EMZETT.AUX
In short: “Auxiliary” — an additional analogue audio port (usually a 3.5 mm jack) through which external devices can input or output sound.
In more detail: AUX ports are purely analogue and don’t transmit a digital signal — unlike HDMI, which carries sound digitally too. They’re found on headphones, speakers, car radios and many other devices as the simplest, universally compatible audio connection.
In Depth
Origin and how it works
The name “auxiliary” (secondary/additional connection) reveals its original role: an additional, universal audio connection alongside a device’s built-in speakers or amplifier — originally intended, for example, on car radios to connect a portable CD or MP3 player. The 3.5 mm jack is a purely mechanical, analogue standard — unlike digital interfaces, there’s no protocol, no authentication, no “fitting” or “not fitting” at the software level; every plug of this size works with every matching jack, regardless of manufacturer or year of manufacture. That makes AUX extremely durable and universal, but also technically limited: it only transmits a pure audio signal as an analogue alternating voltage, no control signal (e.g. no automatic volume control from the source, no play/pause control) and no additional data such as metadata about the currently playing track.
TRS vs. TRRS
Over the years, the classic stereo jack (TRS — tip/ring/sleeve, three contact zones for the left channel, right channel and ground) was extended with an additional contact ring (TRRS — tip/ring/ring/sleeve), to integrate a microphone signal for headsets into the same jack, for example. This enabled headsets with just a single plug instead of two separate ones for headphones and microphone. The problem: there was never a truly universal standardisation of which of the four contacts carries which function — different device manufacturers (especially between PC headsets and smartphone headsets) sometimes used different pinouts, which occasionally led to compatibility problems (microphone doesn’t work even though the plug mechanically fits).
The disappearance of the headphone jack
On modern smartphones, the jack has increasingly disappeared, in favour of Bluetooth headphones or digital audio over USB-C. The reason is partly space savings in an ever-thinner casing, partly technical necessity: an analogue AUX output needs its own digital-to-analogue converter (DAC) permanently built into the device — with USB-C audio, this conversion instead happens in a small chip in the headphones or adapter itself, which saves the smartphone manufacturer space and cost, but requires an adapter or USB-C headphones.
Optical audio as a digital alternative
Alongside analogue AUX, TOSLINK (optical audio) offers a digital alternative via a similarly-looking jack — here the sound is transmitted as light pulses through a fibre-optic cable instead of as an analogue voltage. This avoids electromagnetic interference (hum from power supplies or other cables nearby), a problem analogue AUX connections can definitely have with poor shielding, audible as a quiet hum or noise in the background. TOSLINK is common on home cinema systems and some TVs, but practically never found on mobile devices, since the technology is more sensitive to cable bending and more elaborate in the connector.
Volume and impedance problems
A practical problem with AUX connections is impedance matching between source and destination: headphone outputs are optimised for low-impedance headphones, while an AUX input on a stereo system often expects a different voltage level. In practice this often shows up as the volume still arriving too quiet or distorted even with the source turned all the way up, or noise becoming more noticeable at low volume than with a properly matched digital connection. That’s why the same piece of music can sound audibly different over AUX depending on the combination of source and destination, while digital connections (Bluetooth, USB audio, optical) transmit the signal losslessly as numeric values, where these matching problems don’t arise.
See also: Microphone, USB-C, USB