Bruel & Kjaer - Beat Frequency Oscillator

The Bruel & Kjaer 1022 beat frequency oscillator.was used in the 1960's at the BBC Radiophonic Workshop.
Beat frequency oscillators or BFOs first developed from radio telegraphy. The oscillator was used to convert morse code to audible signals.

How does the BFO work?
It has 2 oscillators. One gives a fixed frequency and the second a variable frequency. These two frequencies are fed into a mixer which produces a sum ( we will call this "f1") & difference ( lets call it "f2")

The difference between f₁ & f₂ always lie in the audio-frequency range.

The demo is in Norwegian (a language I sadly don't understand).
However, I think the video speaks for itself.

Elektronisk urmusikk (8:40 min)

The picture taken below in the BBC shows a device nicknamed the "wobbulator".
The square box with that huge dial.
It's a sine wave oscillator with frequency modulation.... probably a Bruel & Kjaer 1022

The synthesizer hadn't been invented and the BBC along with other studios around the world built their own musical instruments or converted existing electronic test equipment (mainly signal generators and filters) into something that could make sounds.

The central knob sets the frequency of the main oscillator.

BFOs are used  when the need comes to cover a very large frequency range with a single dial rotation.

Due to their complicated circuits BFO's are today considered obsolete and many of their tasks have been replaced by the Wien bridge oscillator.