there's no such "flag", robert.
in crts the vsync and hsync signals are "simple" sawtooth waveforms. at any given time, it's the voltage in these signals that deflects the electron beam from the canon and moves it across the screen's surface.
To achieve interlaced projection, the the level range of the vsync signal (50/60hz) is shifted.
Let's say (and I'm aware this description is highly inaccurate) that, for a PAL signal, we divide the full voltage range that the signal goes through (the amplitude) in 625 discrete points, each representing the discrete V values that the signal would have at the start of each line of a whole interlaced frame.
For the odd field, the aforementioned discrete values of V would be 1, 3, 5, 7... 625 and for the even field they would be 2, 4, 6, 8... 624.
That would cause the fields to be projected at different phisical places of the screen, creating the standard interlaced projection.
For LDTV, it would be as simple as using an identical range for all fields instead.
Analogue video is just a bunch of lines. there's no inherent interlacing in it other than what's caused by the proper sync signals.
In fact, analogue SDTV video is actually a continuous "line", a stream of voltage variances in 1 or more wires, projected as a series of lines on a CRT by moving the electron beam arround the screen some 15 thousand times from left to right and 50-60 times from top to bottom, and that movement is controlled by 2 sawtooth waveforms called vertical and horizontal sync.
Of course there's a lot more detail to it than that, but the insistence of ghitlescu in saying that it's impossible to evade interlacing is just wrong.
What I think that happens is that most capture hardware is designed to assume interlacing, so the sync signal information is not used in full. that is, the only thing it takes from the sync signals is to know WHEN lines and frames start and end. it's internal hardware synchronizes to that and generates it's own clock for the ADC sampler, but disregards the actual voltage values along the lenght of the wave.
This brings me to clarify my earlier interest in your plasma screen that seems to detect LDTV projection, robert.
I don't own anything other than CRTs so far (unless you cout my nintendo ds
), so I have no experience with these, but after learning what LDTV was, I became a bit paranoid about buying some LCD/LED/Plasma/whatever sometime in the future and finding out it can't tell LDTV apart from SDTV. Your plasma being able to distinguish it got me interested
It probably has a good vsync "sensor"