TFT drive waveforms

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I'm attempting to convert a 6.4" TFT controller board from PAL/NTSC in, to RGB in (for VGA).
I've found the decoded signals with R, G & B on them, and am ready to break-in and apply VGA. But one thing is strange, the waveforms each have a 7.8KHz superimposed waveform, ie alternate lines are at a different DC level.
Does the display need this? If so why?

Is it something to do with interlaced to progressive scan?

TIA :)
 
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