The audio interface is at the centre of every modern studio, but how do you choose the right one for you?
Thirty years ago, control rooms contained big mixers and racks of audio processing equipment. Today, the computer is the cold robot heart of the studio, and software has replaced the console and outboard. This change means that one piece of equipment has become indispensable. You can't run a computer-based studio without an audio interface — but how do you know which interface is right for your computer-based studio?
Getting sound into a computer is a two-stage process. First, an analogue signal is converted to a stream of numbers. Then this stream of numbers is fed into the computer. This second stage is the core function of an audio interface, but nearly all of them do both.
In other words, most interfaces can accept analogue signals directly. What's more, they can often take two different types of analogue input. A 'line' input expects signals at a standard studio level such as is generated by synthesizers, mixers or mic preamps. However, we also want to plug microphones and electric guitars directly into our audio interfaces. These put out feebler and less predictable signals, which have to be preamplified before they can be digitised. Most project-studio interfaces thus feature analogue inputs with mic preamps and/or sockets to plug a guitar into. Often, you'll find dual-purpose sockets which combine an XLR for mic input and a quarter-inch jack for line input.
So one of the most basic questions to ask yourself is: how many analogue inputs do I need, and of what type? As a first step towards answering this question, count up the number of sources you're going to want to connect simultaneously. If you only ever record yourself, you might only need one or two inputs, even if you're building complex tracks by overdubbing. If you record bands live, you may require enough inputs to cover a multi-miked drum kit, several other instruments and vocals.
In a studio where the audio interface is the only item of hardware, you'll need it to have as many mic preamps as you're ever likely to want to connect mics. But if you plan to use your audio interface with a hardware mixer, or you only ever record synths, or you have other equipment already that can amplify the signals from your mics, you might well prefer to get an interface that has only line-level I/O.
This only gets us so far, because there are a lot of audio interfaces that seem to offer identical analogue input facilities. If you have to choose between several models with the same features, you should also consider the following:
- The audio performance of interfaces varies, and some of these differences can be important. Read the 'Which Specs Matter?' box for more details.
- Some interfaces have separate mic and line sockets for the same inputs. This allows you to leave mics and line-level sources permanently connected and switch them on the fly as needed, rather than having to re-plug.
- On some interfaces, mic preamp gain is adjusted digitally. This is more precise than using an analogue control and means that settings can be fully recalled and sometimes even stored with your DAW project.
- If you use capacitor microphones, you'll need your interface to offer phantom power. Nearly all do so, but sometimes this is only switchable globally or in groups. This can be relevant if you want to connect things like ribbon mics, which shouldn't encounter phantom.
Audio interfaces perform a comparable two-stage job at the 'back end' of the system, spitting digital audio out of the computer and turning it into an analogue signal. The most basic reason for this is so that we can connect speakers or headphones to hear sound coming out of it! Nearly all interfaces therefore feature at least one pair of line-level outputs and at least one stereo headphone socket.
Again, the key question to answer is: how many outputs are you likely to need? Bear in mind that there are almost no audio interfaces with more than two headphone outputs. For recording more than two musicians at once, you'll thus need a dedicated multichannel headphone amp. This, in turn, will need to be fed from its own pair of line-level interface outputs. Bear in mind, too, that unless you have an outboard monitor controller with built-in speaker switching, you'll also need a separate pair of line-level outputs for each pair of speakers you have.
Remember, also, that outputs aren't only used to feed monitoring systems. To integrate hardware compressors or equalisers into your software mixes, you'll need line-level outputs to feed them and line-level inputs to receive the processed signal. The same goes if you want to mix on an analogue console.
Once again, considering only the raw numbers will probably leave you considering lots of interfaces with the same basic output arrangement. If so, it's time to ask yourself some more detailed questions that will help you find the most suitable interface for your needs.
- Most audio interfaces provide some monitor control, but the features on offer vary wildly. At its most basic, this is a simple level control for one pair of outputs. At its most sophisticated, you might have configurable control over multiple outputs, along with additional features such as talkback, dim, speaker switching, a button for checking your mixes in mono, and so on.
- Headphone outputs don't necessarily all appear as separate destinations in your recording software. Sometimes they duplicate what's feeding one pair of line outputs, though you'll often have some choice about which pair.
- If you use modular synthesizers such as Eurorack devices, you might want to consider buying an audio interface with 'DC-coupled' outputs that can generate a steady-state or DC voltage. These can produce the control voltage (CV) signals used to drive modular synths, allowing you to sequence your synths in software. (See the 'Modular Interfacing' article elsewhere in this September 2020 issue for an in-depth guide to this.)
A little research into larger interfaces will reveal that once you get beyond eight mic inputs, your options narrow dramatically. Yet eight mic inputs is not enough to track a band. What if you need 12, 16 or even more mic inputs?
One option is a mixer that incorporates an audio interface. Products like the Zoom L‑12 and L‑20, Behringer's X series, the Tascam Model 16, Soundcraft's UI mixers, PreSonus's StudioLive consoles and the QSC TouchMix‑30 Pro provide lots of inputs, and can take care of live sound and cue mixing as well as recording. They can be excellent choices for people who work primarily with bands, but there's a lot of variation in the features on offer. A detailed comparison is outside the scope of this article — but very much within the scope of Chris Korff's 'Digital Mixers In The Studio' article elsewhere in this issue!
A second option is to attach more than one audio interface to your computer. Usually, this means buying two interfaces from the same manufacturer, and even then, multiple working is hardly ever supported on USB audio interfaces. Interfaces that can be connected in multiples usually do so using Thunderbolt ports or PCIe slots; not every computer has these, and interfaces that exploit them tend to be more expensive than USB equivalents.
Fortunately, there's a third option: adding more analogue inputs to an existing interface. Interfaces often provide more paths to the computer than they have analogue inputs. These paths are fed by digital inputs, which can accept signals already converted to digital data by some other device. Indeed, some interfaces only offer digital inputs, the idea being that users can assemble a bespoke system by choosing their converters separately.
Digital audio can be encoded electrically, but it can also be represented optically, using pulses of light. The key point is that the encoding scheme and the physical medium are independent of one another. The most common optical connector is thus used for two different, incompatible types of digital audio data — one of which can also be sent over an electrical wire! That data type is S/PDIF.
Connecting an optical or electrical S/PDIF cable from the output of one device to the input of another will carry two channels of digital audio between them. However, the same cables that carry optical S/PDIF signals are also used for a multichannel format known as ADAT or Lightpipe. If you're working at the standard 44.1 or 48 kHz sample rates, a single Lightpipe connection can carry eight channels of digital audio in one direction. In the project-studio world, this makes it ideal for affordably expanding an audio interface, and there are lots of rackmount units on the market that offer eight channels of analogue-to-digital conversion with ADAT connections.
Beyond the project-studio world, you'll find many other digital formats, most of which need not concern us here. Assuming your expansion plans are centred around ADAT and/or S/PDIF, there are a few considerations to bear in mind:
- Optical connectors are usually switchable between ADAT and S/PDIF, but this is not always the case, so do check that optical S/PDIF is supported if you need it.
- Many interfaces carry two pairs of optical connectors. Sometimes these can be used to add two eight-channel ADAT expanders, for a total of 16 extra channels. In other cases, though, the limit is fixed at eight channels, and the second connector is used only at 88.2 or 96 kHz sample rates; at these rates, twice as much data needs to be moved around, and eight channels' worth of audio will no longer 'fit' down a single optical link.
- Not all interfaces allow you to use ADAT and S/PDIF expansion at the same time, even if both sockets are available.
- In order to communicate successfully, digital devices need a shared timing reference, so once you introduce any sort of digital connection into your studio, you'll need to learn about clocking. When only two devices are involved, this is usually straightforward, and it will be explained in the product manuals.
In an ideal world, we'd monitor what's being recorded through our recording software. The problem is that getting the data into and out of the computer takes time, and if it takes too much time, there's an audible lag between playing a note and hearing it on headphones or speakers.
The total time taken for a signal to travel through a recording system, from source to monitor system, is known as the round-trip latency. Some people are more sensitive to latency than others, but once it gets much above 10 milliseconds, most will notice it. Interfaces are supposed to report their latency to the host computer, but many do not do so accurately. Latency is adjusted using a setting called buffer size. The lower the buffer size, the lower the latency — and the greater the demand on the computer's Central Processing Unit.
For any given buffer size, some interfaces will perform better than others, both in terms of the CPU load and of the actual latency they deliver. Thunderbolt and PCIe interfaces often outperform USB interfaces here, but another important factor is the driver software that handles data transfer between interface and computer.
Many USB interfaces are 'class compliant', and can use the Apple driver built into the Mac OS operating system. This is good enough for most purposes, but interfaces that employ custom driver software usually perform even better. This includes some USB interfaces, and all Thunderbolt and PCIe models.
On Windows, recording software uses the ASIO driver format developed by Steinberg. This isn't part of Windows, so you'll always need to install a driver, and the quality of these is quite variable. Many manufacturers of USB interfaces license third-party driver software from developers such as Thesycon, whilst other manufacturers code their own drivers. The latter usually offer better performance, but the situation is complex and it isn't always easy to tell what driver a given interface uses. For more detail and for rigorous measurements of low-latency performance on Windows computers, a visit to Vin Curigliano's DAWbench.com website is essential.
In general, low-latency performance is better today than 10 years ago. But even with the best drivers, a round-trip latency of under 5ms can be hard to achieve, especially on USB interfaces. For this reason, many audio interfaces build in a mixer which allows us to hear input signals without waiting for them to pass through the computer and recording software.
On some small 'desktop' interfaces, this mixer is controlled using a simple knob that adjusts a balance between input signal and playback from your recording software. Where more than a couple of inputs and outputs are concerned, though, manufacturers build in a digital mixer controlled from software.
Manufacturers take varied approaches to the design of digital mixers and the software that controls them. Some build in very powerful and complex mixers with endless routing options. Others concentrate on simplicity and ease of use, offering just enough functionality to cater for typical use cases. Yet others build in not only mixing features but also plug‑in equalisers, compressors, reverbs and other signal processors. Digital mixers in interfaces can sometimes be controlled remotely from tablets and phones, and even sometimes within recording software by the same manufacturer.
Which of these approaches suits you is a matter of personal taste, but be aware that all of them can be implemented well or badly, and it pays to do some research. Read SOS reviews and check user forums online before parting with your cash. This is an aspect of interface design that's easily overlooked, but it will affect your day-to-day experience with the product like nothing else.
Besides the core features I've already described, there are other features that vary between audio interfaces. If you're still struggling to choose, perhaps these considerations will help to tip the balance:
- Small interfaces can often be 'bus powered' through the USB or Thunderbolt cable, and some offer no alternative. Bus powering is convenient and makes the interface more portable, but can limit performance. For example, bus-powered interfaces often cannot drive headphones as loud as mains-powered rivals.
- Some mains-powered interfaces require an external power supply unit. This can be inconvenient, and a lost or damaged PSU will put your studio out of action until it can be replaced.
- Larger interfaces almost always adhere to the 19-inch rackmount format, but smaller ones come in many shapes and sizes, so ergonomic differences might affect your decision. If you are going to use your interface in a rack, do you want all the sockets on the back?
- Level meters provide indispensable information about the amplitude of signals going in and out of your interface. This information is always available in software, but most interfaces also provide hardware meters too. If the reassurance and immediacy of good hardware metering is important to you, be aware that the functionality on offer is very variable, ranging from a single LED on some interfaces to highly configurable, colour touchscreen displays on others.
- Some interfaces include MIDI In and Out ports for digital control of synthesizers.
Finally, always remember that an audio interface requires committed support from the manufacturer, for instance by providing driver updates. Some manufacturers have a better track record than others when it comes to providing this support, especially for discontinued models. Choose right, and your interface should last you through many OS and computer upgrades!
All manufacturers publish technical data about their products. Unfortunately, this often leaves out important measurements such as low-latency performance, but specifications can still be useful in deciding which model is right for us.
In some respects, the technical performance of modern audio interfaces is so good as to be a non-issue. For instance, all of them have a flat frequency response throughout the audible range, so won't audibly change the timbre of sounds going in or out. Many also offer a dynamic range of at least 110dB on both inputs and outputs, which is far more than is needed to capture any real-world signal. So an even higher dynamic range figure, for example, is perhaps best treated as a sign that the manufacturer knows what they're doing, rather than something that will directly benefit our recordings.
Specifications that make a real difference include gain range and maximum input level for mic preamps. The larger the gain range, the wider the range of input signal levels that can be accommodated. The maximum input level gives you a reference point for that versatility. If this is high — say, +23dBu — you'll know that you can safely record drums and other loud sources without fear of clipping. But unless the gain range is also high, it might be a struggle to get a respectable signal level on quiet sources, such as speech recorded with a dynamic mic. There is quite a lot of variation between interfaces, so it's worth thinking about what applications really matter to you. (Gain range is sometimes defined using maximum and minimum values, in which case you can calculate the range by subtracting the minimum from the maximum. If, for example, the minimum gain is -5dB and the maximum is +55dB, the total gain range is 60dB.)
If you record quiet sources, you want to be able to do so without adding unwanted noise. All mic preamps introduce some noise into the signal path, but some perform better than others in this respect. The key measurement here is Equivalent Input Noise or EIN. Look for the largest negative number and be aware that A‑weighted figures look better than unweighted ones. The very best preamps manage about -129dB unweighted, which equates to around -132dB A‑weighted.
On the output side, maximum output level for line outputs can be important if you want to connect your interface to old-school studio hardware. Most professional outboard is aligned for a maximum level of +20 or +24 dBu, but not all interfaces can generate this. This can mean that the outboard won't deliver optimum performance, and in extreme cases, you might struggle to get a hardware compressor to do anything if you can't feed it a strong enough level from your interface!
The built-in headphone outputs on audio interfaces also vary, and some can drive headphones louder than others. Unfortunately, this is a specification that is often presented in different ways or not at all, making it quite hard to compare rival products.
Four main connection protocols are in widespread use for connecting audio interfaces to Macs and PCs, though Ethernet is less relevant to project studios at present. I've summed up the main pros and cons of each in this table.
|USB||Affordable and simple to use, future-proof and available on all computers.||Low-latency performance can be indifferent, interfaces generally can't be used in multiples.|
|Thunderbolt||Usually offers very fast low-latency performance, easy to use, often permits interfaces to be used in multiples.||Not universal on Windows computers, more expensive than USB, cables are costly.|
|PCIe||Generally offers the best low-latency performance, often permits a choice of converters.||Not available on laptops, typically a high-end professional option.|
|Ethernet||Extremely flexible, especially in multiroom installations, allows very long cable runs, multiple computers can share access to system.||Complex and can be hard to set up, low latency requires a dedicated Ethernet card in the computer, rival incompatible standards.|