A growing number of software synthesizers offer audio inputs, which opens up entire new vistas for effects processing. Here, I'll discuss using virtual-instrument plug-ins for processing recorded guitar tracks. You can, of course, use standalone virtual instruments, and you can process real-time input, although you may experience some latency.
Setting up a virtual instrument for effects processing depends on the design of the instrument and the plug-in host application. The instrument must support audio input, and how it allows that input to be routed determines what kind of processing is possible. The instrument and host must have either a sidechain input to route external audio from an audio track to the plug-in or a special “FX” version of the instrument for insertion in effects-plug-in slots. The latter is frequently the case.
Virtual instruments' envelopes usually need to be triggered by MIDI note messages to process external audio, just as they do to play notes. For hosts such as Logic that don't route MIDI note messages to effects plug-ins, you need to use the sidechain approach or limit yourself to virtual instruments that can lock their envelopes on. For the examples here, I'll use Native Instruments' Pro-53 synthesizer, which comes with an FX version and an envelope-hold switch.
Echo of the Past
Modeled after the classic Prophet 5, the Pro-53 has a typical analog-synth signal path: oscillators and a noise generator followed by a multimode filter, an enveloped amplifier, and a feedback-delay effect. External audio enters the signal path at the same point as the oscillator-noise mix and can, therefore, be subject to all the aforementioned processing. Virtual instruments following other paradigms often make other kinds of processing available — examples include frequency and ring modulation, analysis resynthesis, granular effects, and extended multi-effects chains.
The simplest Pro-53 processing is to use its delay effect. To do so, turn all mixer controls to Minimum; set the filter cutoff to Maximum, making sure that the HPF button is off; ensure that the amplifier-envelope's sustain is set at Maximum; and click the amplifier's Hold button on. After routing audio into the Pro-53, use the Ext In knob to adjust the level, turn the delay effect on, and play with its controls.
Delay times can be set from 1 millisecond to 1 second, and there's a built-in LFO for delay-time modulation. Use the left half of the time range together with moderate to high feedback and depth settings for flange, phase, and chorus effects; use the right half of the time range with minimum depth (no modulation) for echo effects. Turning on the Sync button will sync the echoes to tempo.
Next, bring the filter into play by reducing the cutoff and adjusting resonance to taste; the filter is often the best feature of classic synth emulations. At this point, you can play a MIDI keyboard or route a MIDI sequence into the Pro-53 and turn off the amplifier's Hold button. The incoming MIDI notes will then control the filter and the loudness contour according to their ADSR envelope settings.
FIG. 1: Native Instruments' Pro-53 allows external audio to be processed by its filter and delay effect, and offers numerous modulation possibilites.
Alternatively, you can leave Hold on and use the LFO to retrigger the envelopes by turning on its Trig and MIDI buttons (for tempo sync). No waveform is necessary for that, but you can also route the LFO to filter cutoff, in which case you'll need to route Wheel-Mod to the filter and turn the Mod Wheel up. Try that with the triangle waveshape and with LFO triggering still on. Adjust the filter's Env Amt knob to control the relative influence of the envelope and triangle wave.
Finally, you can take a walk on the ugly side by using the Poly-Mod section to route Osc B to modulate filter cutoff. That produces an effect similar to frequency modulation, which you can control using Osc B's tuning and waveshape. Fig. 1 shows the full setup, and Web Clip 1 gives an audio example of each stage in the process.
Orren Merton is the author of Logic Pro 7 Power! (Muska & Lipman, 2004) and Logic 7 Ignite! (Muska & Lipman, 2005).