Amadah, Blaauw & Brooks

"24 hours non-stop through the Internet the music formerly known as: Hippie music, Underground, 60's punk, Flower Power, Mod, Free-Form-Freak-out, Garage music, Psychedelia or Teen Beat, the weirdest, the worst, the most powerful and nastiest ever recorded."

"A customer voice frequency pushbutton signaling scheme involving a new two-group signal code, termed "four-by four", is described. It is shown that the use of this code, with judiciously chosen frequencies, permits the detection of bona fide signals after transmission over any ordinary voice connection and facilitates discrimination against false signals resulting from speech. Apparatus for generating the tones is described, and the principles of reception are discussed"

This 1960 article sets out the why and how of Dual-Tone Multi-Frequency (DTMF - Touch-Tone) in band audio signaling. It has become one of the most widespread sounds of contemporary planet earth.