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Audio Mixers for Video Production

Whether you're shooting on location or dubbing in the edit bay, you'll find plenty of uses for an audio mixer. This survey will help you choose the right mixer to fit your needs -- and your budget.

Video, despite what the word suggests, is not just a visual medium. It's also very much an audio medium.

Audio makes video come alive. Would you watch a music video without the music, or a wedding video in which you couldn't hear the exchange of vows? Pretty dull stuff.

When you skillfully combine the sound and the image, the resulting creation far exceeds the mere sum of its parts.

One of the most useful tools for shaping sound for video is the audio mixer. Just as a video switcher enables you to select and manipulate your images, an audio mixer allows you to do the same with your sounds.

There are three basic kinds of mixers: console models, portable field mixers, and DJ-style mixers. A fourth kind, the A/V mixer, has emerged in recent years as a popular choice among prosumers. They combine both audio and video functions into one unit.

In this article, we'll focus on audio-only mixers that are reasonably priced (under $1,000) and useful for either video editing or field production.

E Pluribus Unum
An audio mixer is an essential piece of gear for any serious-minded videomaker. In essence, it allows you to combine the signals of many distinct audio sources into one signal.

With a mixer, you can control the relative levels of all your sources--and in some cases, even their tonal qualities--before routing them to your camcorder or VCR for recording.

Anything that outputs an audio signal can be used as a source. This includes mikes, CD players, cassette decks, turntables, electronic instruments, TVs and, of course, VCRs and camcorders.

The simplest type of mixer is a "Y cable"--a Y-shaped adapter that merges two signal-carrying cables into one.

Another basic form of mixer is the "junction box." As the name implies, it's simply a box where several cables, carrying different signals, come together to create a single composite output signal. A junction box usually offers a volume control so you can adjust the outgoing signal level.

Both of these approaches offer cheap, though limited, alternatives to a full-featured audio mixer. The problem is, there's no way for you to adjust the relative levels of the individual signals. To do that, you have to get the real thing.

Audio mixers span a wide range of sizes and prices, from small portable models in the $200 to $400 range to mammoth recording studio setups costing many thousands of dollars.

While there are some notable differences in features, field mixers and DJ mixers are basically scaled-down versions of the traditional mixing console. Since there's much more in the way of inputs and controls on console-style mixers, we'll cover those first.

Consoles
Your first encounter with a console-style mixer, with its jumble of knobs and switches, may be an intimidating experience, until you realize that most of the controls are logically organized and highly redundant.

Each input has its own set of controls. These are arranged in a series of rows, or "input modules," across the board. Once you've learned the function of one of these modules, you've learned them for all the other modules.

You connect audio sources to a mixer by plugging them into the input jacks, usually located on the rear panel of the unit. A moderately-priced mixer generally offers anywhere from eight to twelve inputs. Large recording studio consoles can have upwards of 60 or more.

Each module on the mixer represents a single mono input or one set of stereo (left and right) inputs. Each module may also have auxiliary inputs, with a select switch so you can use the same controls for additional sources.

Most mixers accept both mike- and line-level inputs. These may be either two separate jacks or a single jack with a mike/line toggle switch. The former are more common.

Why do mixers segregate the mike- and line-level inputs? Because of the drastic difference in their signal strength. Compact disc players, tape decks, VCRs and nearly all other types of electronic audio and video equipment deliver a line-level signal of 0.8 to 2 volts. Mikes, on the other hand, put out a much weaker signal--sometimes only a few millionths of a volt.

Plugging a line-level source into a mixer's mike input overloads the circuit and results in distortion of the audio signal. Conversely, connecting a mike to a line-level input produces a noisy, barely-audible signal.

Record turntables, once a mainstay of audio production, are quickly becoming a thing of the past. Even so, you may still have occasion to use one. Turntables with magnetic cartridges output a signal that's weaker than a normal line-level signal. For this reason, mixers sometimes provide magnetic-phono inputs as well as the standard mike- and line-level inputs.

Certain kinds of mikes, such as condenser mikes, rely on DC power to function. To get rid of the need for batteries, some mixers incorporate what's known as a "phantom power" supply. The mixer sends the necessary voltage to the mike over the same line that carries the audio signal.

On a high-quality mixer, the mike inputs are almost always 3-pin XLR connectors. Professional mikes use these "balanced" lines because they're less susceptible to picking up electrical interference. Line inputs may be either XLR type, RCA type or (on rare occasion) 1/4-inch phone plugs. Some mikes use "unbalanced" lines, which usually terminate in a 1/4-inch phone plug.

Controls
The controls on a mixer's input modules are usually arranged in a vertical fashion, ending with a sliding fader or rotary pot (potentiometer) for level adjustment.

At the top of the module is often a control to adjust the sensitivity of the input channel. This "gain" or "trim" control, as it's called, helps get rid of distortion caused by a very strong incoming signal by "padding" it, or reducing its strength. It can also be used to increase the strength of very weak signals.

Some mixers offer a three-position switch with 0-, 20-, and 40-dB of padding. Others have a rotary dial with variable gain or signal strength reduction from 0- to 20dB. Still others use a simple switch with a single value of padding. There may also be a small LED light which flashes when an incoming signal is too hot.

Mixing consoles often allow you to route an incoming signal out to an effects unit, such as reverb or echo, and then back into the stereo or mono mix. The module has a rotary pot so you can adjust the amount of processed signal you want to mix with the original signal.

As well as their input and output functions, mixers also have a monitor function. Aside from the main output that feeds your recording VCR, there's also a monitor channel so you can listen to the mix through headphones or external amplified speakers. The monitor line is completely separate from the main output, so adjusting the volume there has no effect on the level of the signal you're recording.

Any mixer worth its salt also provides you with a way to visually monitor your levels. Some have needle-style VU (volume unit) meters; others have LED displays that respond quickly to peaks in loudness.

One of the most useful features on a mixer's control module is the "solo" switch. Pushing this button mutes the sound to the monitor line of all signals except the sound coming from a single channel. This makes it a lot easier to isolate problems such as line buzz or distortion that may be ruining the mix.

Another useful feature is built-in equalization, or EQ. If you know how to use the bass and treble controls on your stereo or car radio, then you should have no problem understanding EQ.

Recorded sounds, particularly music, incorporate a wide range of audio frequencies. Raising or lowering the level of an audio signal at different frequencies alters the tonal quality of the sound.

Boosting the lower bass frequencies, for example, can give music a bigger, more powerful sound. Boosting higher frequencies brings out the shrill, high-pitched qualities. Mid-range frequencies generally influence the clarity of the human voice.

Mixers usually offer EQ control in at least two frequency ranges, most commonly high and low. More expensive models offer control at selectable frequencies. A rotary pot enables you to boost or cut the signal as desired.

Now that we've discussed some of the features you might find on a mixer, let's go over some of the specific models available on the market today.

Some Console Mixers
Mackie's MS-1202 is a 12-channel stereo mixer that retails for $399. This unit comes with four balanced XLR mike inputs with phantom power, four balanced/unbalanced 1/4-inch line-level inputs and four 1/4-inch stereo line inputs. There's also one pair of stereo RCA phono inputs.

The unit's main outputs are two balanced/unbalanced 1/4-inch jacks. Auxiliary outputs include one pair of stereo RCA jacks and one pair of 1/4-inch phone jacks.

Each input channel on the mixer features a rotary-style gain control, pan adjust, two-band EQ and auxiliary send/return controls for routing through effects or signal processing equipment. LED meters display the master output level.

From Comprehensive Video Supply, there's the MX-880E ($359). This eight-input stereo mixer features 2-band EQ, pan controls, auxiliary send/return and LED metering. Inputs one to four are mike- and line-level selectable. Inputs five to eight are switchable between mike, line and phono.

Studiomaster's Diamond Pro Series of stereo mixers offers the 8-input 83-RDP ($759) and the 12-input 123-RDP ($959). Features include selectable balanced mike/line inputs and auxiliary sends.

In recent years, digital recording has taken the audio industry by storm. Only recently, however, have those advances begun to filter down to the prosumer market.

Listing at $1999, Yamaha's 16-channel ProMix 01 is one of the first truly affordable digital mixers. The unit converts all incoming audio signals to digital information with 20-bit AD/DA (digital/analog, and vice-versa) converters, and features two built-in digital effects units and three-band EQ on all input channels. This mixer can also store and recall up to 50 different setups, including fader positions and EQ settings.

DJ Mixers
With the proliferation in recent years of DJs and karaoke at clubs and parties, a new type of specialized audio mixer has come about. Although not designed with videomaking in mind, DJ mixers offer some unique and useful features.

Primarily, inputs on a DJ mixer are stereo RCA phono jacks. This makes sense, since almost all audio equipment designed to play pre-recorded music use this type of connection. Most often, a single fader controls both left and right channels. There are generally only one or two mike inputs.

Common features include a solo or "cue" switch for each input channel and a single cross-fader which gradually lowers one source while introducing another. This can be useful for performing the audio version of a video dissolve.

Also possible with a DJ mixer is a technique called "ducking," which lowers the volume of all channels except the mike. Videomakers may find this feature useful in voice-over narration.

Some DJ mixers, like Audio Technica's AM-200, come with built-in digital sound effects, such as a snare drum hit, bomb blast, UFO and laser fire. Priced at $250, the AM-200 offers four stereo line-level inputs and two mike inputs.

From Rane, there's the MP-24 mixer ($1299). The unit comes with six stereo line inputs, three phono inputs and two mike inputs. Both mike channels come with three-band EQ. There's also a four-band main program equalizer, which operates independently of the mike EQ.

Gemini offers three DJ models. The PDM-1008 ($286) has five inputs: one mike, two line and two phono. The PDM-3008 ($407) features one mike, four line and two phono inputs. You can upgrade this unit to five line inputs and an optional 14-band EQ. Gemini's PDM-7008 ($616) comes with an eight-second digital sampler to store sounds, and battery backup.

Field Mixers
Most mixing consoles are designed for studio use. Their multitude of knobs, switches and auxiliary inputs may come in handy for editing, but in the field, they're just excess weight.

There's no rule that says you can't use a console-style mixer out in the field. Given a choice, however, most videomakers would opt for a smaller, lighter, more sturdy unit.

Portable field mixers pack only the most basic mixer features. They provide more mike inputs than line inputs, and to save space, they usually offer rotary dials rather than faders for level controls. Several models operate on both AC and battery power.

The more videomaking you do, the more you'll realize the limits of a single camera-mounted mike. The best position for a shot isn't always the best one for your sound. And quite often, one mike just isn't enough.

When shooting a wedding ceremony, for example, it's not often possible to get close enough to the bride and groom for good audio. To compound the problem, one or both of the parties is often unusually soft-spoken.

A single external mike is a step in the right direction, but where do you place it? Remember, there's also a singer in the back of the church and readings at two different lecterns in front.

With a field mixer, you can place a mike at each location, adjust all the signals to a suitable level and run the entire mix into your camcorder.

Another instance where a field mixer comes in handy is videotaping a live band. If you've ever tried this with only your camcorder mike, you probably noticed how much the sound quality changes as you move around for different shots. Whichever instrument you're closest to at the time tends to overpower all the others. A couple of mikes strategically placed will give you well-balanced, consistent sound throughout the performance. As an added bonus, you may be able to get a line-level tap from the band's sound board to your mixer for strong, clean vocals.

The MM-4200, from Comprehensive Video Supply, is a stereo field mixer that operates on both internal and external 12-volt DC power. The unit, which retails for $815, offers four balanced XLR inputs with selectable phantom power and two stereo (left and right) outputs.

Each input also features a pan pot and a selectable low-cut filter--an EQ function to reduce low-frequency rumble often caused by wind or indoor air systems. There's also a battery condition test switch and built-in belt clip.

A smaller mono version is also available from Comprehensive. The MM-3100 ($315) has three balanced XLR inputs and one output. An optional carrying case is available for $59.

The four-input Shure M-67 was a long-time video industry standard. The company recently replaced the model with the upgraded M-267, which retains much of the design and style of the original unit. Retailing for about $565, the mono mixer runs on three 9-volt batteries or AC power. The unit automatically switches to battery power should the AC fail.

The M-267 offers four XLR inputs which are switchable to either mike- or line-level. It also features an LED peak indicator and a professional-style VU meter.

Recruit a Sound Man (If You Can)
It's important to remember that as you add more and more sources, sound control increases in complexity. Not only do you have to keep all your levels within proper limits, you also have to keep the overall mix aesthetically balanced. At this level of production, it's a good idea to have a secondary crew member who can run the mixer while you shoot.

But whether or not you have help on location, one thing is certain: an audio mixer can help you improve the quality of your videos. So when it's time to upgrade your gear, be sure to keep the audio high on your list of priorities.


Tips for Choosing the Right Mixer

  • Inputs. Choose a mixer that offers a sufficient number of both mike and line level inputs to suit your needs. Switchable inputs are a great feature, as are inputs which offer variable gain control or "trim."

  • VU Meters. These offer more precise monitoring of audio than peak-level LED indicators.

  • Headphone Output. A headphone jack with a volume control is also a very important monitoring feature. Keep in mind, however, that the volume in your headphones is not an accurate representation of mixer's overall output level. A solo or cue switch is invaluable for isolating noise, interference and other audio line problems.

  • Equalization. The more EQ control you have, the better. Two-band and three-band EQs are the most common and may be sufficient for most applications in the field. It's best not to overdo equalization at the time of recording. When it's time to edit, a separate multi-frequency graphic equalizer can help you sweeten up your sound.

  • Power. Out in the field, AC power is not always handy when and where you need it. A portable mixer that runs on both AC and battery power is your best bet. Even indoors, however, battery power comes in handy as a safety back up.

  • Stereo or Mono? Even if your finished production will not be in stereo, it's more convenient to have a mixer that accommodates the stereo outputs of the CD players, cassette decks and other audio equipment you're likely to be using. Plus, it will come in handy if you ever decide to produce a program in stereo.

  • Faders or Dials? Sliding faders provide smoother and more precise control than rotary dials. This is especially important during editing, where you may need to perform audio transitions such as fade-ins, fade-outs and cross-fades. In the field, the rotary dials on a portable mixer are generally sufficient for setting levels and making adjustments.

--R.B.


Matching Connectors & Signal Levels
Many mixers (particularly professional-quality ones) utilize a 3-pin XLR connector for their main output. Most camcorders' external mike inputs, on the other hand, accept a 1/8-inch mini phone plug. You can use an adapter, but that solves only part of the problem. Your camcorder expects a weak mike-level signal, and these mixers put out a more powerful line-level signal.

Some mixers have a switch to toggle the output from line-level to mike-level. If yours has only line-level out, you need to use an attenuator. This device places a resistor in the line between the mixer output and the camcorder input to reduce the strength of the signal to a usable level.

Also, most camcorders don't allow you to manually adjust the level of your incoming audio signal. To keep things simple, manufacturers incorporate something called automatic gain control, or AGC, which keeps the audio signal at a constant normal level. This feature works great to prevent signal overload in loud, noisy situations. The problem is, in quiet interludes, such as a lull in dialog or a soft passage in a musical performance, AGC boosts the signal. The result is a sudden and annoying increase in background noise. Despite your best efforts setting levels on your mixer, AGC takes over.

The best way around this problem is to send your mixer output to a VCR with manually-adjustable audio controls.

One final note: it may occur to you to use the RCA-type audio line input on your camcorder. Unfortunately, on most camcorders, this circuit functions only when the camera is in VCR mode. It's de-activated when you're using the camcorder to shoot.

--R.B.

Tags:  July 1995
Robert
Borgatti
Sat, 07/01/1995 - 12:00am