The Parts of an Analog Phone System
By John Papiewski
The traditional business service you get from your local phone company is analog. In this system, electrical signals in the phone line vibrate in a manner analogous to sound waves. This is in contrast to newer digital services, which turn the sound of your voice into computer data bits. An analog phone system consists of parts evolved from principles developed in the early 1900s and includes a dialing mechanism, a ringer, a hook switch and a handset.
To signal you have a call coming in, the phone company sends 90 volts AC to your line. In older phones, this voltage rang an actual bell. Modern units have electronic tones, but the phone senses and responds to the same voltage. Note that the voltage is high enough to pose a shock hazard. Unless you are experienced with electrical repairs, avoid working on bare phone wires directly. The line has three basic voltage levels: the 90-volt AC ring signal, a 48-volt DC signal when not ringing and three to nine volts of DC when you're making a call (Reference 2).
An analog phone system uses two kinds of signals to dial a number: a series of pulses or a set of fixed tones. Pulse dialing is by far the older of the two methods, coming from the era of mechanical rotary dials. Because phone companies treat pulse dialing as an obsolete technology, few still support it. Modern phones use the familiar touch-tone dialer that produces fixed electronic tones when you press the buttons. Circuits at the telephone company's office detect the tones and translate them into numbers used to place a call. The use of touch tones opened up capabilities most businesses now take for granted, such as caller ID and automated answering services.
A hook switch is an electrical switch that connects your phone to the line when you lift the handset from its cradle. Electronic analog phones typically don't have a traditional switch; instead, they connect the phone when you press a "Call" button and disconnect with an "End" button. When use the phone to place a call, you take the phone "off hook" and get a dial tone; when you end a call, the phone goes "on hook." The phone rings only when it's off hook
An analog phone's handset contains the earpiece and microphone that picks up your voice and lets you hear the person at the other end of the conversation. The technical terms for these parts are the "receiver," which receives signals from the line, and the "transmitter," which turns sound into electrical signals and sends them into the line.
Chicago native John Papiewski has a physics degree and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."