If you’re about to get into podcasting, multitrack recording, or streaming: you need to take audio seriously. There’s a lot more to creating melodious vocals or jaw-dropping guitars than you might think. It’s time to talk about audio interfaces.
In this article, we’ll explain what an audio interface is, what it does, and what you need to consider when looking for one.
Editor’s note: This article was updated on May 11, 2021 to include information on ingress and egress.
What is an audio interface?
Traditional microphones capture the sound you create in the air and transmit that signal as an analog electrical waveform. For this output to make sense to your computer, it must be converted into a stream of discrete electrical voltages corresponding to 1s and 0s. This is a digital signal. USB microphones handle this process internally. Standard mics use a balanced XLR (male) connector to output the analog electrical signal. There are also microphones that can output both analog and digital formats simultaneously.
See: Best USB interfaces: save to your computer
So how do you connect a microphone’s XLR to a computer? Well, that’s where a handy device known as an audio interface (sometimes referred to by the legacy term “sound card”) comes in. Today we are 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 second hand market. We recommend avoiding them as many are no longer fully supported by the manufacturer or by current operating systems.
How do audio interfaces work?
Audio interfaces provide multiple functions, both analog-to-digital (ADC) and digital-to-analog (DAC) conversions are performed by the interface, and most have 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 fit comfortably in a bag with a laptop.
Each mic channel must be able to supply +48V for condenser microphones and have variable input gain covering at least 60dB. Typical features that join the fun as prices rise include additional headphone jacks and volume controls. Mic preamps can be further advanced with the addition of: high pass filters to eliminate rumble; attenuator pads for loud signals; phase inversion 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 a certain combination. It’s not something that adds much value, so don’t overspend on that.
Audio interfaces make it easy to connect a microphone’s XLR to your computer.
Even basic interfaces should provide one or two high impedance instrument inputs for direct injection (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 multiple sources at once, such as a live drum kit mic, you may need more than the small number of analog inputs available on the basic interfaces. If this scenario is in your future, and it will be if you like the idea of recording tapes, many interfaces will allow expansion via a connector carrying multiple digital inputs or outputs.
What type of interface do you need?
We have already mentioned that some microphones 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 probably won’t need an interface at all. If you’re fixated on a mic with an analog output, read on.
Before choosing an interface, you should consider the following points:
- How many microphones do you need to record at the same time?
- 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 delve into sample rates and bit depth. Instead, we’d just recommend avoiding anything that doesn’t support at least 24-bit at 48kHz, as that’s a realistic minimum performance goal for capturing audio for most applications. .
The number of microphones you need to record at once determines the number of XLR inputs with mic preamps you need on the interface. If you’re using equipment that provides line-level signals, such as outboard mic or guitar preamps, outboard processors, or older synthesizers, count the total number of analog inputs. Since mic preamps generally accept line levels without issue, you need to think about which mic and line input combinations you should consider. Four entries should cover most home recording scenarios.
Although it’s not 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 suffice, assuming you don’t intend to bring a signal back into the analog world for processing (or re-amping guitars) and just need power your monitor speakers. If you are interested in monitoring with a surround system, you are looking for more than eight output channels (DAC). A separate stereo headphone monitor is very handy and will add two to the number of DAC channels you need.
Expandability and portability
Do you plan to undertake larger projects in the future? You may need to pick up more people or more instruments, so you should invest in an interface with additional digital inputs/outputs.
Depending on how many additional channels you want, consider S/PDIF (two channels), the popular ADAT port (up to eight channels per port) or the less common BNC MADI (up to 64 channels), which gives you will allow proof of extensibility without compromising portability. These will allow you to ‘overlay’ an external multi-channel AD converter, which can often also provide additional DAC channels, should you need them.
Portability is an important consideration if you want to take your recording setup on the road, to a jam room, or to a friend’s house. In addition to looking for something light and compact, consider that some simple interfaces are bus-powered, meaning they are powered by the host computer and don’t need a separate power supply. This makes setup faster, easier, and means you can operate where no power outlet is available as long as your laptop battery can keep going.
Theand benefits of Thunderbolt
The last thing to consider is latency: this translates to how much lag there is in your recording system. It will determine the delay you experience when 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 to a host computer. USB interfaces have a limitation on audio processing speed and round trip delay time. Some companies enhance this with proprietary audio drivers with their interfaces, but these tend to be more expensive. The absolute minimum delay is considered to be around 4.5ms for a round trip, but most take longer than that. Some products on the market have a simple workaround that allows you to mix the direct audio from the mic input with the headphone output, bypassing the internal processing lag.
If latency is really important to your application, take a look at Thunderbolt-equipped interfaces, which use the new peripheral connection standard popularized by Apple. This can provide lower latencies (less than 1ms) but less choice in terms of available hardware.
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