Audio Interfaces & the Numbers That Matter

Audio interfaces are essential items for modern music makers. They take the sound of the physical world and translate it for your computer to process inside your Digital Audio Workstation (DAW). They also take computer audio and convert it back into electricity ready to be sent to your speakers and headphones. In this article we'll look in more detail at what audio interfaces do, define common terms you'll encounter, and discuss the numbers and features that make a difference to you when recording or listening to audio in a typical home studio.

What Does an Audio Interface Do?

An audio interface is an electronic device that acts as a translator between the analogue and digital worlds. They can also be referred to as sound cards, but it's an older name that no longer fully encapsulates everything these devices can achieve. Someone talking about a sound card these days is mostly likely referencing an audio interface that plugs directly into the PCIe slot on their computer motherboard.

Audio interfaces use specialised computer chips called Analogue-to-Digital Converters (ADCs) to turn real world sound into ones & zeros. They then use Digital-to-Analogue Converters (DACs) to do the opposite. Depending on the size of the interface, it will have a number of inputs and outputs for that conversion, and may even do some effects processing on the audio before it hits your computer or gets piped latency-free to your monitoring headphones. The quality of these converters is the most important consideration when purchasing an audio interface.

Sample Rate

The sample rate (or sampling frequency) measures the number of times per second an audio interface 'samples' analogue audio to be converted into data. This is like the 'refresh rate' of your television or 'frame rate' of your video game console. Much like these examples, the higher the sample rate, the smoother the audio with feel. This is incredibly important for genres such as classical and jazz, where capturing the subtle nuances of its acoustic instruments is paramount.

Sample rate is expressed by a two- or three-figure number with 'kHz' after it. Hertz measures the amount of times something happens per second. 44.1kHz (CD quality) is 44100 samples per second. The majority of audio interfaces these days support up to 96kHz or 192kHz. More is better with two caveats. Namely, the quality of components is more important than any potential sample rate, and recording at 24-bit 192kHz will take a not-insignificant amount of data (~ 33MB per minute).

Bit Depth

The bit depth defines the fidelity of each sample, or rather, the amount of ones & zeroes your audio interface can use to encode the sample's volume. This is like the 'resolution' or perhaps 'colour depth' of your computer screen. In music, this is known as 'dynamic range' and you'll know you don't have enough when your quiet moments have electrical noise (the noise floor) and your loud moments lack the impact to contrast with them. As with sample rate, more bit depth costs more data on your hard drive.

Bit Depth is expressed in bits, with 1 bit roughly equalling 6dB. 16-bit (CD quality) is therefore 96dB of dynamic range. It also represents 65,536 (2^16) possible data values for encoding. Compare this to 24-bit with 144dB of dynamic range and 16.7 million data values available, and you can see how the latter provides more 'breathing room'. 32-bit 'float' is essentially 24-bit with clipping protection. It's fantastic at a software level but isn't a killer feature in any but the most extreme recording environments.

Channels

Channels are the number of audio streams an interface can process at one time. This allows you to record multiple devices into separate channels in your DAW simultaneously. Two channels is perfect for two mono (guitar + vox) or one stereo (synth/sampler/FX) signal, eight would capture most of the individual outs of your drum machine, while 16 channels is enough to track a live band with a fully-mic'd acoustic drum kit.

It's important to note that the number of channels an audio interface supports may not match the number of inputs that exist on the physical unit. Formats such as ADAT, MADI, and Dante allow you to send multiple channels over a single, specialised connection with the use of an external adaptor. We'll describe these in more detail below, but the basic takeaway is that these are best considered if you need a lot of (8+) channels.

Inputs/Outputs & Preamps

Your inputs and outputs (I/O) are the physical jacks/ports that connect your audio interface to the outside world. This includes analog audio, monitoring, MIDI, digital audio, and word clock. Inputs and outputs are some of the most important things to consider when choosing an audio interface. There's no point recording your jazz trio in 24-bit 192kHz if it's only through a pair of room mics.

Most musical instruments output audio at relatively low volumes (~ -10dB to -60dB). Preamps raise this to the standard operating level of your audio interface (+4dB). This 'magnification' is incredibly sensitive to the quality of components used in the preamp. Higher-end interfaces might favour preamps with enhanced 'clarity' or an attractive 'colour', but they will almost always be flat-out better than a cheaper unit.

Computer Connections

The USB/FireWire/Thunderbolt connection on your audio interface sends digitised audio (and MIDI) information to your computer and receives your software audio in return. USB (1, 2, & 3) is the most commonly-supported protocol as it connects natively with just about any computer. FireWire (400/800) is an older format not found on many PC motherboards and was thus mostly adopted by Mac users. Thunderbolt (1, 2, & 3) is a newer protocol championed by Apple and supported on higher-end PC motherboards.

The most important consideration with computer connections is what you physically have available to you. PC users simply mightn't have a motherboard that supports thunderbolt, while Mac users may have more spare Thunderbolt than USB ports. The transfer speeds of these formats are so high as to be largely irrelevant. The 480Mbps of USB 2 is more than enough for most home studios, and USB 3 is 10x faster at 4.8Gbps. If you need the fastest thing available, Thunderbolt offers up to 40Gbps.

Digital audio connections transmit vast amounts of data over a multitude of channels. They are crucial in professional recording and broadcast studios.

AES/EBU (Audio Engineering Society/European Broadcasting Union) is used to transmit 2 channels of 24-bit 192kHz data over coaxial or fibre-optic cable.

S/PDIF (Sony/Philips Digital Interface) is the commercial variant of AES/EBU and is mostly found sending compressed 5.1/7.1 surround sound to home theatre systems.

ADAT (Alesis Digital Audio Tape) uses an optical cable and supports up to 8 channels of 24-bit audio at 48kHz or 4 channels at 96kHz.

MADI (Multichannel Audio Digital Interface) is a flexible system that sends 64 channels of audio at 48 kHz, 32 channels at 96 kHz, or 16 channels at 192 kHz over optical cable.

Dante (Digital Audio Network Through Ethernet) is a highly-scalable system that supports up to 512 channels, 196kHz, and 32-bit float on up to 10Gbps ethernet cables.

Word Clock

Word Clock is the clock signal used to sync the samples which comprise an audio signal. All digital audio devices have a word clock which defines when samples are recorded or played back. If you have multiple, unsynchronised devices in your studio, your samples and their component waveforms can get cut off, creating clicks and pops. High-end audio interfaces typically have word clock send and receive, which allows them to be the master clock for your setup or to be 'slaved' to a dedicated unit. Word clock shouldn't be a consideration for most home studio setups with a single audio interface, an analogue mixer, and a bunch of instruments.

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MIDI

Audio interfaces may include MIDI I/O. This allows you to sync instruments you're tracking to your DAW. It should be noted that external clock sources are going to be more stable than computer-generated ones. This is due to the fact that CPUs don't prioritise the DAW processes running the clock. It's better to clock both your DAW and your instruments to a semi-decent drum machine, sampler or synth. This isn't a critical feature if one of your aforementioned devices has a USB MIDI connection.

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DSP

Digital Signal Processing is the process by which some audio interfaces apply software processing to your signal before it hits your DAW/Speakers. This frees up CPU resources, gives you a self-powered plugin collection, and provides latency-free monitoring that's closer to your vision than a bone-dry vocal. Audio interfaces that support DSP usually either come bundled with more than you'll ever use or charge hundreds of dollars for near-perfect emulations of hardware that costs more than your car.

Price

Price is an especially important consideration for audio interfaces. The three most important purchases you can make as a musician/producer are your main instrument/controller, studio monitors, and audio interface. These are central to your workflow and its advisable to spend as much as you can reasonably afford.

Increased cost should either translate into a product with higher-value components or an expanded feature set. Now, while the latter is nice to have, there's no point paying for something you aren't going to use. A simpler interface at the same price point is probably packing better converters and preamps.

When selecting an audio interface, try to identify the features you can live without. This could be extra channels, specialised connections, or DSP. Then look at products in the upper bracket of your price range that only do the things that you need them to. These should provide the best value in terms of sound quality.

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Audio Interfaces & the Numbers That Matter

What Does an Audio Interface Do?