RCA jack

An RCA jack, also referred to as the inch phono jack, is a type of
electrical connector which is commonly used in the audio/video market. It
originally started replacing the older phone jacks used in the audio world
when cassette tape players started becoming popular in the 1970s. Today it
is one of the few truly universal connectors.

Cables have a standard plug on each end, consisting of a central male
connector, surrounded by a ring. The ring is often segmented for
flexibility. Devices mount the jack, consisting of a central hole with a
ring of metal around it. The ring is slightly smaller in diameter and longer
than the ring on the plug, allowing the plugs ring to fit tightly over it.
Connections are made by pushing the cable's plug into the jack on the device.

The jack has a small area between the outer and inner rings which is filled,
typically with plastic. They are often color coded, yellow for composite
video, red and white for left and right stereo audio. This trio (or pair) of
jacks can be found on the back of almost all audio and video equipment.

One problem with the RCA jack system is that each signal requires its own
wire. Even in a simple case of attaching a cassette deck you'll need four of
them, two for input, two for output. In any common setup this quickly leads
to cable spaghetti, which is made worse if you consider more complex signals
like component video (another three cables each way). There have been
numerous attempts to use combined connectors in both the audio and video
world, but none of these have ever become universal.