When someone presses the call button on your video door phone it activates its camera and alerts the indoor monitor. You then talk to them while you view their image. You can even press an open lock button if you have an optional electric door lock installed.

Power Line Carrier Systems

Power Line Carrier (PLC) intercom systems communicate using a very low FM frequency over your house’s existing 110 volt AC house wiring that supplies power to receptacles and light fixtures. You just plug them in anywhere you have an outlet and they’re ready to go.

While they sound like the ideal solution for basic applications, most users are typically not happy with these intercom systems.

These units are very susceptible to interference from both inside and outside the house or business. You may experience buzzing, poor audio, or they may work perfectly well for you. They are very low featured, but they are also very inexpensive. These systems are not recommended for most people.

2-Wire And 4-Wire Systems

In your search for an intercom system, you’ll see a many systems listed as 2-wire or 4-wire. Years ago the differences between the two were significant.

In voice communications there are half-duplex and full-duplex methods of transmission and receiving. Half-duplex communication is like push-to-talk “walkie-talkie” radios or CB radios. When you want talk to someone else with a compatible radio, you have to press a button to talk, let go, and then wait for a reply. The person on the other radio does the same to respond to you. Neither party can talk at the same time.

Full duplex communication is what a telephone uses. All parties on the call can speak at the same time.

Years ago 2-wire intercoms were half-duplex only. With only one pair of wires, they had to be used for both transmit and receive. When one party was talking, the wires were being used to transmit the voice to the other party. Then the wires were used to receive the voice from the other party, and vice versa. Some very basic intercom systems still use this method of half-duplex communication.

4-wire intercom systems were full duplex. Since there were four wires, one pair could be used for receive and the other pair to transmit.

New developments in technology made it possible for even two wire systems to have full-duplex operation. So the choice between two-wire and four-wire is more about whether or not you already have wire in place and how many conductors it has.

If you already have two wires in place, then it would obviously be better to choose a two-wire system. If you have to install wire, then you should always install a cable with more wires than you currently need. Then you’ll be ready for future applications and you can choose any wired intercom system you want.

Cat-5 Systems

Which brings us to a third type of wired system: the Cat-5 system. Cat-5 is short for Category 5 and it is a specific type of bundled wires. It is typically used for data networks and it consists of four twisted pairs of copper wire. It supports frequencies up to 100 MHz and speeds up to 1000 Mbps.

Intercom systems that use Cat-5 wiring communicate digitally between stations and therefore may offer more features. Cat-5 is usually used by the more expensive systems that are professionally installed. The use of Cat-5 is relatively new so there are fewer, but a growing number of choices for these systems.

But even if you don’t install a Cat-5 system now, we recommend you install Cat-5 wiring and use that for your 2- or 4-wire system. Then you’ll be ready if you want to upgrade later.