How to Combine Multiple Signals and Still maintain Total Sound Control

Controlling your audio is a breeze when you’ve got just one signal to work with. One mike input, one mike cable, one mike. No problem.

But what happens when you need to combine multiple signals? What if your latest masterpiece requires you to mix audio from a mike, CD player, source VCR or two and cassette deck? You’ll notice your record deck or camcorder doesn’t have audio inputs for each of these.
You need an audio mixer.

Audio mixers come in all sizes, from two-channel battery-powered boxes small enough to fit in your pocket, to room-size behemoths with computer control and literally thousands of knobs and switches. In this article, we’ll consider mixers under $1000.

What Mixers Do

In a word, mixers mix sounds. Well, not really sounds, but the electrical signals that used to be sounds. These signals can be either microphone level (very weak, like 1/100,000 of a volt) or line level (generally 1 volt).

You can mix speaking voices, singing voices, musical instruments and sound effects, whether live or prerecorded. Depending on the type of video you’re making, you may use a simple mixer, or you may want a larger, complicated console, or “mixing desk.”

Each sound source connects to the mixer by way of a plug (on the incoming wire) which fits into a jack on the mixer. Each line into a mixer is called an input, or channel; inputs are either single (mono) or dual (stereo). The output which feeds your camcorder or VCR can also be mono or stereo, with your choice of mike or line level as required. Always match the mixer level to the recorder’s needs; otherwise you’ll end up with either too much signal (distorted beyond use) or not enough (too weak and noisy to enjoy).

When to Mix

For many videomakers, audio takes a back seat to video. But capturing good sound is as important as capturing good images. No matter what the shoot, you must pay as strict attention to the audio possibilities and you do the video possibilities.

Let’s look at several common shooting situations from the sound mixer’s point of view: Weddings. First there’s music (a piano or organ, a guitarist, small combo or orchestra). Next, add a couple, saying their vows, and an official conducting the ceremony. Finally, there’s the audience, coughing, crying and congratulating the happy couple. These are the sound sources the sound mixer must receive and mix. Given these sources alone, you could employ five mikes or more. Such a setup might intimidate a beginner, but the serious videomaker would consider it worth the effort.

Live musical performances. Whether you’re shooting a class recital or a nightclub act, mixing can: 1) make up for lousy room acoustics, by getting you in closer to the good sounds; and 2) eliminate unwanted noise, like a rowdy audience or a passing freight train.

Events. Events–from softball games to natural disasters–feature a host of natural sounds. In such instances, the on-camera mike can work fine. But you can make the audio even better by adding your own live narration, calling out a play-by-play of the big game, or describing the course of newsworthy events as you witness–and record–it.

Here you might use a a small mixer, clipped onto your belt or attached to the camcorder, to combine the headset mike with the camera mike. This allows you to record both the sounds of the event and your narration.

Types of Mixers

In its simplest incarnation, a “mixer” is only three wires twisted together and wrapped with tape (or you can buy a “Y” adaptor). Two signals combined in this manner give you just a little extra flexibility for almost no expense. This is called passive mixing.

Say you’re videotaping a wedding, and want to hear both bride and groom say their vows clearly. A small lapel mike concealed on each person, mixed together with a “Y” adaptor, may accomplish the task.

Several drawbacks pop up, though, when you mix this way. First, both mikes will combine at equal level. If the bride’s voice is softer than the groom’s, you’ll have no way to compensate, or balance the two.

Second, unless the microphones are identical in design (matched impedance and sensitivity) the mike with the stronger signal will “load down” the other, dissipating some of the weaker mike’s signal; this will lower the volume so much you’d be better off with just one mike, after all.

You’ll have the same problem with line-level signals from the audio outputs of VCRs or camcorders. Unmatched audio lines never mix equally. They can terrorize your signal with distortion and add unwanted background noise.

Active mixing to the rescue: real mixers add a few electronic components (transistors or integrated circuits) to isolate each incoming mike or line from one another and boost each signal when necessary.

At the same time, you take control over each source with a fader or pot (short for potentiometer). A fader adjusts the relative loudness of each source, from all the way off to full volume and everywhere in between.

Keep Your Balance

In the audio world, there are two different cabling schemes to carry audio signals. One is the common conductor/shield arrangement used with most line-level signals. We call this type of cabling unbalanced. A standard RCA-style cable, like those used to interconnect two VCRs, carries an unbalanced line-level signal.

The other scheme is called balanced wiring. In addition to the normal audio signal flowing inside the shield, balanced wiring adds another signal on a second conductor. This added conductor carries the electrical inverse of the actual signal.

Without getting into to much technical mumbo-jumbo, this method of carrying audio allows the receiving device to cancel out noise picked up along the way. The more susceptible the signal, the greater the advantage of balanced wiring.

Which is why good microphones use balanced cables. A mike puts out a tiny signal, one that needs all the help it can get to survive our electronic jungle. Many inexpensive mikes use unbalanced wiring, which limits the distance you can safely route their signal.

Some decent commercial recordings have been made with 50-foot unbalanced mike cables and several hundred feet of line-level cabling. But with lamp dimmers, power transformers or a radio/TV transmitter nearby, you may need the advantage of balanced audio.

Consult your equipment manuals to see what’s balanced and what’s not. Balanced mikes (and lines) commonly connect with a 3-pin plug known as an “XLR” connector. The unbalanced variety use either a mono one-quarter inch phone plugs or a 3.5mm mini-plug. If you are connecting a balanced cable directly into your camcorder or VCR you may need to use an adaptor to match your source’s cable to your destination.

In both consumer and professional gear, RCA phono plugs are for line-level audio. In consumer video, the RCA plug is also for the compositet video signal in and out; so be careful when you connect up since they look identical. Observe the color code: yellow usually means video, while red and white are reserved for audio.

Sadly, there are no national or international standards regarding plug types, sizes, colors or wiring polarity. If you use an assortment of audio stuff from different manufacturers, you will eventually find out that things don’t always match up. Ask your friendly audio salesperson to fill you in on the necessary adaptors and wiring diagrams.

You may ask: why not make everything balanced and standardized and be done with it? Guess what? Balanced costs more. XLR connectors are both larger and more rugged than mini-plugs, taking up to 10 times more space and costing 10 times more money.

Mixer and recorders require extra circuitry to accept the balanced feed. It’s a design found only on pro-quality equipment.

The question of impedance comes up every so often, but in the 90s it’s almost a forgotten science. Most microphones are low impedance (50-600 ohms, or units of electrical resistance). Most line-level sources are medium impedance(1000-5000 ohms), whether balanced or not. Any mixer made today will match these figures just fine.

If you should come across an ancient Altec or Western Electric mixer, the kind with tubes, it will be high impedance with optional transformers to accept lower impedance sources. It will also be very valuable to collectors.


After all these different signals start coming in to your mixer, it’s not too much trouble to make some adjustments to the incoming sound to increase its quality. First consider some tone control, known to sound engineers as equalization or EQ. Each channel can have its own bass, mid-range and treble controls.

EQ can either increase or reduce selected frequencies. You can cut such sounds as unwanted hum (from a guitar amp), the rustling of wind (outdoor sports) or rumble (from air conditioning or a moving car). You may want to boost certain frequencies to add clarity to narration or give a sound effect more impact.


The next step up from simply changing the tonal balance of a sound is to add some special effects, the most common being echo or reverberation (reverb). Though often confused and mislabeled by engineers and equipment manufacturers alike, by definition the two effects differ.

Reverb is the smooth decay you hear in a large room, a diffused sound which dies out slowly after the source of the sound stops.

On the other hand, echo is a distinct repetition (HELLO Hello hello…) which can die out slowly or just disappear after one or two repeats. Before you buy a mixer with reverb or echo, listen to the effect to make sure it’s the one you have in mind.

When do you use reverb? The most common use, according to professionals, is to augment a vocalist. Reverb “fattens” up a thin, weak voice; it makes a good singer sound even better. Reverberation also adds “depth,” a three-dimensional quality. Dial in the amount of reverb desired; you can sound like you’re in the Grand Canyon or at New York’s Grand Central Station.

Some musical instruments sound good with reverb, some do not. So it’s good to adjust reverb intensity on each input separately. Speaking voices, sound effects and some instruments simply sound odd when you add echo. Think science fiction here, or melodrama, or perhaps a dream scene. Play it up for comedy, or as an attention-getter. But remember: too much, too often can be very, very annoying.

Dee Jay Mix

One unique product in the world of audio mixers is the DJ mixer. This design is especially useful in post-production, when you’re editing video while dubbing audio from multiple sources.

Several things set DJ mixers apart from other types. First, they’re more likely to have a cue channel. This lets you monitor a channel through headphones without sending the signal into the mix to your recorder. Disk jockeys use this feature to cue up a record without the audience hearing it. Videomakers make use of this function to isolate problems in a mix (such as hum from a loose wire, an off-key musician or an announcer who’s not quite ready to speak).

Next, the DJ mixer has a cross-fader. This control lowers one input while raising another–the equivalent of a video dissolve. It’s often used to slowly segue between two pieces of music, a seamless transition from one song to the next. It’s very useful when making up background music for film-to-video transfers or travelogue scenes.

Designed for the DJ, DJ mixers usually have one mike input with built-in special effects. The most useful of these is automatic voice-over, sometimes called “ducking.” Push a button and the microphone channel instantly overrides all other inputs, and adjusts itself to exactly the right level, turning down (“ducking”) the other channels to a lower level. This can be great for narration.

Some other special features: the deejay speaks, simultaneously turning a special knob and lowering the pitch of his or her voice. Or shift it to sound like a totally different person. Another set of switches lets you create instant digital sound effects such as bombs, laser blasts, drums and UFOs.

Deejay mixers are made for stereo: all the inputs (except the mike) are left and right channels controlled by one fader. This is important if you are mixing a video sound track in stereo. Other mixers (even the pro models) usually have mono inputs, so you need two for each stereo source. If you’re mixing from CDs, stereo tape and other stereo sources, you’ll soon run out of fingers to control all the faders–unless you have stereo inputs.

Typical Mixer Close Up

Let’s look at a pro-quality mixer up close. There are quite a few in the under $1000 category that offer flexibility with and a myriad of bells and whistles. Remember, not all of these features are necessary and not all come on every make and model (see Figure 1).

An input module–a strip of controls with the input fader at the bottom–handles each sound source. This fader can be rotary (a round knob) or linear travel (a square knob on a sliding lever). Linear faders are also called sliders or slide pots.

Sliders are nice if you have the extra money (cost is a few dollars more per channel) and the room. They take up more space on the mixer but are easier to control. A quick glance tells you which inputs are on and how loud they are. Slide faders are also better if you suffer from arthritis or have other difficulties with your fingers.

Next we have an input trim and/or a mike-line switch. This compensates for higher output sources (like tape decks) and accommodates both louder and softer sounds. You match the input trim to the input signal level; then you have the proper level to mix. Input trim reduces (attenuates) larger signals, and lets you boost weaker ones.

Some mixers have both mike and line inputs, with separate jacks for each. If they do, the mike is usually balanced and the line is unbalanced. Be aware that some cheap, low-level mixers use phone jacks to accommodate unbalanced low impedance mikes. With two sets of jacks you must have a mike/line switch for each module, to convert the input sensitivity from low to high when you select the desired input.

The input peak LED is a red LED that flashes when the input level comes dangerously close to overloading. Peaks in audio refer to the highest voltage at any instant. They can last only a fraction of a second, but that’s all it takes to cause audible distortion. In this case the danger is not physical; it won’t catch on fire and blow your mixer away. But it does make for a fuzzy, abrasive sound.

The cue (or solo) button takes the input signal and routes it directly to a special output line (without affecting the mix in progress). As we said earlier, DJ mixers have this feature to help cue up records. Pro mixers (with many channels active at the same time) have this so you can quickly find out which channel has what.

Push cue while listening through headphones and it temporarily silences all the inputs except the one selected. An invaluable function when searching for a problem such as a dead mike or broken cable, or when you can’t remember which input you plugged something into.

You’ll also find aux send, EFX send, or reverb send on your mixer. All do the same thing: send a portion of each input to an external effects device such as an echo/reverb unit, or a separate tape recorder, headphone or loudspeaker system. Sends can be pre-fader, which means their level is not affected by the individual channel fader; or post-fader, where the signal varies with the fader setting. With the latter, no signal goes to aux/EFX/reverb send output when the fader is off.

The EQ is the frequency equalizer or tone control we mentioned earlier. It gives you a variable boost or cut in certain frequencies. The knobs are calibrated with marks so you can write down your settings to duplicate a certain sound again later.

On to the pan (panorama) pot, which places a sound in the stereo soundfield. Once known as the “angular position” control, this variable control allows you to put a voice or sound effect anywhere between the far left and far right. You can rapidly twiddle this knob, causing the sound to bounce back and forth.

On mixers with more than one output channel, the output bus assign allows you to assign each input to any, all or just some of the outputs. This way you don’t have the exact same content of the mix going everywhere,

Mixers made for multitrack recording can have four or eight output lines called buses. For video, these extra output buses can simultaneously feed several VCRs, a room sound (P.A.) system and an audio recorder. Or a local TV station via phone lines, microwave or satellite uplink. These small mixers let you think big.

Some microphones require phantom power, external power in the 12 to 48-volt range. Rather than employ a separate power supply box on the floor somewhere (more wires and gadgets to trip over or break down), some mixers can send the needed power right down the balanced mike cable. Condenser mikes (top quality, mostly expensive) often run only on phantom power–no batteries required.

Outputs and Beyond

After the input channels are summed, or mixed, they go to the output section. Here you may have EQ which affects the entire mix. Here also you can put back into the signal any audio effects you routed signals to with the EFX send. Or mix in some echo from the internal reverb, if there is one.

The outputs should have some kind of volume indicator to tell you how “hot” your signal is going out. Tradition needle-style VU (Volume Unit) meters were once common. Today, you’l more likely find a set of LEDs (sometimes all red, sometimes green/yellow/red); these are faster to react to peaks in the audio when compared to a VU meter’s waving needle.

Either way you get some idea of what your mixer is sending to your VCR or camera. Here’s where you make sure the mixer’s output level matches the inputs on the unit you’re feeding. If you’re feeding a signal of line level strength into a camcorder with only an external mike jack, you’ll need an adaptor called an attenuator. This drops the mixer’s higher line level down to the weaker mike level.

Some mixers include a special “mike level output” which spares you the adaptor.

Good Audio Now

Video is sound as well as images. No matter what kind of video you make, you can certainly improve your soundtracks when you plug into a mixer.

Why wait?

10 Features Your Mixer Should Have

Here’s what to look for when buying an audio mixer:

1. Quality. You’ve made the decision to invest in a mixer. Now make the decision to invest in one that offers you the best sound quality you can afford.

2. Ruggedness. If you’re running though jungles or shooting on the football field, choose a mixer with a metal case and protected controls. The average mixer, with its plastic knobs and case, may not be rugged enough for on-location shoots.

3. Stereo or Mono. If you’re recording or editing mostly music, especially from CD or tape sources, a stereo mixer will help–even if the finished product is a mono track on your video.

4. Faders. What type of fader fits your budget? Your space? Your manual dexterity? Rotary are cheaper and smaller than sliders, but they can be difficult to control if you have many inputs active at the same time.

5. Inputs. Just adding narration? Two inputs is probably enough. A wedding might need four to six; small music groups, eight inputs. That 40-piece band might need a few more…

6. Mike vs. Line In. Will you be mixing mostly amplified signals (VCR, cassette, CD player) or live mikes? A microphone cannot plug into a line input. But you can adapt line-level sources to mike jacks. Or look for a mixer with convertible inputs.

7. Battery vs. AC. Do you need to go completely portable, or can you plug into power lines? Size counts here also. Bigger mixers take more power than batteries can practically supply. If you do go with batteries, be sure to calculate their running time and take along plenty of fully-charged spares.

8. Effects. Do you want built-in echo? Do you need a compact mixer without accessories dangling nearby? Or do you have outboard gear like synths, samplers, limiters and reverbs that require aux sends? You may end up with more features now than you need, but in a year or so you could wish you had more.

9. Deejay. If you only need one mike input and mix a lot of prerecorded music, consider the advantage of all stereo inputs and cross-fade controls.

10. Size. Although determined mostly by the number of inputs, you do have a choice of full-size vs. compact designs. Will you be back-packing with your gear, or transporting it in a truck? Or are you outfitting a nice comfortable edit suit, sharing a desk with your typewriter and answering machine?

(Sidebar #2)
Digital Audio

No one witnessing the onslaught of CD, digital compact cassette (DCC) or MiniDisc (MD) would deny that digital audio storage is here to stay. But few would have imagined that an affordable, fully digital mixer would hit the market so soon.

Yamaha’s revolutionary new 16-channel ProMix 01 converts all incoming audio to 20-bit digital for better-than-CD audio performance. The board includes two built-in digital effects units and four assignable stereo compressors. Each input channel has a 3-band digital EQ.

Doing all audio processing in the digital realm has advantages besides fidelity. The ProMix 01 will instantly store and recall 50 different mixer setups, including all fader positions, EQ, compressor and effects settings. MIDI supports allows dynamic changes of any parameter mid-program.

Slated to ship in August, the Yamaha ProMix 01 should carry a list price tag of under $2000.

The Videomaker Editors are dedicated to bringing you the information you need to produce and share better video.

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