The idea of generating a key for speech encipherment in this way was not entirely new.
The X-system was not always used with one-time gramophone records for the key.
There was an alternative, called 'the threshing machine'.
But this only had to produce a stream of digits at a rate of 300 a second, and was only used for testing or for low-status signals.
The Delilah was much more demanding.
The generator had to be electronic, and the basic unit he used was the 'multivibrator'—a pair of valves possessing the property of locking into an oscillation between 'on' and 'off, with a length which would be some integral multiple of a basic period.
His key generator made use of the outputs of eight such multivibrators, each locked into a different mode of oscillation.
But that was just the beginning.
These outputs were fed into several circuits with non-linear elements, which combined them in a complicated way.
He had worked out a circuit design which ensured that the energy of the output would be spread as evenly as possible over the whole frequency range, and he explained to Donald Bayley with the aid of Fourier theory that this would endow the amplitude of the resulting output with the necessary degree of 'randomness' for cryptographic security.