| That's the color burst. NTSC faced the problem to add color to an existing monochrome broadcast system while a) staying compatible and b) using the same bandwidth as the previous signal. The solution was to modulate the color difference signal (two, actually, through quadrature amplitude modulation) to the existing signal, which was from then on called "luminance" (basically the "brightness information" of the picture). The color burst not only indicates that the signal is color, it is also what the receiver locks onto to determine the phase of the color information modulated on the luminance signal. Quadrature amplitude modulation is "carrierless", so you need the color burst as a reference. In NTSC, if you watch the signal live on an oscilloscope, the color burst usually appears as a static shape. In PAL, it's a blurry mess, because it purposefully not only flips 180° with every line, it also gets shifted a bit. That's the result of some schemes added from NTSC to PAL to make the picture more palpable. Since the color signal (mostly[1]) occupies the same bandwidth as the luminance signal, there is noticeable crosstalk between the two. It also works against phase errors: People who grew up in the US will know the "tint control" on the TV used to correct the hue of the picture. In PAL-land, that was not a thing anymore. [1] The color signal tries to occupy the gaps in the spectrum of the luminance signal, which in most realistic images are the result of the line-based nature of video signals. However it can never be perfect, there will practically always be overlap such that luminance and chrominance cannot be fully separated. |