Sound Reasoning: The Mixer Method

Consider this scenario: the band down the street has heard that you're a fledgling videomaker and they've approached you about making a video for them. You jump at the chance--what a great way to get some experience, as well as some footage for your sample tape.

The day comes. You gather up every mike you can find or borrow. You mike the singer, the drums, the amplifiers--and then you freeze in horror. You're staring at the single input on your camcorder. How do you get all those mike signals into your single input? What to do?

You need an audio mixer. Just as a TV station combines multiple camera signals with a camera switcher, you combine multiple audio sources with the audio mixer. Audio mixers are useful in a number of different videomaking situations, from shooting at remote locations to editing in the post-production suite. They're an essential tool in the videomaker's arsenal.

So let's take a look at audio mixers. We'll look at what they do and how they do it. We'll also consider some of the features you should look for to match an audio mixer to your particular needs.

Basics
While mixers come in many different levels of sophistication, there are three basic jobs all mixers perform. These are: balancing the levels of the various inputs; setting levels for recording; and combining multiple audio sources into a single output. (See figure 1 to get an idea of how the how the sound signal travels through a mixer while it does all of these jobs.)

First, they allow you to balance the levels of incoming audio signals. Mixers make this possible by giving you separate controls for each incoming signal. For example, you may be shooting a discussion between two people who are both wearing microphones. One person may have a loud, booming voice while the other speaks very quietly. Using the individual controls for each mike, you can lower the strength of the loud person's mike signal and raise the strength of the quiet person's mike signal. This results in a balanced volume between the two speakers.

The second job a mixer must do is allow you to set the level of each incoming signal for proper recording. Each channel has a level control for the incoming sound source. This may be in the form of a sliding control or a knob. From here, the signal travels to a level meter or indicator. The meter might be in the form of a needle moving across a gauge, or a row of light emitting diodes (LEDs). As you slide the control or turn the knob, the meter indicates an increase or decrease in the signal strength. Regardless of the type of meter, there will be an area marked in red which indicates too much signal; if you record with the meters in the red, the audio will distort. The general idea is to set the levels for the highest indication you can without going into the red. Of course, you may want some sounds to be quieter than others, so you will set those levels accordingly.

Depending on the sophistication of your audio mixer, there may be a meter or indicator for each channel. On many simpler units, there's only one for the whole mixer. In this case, there may be a switch so you can assign each channel to the indicator separately.

Setting audio levels is often an ongoing process. Your talent might raise or lower his or her voice as you shoot. Or you may want a change in the strength of any line-level signals you're using, such as music from a CD player. In this case, you'll have to pay constant attention to the levels. This is especially true when you use an audio mixer to add music or sound effects while editing your tapes.

It's always important to monitor the sounds you're mixing. Only the cheapest mixers don't have a headset output. Regardless of how precise your indicators may be, the headset is the best way to be sure of what your mixing result really is. For example, you may set the level on one signal to be quieter than the others. You can see on the meter that the signal is there. However, the other signals may be so loud that when you listen on the headset, you can't hear the quiet one. What you hear is what you get. Use the headset.

The third thing a mixer does is combine all of your audio sources into one program available at the mixer's output. Audio mixers can have anywhere from one to multiple outputs; these outputs can be mono or stereo. A mixer with a stereo output usually has a switch for mono use. Some mixers with multiple stereo outputs have a separate mono output as well.

One of the benefits of this is the combination of mike-level and line-level signals into a single output. Unlike mike-level signals, line-level signals are amplified by the equipment that originates them. As a result, they're much stronger than the signal from a microphone.

For example, the audio output on your camcorder is a line-level signal. CD player and audio tape player outputs are line-level, too. Line-level signals are too strong to plug into a microphone input without causing massive signal distortion and possibly even damage to your mixer.

Manufacturers use line-level signals a lot in electronics gear because they require less circuitry than mike-level signals. Also, as the higher signal strength is more resistant to noise, it generally produces a cleaner sound.

The input of each channel of an audio mixer has a pre-amplifier, which boosts the strength of weak mike signals up to line-level strength. But when you connect a line-level signal to your mixer, it bypasses the pre-amplifier section. In this way, the audio mixer "sees" both mike- and line-level signals as equal in overall strength. The result is that you can use the controls of each channel on your audio mixer to get a good balance between all the inputs, line-level or microphone.

Many mixers offer stereo line-level inputs for each channel, while some professional mixers treat the right and left stereo signals as separate channels.

Mike- and line-level signals usually require different types of connectors. See Videomaker Magazine's April 1995 "Sound Reasoning" column for examples of these connections.

Now that we've covered the three basic jobs that mixers perform, let's get acquainted with the various types of mixers that are available to the videomaker.

Types
There are several types of audio mixers. There's a considerable amount of flexibility in each of these types, but some are more adaptable to videomaking than others.

Probably the most important mixer for videomakers is the production mixer. This type breaks down into two divisions: field production and studio production models.

First is the field production mixer, also called a remote mixer. As the name suggests, this type of mixer is portable, so you can use it for on-location shoots. They're small, no-frills mixers with usually six or fewer channels. Many are capable of operating on battery power, which is a great advantage. Usually, there are only one or two level indicators (sometimes none). Most provide only one output, which may be switchable between line-level, mike-level, mono and stereo settings.

Second is the studio production audio mixer. These are the monsters you see in TV and music studios with multiple channels and more knobs and controls than the space shuttle. They can be frightening to look at, but they basically do the same job the small remote mixers do, only with more channels and features available. (See figure 2 for an example of using a production mixer with a camcorder.)

These categories are by no means exclusive. Smaller studio production mixers may not have the advantage of some remote units that run on battery power, but if A/C power is available at your remote location, a studio mixer will work just fine. Also, you can use a remote production mixer in a small studio provided it has enough inputs to meet your needs.

Many audio/video mixers are now on the market. These are small combination units which offer video switching and mixing together with audio mixing in one easy-to-use package. They may fit perfectly into a small home editing setting. Unfortunately, there's a limit to their audio mixing versatility (and mike inputs), making them a poor choice for remote or studio production.

Finally, there's the DJ (disk jockey) audio mixer. You use a DJ mixer for mixing and playing of pre-recorded entertainment. As a result, the DJ mixer usually has many line-level inputs with only one or two mike inputs. While such a mixer might fit well into an editing suite (where most signals in use are line-level), it's rather useless in a production setting where microphones abound.

So we see that the production mixers (both remote and studio style) adapt themselves best to actual production settings. The greater number of mike-level inputs and the capability of some field production mixers to run on both A/C and batteries offers serious production advantages over other mixer types.

Features
Now let's look at some of the features found on these mixers.

Many mixers offer an equalizer (EQ) circuit, which controls the levels of specific audio frequencies within the signal. The best mixers place an EQ circuit at each input, while others have a single EQ at the output of the mixer. You can use them to punch up the sound of your talent's voice, enhance the bass or treble range of music or sometimes eliminate hum or other noise you don't want. While there are several types of EQ, the most common found in audio mixers is the parametric equalizer. These usually come in the shape of knobs, sometimes stacked one above the other, with settings that allow each knob to boost or cut a narrow band of frequencies.

Other features include a cue channel, which allows you to hear a sound before you commit it to the program output; send/return circuitry, which allows you to send a channel (or channels) outside of the mixer to an external echo unit or other effect for further processing; and a cross-fader, which allows smooth transitions between two channels. (Cross-faders are common on DJ mixers.)

Pan pots are an important feature found on audio mixers with stereo outputs. Found at each channel, a pan pot allows you to assign one side of a stereo signal to either the right or left channel of the output, or anywhere in between. You can set them ahead of time, or use them to make a sound "move" between the right and left output during recording.

Low-cut filters and bass rolloff switches both reduce very low audio frequencies. You can use them to eliminate mike wind noise and other undesirable low frequencies. The bass rolloff switch has a more mild effect than the low- cut filter.

Master output level controls are the norm on most mixers. They offer control over the strength of the overall output of the mixer, and they generally have their own level indicators so you can monitor the level of the output.

Other mixers may offer further features for processing your sounds. The above features are the most common.

Buying a Mixer
The basic idea in buying an audio mixer is knowing what you want it to do for you. Running out and buying the most awe-inspiring unit you can find will be a mistake. It may weigh forty pounds and run on A/C power only, making it a real pain on remote shoots.

Before you buy, ask yourself a few questions. What are you going to do with it? Will you use it for remote shoots only, or are you going to bring it back to your studio for editing work?

Regardless of your intended use, here are the features you should consider carefully in a mixer:

Inputs (stereo or mono). This depends on how many signals you will be mixing at any given time. If you're going out to shoot people, you might get away with four mike channel inputs. If you plan to edit with your mixer, you will need at least two or three stereo channel inputs for your source deck, music player and effects device. If you plan to do A/B roll editing, you'll need another audio input for the second source deck.

Level controls. Rotary knobs are sometimes cheaper and often found on remote production mixers. However, they're more difficult to use than sliding faders, especially when you need to control multiple signals.

Battery power. A number of high-quality mixers are available with battery power. Since A/C power is often unavailable at remote locations, they're an excellent choice for a remote production mixer. Most also offer A/C power capability. But battery-powered mixers seldom offer many inputs, which may limit double-duty as an editing mixer. If you want the advantages of battery power, you may want to consider a second mixer for your editing suite.

Effects. Do you need equalization, reverb or other effects? Maybe not at a remote location, but probably so while editing. Most smaller remote production mixers don't have these features, or even connections to add external processors.

Size and durability. Mixers come in every size imaginable. Obviously, the smaller a unit is, the easier it will be to tote along on remote shoots. But remember that another concern with small mixers is durability. Sooner or later, a remote mixer is going to bounce around in your car, fall off rocks, get rained on and maybe even take a swim in your local creek. Make sure it's tough.

Quality Counts
We'll close this article with the last, but most important thing you should consider in an audio mixer--quality. You're buying a mixer to improve the sound of your videos, and you're spending your hard-earned cash to do so. Buy a mixer that offers the best sound quality you can afford. Consider only brand names and have the salesperson let you test it (listen to it) before you part with your precious greenbacks.

You should now have a good idea of what a mixer does and what to look for when choosing a mixer to fit your needs. Happy mixing!