The input signal combines video amplitudes (positive voltages) and sync pulses (negative voltages) which tell the CRT when to move the beam around. Technically

The input signal combines video amplitudes (positive voltages) and sync pulses (negative voltages) which tell the CRT when to move the beam around. Technically composite video is AC-coupled so I'm not sure how sync pulses are actually identified (but video brightness is relative to the porch voltage right after a sync pulse). Not sure if antenna TV RF signals are DC or AC coupled.