As I understand it, telephones take the sound, digitalize* it, send it through the wires, then turn it back into sound and spits it out at the other end. Where do they get power from? Does the signal just get progressively weaker through each transformation, or is there a small power line in phone jacks?
IIRC the telephone line is powered; I think its 48 volts DC or thereabouts.
Telephones are analog contraptions, they don’t digitize[sup]1[/sup] anything. They are powered by a small DC current[sup]2[/sup] from the phone company’s central office downtown.
[sup]1. There may be digital encoding at the switch where your call is routed, but the handset itself remains an analog device.[/sup]
[sup]2. But AC current is used to encode the audio signal and power the ringer.[/sup]
Ringer voltage is typically around 90 volts AC, which is why the hand-cranked army field telephone has been an improvised interrogation mainstay for nearly a century.
You can get such a phone at an army surplus store, wire it into a house jack. It’ll work just fine and give you home survivalist chic. Don’t crank the handle, though. The phone companies prefer any voltage on the line come from them.
There are “digital” phones, though, typically wired to a digital PBX system installed in an office building. The voltage to run the phones (and the little LCD display windows many models use) comes from that unit, though the unit is ultimately connected to one or more outside phone lines that are (typically) analog, with power supplied by the phone company.
People just don’t appreciate the sheer genius of the telephone. In the early days, when wire was expensive and electronics were really, really crude, they figured out how to get a ring signal, power, and voice signals in both directions all onto 2 itty bitty wires.
The telephone company provides 48 volts DC at a fairly low current. Your telephone is sitting there with the 48 volts basically disconnected. The phone company sends a 90 volt AC ring signal down the line when the phone rings. When you pick it up, it makes a connection and current from the 48 volts DC begins to flow. In the olden days this triggered a relay which let the phone company equipment know you had picked up. The phone company would vary the current going through the phone line, and the resulting variations in current would make sound come out of the earpiece. The mouthpiece varied the same current, and a very creative couple of coils of wire were used to make sure that the sound from the mouthpiece wasn’t blaringly loud in the earpiece.
Extremely creative considering it was basically all done with some coils of wire and some batteries.
The simple coils of wire and such have long since been replaced with much more modern circuitry, but the basic theory of operation is still the same. You still get 48 V DC at a low current level to provide a small amount of power to the phone. If your phone needs more power than that (like phones with an integrated answering machine and such) then it require a seperate AC adapter for power.
Modern digital phones like those often found in businesses work a little differently. They also get their power from the phone line, and digitize your voice into data packets which are transmitted down the “phone” line to the system’s main computer. In this case, even through they are using the same type of cable as a standard phone line, the signals on it are really dedicated computer network type signals. They aren’t standard phone signals. The signals may end up getting switched back to analog before it leaves the building, or they may be transmitted digitally in something like a voice over IP (VOIP) system.
As for signal loss, in the old days your voice signal had to travel down a long piece of wire to the phone’s switching station. From there, the signal might get amplified as it was switched onto a dedicated line for transmitting it to another switching station. The signal might go through several switching stations. Eventually, it ends up being amplified again and converted back to a plain old ordinary voice signal whcih goes to the receiving phone. In the old days of analog phone systems, you didn’t have to worry too much about the signal getting too weak. You just added amplifiers as needed. However, every step along the way, if there was any electrical noise of any type (which there always is at least some in analog circuits) then that noise would end up getting mixed in with the voice signal and once it’s mixed in there’s no way to get it back out. So, the more things your voice had to go through, the crappier it sounded on the other end.
These days, even on an old fashioned analog home phone line, your voice gets digitized fairly quickly in the process, often at the first switching station it goes through. The great thing about digital signals is their huge immunity to electrical noise. Unless you get enough noise to flip a bit from a 0 to a 1 (or vice versa) then the data gets through intact. You can re-amplify it and retransmit it around as much as you want with no loss of signal whatsoever. The only noise that ends up in the voice is the noise in the analog to digital converter where your voice gets converted to digital, and the noise in the digital to analog converted that puts it back into a voice signal for the receiving phone.
Random question: What do they call the old fashioned, kinda cube-shaped phones you used to see all the time? I’ve been meaning to get one, because I can’t stand newer phones.
There was the 500 series (dial) and the 2500 series (touch-tone) manufactured by Western Electric.