If you see the term ‘I/O’ a lot when browsing for audio interfaces, it stands for inputs/outputs. And when looking at audio interfaces, you may also notice that these inputs and outputs come in two mediums; analogue and digital.
Analogue I/Os can have different connection types that are compatible with microphones or instruments. The two typical inputs you’ll come across are ‘XLR’ (balanced) and ‘line/instrument’ inputs (unbalanced). When it comes to outputs, though, they can take a few different forms. ‘TRS’ jack outputs (balanced) are the most conventional, although some interfaces also have XLR outputs. Smaller interfaces can also have ‘RCA’ outputs too.
The digital I/Os that you’ll find on some audio interfaces are either ‘S/PDIF’ or ‘optical’. These can be used for plugging in external units that also support these systems, such as additional preamp expanders or other forms of studio outboard.
It’s important to bare in mind that I/Os do not directly correlate with the amount of things that you can record with that interface alone. For example, a sophisticated audio interface may have 18 inputs and 20 outputs, but this does not necessarily mean that you can plug 18 microphones in. In this instance, you may have 8 microphone preamps available on the interface, while the other connections are made up of line/instrument inputs and digital inputs. The same applies for the amount of outputs available. You can’t plug in 10 pairs of monitors!