Connecting a mic to a computer for recording sounds like an easy task: just plug the cable into the mic input on the computer and you’re good to go, right?
It’s not that simple, unfortunately; there is a very big difference between consumer and professional audio electrical standards, and mixing them up can create problems that vary from a distorted and noisy sound, to no signal at all, to actual damage to your equipment.
The standard consumer mic input on most computers (usually looking like the one in the above picture) is a rather budget affair, the whole circuit usually costing a few pennies. It tends to have very low gain, wildly variable impedance, and a small amount of DC power (normally 5 volts) supplied in a way that can damage some professional microphones, whilst remaining too low to power others. The incoming signal is then fed into an inexpensive analogue-to-digital converter that can further degrade the sound during the critical task of converting the signal into a digital stream to be sent to your recording software.
This input expects the kind of microphones that can be bought in computer stores. Consumer computer microphones will use a mini-jack connector; they will tend to have high output levels, high distortion and noise ratings, and uneven, spiky frequency response and polar patterns. In other words, they sound pretty bad!
Professional Mic Inputs
Professional microphones will still vary in electrical specifications, but the differences are small enough that they’re all mostly inter-compatible. All pro mics (with the exception of portable wireless systems and some vintage-style tube designs) will use a male XLR socket; they are designed to have the best possible sound quality and lowest noise level, at the expense of overall signal strength (which can be up to 1000 times lower than that of consumer mics). To make up for this, microphone inputs have internal amplifying circuits called preamps, which correct and raise the signal to a standard “line” level.
|Sennheiser e965 and its male XLR socket||
A pair of female XLR sockets
Condenser microphones like the Sennheiser e965 above have internal circuitry that requires phantom power (usually 48 volts, although it’s often a bit less), whereas dynamic designs don’t require it. Microphone inputs on a professional device will normally have a phantom power switch, and are kitted out with female XLR sockets like the above, so you can use industry-standard XLR leads with them.
Connecting Microphones via Audio Interfaces
So, how does one plug these microphones in? Well, some motherboards have line inputs into which a stand-alone preamp can be plugged, but even then you might run into noise and operating level problems. The low quality of analogue-to-digital (A/D) converters also remains an issue, often giving you a diffuse, watery sound that can defeat the purpose of improving the rest of your gear.
By far the best way to connect professional microphones is through an external sound card or audio interface – basically a box that takes over all the audio signals going in and out of the computer, including headphone and speaker outputs. Audio interfaces have the circuitry required to work with many other types of signal, like synths, electric guitars, MP3 players, and tape recorders.
They have high-quality electrical components and very clean converters, which convert the inputs from analogue to digital, and the speaker and headphone signals from the computer from digital back into the analogue realm to ensure maximum quality when recording as well as when listening to the playback. They are usually plugged in through USB, Firewire or Thunderbolt, and upon being selected on the computer these ins and outs effectively supersede the built-in connections.
Audio interfaces come in many different sizes, depending on how many separate inputs and outputs are required. Some, like the Focusrite Scarlett 2i2 above, offer a simple, no-frills experience, with only two inputs and a stereo speakers and headphone output; the largest ones by Avid or Apogee have complex routing options and input numbers that can go well into the triple digits.
In our next instalments we will be taking a closer look at audio interfaces and what to look for when purchasing one, as well as differences between vocal microphones and their relative uses. Until then, happy recording!
by Pablo Bellinghausen