Choosing an Audio Mixer

Ask any audio engineer about audio in video and he'll probably tell you that in many video productions, audio gets treated like the poor, ugly stepchild. People spend hours perfecting how a video looks, and only a few minutes or even seconds checking how it sounds. The result: a video that looks great but lacks total entertainment punch because of weak sound tracks.

Most of the time, in terms of the topics covered in this column, we talk about how you can use different editing tools to improve how a video looks. Titlers, edit controllers, monitors--they're all things to help you make better pictures in an edit session. It's time we give audio it's due and talk about editing tools to make your video sound better, too.

This month, the subject is mixers, and how to pick the right one to help you edit a better sounding video. We'll start by covering some of the features you don't want to miss in a mixer, and then suggest how you might pick one to fit your budget and your videomaking style.

All audio mixers, from the simplest to the most complex, share one structure: an input section, a control section and an output section. What separates the advanced mixers from the basic ones are some added features and functions in each mixer section.

The mixer's input section is where you connect the sound sources you want to add to your video project. Signals from things like microphones, CD players, cassette decks, or electronic keyboards enter the mixer at the input section. Mixer inputs are also called channels.

A mixer may have anywhere from two to 48 inputs or channels. The number depends mostly on price. Mixers also have two types of inputs: low or mike level, which is for microphones, and high or line level, which is for CD players, VCRs and tape decks. You can't plug a microphone into a high-level input, or a CD player into a low-level input and get good results.

You can connect sound sources to the mixer using two types of cables: balanced or unbalanced. The RCA cables often used to carry audio between two VCRs are unbalanced. All mixers can use unbalanced cables. But a mixer must have balanced input connectors to use balanced cables. Balanced cables provide better sound quality, because they compensate for electrical and radio-frequency (RF) interference. Most of the more expensive mixers have balanced input connectors, but cheaper models may not. Mixers that support balanced cables will usually have three-pin XLR input connectors.

The plugs on unbalanced cables come in a few different shapes and sizes. These cables have either RCA phono style plug, (like those that appear on many camcorders and VCRs), a quarter-inch plug or an eighth-inch phone plug. Stores like Radio Shack sell plenty of adapters to help you convert a plug before connecting it to the mixer.

The control section of the mixer is where you blend the various sound sources into one mixed sound track. You adjust the volume of each incoming sound by turning knobs (called potentiometers or "pots") or by sliding slide controls (called faders).

The mixer sends the blended sound to the output section. A cable carries the signal from the mixer output jacks to the audio inputs on your edit or record VCR.

On a simple, inexpensive mixer, the output section may be nothing more than an "audio mix out" jack. Better models give you a master fader, which controls the output volume of the mixed sound signal. With a master fader, you can slowly fade the entire mix in and out, or adjust its level during editing. Without it, you must use the edit deck's audio input level controls to fade sound in and out. If your edit deck doesn't have input level controls, you can only cut mixed sound in or out.

Once you've edited a handful of projects, you realize there's more to working with sound in a video than just varying the volume level. Sometimes you need to adjust the tonal quality of the sound as well. Maybe there's some annoying tape hiss coming from the cassette you used for the background music. Perhaps the lavaliere mike sounds a bit too "boomy" in the low-range frequencies. A basic mixer with level faders won't fix these problems.

Instead, you'll need a more advanced mixer that provides an equalizer, or EQ, for each fader in the control section. An equalizer lets you pinpoint a specific frequency range, and either boost or reduce its presence in that particular channel.

For the tape hiss problem, you could "roll off" the high frequencies from the cassette deck to reduce the hiss. To clean up the low-range "booming," you can use the EQ to reduce the bass frequencies coming from the microphone.

Because many camcorders record stereo audio, and many televisions can play stereo audio, better mixers let you create your sound tracks in stereo. (Note: to record a stereo sound track, you'll need two available audio tracks on your master video tape.)

On a stereo mixer, each fader has a pan control whose function is similar to the "balance" knob on a home stereo receiver. It determines where the sound appears between the left and right channels. With the control turned completely to the left (counterclockwise), the sound will come only from the left channel. With the control at the center or "twelve o'clock" position, the sound will come from both channels at equal volume.

Better mixers will also have volume-level meters of some kind to show you the volume of incoming and outgoing sound signals. Most mixers offer VU Meters, which provide an average sound- level reading. Other mixers add a peak level indicator, which shows you the maximum sound level. Either feature alone is an adequate tool for measuring sound levels. Having both ensures you'll know exactly how much sound your getting from your sources, and how much you're sending to your tape.