Pulse code modulation: invented for microwaves, used everywhere

The inventor of PCM, Alec H. Reeves, was seeking a modulation technique which could match the capabilities, and the limitations, of the newly-developed microwave channels. By a deep irony of technological history, PCM in its basic form turned out to be ill-adapted to the microwave channels which emerged from pioneer investigations into practical use. It was first employed in line communication, and only after widespread penetration into terrestrial line networks did it make much headway into radio systems. The great stimulus to the invention of PCM was the collateral development of microwave radio: but by the time that PCM became a practical system for telephony, microwave radio had for good reasons developed along analogue lines. There was however an obvious application for PCM on cable, which led to near-universal implementation on transmission routes and ultimately to the digital integrated network. Digital techniques spread back into radio systems only after widespread usage elsewhere, and much technical development to adapt them to the medium.