GearCast - 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 interfaceDefinition:
A device that adds I/O capability to another device. Common would be a MIDI, audio or video interface for a computer. you already have by adding another audio interfaceDefinition:
A device to convert audio signals into digital code suitable for use by a computer; typically communication occurs through a port such as USB, Thunderbolt, FireWire, etc. , or using an interface’s ADATDefinition:
(Alesis Digital Audio Tape): An S-VHS tape-based, 8-track digital audio recording format, syncable with additional units to add tracks in groups of 8. 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 MIDIDefinition:
(Musical Instrument Digital Interface) A hardware/software standard for communication of musical data digitally among electronic instruments, effects, and computers. However it is also used for to control lighting, pyrotechnics, theatrical displays, and mechanical devices. For example, the fountains at the Bellagio in Las Vegas is controlled by MIDI. Also called MIDI 1.0. Setup application, and then choose the Audio Devices window. ClickDefinition:
To press one of the switches on a computer's mouse to initiate a function or modify a parameter as-in a "mouse click". the little + sign in the lower-left corner; an AggregateDefinition:
The ability to use multiple audio interfaces with a single computer. Device box appears, and you’ll see a list of available I/ODefinition:
(Input/Output): 1. A pair of connections that provide input(s) and output(s).  2. The connections that link a computer to devices outside the computer.. Check the interfaces you want to aggregate, and then choose the desired clockDefinition:
The master timing reference used in an electronic system. The clock typically emits a fairly high-speed stream of voltage pulses or software timing messages. Each message signals some part of the associated system to proceed to its "next state" condition. 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 syncDefinition:
To make two or more pieces of equipment or signals synchronize with each other. between the interfaces (i.e., there’s no word clockDefinition:
A series of regular, highly accurate timing pulses equal to a digital system's sample rate, which provides a timing signal for interconnected digital equipment. It may be a separate device (e.g., clock generator) or embedded in a digital stream. Typically uses a BNC connector. 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.

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 driverDefinition:
A system software routine that provides a means of communication between a computer and an associated peripheral (audio interface, hardware controller. etc.). Drivers are updated often for improved efficiency, or compatibility with newer operating systems. for pro audio is ASIODefinition:
ASIO (Audio Stream Input/Output): A computer driver developed by German software company Steinberg to link hardware inputs and outputs with audio-based recording and playback programs. Used primarily with Windows operating systems., which generally gives lower latencyDefinition:
The amount of delay a signal, data, sound, video, or control signal acquires within a device, system, or long-distance connection. 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., FireWireDefinition:
Apple protocol (also IEEE-1394), developed with other companies including Sony and Panasonic, for digital data transfer at rates of 100-800 Mbps or ThunderboltDefinition:
A computer interface port or cable that multiplexes PCIe and DisplayPort data lines into two serial signals (along with DC power), which is then demultiplexed into separate PCIe and DisplayPort ports by the Thunderbolt controllers at receiving devices.).

However, Windows’ native drivers like WDM/KS, WASAPI, and WaveRT can indeed handle multiple interfaces if the interfaces are compatibleDefinition:
Capable of operating with a particular selection of hardware or software. 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).

Expanding Interfaces via the ADAT Port

The ADAT optical interface was invented to provide a way to stream 8 channels of digitalDefinition:
A signal or data expressed as series of the digits 0 and 1, typically represented by values of a physical quantity such as voltage or magnetic polarization. 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 USBDefinition:
(Universal Serial Bus) A standard, bi-directional serial connection between computers and peripheral devices. Duo, TASCAM 102i, PreSonus Studio 1810c, PreSonus Studio 192, TASCAM 208i, Behringer UMC1820, MOTU 828mk3, and several others, offer one or two ADAT inputs.

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 analogDefinition:
A signal that is continuous in nature, as opposed to being defined as a series of discrete numbers (or elements) as found in digital signals; electrically "analogous" to an acoustical signal in the air. 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.

Some units can serve as either an interface or stand-alone set of mic preamps. For example, the TASCAM US-20x20 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 gainDefinition:
The amount of amplification provided by an amplifier circuit, expressed in dB or numerically as in "3X" = "3 times". directly from the US-20x20’s front panel controls, so it’s not necessary to load its mixerDefinition:
Combiner, controller, and router for multiple audio or video signals. 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.

Note that unlike aggregating another interface, adding preamps with an ADAT output doesn’t take up USB or other busDefinition:
1. In audio, a common point that combines multiple signals, as in a "mix bus."  2. In computers, a path that interconnects various elements for transferring data among those elements. bandwidthDefinition:
A range of frequencies that a device can generate, reproduce, or process. This usually implies little or no alteration within the range., 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-20x20’s Mic Pre mode).
2. Patch a TOSLINKDefinition:
   (Toshiba Optical Link) A connection for stereo S/PDIF and 8 channel ADAT that uses optical cables. optical cable from the expanderDefinition:
   The inverse of a compressor in that it increases dynamic range. It can be used as effect, or in tandem with a compressor to reduce noise in a medium. See Compressor. 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).

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

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 kHzDefinition:
  Abbreviation for Kilohertz: 1,000 Hertz, or one thousand cycles per second. sampling rates. With the ADAT S/MUX mode that allows for higher sample rates, the channelDefinition:
  1. In radio, television, citizen's band radio, and other wireless communications modes, a specific frequency for communication.  2. In mixers and video, an input or output signal path with controls.  3. In MIDI, one of 16 data paths. 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 rateDefinition:
  The number of times per second that an analog signal is sampled prior to conversion into digital via an analog-to-digital converter. See Nyquist Frequency, Analog-to-Digital Converter. 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 rackDefinition:
  A frame for holding gear that meets an industry standard for enclosures. This specifies a 19 inch width, and a height that is some multiple of 1.75 inches. A rack frame can be open or enclosed, and has rails drilled with standardized screw holes for attaching rack equipment., and not have to run a bunch of cables from the various keyboards to the main interface.