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  1. I noticed that with the requirement of the AM modulation technique being VSB for NTSC video broadcast, the chroma subcarrier in the lower sideband of the signal is cut off. This will cut in half the amplitude of the chroma signal (a TV receiver, after demodulation of the RF carrier and separating of the luma and chroma signals, will end up with a chroma signal that's half the amplitude of the chroma signal generated by the camera that was connected to the transmitter). For example, the amplitude of the color burst in the baseband signal is supposed to be 20 IRE. A demodulated NTSC TV broadcast would create a baseband signal in which the color burst amplitude was only 10 IRE. I think this would make it hard for the chroma synchronization circuits to detect the presence of the color burst, and thus make it hard to properly decode the color image.

    How is this compensated for in actual NTSC broadcast systems? Does the baseband composite video signal output from professional TV cameras (intended to be used in TV broadcast) actually output a composite video signal with a chroma amplitude twice what the specs say the amplitude should be for baseband, in order for the transmitted signal to carry the chroma at the correct level? Or does the RF video modulator in the transmitter itself contain a circuit to separate the chroma from the luma, double the amplitude of the chroma, and then recombine them into a new composite signal ready for transmission? Or do most TV receivers just contain a circuit after the RF demodulator and chroma-luma splitter circuit, which is designed to double the amplitude of the received chroma signal back up to NTSC baseband specification, before further processing of the chroma and luma signals into RGB?
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  2. Member Cornucopia's Avatar
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    The amplitude of colorbust is 40 IRE (20 above, 20 below).
    IIRC, the filter tunes in the color carrier frequency, phase locks with it, uses it to extract (usually via comb filtering) the color signals (I & Q or U & V), AMPLIFIES IT (comparing recovered colorbust w reference), then continues the processing chain (whether component, or composite, etc).


    Scott
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