Hardware devices always have their outputs limited by such factors as the number of jacks you can actually fit in a box and still hit a target price, but virtual instruments have no such limitation— with an instrument like a drum kit, you could literally have every drum on its own mixer channel.
THE VIRTUES OF VIRTUAL
Here’s how multiple outs work in today’s virtual world. Inserting an instrument with multiple outs into a host like Logic, Cubase, Digital Performer, Sonar, etc., creates a separate audio track for each out. This also applies to instruments that are rewired into the host—but note there might be a lot of outputs. For example, rewire in Propellerhead’s Reason, and your sequencer will sprout another 64 tracks. As a result, when inserting a virtual instrument, many programs give you the option to either create a track for as many outputs as you want, a track for the main output (usually a stereo mix), or both.
These audio tracks work like any audio track: You can insert effects, mix, pan, automate, etc. Few programs display a corresponding waveform in the track, although you can always see the output in the mixer meters.
PUSHING THE CPU’S LIMITS
With a multitimbral synth or sampler, each instrument would ideally have its own stereo output. But with a 16- channel multitimbral instrument, that means 32 outs. Not only are instruments with 32 outs rare, they devour CPU clock cycles.
One workaround is to insert more than one instance of the synth, but use fewer instruments from each— for example, four instruments from one instance, and another four instruments from another. Although that might seem like a worse option (inserting more instruments stresses out the CPU more), it allows for a CPU-saving workaround: Selective instrument “freeze.”
In case you’re not familiar with the freeze function, it essentially converts the virtual instrument track into a more CPU-friendly hard disk audio track, then “disconnects” the instrument from the CPU. As long as you retain the MIDI track that drives the instrument, if you decide the frozen track needs further editing, you can always “thaw” it, edit the MIDI track, then refreeze. So, to save CPU power, you can freeze one of the instruments while you’re playing/editing the other one in real time.
One big advantage of a virtual instrument’s multiple outs—being able to add effects within the host—has been mitigated somewhat by the many instruments that now include built-in effects. However, these are usually optimized to keep CPU power consumption under control, so you can often get better quality with “outboard” plug-ins.
With effects like reverb, you don’t really need an instance on each instrument. So, another option is to use a higher-quality instrument effect like onboard convolution reverb in an instrument aux bus (if that option is available), and send it to the instrument’s stereo output—see Figure 1. Or for more flexibility, you can instead disable any effects in the instrument, assign a really good reverb to an aux bus in your DAW’s mixer, and use each instrument channel’s send control to apply some signal to the aux bus. In this case, the reverb is set for processed (wet) sound only, then brought back into the mix.
Using reverb this way is a fairly traditional example, but sometimes using a processor like distortion or overdrive as an aux bus effect can add a wonderful quality to the sound—when added selectively to tracks.
Separate outs have other creative uses. For simulated stereo imaging, load the same instrument into multiple channels, and restrict each one to particular note range, panned somewhere in the stereo field. You could pan them so that, for example, lower notes appear from the left, and higher notes from the right. Or, load the same instrument into two channels, but don’t restrict the key ranges; instead, tune one a few cents sharp, and the other a few cents flat. This can produce a huge stereo image, but check mono playback— you may need to edit the detuning to avoid “beating” or signal cancellation.
Get the idea? Multiple outputs allow for all kinds of interesting options—try them out.