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What is an audio interface?

In simplest terms, an audio interface connects your microphones and other sound sources to your computer — it bridges the gap from analog to digital. Audio interfaces are commonly equipped with mic preamps, line inputs, and a variety of other input options.

You might be asking yourself, "If audio interfaces are often equipped with preamps, why wouldn't I just buy a channel strip or preamp?" Well, the answer to that question lies in the analog-to-digital conversion. Traditional preamps and channel strips send out an analog signal, and for audio to be usable by a computer, it needs to be digital. The audio interface converts the analog signal to a digital output, which is then sent into the computer.

"Why isn't my computer's built-in sound card sufficient?"

If you're serious about recording audio, you'll quickly become dissatisfied with your computer's limited audio capabilities. Most consumer sound cards aren't equipped with high-quality converters. They're fine for gaming or listening to MP3s, but consumer sound cards lack the headroom and the power to preserve the signal integrity. It is important to note that poor A/D conversion is largely what led to early forays into digital recording to be regarded as "harsh."

Another drawback of consumer cards is that most of them only support two channels of simultaneous audio, making it nearly impossible to track drum kits or entire bands without a LOT of submixing and creative bussing.

Aside from the poor-quality conversion and limited channels, consumer cards are also prone to excessive latency, jitter (errors in timing that "smear" the audio signal), and overall inferior sound quality.

Inputs and outputs

With the exception of computer connectivity, no other feature set plays as big a role in choosing an interface as the number and type of inputs and outputs. From just two channels to 18 or more, there are audio interfaces to meet almost any I/O need. If miking a full drum kit with close mics everywhere, an interface with eight inputs might be a good choice. Conversely, if your only intentions are to record your voice and an acoustic guitar, two inputs will do nicely.

You'll also need to consider the interface's output options. At least one output for headphones is usually needed, as is a proper connection to one or more sets of studio monitors. Another consideration is that some interfaces offer outputs for sending audio from the interface to external processing devices, and back again. If you've got analog compressors, reverb units, etc., this may be an important feature to have.

In addition to analog inputs and outputs, many interfaces have digital I/O. These connections usually comes in the form of S/PDIF or ADAT lightpipe formats. Digital inputs are useful when connecting instruments with digital outputs, or studio monitors with digital inputs. S/PDIF connections carry one or two channels, while ADAT lightpipe is capable of carrying eight channels of up to 48kHz audio via a single cable. (This is often a handy tool for increasing your interface's number of simultaneous inputs.)

Manufacturers have also created specialized interfaces for different types of performers. There are now many application-specific devices for guitarists and keyboard players. Certain interfaces include both mic preamps and hi-Z guitar inputs, and also offer emulations of amps, guitars, effects, and vintage studio gear, perfect for axe-slingers with recording aspirations.

Computer compatibility

Are you Mac or PC? What software do you intend to use with your interface? Compatibility between your hardware and software is essential for a stable system. Some interfaces and DAW software are designed to run only on the Mac platform or Windows platform. Fortunately, most current audio interfaces are cross-platform (meaning that they will run with either Windows or Mac computers) and are supported by most audio software.

There are often exceptions to compatibility, as well as important computer system requirements. Make sure to discuss your setup with your Sales Engineer before making a final decision on an interface.

Computer connectivity

Computers today are available equipped with ports to accommodate a whole host of digital audio connection options. FireWire is a powerful format for transferring data between two or more devices such as an interface and a computer.

Another fast protocol, USB 2.0 supports multi-channel audio and is found on a variety of interfaces. The great thing about USB is that while FireWire isn't available on every computer (without the purchase of a FireWire card, that is) virtually every modern computer is equipped with USB 2.0. (The USB 1.1 format can easily handle two channels at a time, and is found on many smaller interfaces.)

Even faster data transfers can be found in the PCI and PCIe protocols. PCI is a high-bandwidth and processor-independent data path between the CPU and high-speed peripherals. It allows for transfers of up to 132 megabytes per second at a bus clock speed of 33 MHz.

PCIe (or PCI Express) is an even faster communications protocol than PCI. The higher speeds of PCI Express (ranging from 250Mbps to 4,000Mbps) allow it to replace almost all existing internal busses, including PCI.

Though it's becoming less prevalent due to the increased availability of FireWire-equipped notebook computers, PCMCIA is another data-transfer protocol for laptops. Originally the PCMCIA Card (or PC Card, as it is commonly called) was developed as a memory device that could be hot-swapped in and out of any computer with a compatible slot. Later, other applications such as modems, networking, audio and video recording, and playback were applied to the technology. These cards provide laptop owners the power and sound quality often found in desktop systems. Many PCMCIA cards have breakout boxes that connect to the card as a means of audio input.

Software control

Most interface purchases include some form of DAW (Digital Audio Workstation) application for recording and mixing. Some also include software which allows you access to more complex routing of audio to and from the interface and the computer. This software typically allows you to use your interface like a mixing console, bussing audio to external devices and back again.

Do bit depth and sample rate matter?

Yes and yes. Bit depth and sample rate are very important considerations when it comes to choosing an audio interface. The number of bits determines the theoretical maximum dynamic range of the audio data regardless of sample rate. Each additional bit adds 6dB to the dynamic range of the audio. More bits help capture quieter signals more accurately. On the sample rate side of things, think of it as snapshots. At 44.1kHz, the computer is taking 44,100 "pictures" of the audio every second as it enters the computer. The sample rate is part of what determines the frequency response of the system. As a general rule of thumb, the highest frequency a digital system can record or play is half the sample rate.

If your only expectation is to produce demo-quality CDs to hand out to friends and family, or maybe sell at a local show, 16-bit/44.1 kHz (commonly known as "CD quality") will be fine. Conversely, if your objective is to record string quartets, an audio interface that is capable of doing 24-bit/96kHz or even 192kHz is desirable. With DVD video and audio capable of utilizing 24-bit/96kHz audio, today's market is full of devices capable of reproducing high-resolution audio. Ask any group of engineers and you'll get many different answers concerning bit depth and sample rate, but keep in mind that the bit depth provides more headroom when recording, so greater bit depth is more desirable than high sample rate when it comes to basic music recording. Plus, stepping up to a higher sample-rate sometimes comes as a trade-off, reducing track count.

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