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brightness. Moreover the polarity of the picture signals that is
correct for the cathode normally happens also to be right for dealing
with the synchronising signals. So cathode modulation of the beam
is usual, the grid being reserved for control of general picture
brightness by the viewer; there is no scarcity of voltage for that.
Because of the polarity of cathode modulation just referred to, the
vision detector and v.f. amplifier must be arranged so that an
increase in radio signal amplitude makes the cathode more positive,
the 75% (black-level) amplitude being just enough to extinguish the
spot altogether. The sync, pulses then mak