More than just a box with knobs, faders and pretty lights, a mixer is the heart of any audio-for-video system.
Videographers who want to improve the sound of their videos have probably given some thought to the importance of an audio mixer. If this describes you, youre on the right path. In most editing systems–even modest ones–an audio mixer is the key tool for controlling and enhancing sound. Without one, youre not getting all you can from your audio.
Mixers come in two primary configurations for editing, and one thats more suitable to shooting. Well explore each of these mixer styles, concentrating on their features, applications and how to integrate them with your existing equipment.
The Production Mixer
The production mixer allows you to combine multiple sound sources or microphones while shooting. In its simplest form, the production mixer is a passive device that attaches directly to the camcorder. It will usually accept one or two external mikes on standard 1/8-inch minijacks; a stereo minijack accepts the headphone output from a portable cassette deck or CD player. This allows you to add music or sound effects to your production as you shoot, eliminating a step from the editing process. These small mixers usually have faders or knobs to adjust the relative level of all inputs. A 1/8-inch minijack cable (stereo or mono) carries the combined audio to your camcorders external microphone jack.
More flexible (and expensive) are the production mixers designed to sling around your waist. Also called "field" mixers, these units run on batteries and will accept two or more professional mikes with large XLR connectors. Some offer phantom power, allowing you to use high-quality condenser mikes that dont have a built-in battery. (For a more-detailed discussion of condenser mikes and phantom power, see The External Mike article in the July, 1997 issue of Videomaker.) Often, production mixers boast a built-in audio limiter. This keeps unexpected loud sounds from distorting the camcorders audio input.
Most production mixers offer good metering so you can set your levels with precision. Many offer a panpot for each input, allowing you to adjust the stereo placement of the various mikes. Some have additional stereo inputs, useful for integrating pre-recorded sources. Finally, most production mixers offer a headphone output for monitoring the signal.
The on-camera mixer is handy for simple shooting tasks where lots of control is not needed. If your videos justify the expense, a true production or field mixer gives you a high degree of control over your audio as you shoot.
Im Mike Line
The mike/line mixer gets its name from its ability to combine microphone signals with line-level signals (from a CD player, VCR, cassette deck, etc.). The mike/line mixer is most useful after you shoot, when youre combining various audio sources into a cohesive soundtrack. It offers the greatest degree of control over your sound, but also has the steepest learning curve.
We usually distinguish mike/line mixers by the number of inputs (or channels) theyll combine. A 16-channel mixer, for example, will combine up to 16 discrete audio signals; a 4-channel mixer just four.
If youre in the market for a mike/line mixer, you should decide first how many channels you need. Remember that a stereo source takes up two inputs (for the left and right signals); mikes take up just one, as do any mono sound sources you may use. Lets say your soundtrack will combine audio from two narrator mikes, a stereo CD player, two mono source VCRs and one stereo cassette deck. If you add all these inputs up, youll find that you need at least an 8-channel mixer.
The sea of knobs found on all these channels is less intimidating when you realize that each channel has the exact same controls. Once you learn what the knobs do on input 1, for example, you know how to operate inputs 2 through 16.
Starting at the top, most mike/line mixers offer an input trim knob. This brings the strength of the input signal up to a level that the mixer can work with. Youll often find a switch that toggles between mike and line inputs in this area. After the input stage comes equalization (or "EQ"). This is where you can change the tonal balance of the sound on that channel, making dull sources sound brighter and thin sources more full, for example. The simplest EQ offers just a high and low control, which operate much like the bass and treble tone controls on a home stereo.
In most mike/line mixers, the next controls are the auxiliary sends. These allow you to divert a specific amount of signal from each channel to a special output on the mixer. Plug this output into an audio-effects box, and you can add reverb, echo or any other effect to a given channel simply by turning up the appropriate aux send.
The next control is usually the pan knob, which allows you to "place" a sound wherever you wish in the stereo “space.” Turn the pan knob all the way to the right, and the input signal will appear only in the right speaker. Put it just to the left of center, and the sound will appear to come from that position between two stereo speakers (or headphones).
Finally, each input channel has a fader or knob to control how strong that input is in the overall mix. Faders take up more real estate on the mixer than knobs, but are easier to move smoothly. With knobs, its almost impossible to change levels on more than just one or two channels at the same time.
When shopping for a mike/line mixer, leave yourself room to expand by buying a unit with about 50% more inputs than you need right now. Look for a model that offers the amount of control you feel comfortable with. Lastly, a mixer with plenty of metering makes it easier to set record levels as you edit.
Not Just For Spinning Discs
The third style of mixer–the DJ mixer–is also useful for video soundtracks. The DJ mixer is so titled because its used primarily for blending pre-recorded sounds or music with the DJ’s voice. DJ mixers work with stereo sources almost exclusively, allowing control of both left and right stereo signals with a single fader or knob.
Though a DJ mixer may accept between four and eight stereo sources, it will not combine more than two at the same time. Instead, most DJ mixers allow you to assign stereo input signals to both sides of a special crossfader. By moving the crossfader (usually horizontally), you can smoothly move from one stereo source to the other. Only when the fader bar is in the middle of its travel does the DJ mixer actually mix two sources. Inputs not assigned to the crossfader sit idle.
A separate fader usually controls the level of the mike in the overall mix. A nice feature found on some DJ mixers is a ducking circuit, which automatically lowers the level of the music as you speak into the mike. When you stop talking, the ducker smoothly restores the level of the music.
Most DJ-style mixers offer a cue feature that allows you to listen in on any of the input sources with headphones. This cue circuit doesnt affect the mixers output in any way, and may be more sophisticated than those found on mike/line mixers at the same price point.
DJ mixers differ from mike/line mixers in several other ways. First, DJ mixers rarely accept more than one mike input. A DJ mixers mike input may not even have a trim control, and it rarely offers any sort of equalization. Still, the DJ mixers mike input is very useful for adding narration.
Whereas a mike/line mixer usually offers EQ on each input, most DJ mixers offer simple bass/treble controls (or a small graphic EQ) on the master output only. Metering is usually much simpler on DJ mixers, and they dont have an aux send system for adding effects. Some DJ mixers do offer a few advanced features, like a sampler for recording and replaying short clips of audio, built-in sound effects you can trigger with buttons and a reverb/echo effect for the mike input.
DJ mixers work well for video, primarily due to their simplicity. If the sight of all the knobs on a mike/line mixer causes you anxiety, take a DJ mixer for a test spin. DJ mixers are usually more affordable than mike/line mixers, though the falling cost of the more flexible mike/line mixers is closing the gap. DJ mixers generally have a smaller footprint than mike/line mixers, which can be a real plus when youre cramped for space.
Wiring it In
Integrating a mixer with your existing equipment is easy. Simply think of the mixer as the hub that everything plugs into (see figure 1). The outputs from all your sound sources go to inputs on the mixer; the mixers main output goes to your record deck. If youre listening in on your mix with external speakers, most mixers offer an additional output you can run into a home stereo receiver or power amp.
If youre adding effects with a mike/line mixers aux sends, route the mixers aux output to the input of an effects processor. If the mixer has dedicated effects returns (and many do), plug the effects units outputs in there. If not, return the effects outputs to a pair of mixer channels.
With everything wired in to your new audio mixer, youll wonder how you ever got along without one.