Audio interfaces for Home Recording Studio

Once you know the physical configuration of interfaces, it's time to look at the different types of interfaces available from a functional perspective (i.e., the different tasks they perform). An interface, regardless of whether it's a soundcard, breakout box with card, USB, or FireWire, can perform a variety of duties involving audio, MIDI, and other digitally controlled signals (such as synchronization). Also, many devices can perform all these tasks, which could make your interface a one-stop shopping solution for all your signal-routing and processing needs.

Analog Audio Interfaces

Analog audio is what comes out of your mic or instrument cable. It's what you would plug into a P. A. system or an amp, if you were playing live. But now we're in the digital recording world, so this signal needs to be converted to digital data to be recorded onto a computer. Inside an analog audio interface are analog-to-digital converters, chips and circuitry that turn continuously fluctuating voltages into discrete digital chunks. When audio is converted into digital data, two specifications are considered: the sample rate and the bit depth. The sample rate is how many times the converter samples (takes digital snapshots of) the incoming signal. Just as film projection involves moving a series of still photos past the eye fast enough to simulate continuous motion, so too is digital audio playback a series of frozen sonic images that, played sequentially at a fast enough rate, appears continuous.

Several sample rates exist, but the CD spec is 44.1 kHz (kilohertz), or 44,100 samples per second. Once these samples are captured, the next spec that comes into play is how to store them. A single sample at 16-bit resolution (16 bit means 2 to the 16th power) has a range of 65,536 different places to represent that signal. Higher sample rates and bit depths exist, and musicians are pushing to get these better formats as part of our available listening choices, but for now the CD spec of 44.1 kHz/16 bit is what all music must eventually become (even if it was originally recorded at higher and better sample rates and bit depths) if it is to be listened to on a conventional CD.

An audio interface, at the very least, must be able to convert analog audio into a format of stereo channels (separate paths) at 44.1 kHz/ 16-bit digital audio. The interface also must have circuitry to perform the converse operation—taking digital data and converting it to the analog audio understood by speakers, headphones, and other analog equipment (such as cassette decks). This twin conversion circuitry is known as an A-D/D-A converter.

Digital Audio Interfaces

Not all the signals you'll deal with in your recording activities will necessarily be analog. Sometimes the analog-to-digital conversion will be handled for you, before you even get to the interface or computer. For example, if you use a digital mixer or multitrack digital tape machine (such as the Alesis ADAT or TASCAM DA series) to gather and wrangle your signals, you may be presenting your computer with an already-converted signal. DAT decks and some CD players feature digital outputs as well as analog ones, and it's more convenient and hassle free to simply transfer a digital signal to a computer than to try to record it in the analog fashion (where you must consider and set recording and playback levels).

For situations where your signal has already been digitized, you'll use a digital-audio interface. These don't feature the familiar 1/4" jacks and mini-jacks that are seen for analog connections. Instead, you'll see RCA jacks (like those used on stereo systems, but these handle digital signals), a fiber-optic connector called Toslink, and computer-style D-sub connectors (those multipin, trape-zoid-shaped plugs).

Analog/Digital/Clock

It's much more common in audio interfaces to find a combination of analog and digital connections, because most recordists' setups include both analog and digital sources. If you have digital connections, you must have a sync port, and that can be in the form of a Word Clock connection (a separate signal that ensures two machines will lock together) or sync ports, which talk to the machines' internal synching signals. An analog/digital interface will contain analog connections (in the form of 1/4" and RCA jacks), digital audio connections (RCA, computer-style D-Sub, or Lightpipe), and Word Clock (BNC connector) or sync ports.