Audio Interface Connections

Audio Interface Connections

What do all the jacks and ports on this thing do?

by Phil O'Keefe

Audio interfaces for computers, tablets, and smartphones offer a variety of different formats, with a wide range of features. Their common goal is transferring audio signals into, and/or bring them out of, your computer or other device.

An audio interface is necessary because audio signals from electric guitars, keyboards, and microphones are analog signals that need to be converted into digital form that your computer, smartphone, or tablet can record, process, and manipulate. Then, that digital information needs to be converted back into an analog signal for playback. An audio interface provides the required conversions. 

First There Was MIDI

Years before we were using computers to record and play back audio, musicians were using MIDI. MIDI is the Musical Instrument Digital Interface, an industry-standard protocol that allows equipment from different companies to exchange data and work together. Commonly found on keyboards (as well as many other pieces of gear), MIDI ports carry performance data like what keyboard notes are played, pedal position, and the like - not audio. However, it's not uncommon to find MIDI input and output ports on a modern computer audio interface. Originally MIDI ports used 5-pin DIN connectors, although today many devices send MIDI data out of a dedicated USB connector instead. This connects directly to the host computer, just like any other USB peripheral.

                        Let's break each of the three categories down a bit further.

• Today the printer port connectors and internal sound cards that were popular in the past have largely been superseded by interfaces that connect to the host computer over standard USB, Firewire, or Thunderbolt connections.
• While there are still a few USB 1.x interfaces on the market, they can transfer only a limited amount of data so most modern USB devices connect to a USB 2.0 port. USB 3 interfaces are now starting to become available and they require a USB 3 port on the host computer. Interfaces equipped with USB 2 connectors will work with USB 2 or USB 3 computer ports.
• Firewire comes in both Firewire 400 and Firewire 800 versions and these two use different cables, although adapters are available that let you plug a Firewire 400 cable into a computer with a Firewire 800 port. Firewire interfaces are far less common today than they once were.
• The latest interface format is Thunderbolt, and like USB 3 it is a much faster interface than can carry more data than the earlier USB and Firewire interfaces. However Thunderbolt is still relatively rare on PCs - you're more likely to find them in Apple computers.

Let's look at some more of the specific connectors that transfer audio and MIDI data in and out of your interface and by extension, your computer. Typically you'll have a combination of some of the following - sometimes all of the following - it just depends on the features of the particular unit:

    •    XLR microphone inputs. (Fig. 2)

    •    1/4" line inputs (Fig. 3) - sometimes in combination with the XLR mic inputs on combo XLR-1/4" inputs (Fig. 4). Occasionally you'll also find XLR line inputs and outputs, and even RCA line inputs / outputs (Fig. 5), but these are relatively rare compared to 1/4" line inputs and outputs.

    •    High impedance (High-Z) inputs on 1/4" jack for recording guitar / bass "direct." Trying to plug a guitar or bass into a standard mic or line input will degrade the sound quality; a high impedance input maintains the instrument's fidelity.

    •    S/PDIF Digital I/O on RCA-style coax jacks

    •    ADAT lightpipe multi-channel digital audio I/O (Fig. 6)

    •    L/R main (or monitor) outputs. Often on 1/4" jacks, but occasionally you'll see XLR line outputs or even RCA jacks used instead.

    •    Multiple line output jacks - usually on 1/4" TRS jacks, but sometimes on XLR jacks

    •    One or two headphone outputs (Fig. 7) - usually using a 1/4" TRS stereo jacks, but also occasionally 1/8" TRS stereo jacks.

Below are some pictures using TASCAM US-4x4 and US-20x20 audio interfaces to illustrate the various types of connectors found on a modern audio interface.

While the various types of connectors listed in this article cover the vast majority of ports you're likely to encounter on a modern audio interface, it's not a comprehensive list of every conceivable type of connector. For example, some audio interfaces have multiple inputs and outputs mounted on DB25 connectors, and some units have BNC word clock and other synchronization ports and jacks, but such units tend to be high-end products designed for professional studio use, so musicians are less likely to encounter them unless they're heavily into high-end recording technology.

Hopefully we've covered the basics and this will give you a firm foundation to build on, but remember - if you don't know what is supposed to plug into a particular jack, or you're not sure where to plug that cable, read the manual or ask someone. While most connections are fairly "bulletproof," it is possible to break something if you accidentally plug the wrong plug into the wrong jack- even if they match, they aren't always used for the same function. So don't try to plug the 1/4" speaker output from your guitar amp straight into the 1/4" input on your audio interface, or you'll be sorry! -HC-

If you have questions or comments about this article, or run into a connector you're not sure about, join in the discussion in this thread right here in the Studio Trenches forum on Harmony Central!

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Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.