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Old 02-24-2018, 09:57 AM
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old_tv_nut old_tv_nut is offline
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The oscillator in the VIR sets locked to the regular burst as usual. The VIR made a secondary adjustment of phase. The goal of the VIR was to provide a secondary signal that suffered all the phase and amplitude perturbations as the video chrominance, all the way from the original encoding at the studio. VIR and the burst would always have the same frequency, since the local proc amp sync and burst reinsertion was locked to the incoming signal from the network. VIR was never intended to be a frequency reference.

Later, when the use of frame synchronizers became common, even the local frequency could be different from the incoming. VIR could no longer be carried all the way through the chain.
[Edit - maybe it could be - have to think about that]

For another thing, some stations began using local VIR insertion to adjust transmitters in an automatic closed-loop system, and this broke the chain as well.

Before frame synchronizers, the major networks had established atomic clock references to which everything was synced. These were so stable that you could sync a receiver to one network it displayed the picture from another (which I actually did for an experiment when I worked at Zenith). The National Bureau of Standards published a paper on using the network color burst as a better frequency reference than WWV.

As far as putting the burst on a pedestal:
1) The reason for the pedestal in VIR is so that any differential phase incurred along the chain would be compensated precisely nearer to skin tone luminance.
2) The reason for not putting the burst on a pedestal in the original NTSC/FCC specs was to prevent making it visible during retrace in sets that didn't have power retrace blanking. There was a proposal to put it on a pedestal to avoid possibly clipping it in studio amplifiers or transmitters, but this was rejected. Adding the pedestal after the standard was established would have required expensive surveying of the existing receiver population to make sure retrace was no longer a problem.
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Last edited by old_tv_nut; 02-24-2018 at 10:13 AM.
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