| 摘要 |
A video imaging apparatus includes a cathode-ray tube including a focus electrode. A source of a first parabolic signal at a frequency related to a deflection frequency, selected from a plurality of frequencies, has an amplitude determined in accordance with the selected frequency. A control circuit has an input coupled to the source of the first parabolic signal for generating an output signal. The output signal is for maintaining the amplitude of the first parabolic signal for the plurality of deflection frequencies. An amplifier, that is responsive to the output signal, is coupled to the focus electrode for amplifying the parabolic signal to generate a dynamic focus voltage at the focus electrode.
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