How did old dial phones figure out where the call is going without having a computer inside? What is the mechanism behind the dial?
In: Technology
The old, rotary dial telephones were just a small part of a much more complicated machine – the local telephone switch (local being local to a small town or several large neighborhoods.)
As part of the greater machine (local switch), and in order to minimize the number of individual wires connecting the handset (rotary telephone) to the larger switch, a signalling mechanism was used involving completing and interrupting a circuit with multiple voltages, both AC and DC.
When a call was received AND the phone was “on hook” (such as the receiver being in the cradle), the local switch would send a high voltage AC signal to activate the bell[2] , at around 20Hz.
Once the receiver was picked up (the phone was “off hook”), the local switch would detect that and stop the ringer signalling. The mouthpiece in the receiver (the part you spoke into) was a “carbon microphone”[3] that would change the electrical resistance with the pressure waves of the sound impacting on it. This change in resistance would produce the analogue electrical signal representing the voice of the speaker.
Conversely, the sound from the other end was impressed on the pair of wires at a higher frequency (than the ringer). A system of passive electronic filters made of inductors, capacitors, and resistors separated the mouthpiece signal from the earpiece signal.
In order to make a call, the receiver would be lifted off the cradle, changing the phone from “on hook” to “off hook” state, completing the DC circuit. The local switch would detect this (and would usually provide feedback as a low frequency buzz.) The local switch would connect the phone circuit to a “stepper” switch.
Operating the dial would (quickly) switch the DC circuit at a fixed rate (frequency.) This frequency was low enough that it was possible for some agile people to actually make phone calls by tapping the cradle “hook” button fast enough, with the right timing. (A parlor trick for nerdy teenagers.)
This fixed rate of on-off switching is the “pulses” of pulse dialing[4] . In the original design, it caused the stepper switch[5] to step through it’s various positions and then (usually) proceed to connect to another stepper switch.
These stepper switches were electro-mechanical and hideously complex (for the time – and even for now.) The sheer physical size and number of these stepper switches needed for each subscriber line, the number of cross connects needed, etc. meant that most of these switches didn’t support more than 5-digit dialing.
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