Connection Type and Latency
Latency - the delay caused by the time it takes audio to pass from the input to your audio interface through the ADC (analog to digital converter) through your DAW and then out via the DAC (digital to analog converter) - is one of the major factors engineers consider when choosing a new audio interface. Largely dictated by the connection type between the interface and the computer, this is a particularly important consideration if you plan to record with plug-in effects and processors or if you're dealing with high track counts. In general, the faster the connection, the better performance you can expect from your audio interface.

USB Audio Interfaces
While you might encounter an old USB 1.1 audio interface, USB 2.0 is the most common connection type on the market, providing moderate speed and near universal compatibility on Mac and Windows PCs. To overcome latency issues, many USB audio interfaces include "latency-free" monitoring options, which range from analog routing options that let you simply monitor the live input to more advanced onboard digital mixers that may even include DSP effects/processing. USB 3.0 is over 10 times faster than USB 2.0 and supports higher track counts with far less latency. Class-compliant USB audio interfaces, such as the Antelope Audio Orion 32, can provide up to 24 channels of iPad recording via the Apple Camera Connection Kit.

FireWire Audio Interfaces
There are two types of FireWire found on audio interfaces: the older FireWire 400, which is the same speed as USB 2.0, and FireWire 800, which is almost twice as fast. For years, FireWire was the standard for high-speed audio interfaces and was found almost exclusively on Mac computers. Most modern computers connect to FireWire audio interfaces via FireWire to Thunderbolt adapters. FireWire delivers ample performance for most project studio needs, and FireWire audio interfaces have the benefit of being extremely affordable while providing high channel counts.

Thunderbolt and PCIe Audio Interfaces
Audio interfaces connecting via Thunderbolt offer connectivity speeds twice as fast as USB 3.0 and over 12 times as fast as FireWire 800. While Thunderbolt ports are universally found on modern Macs, they're uncommon on Windows PCs, and many popular Thunderbolt audio interfaces don't support the Windows OS. PCIe audio interfaces boast specifications in line with Thunderbolt audio interfaces but require PCIe slots for installation. PCIe systems - such as Avid Pro Tools | HDX and Apogee Symphony - are typically intended for high-volume professional audio-production applications.

Analog and Digital Connections
Analog connections on audio interfaces come in a variety of formats, such as XLR, 1/4", and RCA connectors, or they sometimes opt for the smaller DB-25 connectors, which pack a lot of I/O in a small amount of space. Onboard microphone preamplifiers can greatly eliminate the need for additional recording hardware, and models such as the Focusrite Clarett line include enough onboard preamps to track entire bands. Audio interfaces may include more analog connections than channels of digital conversion, so you'll want to make sure you have enough channels available to accommodate your recording and monitoring needs. Likewise, digital I/O such as 8-channel ADAT lightpipe may allow you to easily expand your audio interface without outboard preamps, and some interfaces even include MIDI I/O for connecting keyboards and controllers.

Choosing the Right Format
Audio interfaces come in a variety of shapes and sizes, but they're generally intended for either desktop or rackmount use. Desktop interfaces tend to be smaller with fewer connections, but they put all of the hardware controls you need within easy reach. Some desktop audio interfaces are bus powered, which makes them ideal for mobile use. Rackmount audio interfaces are often permanently installed in 19" equipment racks, providing more I/O but sometimes at the expense of hands-on control.

Onboard DSP
Many audio interfaces come with onboard digital signal processing and DSP-based mixers, providing built-in effects, dynamics, equalization, and monitor mixing. This allows you to put reverb or delay on vocals for monitoring, or sometimes to record with EQ and compression, all without adding latency to your system. Onboard DSP can be either fixed or expandable, depending on the system. Fixed systems such as MOTU's CueMix FX include a number of effects and other processors, which you can access via dedicated software; whereas systems such as Universal Audio's Apollo line include DSP that can power special plug-ins.

Finding the Right Audio Interface
Between connection types, I/O configurations, format, and more, there's a lot to keep in mind when looking for the right audio interface to suit your needs.

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Analog/Digital Converters