To recap, Quindar tones are a form of in-band signaling, with a 2,525-Hz pure sine wave intro tone that signaled the transmitters connected to Mission Control in Houston over leased telephone ...

I try to keep the piezo signal as an acoustic baseline using perhaps a pre-amp and some compression to bolster the tone, and maybe a Boss OC-5 Octave in Poly mode for some bass in the bottom end.

each key pressed would switch two different tapped inductors into the circuit to produce the dual-tone audio signal. Other switches controlled power to the oscillator circuit as well as attenuatin ...