VIRTUAL MIXER BASICS

Today's virtual mixers in computer-based recording programs have functionality hardware mixers can only dream about

by Craig Anderton

Today’s digital audio recording software is marvelous. For a few hundred dollars and a computer, you can have a system which would have costs tens, if not hundreds, of thousands of dollars not that long ago—and record as many tracks as your computer can handle.

While multiple tracks add flexibility, they also require mixing—the process of combining all these tracks, and doing any necessary processing, for the best possible blend of sound quality and “transportability” (ability to sound equally good over any playback system, from earbuds on a smartphone to a sophisticated audiophile setup).

THE VIRTUAL MIXER

Most recording software has a mixer section based on the hardware mixers that existed prior to computer-based recording, and are still used for live performance. A mixer is the "audio traffic director" that combines signals (each with its own level control), routes them to appropriate destinations, and provides the mixed (usually stereo) output signal. Mixers may appear intimidating, but they consist of many identical modules; learn one, and you know how 90% of the mixer works.

To illustrate basic mixing principles, consider a basic song with lead vocal, harmony vocal, piano, and guitar. The four tracks feed four mixer inputs. These go through four controls that set each signal's level, then the combined signals feed a common output stereo master bus, which goes to your monitoring system. Think of the input signal path as a vertical, downward flow into the mixer, and the bus (output signal path) as a horizontal flow from left to right from each channel to a master channel (Fig. 1). This master channel feeds your monitoring system.

Fig. 1: A mixer setup in Cakewalk SONAR X3. The four inputs are to the left. The left-most input has additional signal processors which can optionally “fly out” from the main channel for editing, then folded back in to save space. The master bus is the second channel in from the right; the right-most channel is a bus dedicated to reverb, as explained later.

At the junction of each input and the bus going to the master channel, you'll find a fader to set the level. Because the output is usually stereo, rotating the panpot for each channel places the signal anywhere in the stereo field (left, right, or center).

With software mixers, you don’t have hardware limitations so you can construct mixers with as many channels as you have tracks, assuming your computer has enough processing power to handle all those tracks.

GETTING ON THE BUS

In the days of hardware, only the simplest mixers restricted their buses to a single master output bus. Typically, there would be several buses (8-bus mixers were common), and you could set up different mixes on these different buses. The extra buses could be used to create separate headphone mixes for the musicians doing overdubs (for example, the bass player might want to hear more drums in the mix, while the singer might want to hear more vocals), surround-sound compatible recordings, or adding processing such as reverb (more on this later).

With hardware, the number of buses you could have were limited by hardware—you can fit only so many knobs on a mixer’s front panel. Virtual mixers let you have a virtually unlimited number of buses. You can even send buses to buses.

Most virtual mixers send signal to buses through a drop-down menu selector or switch that chooses the destination bus, and a send level control that controls the signal level going to the bus. You’ll also find a pre-post switch. This chooses whether the signal going to the bus comes from before a channel’s fader (pre), or after (post). In the pre position, the signal going to the bus remains constant regardless of what’s happening with the main mix. In the more common post position, the signal going to the bus depends on the main channel fader as well as the send level control.

In Fig. 1, there’s a separate bus for sending signal from each channel to a reverb. The vocals send the most amount of signal and therefore have the most reverb, while the piano and guitar send less so there’s less reverb on those signals. The reverb bus appears as another mixer input channel, and as with the other inputs, feeds the master channel.

INPUT MODULES

Each mixer channel has its own input module for processing or routing a signal before sending it to the output bus. Typical features include a gain trim, clipping indicator that lights if the signal exceeds the mixer's available dynamic range, the send control(s) that route the audio to the various buses mentioned previously, panpot to control the stereo image placement, a fader to control overall level, meters, etc.

Most hardware mixers also included equalization (EQ) on each input module; think of EQ as a fancy, flexible tone control. A basic EQ might offer boost and cut for treble and bass. A more sophisticated version could have separate controls for frequency, boost/cut, and width so you could dial in a specific frequency and boost or cut the frequency response. For example if the sound was muddy, you could trim the bass but if it was not bright enough, you could increase the level of higher frequencies.

However with computer-based recording, we now have plug-ins that allow inserting a huge variety of processors, not just EQ, into an input channel. Some virtual mixers have a built-in set of processors with the option to add more, while others simply include a place to insert effects so you can customize the roster of processors as desired for every channel. In Fig. 1, each input has an “FX bin” to insert effects; reverb is inserted in the reverb bus’s FX bin. However like many other virtual mixers, SONAR includes its own processors, which you can “fly out” from a channel in the console view, or view in a separate inspector. Fig. 2 shows the processing included in each channel strip for the mixer in Propellerhead Software’s Reason.

Fig. 2: Part of the mixer in Reason. Each channel has a compressor (at the top), with EQ below it, then inserts and sends. You can also see the wider master bus. The graphic to the right lets you focus on specific parts of the mixer; you can also show or hide specific modules to reduce visual clutter.

As to how you would use EQ, while mixing various sounds sometimes occupy the same part of the audio spectrum and "mask" each other. EQ can separate instruments by shifting the emphasis from one part of the frequency spectrum to another. For example, if background ambience interferes with narration, reducing the response of the ambience at speech frequencies creates more audio "space" for the narration.

Other effects you might want to insert on an “as-needed” basis include compression to even out variations in dynamic range for a "smoother" sound, noise gate to removes hiss, reverb to create ambience effects such as concert hall simulations, and the like.

Other common input module features include a solo button, which mutes non-soloed input modules. This is handy for making subtle changes in one track that would normally be overwhelmed by the other tracks. A mute switch does the opposite: it automatically cuts out (or mutes) its associated channel. Mute switches can also be used to “de-clutter” arrangements by taking certain tracks out of the mix at selected times.

MASTER FADER CONTROLS

The master bus will have a set of controls that’s similar to individual channels, including the option to add processors that affect the entire mix.

AUTOMATION

Automation lets the computer “remember” your mixing moves—like moving a fader up or down, changing the panning, varying EQ, and like. This lets you create mixes, listen to them over a period of time and/or different systems, and edit the automation for a perfect mix. For most mixers, the process is as simple as enabling automation record while you’re mixing, then enabling automation read during playback. Editing the automation usually involves just clicking on the fader or other parameter and writing the new moves.

CUSTOMIZING THE LOOK

Hardware mixers tended to be pretty big. It can be challenging to fit all the controls found on analog mixers, along with extras from the digital world, on a computer screen. So, most programs let you show and hide particular elements, or change sizes of various elements—like making channel strips narrower (at the expense of showing fewer controls) so you can see more channels simultaneously onscreen.

BEYOND THE MIXER

There's more to mixing than the mixer, like your monitor speakers and the acoustics of the room where you’re doing your mix. Ideally the mix would sound acceptable over all systems, rather than sound great on one set of speakers but terrible on everything else. In smaller studios, near-field monitoring is popular; this technique uses small speakers at close range (a few feet from the ears) to minimize influences from room acoustics. Like mixers, dozens of companies make monitors; for near-field monitoring, speakers from KRK, ADAM, Yamaha, and several others are popular in mid-line studios.

Mixing is a science and an art. Being able to produce a mix that sounds clear, distinct, and well-balanced over any system is a real challenge, but fortunately, today’s virtual mixers make the process easier than ever.

Craig Anderton is Editor Emeritus of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.