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What is an audio interface?
An audio interface is a piece of hardware that expands and improves the sonic capabilities of a computer. Some audio interfaces give you the ability to connect professional microphones, instruments and other kinds of signals to a computer, and output a variety of signals as well. In addition to expanding your inputs and outputs, audio interfaces can also greatly improve the sound quality of your computer. Every time you record new audio or listen through speakers and headphones, the audio interface will reproduce a more accurate representation of the sounds. They’re an absolutely essential component in computer-based audio production. They’re used for recording music and podcasts, and in video post production for recording voice-overs and sound design.
Audio interfaces are used when more a professional level of audio performance is required from a computer, and when one or more professional microphones, instruments and other kinds of signals need to be routed into or out of a computer.
When an audio interface is used with a computer, it acts as the computer’s sound card. In this sense, an audio interface is very similar to a consumer sound card. However, the similarities end there. A good audio interface not only enables you to connect an assortment of different analog and digital signals, it also provides a more accurate digital clock and superior analog circuitry that improves the overall sound quality. You can achieve an entirely different level of audio than you would by just using the stock sound card that comes with a computer.
Some audio interfaces connect to computers through common USB ports, while others use more esoteric connections like PCMCIA slots. When you’re choosing an audio interface, it’s very important to determine the specific kind of port that’s available on your computer. This will help you find an audio interface that will be compatible with your computer, and narrow down the number of possible models from which you can choose.
There are lots of audio interfaces available that connect through the USB 1.0 and USB 2.0 ports. There are also many audio interfaces that connect through FireWire ports. If you’re using a notebook computer, there are interfaces available that connect through various kinds of ExpressCard slots, and if you’re using a desktop computer, there are models that connect through a variety of PCI card slots. If you know what kind of port you’re going to use on your computer, you can start shopping for the ideal audio interface to suit your needs.
This depends on your specific needs. If you plan on tracking and overdubbing with multiple microphones or instruments simultaneously, you’re better off using a high-speed port such as FireWire. If you don’t plan on recording with more than two microphones at a time, you’ll likely be fine just using a USB 1.0 interface. The more demanding your needs, the higher the bandwidth of an interface you’re going to need. The hierarchy of interface bandwidth speeds from lowest to highest goes from: USB 1.0, USB 2.0, FireWire, PCMCIA/ExpressCard, PCI.
That depends entirely on the kind of work you want to do with your audio interface. If you plan on recording with multiple professional microphones, you need to look for an audio interface with multiple XLR microphone inputs. If you’re going to be recording voice-overs for video production, you may need an audio interface with only a single XLR input. If you’re going to DJ with a computer, it’s a good idea to choose an audio interface with four line-level outputs (two outputs are used to send your stereo mix to the house sound system, the other two outputs are used to privately cue songs).
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If your primary need is the ability to connect microphones to a computer, you should look for an audio interface with XLR microphone inputs. Professional microphones connect with three-pin XLR jacks. XLR connectors are desirable because they lock into place and provide a more secure audio connection. An audio interface outfitted with microphone inputs will typically come with anywhere from one to eight XLR inputs.
Many audio interfaces come with jacks called “combo” inputs. This kind of jack combines a three pin XLR input with a 1/4” TRS input in one socket. Combo inputs tend to confuse people, because they look different than XLR and 1/4” TRS inputs, yet they accept both kinds of plugs. It’s important to familiarize yourself with combo inputs, so you know what they are when you’re deciding which interface to purchase.
Some microphones require a little flow of electricity in order to operate, while other kinds of microphones are capable of picking up sound without any power at all. Certain kinds of microphones run on batteries, while other kinds of microphones are fed power from the device that they’re plugged into. It’s called “phantom power” when the device that the microphone is plugged into supplies it with electricity. Most audio interfaces that feature mic inputs will also supply phantom power. Because only certain kinds of microphones require phantom power, audio interfaces have a switch to turn it on and off. Phantom power tends to intimidate beginners because it just sounds spooky. Fear not. Using phantom power is about as complicated as flipping a light switch to turn on a table lamp. Besides being called phantom power, it is also referred to as “+48V.”
Line-level inputs and outputs can be very useful; however, to use them properly you must first understand the distinction between mic-level and line-level. Microphones output a very weak signal. The signal is so weak that it needs to be boosted up by a preamp when connected to a mic input. Line-level audio signals are much stronger than mic-level signals, and require no additional amplification. Therefore, line-level signals need a different kind of input than microphones do.
Line-level inputs and outputs on audio interfaces usually show up as 1/4” TRS jacks or 1/4" TS jacks. 1/4” is the diameter of the plug and TRS stands for Tip, Ring and Sleeve; TS for Tip, Sleeve. TRS connections are desirable because they provide a balanced (grounded) connection, which is better at rejecting noise that long cable runs can pick up, or reducing "ground" hum. An example of when you would use line-level inputs is when you’re recording the audio from a keyboard. Most professional keyboards have stereo line-level outputs. You can connect these directly to the line-level inputs on an audio interface. When you’re connecting studio monitors (powered speakers) to an audio interface, you plug them into the line-level outputs. You can also use line-level inputs and outputs to connect external effects, compressors, limiters and all kinds of stuff. Other connectors include 3/8" mini and RCA (phono) connectors.
MIDI ins and outs are found on many audio interfaces. They allow you to send MIDI information into and out of a computer. If you’re not familiar with MIDI, just think of it as a simple language that enables pieces of music-oriented hardware to communicate with each other. For example, if you connect the MIDI Out of your audio interface to the MIDI In on a digital piano, you can send a command from your audio software on your computer that tells the digital piano to play a C flat.
People use MIDI ports for all kinds of things. Like in the example mentioned previously, they’re often used to connect external MIDI instruments. You could create a MIDI sequence on a synthesizer, and then bring this sequence into your audio software with the MIDI interface on your audio interface. The beauty is that the MIDI sequence is just a series of commands, so when you record it into your DAW you can completely change it and turn it into something new. MIDI ports are also used to connect hardware control surfaces, keyboard controllers and a wide range of other equipment and devices.
S/PDIF is simply a digital audio format. Just think of it as a digital version of an analog audio connection. S/PDIF stands for Sony/Phillips Digital Interconnect Format. One of the reasons S/PDIF tends to confuse people is that it’s used on different kinds of jacks. The most common kind of S/PDIF connector is a coaxial jack. Unfortunately, this just adds another layer of confusion, because a digital coaxial jack looks exactly like a common analog RCA phono jack. It gets more confusing because a single analog RCA jack can only pass a mono audio signal, while a single coaxial S/PDIF jack can pass a stereo signal. If you weren’t confused enough, the S/PDIF format can also be sent through optical TOSLINK connectors, which look nothing at all like coaxial RCA jacks.
The good news is that you don’t have to worry about any of this stuff. S/PDIF connectors are found on many audio interfaces, and they can be really useful. S/PDIF jacks usually come in pairs, with one for input and the other for output. In order to put them to use, you just need other equipment with S/PDIF input and outputs to connect to them. For example, using S/PDIF inputs and outputs is a common way to connect external effects modules.
Like S/PDIF, ADAT is just another kind of digital audio format. While S/PDIF is limited to passing stereo digital audio signals, ADAT is capable of passing eight independent channels of digital audio. This is what makes ADAT inputs and outputs such a useful thing to have on an audio interface. If an interface has an ADAT input, you can buy a separate piece of equipment that will give you eight additional XLR microphone inputs into your computer. The components to which you connect ADAT ports are sometimes referred to as “Lightpipe expanders.”
ADAT inputs and outputs tend to confuse people, too. The term ADAT used to refer to a specific kind of digital-audio media. In the 1990s, ADAT machines were a very popular kind of multi-track recorder. They recorded digital audio onto Super VHS cassettes. ADAT tapes and machines have completely disappeared from the pro audio world, but thankfully the digital format that they utilized lives on in today’s audio interfaces.
An ADAT jack is an optical connection, and fiber optic cables are used to connect ADAT Lightpipe expanders. It should be noted that the terms ADAT, Lightpipe and TOSLINK are sometimes used interchangeably. Some interfaces allow you to switch a TOSLINK connector between ADAT and S/PDIF.
All digital audio equipment runs on an internal clock. The confusion about this technology kicks in right away because the clock isn’t a physical device; it’s just a series of digital pulses. Even though clocks are just digital pulses, they are not all created equal and like regular clocks, run at slightly different rates from each other. When connecting two digital devices, both need to see only one clock to avoid problems that manifest as clicks and pops in the analog audio. Higher-quality audio equipment will often come equipped with a more accurate clock. An accurate clock makes a piece of equipment sound better. Higher-quality audio interfaces will sometimes feature Word Clock inputs and outputs, so you can sync them with other equipment. Word Clock sync is not the same thing as SMPTE time code sync. The reason you sync Word Clock from device to device is to maintain the collective sonic performance of the gear. It has nothing to do with syncing sound to picture.
If you have a piece of gear that has a superior clock and a Word Clock output, you can attach it to your audio interface’s Word Clock input and assign the external clock as its master. Several pieces of gear can be synced to a single master clock. This will make every digital operation in the separate devices fire in perfect unison, and make your recordings sound better. Here’s an analogy. Picture a large farm with a massive sprinkler system running to irrigate its crops. If the sprinklers are not synced to a master clock, they will all be rotating and spraying water randomly. If the irrigation system is synced to a master clock, every sprinkler will rotate and shoot out water in prefect sync. If you had a really fine-tuned clock as the master, every droplet of water would hit the plants at the same exact moment. It’s not uncommon for an audio devotee to spend over $1,000 just on a dedicated clock for their studio.
Any time you record sound into a computer with an audio interface, you are going to experience some degree of “latency.” If you’re not familiar with latency, think of it as the delay in time from the moment you make a command until the moment your command is carried out. If you strike a bell with a mallet, you will hear the sound of the bell ringing instantly. However, when you need to pipe commands through a computer, things don’t happen as immediately.
When you plug a microphone into an audio interface and say “Check 1-2-3,” that sound has to travel on a long journey before you can hear it in your headphones:
That’s a pretty long trip just so you can hear “Check 1-2-3” in the headphones, right? The resulting latency can sometimes distract musicians and make it difficult for them to perform. This is where the direct monitor knob comes in. When you use direct monitoring, you hear the analog audio that is being plugged directly into the interface, as opposed to hearing it after it’s been sent out to the computer and back. This nearly eliminates the latency, and makes the musician happier. Direct monitoring is usually only found on USB 1.0 audio interfaces, because their slower speed makes them more latency prone. Unfortunately, this functionality isn’t referred to as “direct monitoring” by every manufacturer. Some interfaces have direct monitoring controls, but call it by another name. If you see a USB 1.0 interface with a knob that has “mix” on one side and “computer” on the other, then it has a direct monitoring feature.
Audio interfaces often serve as the heart of a recording studio. Most of the essential tools used in a studio will be connected to the interface directly and indirectly. Of them all, powered studio monitors tend to be the most common tools used with audio interfaces. The cables will vary in length, depending on your setup, with terminations that are appropriate for each item. These might be ¼” TS to ¼’ TS, ¼” TRS to ¼” TRS, ¼” TRS to XLR, XLR to XLR, etc.
With powered monitors in place, you’ll be able to properly hear what you’re working on. When you need to monitor your work privately, a good pair of studio headphones is an essential tool.
The need to plug professional-quality microphones into a computer is the most common reason people purchase audio interfaces. Naturally, having a few good studio microphones to use with your audio interface is a good idea. Mix it up and buy a variety of mics. Having a solid dynamic microphone is a great place to start. Adding a large diaphragm condenser microphone will really expand your sonic palette and let you make good use of your phantom-power switches. Small diaphragm condenser microphones are really great for capturing cymbals and various instruments. And a ribbon mic will round out your mic collection with its ability to capture smooth mid frequencies.
The cable that you use to connect the microphone to the interface can make a difference. Spending a little more on a nicely made XLR cable usually proves to be a wise long-term investment (providing that you don’t abuse it too much).