TV set inside out then a more accurate frequency is the
>        colour burst frequency that is locked into the TV stations accurate
>        source.(WARNING don't go there if you don't know what you are doing)
>
>I've heard that the days when the color burst was locked to a network
>Rubidium standard are long gone and that the local Broadcast station is
>generating its own burst reference from a master crystal timebase. It's
>still going to within a Hertz or two of standard, but not so close that you
>can use it to calibrate your frequency counter timebase.
>
>It's been a long time since I've set foot in a TV transmitter plant, so
>perhaps someone knows the answer to this for sure.
>
>In any event, the horizontal sweep frequency is certainly plenty accurate to
>check the time base and horizontal linearity on an analog 'scope.
>
>Jack
>
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