Audio Interfaces

What Is an Audio Interface Used For?

The audio interface is an integral part of any studio. An interface has the same responsibility in a studio of any size, from a spare bedroom to a sprawling, multi-million dollar facility. The job of an audio interface is to accept an incoming musical performance (MIDI or audio), send the performance into a DAW (Digital Audio Workstation) via the computer, and send the processed performance to a pair of speakers or headphones.

In this exploration of audio interfaces, we’ll cover:

  1. Bit depth and sample rate
  2. Data connection
  3. Inputs and outputs
  4. Compatibility
  5. Quality of the mic preamp(s)
  6. Analog to digital conversion

Let’s start by following the path of two different performances and examine how they interact with the interface.

If we want to record a vocal, the musical performance would be the voice. The voice is captured by a microphone that is connected to an audio input on the interface (most likely an XLR input). The vocal then passes through the interface’s mic preamp (more on that later) and is sent into the computer’s DAW via a data cable. When we create an audio track in the DAW, set the track’s input to receive the vocal from the audio input and press the record-arm button, we see and hear the voice reaching the track and outputting to the speakers or headphones.

If we want to record a MIDI performance, like a drum pattern or a synth melody, we would create an instrument track in the DAW and add a software instrument (drum machine or synthesizer) to that track. When we record-arm the instrument track and press a key on a MIDI controller, MIDI information travels from the controller to the interface via a USB or MIDI cable, is sent to the computer’s DAW via a data cable, reaches the software instrument and triggers sounds or samples stored in the instrument.

Which interface we select for our studio should be based on several considerations: the supported bit depth and sample rate, the type of data cable used to connect the interface to the computer, the type and number of inputs and outputs, the compatibility with the DAW, the quality of the mic preamps inside of the interface and the quality of the analog to digital conversion. In order to understand how to identify which interface would be best for any studio, let’s examine each of those considerations in detail.

Bit Depth and Sample Rate

Bit depth and sample rate are the quality of audio being recorded. Standard “CD Quality” is 16-bit, 44.1kHz. During production, audio can be recorded up to 24-bit, 192kHz. When working in the field of stereo music production, using a sample rate of 192kHz is uncommon. A sample rate of 192kHz is much more commonly used when recording sound libraries, such as orchestral strings, brass or percussion. Bit depths are commonly kept at 24 bit (unless the still-developing 32-bit float is desired). Stereo music production usually uses 24-bit, 48kHz wave files.

With this information in mind, not all audio interfaces can support 24-bit, 192kHz recording. However, not all recordings need 24-bit, 192kHz parameters. An interface that may not be suitable for recording orchestral samples may be well-suited for recording stereo music. Remember to keep the objective of your studio in mind; don’t spend extra money on an interface just because it supports a higher sample rate that you don’t intend to use!

Which interface we select for our studio should be based on several considerations: the supported bit depth and sample rate, the type of data cable used to connect the interface to the computer, the type and number of inputs and outputs, the compatibility with the DAW, the quality of the mic preamps inside of the interface and the quality of the analog to digital conversion.

Data Connection

The data wire that an interface uses to connect to the computer comes in the form of either USB, Thunderbolt or Firewire. This is how the audio or MIDI performance travels from the interface into the DAW (from the interface, though the data wire, into the computer, then into the DAW). Be aware of the type of data connection that an interface uses with regard to open ports on the computer and ever-changing technology.

Let’s say the interface we are considering uses a USB data connection. Audio interfaces tend to not play well with USB hubs. If the computer being used for production does not have enough open USB ports to support all of the required devices, consider making the necessary adjustments or going with a Thunderbolt interface. This also applies if the interface being considered is Thunderbolt: if all of the computer’s Thunderbolt ports are otherwise occupied, consider a USB interface.

As for the actual type of connection (Thunderbolt, Thunderbolt 2, USB 2, USB 3, USB-C/Thunderbolt 3, etc), consider modern technology. Purchasing a Thunderbolt 2 interface to use with a computer that has an original Thunderbolt port will not yield the full performance of Thunderbolt 2 speeds. Currently, most new interfaces are shipping with USB-C/Thunderbolt 3 data connections. Beware of purchasing an interface that uses a dated protocol such as USB 2, Thunderbolt, Thunderbolt 2 or any type of Firewire connection. The dongles required to use these older interfaces can become expensive and new technology will soon render these type of older connections obsolete. Purchasing an audio interface with an older data connection will be an uphill struggle from the start that only promises to get worse over time.

Inputs and Outputs

The type and number of inputs and outputs (abbreviated I/O) are directly related to the interface’s ability to record or monitor a performance. There are two types of inputs and outputs: analog and digital. Each of these types of I/O has different responsibilities and various styles of connections.

Analog inputs are used for recording a microphone, an instrument or a sound module. When using a microphone, an XLR cable is wired from one end of the microphone into an XLR input on the interface. An instrument, like a guitar or keyboard, uses a ¼”-style connector (also referred to as a “TS” or an instrument cable) wired to a ¼” input on the interface. A sound module, like a drum machine or rack-mount synthesizer, uses a “balanced” ¼”-style connector (also referred to as a “TRS”) wired to a ¼” input on the interface.

Analog outputs are used for wiring the interface to a pair of speakers or headphones. An XLR or TRS connector is wired from the left and right output jacks on the back of the interface to each speaker. The headphone output is often located on the front of the interface and is a female TRS connection that connects with the male end of the TRS from the headphones.

Digital inputs often come in the form of Lightpipe (ADAT/Optical), S/PDIF or AES/EBU. These type of inputs can not be used to connect a microphone, XLR, TS or TRS to the interface. Digital inputs exist on most interfaces and are often used to record performances from a hardware unit to avoid an analog-to-digital conversion (for more on this, keep reading). These hardware units may include the very same keyboards and sound modules as previously mentioned, but the digital output of the unit is wired to the digital input of the interface and doesn’t use any sort of analog connection.

Digital outputs are not available on all interfaces and are not commonly used. One use for digital outputs may be to wire the interface to a mixing console for a live show or performance.

MIDI input from a MIDI controller is commonly wired via USB, either directly to the computer or through a MIDI or USB port on the interface. If a MIDI output exists on an interface, it uses the traditional five-pin MIDI cable and allows musical information to be sent out of the DAW into a hardware unit.

Use caution when researching the amount of I/O listed on an interface’s description before considering it for purchase. The description of the interface may say something like “20 inputs and 10 outputs.” This does not necessarily mean that we can use twenty microphones and record a live drum kit. Figure out how many inputs are analog and how many are digital, then find out which type of connectors each of them uses. The same cautious approach should be applied for outputs and will enable you to make an informed decision as to which interface will work well in your studio.

The quality of the mic pre being used matters just as much, if not more than the actual microphone.


Modern technology and competition between manufacturers have allowed most interfaces to become compatible with most any DAW. After the decision of which DAW to use has been made (Logic, Pro Tools, Ableton, FL Studio, etc), the compatibility question becomes how well the interface interacts with the DAW, rather than if it is compatible with the DAW. The compatibility consideration is most evident in the input and output routing area of the DAW. For example, let’s say the interface we are using has two analog inputs and four digital inputs. Depending on compatibility, the DAW will recognize these inputs in one of two ways. The first way is that all of these inputs are recognized in order and listed as inputs 1-6. The second way is the DAW only recognizing the analog inputs, therefore only providing options for input 1 and input 2. If this second scenario is the case, the DAW’s I/O setup must be configured and pointed to the digital inputs so they are recognized and accessible.

Taking the extra time to configure inputs and outputs can be a cumbersome process, especially if you are making your first foray into recording your own music. To this point, software companies such as Avid (Pro Tools) have agreements in place with hardware companies such as Apogee (Duet) to issue specific versions of interfaces that are tailored to a particular DAW and its operating functions (an example of this is the “Pro Tools Duet” interface). Researching interfaces for their compatibility with your selected DAW will help ease the learning curve and allow you to begin recording more quickly.

Quality of the Mic Preamp(s)

Every microphone needs to pass through a preamp in order for it to be heard and recorded. There is no exception. I can not stress this enough:

The quality of the mic pre being used matters just as much, if not more than the actual microphone.

Some high-end interfaces don’t have any mic pres at all: it is expected that the microphone will have already passed through a mic pre before it reaches the interface. Low to moderate level interfaces often contain mic pres. These type of interfaces vary in mic pre quality. It is safe to say that more expensive interfaces have better built-in mic pres.

Regardless of the interface you choose, make sure you are getting the best mic pre available. It will greatly enhance the sound of your recording.

Analog to Digital Conversion

Anything being recorded through the analog inputs on the interface needs to be converted to digital binary code (1’s and 0’s) for sophisticated editing purposes in the DAW. This process is called Analog-to-Digital conversion (abbreviated A/D). Not all interfaces are created equally when it comes to A/D conversion.

The internal components of the converter matter greatly, but again, it is a safe bet to say that the higher-end interfaces generally have better converters. When the A/D process is performed by inferior converters, the performance will lose sonic integrity very quickly and sound bland before it even reaches a track in the DAW.


With so many options from which to choose, how do we begin the process of finding the right interface for our studio? Depending on the objective of your studio (what you are recording) and your budget, make the decision to go with a starter level, intermediate or premium interface.

An Apogee Duet ($595) or Universal Audio Apollo Twin ($899) provide solid solutions for the starter level project studio. Each of these interfaces has different data connections, mic preamps, compatibility features and conversion quality. Research will reveal which is the right choice for your studio.

An option for the intermediate studio is the Universal Audio Apollo 8 Duo ($1,999). There are better converters, Unison brand mic pres, 18 total inputs, 24 total outputs and they can be daisy-chained with additional Apollo interfaces (connected in series, one after another).

A premium interface that meets the demands of the high-profile studio is the Antelope Orion 32HD ($3,495). It has a whopping 32 channels of analog I/O, Lightpipe I/O, excellent converters and a master clock to regulate sample rate. However, be aware there are no mic pres on this interface: outboard or external preamps will be required (and will be an additional expense).

Final Notes

Many interfaces from any level often ship with companion software. This software can be used for basic functions, such as applying phantom power, changing the voltage an analog input is accepting or flipping the polarity of an input or output. Some companion software offers advanced functions, such as internal routing and additional gain stages.

When the time comes to consider an investment in recording equipment — whether it’s $800 or $8,000 — make sure an audio interface is a line item in your budget. After considering the objective of your studio and researching which interfaces fit the budget, examine all of the aforementioned considerations. Making a smart investment in an interface will accelerate your development and provide room to grow in the future.

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