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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.

Fig. 1 The MIDI input and output jacks on a TASCAM US-20x20 audio interface

Then Came the Sound Card...

The earliest audio interfaces were typically computer cards that were installed into an ISA (Windows), NuBus (Apple), or PCI slot on your computer motherboard. These "sound cards" gave the computer basic line-level audio input and output, and while they often had a "mic input," these were never really up to professional standards. Some early audio interfaces were in external boxes that connected to the host computer over a cable connected to the printer port; these have become obsolete as fewer and fewer computers came equipped with old-style RS-232 printer ports.

Many devices today have the equivalent of a basic sound card built in. For example, your laptop, smart phone, and tablet probably have a headphone output, and some also have line input jacks.

While some high-end audio systems still use computer cards (often in combination with external "break out" or "I/O" boxes that contain additional jacks), for the most part the days of the "sound card" have passed. Many modern computers lack card slots and printer ports, so today you're more likely to encounter external audio interfaces that connect to your computer via USB, Thunderbolt, or Firewire cable connections.

The Three Main Purposes of Modern Interfaces

There are three main types of connectors on a modern computer audio interface, and three main tasks that those interfaces perform:

    •    Connections for sending audio (and often MIDI) data into the interface and therefore, into the computer
    •    Connections for transferring audio & MIDI data from the computer out of the interface
    •    Connections for hooking the interface up to the computer itself

Many, but by no means all, interfaces also have a power connector. Although audio interfaces need power, some audio interfaces are bus-powered, and receive the power they need to run over the cable that connects them to the computer.

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.

Figure 2: A female XLR jack (the three-pin connector on the left) - usually used for microphone inputs, but also occasionally for +4dBu balanced line inputs

Figure 3: A 1/4" line input jack. A 1/4" line output jack and a 1/4" headphone output jack looks identical to this, so it's important to pay attention to how the jack is labeled

Figure 4: A combination XLR / 1/4" input - it can be used with either 1/4" cables or XLR cables, and is usually used for combination mic / line inputs

Figure 5: RCA jacks. These are sometimes used as consumer level or "semi-pro" -10dBV line inputs and outputs, but they are also often used as stereo S/PDIF digital audio connectors, so again, make sure you read the labels carefully

Figure 6: Multi-channel digital ADAT "lightpipe" connectors transmit 2-8 channels of digital audio over TOSlink fiber-optic cables

Fig. 7:  Headphone output jacks are wired for stereo, and accept 1/4" TRS headphone plugs. Some use 1/8" TRS jacks and plugs instead

Special Cases

USB microphones are a different kind of animal, because they don't connect through an audio interface. USB microphones, like USB MIDI interfaces, connect directly to your computer's USB port, just like any other USB peripheral. They may look like ordinary microphones but actually have all the basic features of an audio interface built into them - including a mic preamp and level control, the analog-to-digital and digital-to-analog converters, and a headphone jack and volume control for monitoring and playback.

Some modern interfaces have extra USB ports on them. These act like a USB hub and provide extra USB connections for the host computer system. They can be a very good place to connect your devices with USB MIDI outputs, but again, any USB port on the host computer can be used for this, without the need for an external 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!



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.  

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