The flyback converter is a part of the power supply for the CRT, not the beam steering mechanism. For the electron gun to work, it heeds high voltage (on the order of 15kV, depending on the size of the screen). This voltage is generated using a type of DC to DC power converter, called a flyback converter. The converter must be run at high frequency to make the power supply small and light enough to be practical, but the technology did not exist  to run them outside of the audio frequency band until much later than the variants shown in this video. On the output of the transformer is a rectifier and a capacitor, and the output of the flyback circuit is attached to the anode of the tube. The control grids, the filament, and the steering magnets or plates are independent of this voltage source, though they may use the scanning frequency as a convenient switching synchronization source (even though this would sync the converter ripple to areas on the screen).<br><br>Modern power converters are run outside AF range, some even into gigahertz range. Increasing frequency tends to shrink the.magnetics, but increases switching loss. Even so, the flyback converter in an iPhone charger runs at around 500kHz l, which is why it doesn't whistle.