Bing Tone Generator

Keith Wicks writes: I'm looking for information on bing tone. I remember we used to use this slowly pulsating signal before startup of the main radio networks in the early 1960s. I think it was described as the least disturbing signal to have to listen to for long periods.

Does anyone know what the frequency and waveshape were, and are there any publications on the subject? I ask because I occasionally have the need for something similar and thought I'd build a generator. I'm hoping to find a circuit of a transistorised Bing Tone Generator, if such a thing was ever made, but a valve circuit would be of interest too.

Any help would be much appreciated.