First a radar doesn’t use sound waves, and their output was not digitized at the start either. **Rad**ar uses **rad**io waves to detect objects, not sound. To digitize something means to convert into a digital form such as for processing by computers, and radar didn’t do that originally.
Instead the first radar displays were basically oscilloscopes. A beam of electrons was shot at a screen with phosphorous which would glow when excited by the beam, and the beam could be deflected in various directions with the use of magnetic fields. The beam would be swept across the screen at a known, steady speed and the input signal would affect the vertical movement. What would result is a viewable graph of the oscillation of the signal across the screen. To produce only a dot for a specific return could be done by placing a limit on how strong a signal needed to be in order to register on the display.
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