How to Expand Audio Interfaces

Plug into the easiest, most affordable options for adding input channels

They say you can never be too rich or too thin…but when it comes to audio interfaces, sometimes it seems you can never have enough inputs. Projects expand, mics multiply, and all of a sudden, you’ve run out of inputs. Or perhaps it’s a matter of convenience and efficiency, where you want enough inputs to be able to eliminate patch bays, and just be able to choose whichever input you want, whenever you want, at your recording software’s input.

So—do you need to buy a new interface? Maybe, but there are ways to expand the interface you already have by adding another audio interface, or using an interface’s ADAT optical input (if present) to expand the available number of inputs.

Adding Interfaces with Mac OS

Apple’s Core Audio protocol for handling audio was designed from the ground up to accommodate multiple interfaces—a process called aggregation. Open the Mac’s Audio MIDI Setup application, and then choose the Audio Devices window. Click the little + sign in the lower-left corner; an Aggregate Device box appears, and you’ll see a list of available I/O. Check the interfaces you want to aggregate, and then choose the desired clock source—most of the time whatever’s selected when you aggregate the interfaces will work (if not, try a different clock source).

If there’s no hardware sync between the interfaces (i.e., there’s no word clock connection), check the Drift Correction boxes (Fig. 1). Now all input and output options from the aggregated interfaces will be available in your host program.

1_Aggregation with Mac
Figure 1: Thanks to the Mac’s Core Audio drivers, PreSonus and TASCAM audio interfaces are being aggregated to create a single 38 input / 48 output interface for 96 kHz operation.

If you encounter any problems, go to the Audio MIDI Setup’s Help, and search on Aggregation. Choose Combining Audio Devices, and follow the directions.

Adding Interfaces with Windows

The most common Windows driver for pro audio is ASIO, which generally gives lower latency than Windows’ native audio drivers. However, it was not designed to handle multiple audio interfaces. Some manufacturers (e.g., RME, Roland, PreSonus) circumvent this by creating drivers that recognize some interfaces as one big interface instead of multiple interfaces. This can work quite well within whatever constraints the manufacturer imposes—the main one being that it works only with their interfaces, or only with their interfaces that use certain transfer protocols (e.g., FireWire or Thunderbolt).

However, Windows’ native drivers like WDM/KS, WASAPI, and WaveRT can indeed handle multiple interfaces if the interfaces are compatible with those drivers. WavesRT is the most modern Windows audio driver and offers performance that’s almost equal to ASIO. When you select a track’s input or output, all the interfaces that support the selected driver will show their available inputs or outputs respectively in your application (Fig. 2).

2_Aggregation with Windows
Figure 2: Acoustica Mixcraft, when set to Windows drivers instead of ASIO, shows all available drivers when selecting a track input for recording. You can also select a default driver. In this example, the Line 6 Helix’s audio interface is being selected as the track input.

Expanding Interfaces via the ADAT Port

The ADAT optical interface was invented to provide a way to stream 8 channels of digital audio to or from the Alesis ADAT digital tape recorder. Although ADAT tape recorders are long gone, the ADAT optical interface lives on because it’s a simple, efficient, inexpensive way to transfer audio, in real-time, from one digital device to another. Because combining different interfaces via ASIO from different manufacturers can be problematic, it’s fortunate there’s an alternative: interconnecting ADAT-compatible inputs and outputs.

Setup

Suppose you have an interface with eight mic/line inputs, which may not be enough to handle the multiple outputs available on various hardware synthesizers, multiple microphones, instruments, and the like. However, if the interface has an ADAT input, then various ADAT-compatible mic preamps and interfaces can send their outputs to the interface’s ADAT input, essentially turning the original interface into one with up to eight more inputs. Many audio interfaces, like the Focusrite Scarlett 18i20, (Fig. 3), Steinberg UR816C, RME Fireface UC, Universal Audio Apollo Twin USB DuoTASCAM 102i, PreSonus Studio 1810c, PreSonus Studio 192, TASCAM 208i, Behringer UMC1820, MOTU 828mk3, and several others, offer one or two ADAT inputs.

3_Focusrite 18i20
Figure 3: The Focusrite 18i20’s rear panel has dual ADAT input and output ports (outlined in white). 

Regarding ADAT signal sources, units like the MOTU 8pre (Fig. 4), Focusrite Scarlett OctoPre, PreSonus DP88, and others provide mic/line preamps with an ADAT output. Note that the signals carried over ADAT are digital, so there’s none of the audio degradation that could occur with an analog sub-mixer—the signal appearing at your recording software from the expansion mic preamp is no different from the signal appearing at the expansion mic preamp’s input.

4_MOTU 8pre
Figure 4: Although MOTU’s 8pre can serve as a primary audio interface, it also has ADAT outs to send eight preamp outputs to ADAT inputs. The two ADAT ports allow operation up to 96 kHz (as described later), if the main unit has dual ADAT inputs to accommodate the higher sample rate.

Some units can serve as either an interface or stand-alone set of mic preamps. For example, the TASCAM US-20×20 interface has an ADAT optical output but also the option to switch it into a Mic Pre mode. It then no longer needs to connect to your computer via USB, and the eight mic inputs feed both the analog and ADAT outputs simultaneously (Fig. 5). You can control gain directly from the US-20×20’s front panel controls, so it’s not necessary to load its mixer application. Interfaces that don’t have a mic pre mode are still suitable, as long as you can route their preamps to an ADAT-compatible output.

5_TASCAM Mic Pre Mode
Figure 5: In Mic Pre mode, as soon in this applet, the US-20×20’s mic preamp outputs go to both analog and ADAT digital outputs.

Note that unlike aggregating another interface, adding preamps with an ADAT output doesn’t take up USB or other bus bandwidth, as would be needed by a second interface.

Interconnecting an Audio Interface and Expansion Unit

The process for interconnecting two units with ADAT interfaces is pretty painless.

  1. If the unit intended to expand your interface has a special mode for sending its inputs to the ADAT output, select it (e.g., the TASCAM US-20×20’s Mic Pre mode).
  2. Patch a TOSLINK optical cable from the expander unit’s ADAT out to your main interface’s ADAT in.
  3. Open up your main interface’s setup or mixer application, and (important!) choose ADAT as the sync (or clock) source (Fig. 6).
6_UC set to ADAT
Figure 6: Choosing ADAT as a clock source (outlined in white) in PreSonus’s Universal Control application allows a compatible interface to sync properly to an ADAT-based expansion device.

The output signals from the expander will now be available. You may need to enable these in a preferences menu as new potential inputs, or assign the expander’s outputs to your recording software’s inputs (Fig. 7).

7_Pro Tools IO Setup
Figure 7:  In Avid Pro Tools, the eight channels of an expander appear as optical inputs 11-18, which are linked to specific inputs within Pro Tools. For example, if you choose “Mic Narration” as an input for a channel in Pro Tools, it will record the expander signal coming in on optical input 12.

Although that’s pretty much all you need to do, there are a few fine points.

  • A single ADAT optical connection defaults to carrying 8 channels at 44.1 and 48 kHz sampling rates. With the ADAT S/MUX mode that allows for higher sample rates, the channel count goes down (4 channels at 96 kHz, and 2 channels at 192 kHz). Note that not all ADAT-compatible devices support S/MUX.
  • Units with dual ADAT inputs can expand to up to 16 channels, or have a higher channel count with higher S/MUX sample rates, that units with a single input.
  • You won’t be able to set a sample rate higher than 44.1 or 48 kHz if the main interface syncs to the ADAT clock, and the expander module can’t do S/MUX sample rates.
  • The expansion interface doesn’t have to be next to your main interface; the length is limited only by the optical cable. The ADAT spec assumes a maximum length of about 16 feet, but with high-quality optical cables, this can extend much further. So you could have, for example, the “expander” next to a keyboard rack, and not have to run a bunch of cables from the various keyboards to the main interface.

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