When you pick up a landline phone, throughout the USA there is a consistent "dial tone" you hear in the earpiece.<p>I haven't heard it in a while, but IIRC it's two synthesized tones roughly a "major third" apart (https://en.wikipedia.org/wiki/Major_third), slightly buzzy-sounding, which means they probably weren't a "just" major third (https://en.wikipedia.org/wiki/Just_intonation).<p>Once you started dialing, the dial tone would go silent. I think the dial tone would remain in place if you spoke, but I don't remember for certain.<p>Does anyone know how this worked? What generated the dial tone? Presumably there must have been a network of machines responsible for processing people's dialing and routing calls appropriately; did these machines also generate the dial tone? If so, were they all made by the same company, Bell-something-something? The dial tone was/is so consistent in its sound and behavior that it seems likely.<p>Just curious if anyone knows or has links!
The tone was a continuous 350 Hz and 440 Hz sine wave combination, and was generated at the Central Office (likely by a Western Electric No. 5 ESS). It started when the telephone went off-hook and stopped when you started dialing a number.<p>Dialing a number was done by DTMF dual-tone, multi-frequency tones or by pulses (interruptions of the circuit).<p>And to answer your likely next question: No. The ringing tone was not synchronized with the bell on the target telephone.
Wikipedia has a couple of photos of the units that used to generate dial tones in the German PSTN:<p><a href="https://de.wikipedia.org/wiki/Ruf-_und_Signalmaschine" rel="nofollow">https://de.wikipedia.org/wiki/Ruf-_und_Signalmaschine</a>
Eh? What.<p>There is actually a picture of the dial tone generator in Flickr. Motor was moving generators on various speeds. <a href="https://www.flickr.com/photos/9479603@N02/1814546737/in/photostream/" rel="nofollow">https://www.flickr.com/photos/9479603@N02/1814546737/in/phot...</a><p>As regards to logistics of tones and stuff. You dont want to know. It involved multi-level signalling of currents and voltages.<p><i>I am the ultimate expert. I wrote a thesis on Coin-operated Telephone signalling in 1977.</i>