Choices, choices choices

Choosing the right number of inputs and outputs is perhaps the most important decision you have to make when selecting an interface, and it all depends on what you plan on recording. If you record just two tracks at a time—or if you produce music using strictly loops, samples and soft synths—a basic two-channel interface will be more than sufficient. The M-Audio® Fast Track® Pro and Digidesign® Mbox® 2 both feature standard XLR microphone inputs that allow you to record vocals and acoustic instruments using dynamic microphones. Recording drum kits usually necessitates at least four inputs, and tracking a live band requires a minimum of eight, each with individual mic preamps. Large-scale project studio owners will need an interface that offers flexible I/O options for connecting with a variety of outboard analog and digital equipment.

Line Inputs
The most common type of analog connections are called “line level,” and are used to connect keyboards, synth modules, signal processors, CD players and other gear at a standardized signal level. Line level signals require amplification in order to have enough strength to be heard over speakers. You can use 1/4” line-level connections to hook up the outputs of an analog mixer to your audio interface. This allows you to utilize the mixer’s preamps and EQ before sending the signals to your interface.

Instrument Inputs
Most guitars and basses, on the other hand, are basically simple transducers that don’t even put out enough juice to attain line level. In order to record guitar, bass or other “high impedance” (Hi-Z) signals, you either need a direct box (DI) or an audio interface with one or more dedicated Hi-Z instrument inputs. An instrument input functions like a mini direct box, converting the high-impedance instrument signal to a level that can be handled by pro-level recording gear. This will enable you to record guitar or bass without using an amplifier—for which your neighbors will be grateful. Most M-Audio interfaces—including the entire Fast Track series—provide at least one instrument input, eliminating the need to purchase an extra DI box.

Tip: It’s usually a good rule of thumb to get a few extra inputs and outputs, since you never know what projects you might be working on in the future. Not enough I/O is actually the number one reason for having to upgrade audio interfaces!

Recording a band usually requires an interface with at least eight inputs

Microphone Preamps
Microphones also generate weak audio signals that must be amplified in order to match the level of recording gear. A preamp allows you to adjust the amount of gain and boost the microphone output signal to the desired level.

Some preamps are designed to offer clean, transparent results—while others are meant to “color” the audio and provide a distinctive character to the sound. Either way, you want preamps that deliver high gain without introducing unwanted noise and hiss from their internal circuitry. This will allow you to achieve clean, natural sounding audio without having to crank the preamp all the way up. Switchable pads help you record exceptionally loud sources by decreasing the signal by a given amount, usually -20dB.

Signal-to-Noise Ratio
When comparing preamps, signal-to-noise ratio is one of the most important specs to consider. It indicates how much noise is introduced by the preamps’ internal circuitry. The lower the ratio, the better. For example, a signal-to-noise ratio of -100dB indicates that the noise is -100dB quieter than the maximum level of gain before distortion. The flagship M-Audio ProFire™ 2626 interface delivers an excellent signal-to-noise ratio of -109dB.

Truth in Specs—Avid vs. the Competition
Beware when comparing signal-to-noise specs from different manufacturers! Some companies fudge their specs by publishing the noise introduced by the preamps and ignoring the round trip signal path. This results in a signal-to-noise ratio that is misleading. Avid publishes only “real-world” specs so you know how your interface will perform before you make a purchase.

Phantom power provides needed voltage for condenser microphones

About Phantom Power
All condenser microphones (except electrets) need DC power in order to operate properly. While some high-end and vintage microphones draw power from an external dedicated power supply, most condenser mics are designed to receive “phantom power” supplied by the preamp or mixing desk. Phantom power provides between 12 and 48 volts of power along the same XLR cable that carries the audio signal. If you plan on recording with condenser microphones, make sure to choose an interface that offers phantom power.

For more information on choosing an audio interface, download Overdub Vol 2.