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What is an audio interface?
If you’re about to get into podcasting, multitrack recording, or streaming: you need to get serious about audio. There’s a lot more that goes into making mellifluous vocals or kick-ass guitars than you might have thought. Time to talk audio interfaces.
In this article, we’ll explain what an audio interface is, what it does, and what you should consider when looking for one.
Editor’s note: this article was updated on May 11, 2021, to include information about inputs and outputs.
What is an audio interface?
Traditional microphones capture the sound you create in the air and pass this signal as an analog electrical waveform. For this output to make sense to your computer, it needs to be converted into a stream of discrete electrical voltages that correspond to 1s and 0s—this is a digital signal. USB microphones handle this process internally. Standard mics use a (male) balanced XLR connector to produce the analog electrical signal. There are also microphones that can output both analog and digital formats simultaneously.
So how do you connect a mic’s XLR to a computer? Well, that’s where a handy peripheral known as an audio interface (sometimes referred to by the legacy term “sound card“) comes in. Today we’re going to discuss USB and Thunderbolt audio interfaces in general terms. Firewire audio interfaces were quite common until recently and are readily available on the used market. We recommend avoiding these as many are no longer fully supported by the manufacturer or by current operating systems.
How do audio interfaces work?
Audio interfaces provide several functions, both analog to digital (ADC) and digital to analog conversions (DAC) are carried out by the interface, and most feature multiple channels of each. The most basic examples have one or two inputs for line levels or microphones, contain microphone preamps, and a stereo output to feed monitor speakers or headphones. Something simple like this will be compact enough to sit fit comfortably in a bag with a laptop.
Each mic channel should be able to provide +48v for condenser microphones and have variable input gain covering at least 60dB. Typical features that join the party as price increases include extra headphone jacks and volume controls. The mic preamps can get more advanced with the addition of: high-pass filters to eliminate rumble; attenuator pads for strong signals; phase invert switches (handy when recording multiple mics); and input impedance selection (for better matching and signal transfer). Some mic channels add buttons to engage magical secret technologies involving iron, tubes, or “air,” in some combination. This isn’t something that adds a lot of value so don’t overspend because of it.
Audio interfaces make it easy to connect a mic's XLR to your computer.
Even basic interfaces should provide one or two high-impedance instrument inputs for direct injecting (DI) guitars and basses. Additional inputs and outputs will be at line level and carried on balanced TRS connectors (jack sockets).
If you’re recording many sources at once, like mic’ing a live drumkit, you may find you need more than the small number of analog inputs available on basic interfaces. If this scenario is in your future, and it will be if you like the idea of recording bands, many interfaces will allow expansion via a connector carrying multiple digital inputs or outputs.
What type of interface do you need?
We’ve already mentioned that some mics have a USB output. If you only need to record one voice at a time, a USB mic will do what you need, and you likely don’t need an interface at all. If you’re fixed on a mic with an analog output, read on.
Before choosing an interface you should consider the following:
- How many microphones do you need to record at once?
- How many analog line inputs do you need to record at once?
- How many line outputs will you need?
- Should it be expandable?
- How important is portability?
- Latency and the benefits of Thunderbolt
We won’t dive deep into sampling rates and bit-depth—instead, we just recommend you avoid anything that doesn’t support at least 24bits at 48kHz, as that is a realistic minimum performance target for capturing audio for most applications.
The number of microphones you need to record at the same time dictates how many XLR inputs with mic preamps you need on the interface. If you’re using equipment that provides line-level signals like external mic or guitar preamps, outboard processors, or old synthesizers, count the total number of analog inputs. Since mic preamps usually accept line levels without issue, you need to think about what combinations of mic and line inputs you should account for. Four inputs should cover most home recording scenarios.
Though it may not be the first thing you think of, the third question is important and will vary greatly depending on your application. For most people working in stereo, a pair of outputs will do—assuming you don’t intend to bring any signal back into the analog world for processing (or for re-amping guitars) and just need to feed your monitor speakers. If you’re interested in monitoring with a surround system, you’re looking for upwards of eight output (DAC) channels. A separate stereo headphone monitor is very handy and will add two to the number of DAC channels you need.
Expandability and portability
Plan to undertake larger projects in the future? You might need to mic more people or multiple instruments, so you should invest in an interface with additional digital inputs/outputs.
Depending on how many extra channels you want, consider S/PDIF (two channels), the popular ADAT port (up to eight channels per port) or the less common MADI (up to 64 channels) BNC connector, which will allow you future-proof expandability without compromising portability. These will make it possible to “piggyback” an external multichannel AD convertor, which can often provide additional DAC channels as well, should you need them.
Portability is an important consideration if you’re keen to take your recording setup out on the road, to a jam hall, or to a friend’s house. As well as looking for something light and compact, consider that some simple interfaces are bus-powered, meaning that they get their power from the host computer and don’t need a separate power supply. This makes setup faster, simpler, and means you can operate where no power outlets are available for as long as your laptop battery can keep things running.
Latency and the benefits of Thunderbolt
The final thing to consider is latency: this translates to how much lag there is in your recording system. It will determine the delay you experience as you live-monitor the audio being recorded. Too much lag can be very distracting for performers monitoring themselves during the recording process.
USB interfaces are convenient but struggle with latency.
The vast majority of audio interfaces on the market use USB as the interface with a host computer. USB interfaces have a limitation on how fast the audio is processed and on the round trip delay time. Some companies improve this with proprietary audio drivers with their interfaces, but these tend to be on the more expensive end of the scale. The absolute minimum delay is considered to be around 4.5ms for a round trip, but most take longer than this. Some products on the market have a simple workaround that allows you to mix the direct sound from the mic input with the headphone output, bypassing the internal processing lag.
If latency is of real importance to your application, take a look at Thunderbolt-equipped interfaces, which use the newer peripheral connection standard popularized by Apple. This can provide lower lag times (less than 1ms) but less choice in terms of the hardware available.
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