We’ve all been here before: We record a scratch guitar track in pre-production with a single mic, only to realize at mixdown that it’s an irreplaceable keeper take. Problem is, it sounds too narrow to be a featured track. You wish you’d recorded it in stereo. How can you make it sound wider?
Many plug-ins provide mono-to-stereo processing, but they still sound just that: processed. That’s often inappropriate for acoustic instruments such as guitar, banjo, mandolin, and harp, or even for electric guitar for some genres of music. How can you make a mono track sound like it was recorded in stereo without adding audible echoes, reverb, or modulation such as chorusing, flanging, or doubling?
Three plug-ins give you the mojo you need. Brainworx bx_stereomaker, iZotope Ozone 7 Imager, and PSP PseudoStereo can upmix mono tracks to convincing, natural-sounding stereo free of polluting effects. Let’s give ’em a spin!
The owner’s manual for bx_stereomaker describes its mono-compatible upmix processing as “slicing the source signal’s spectral energy.” A Tone control selects “the slicing frequency interval in 11 steps in order to adapt to different sound sources.” Cryptic, but the results sound great!
Fig. 1. Brainworx bx_stereomaker includes controls for centering bass frequencies, reducing highs in and panning the differential signal, panning the mono source, and adjusting the tone and width of its stereo effect. As you adjust bx_stereomaker’s Tone control (see Fig. 1), you’ll hear subtle changes in the stereo image and timbre (for example, acoustic guitar might sound very slightly softer or harder); adjust the control by ear to find the best setting for sitting the track in your mix. You can click the L<>R button to swap left and right channels in the stereo image. If bass frequencies sound too spread out and unfocused, raise the Mono- Freq control to center them. Turning up the Hi-Damp knob progressively rolls off the upmixing effect above 5 kHz; that could theoretically be useful to prevent the stereo expansion from sounding tinny (I’ve never found that to be an issue), but it also reduces its perceived width.
You can make bx_stereomaker’s effect sound even wider by cranking the Stereo-Exp (expansion) control, but keep an eye on the included correlation meter as you do so to ensure you’re not creating an out-of-phase effect; the meter should display positive values at virtually all times. If the image doesn’t sound like it’s quite right, use the Tilt/Pan knob to correct it: In Pan mode the knob pans the mono source’s center image to the left or right, while in Tilt mode it pans only the differential, or spatially enhanced, part of the signal. As long as the Tilt/Pan knob is set to its default noon position, mixing the plugin’s output down to mono will fully restore the original mono signal. An included Balance meter guides your hand by showing the perceived position of the plug-in’s output signal in the stereo field.
Fig. 2. The Stereoize slider in iZotope Ozone 7’s Imager module and plug-in works in conjunction with multiband width controls to produce a natural stereo effect from mono input signal. The width controls’ crossover frequencies can be adjusted by dragging vertical bars situated in the top section of the GUI.IZOTOPE OZONE 7 IMAGER
While not a dedicated upmixer, the Imager module and component plug-in in iZotope Ozone 7 each provides an inconspicuous on/off button and slider that do just that: Activating the Stereoize function upmixes mono input to naturalsounding, mono-compatible stereo (see Fig. 2). Moving the Stereoize slider to the right or left subtly changes the character of the resulting stereo image.
Imager lets you adjust the width of the stereo image independently for four frequency bands with adjustable crossovers. Three different vectorscopes— Polar Sample, Polar Level, and Lissajous— display the stereo output signal’s correlation in turn.
Fig. 3. PSP PseudoStereo’s controls adjust the perceived width and relative amount (vs. mono input) of its stereo effect. The yellow square seen here at the top of the GUI expands horizontally and moves to the right or left as the plug-in’s output becomes more correlated (right) or decorrelated (left).PSPAUDIOWARE PSP PSEUDOSTEREO
PSP PseudoStereo uses a comb filter, delay, and polarity inversion to separate mono input signal into left- and right-channel components. The Freq (Base Frequency) slider sets a frequency at multiples of which the comb filter will resonate (see Fig. 3). You’ll typically want to set the Freq slider close to its center position; moving it to the right of center widens the stereo image further but can introduce very short (roughly 50ms) echoes at extreme-right settings. Moving the Freq slider to the left narrows the output’s stereo image.
To preserve mono compatibility fully, set the Emph (Treble Emphasis) slider fully to the right; if you hear unnatural artifacts when processing a bass-heavy track, move this slider to the left to wean bass frequencies from the comb filter and reduce artifacts. The Depth (Effect Depth) slider adjusts the amount of wet signal at the plug-in’s output; set it close to its mid-point position to achieve a good blend of mono input and stereo output.
As you drag the Depth and Emph sliders, try to make your adjustments restrict the included correlation meter’s horizontal yellow bar to a narrow width and positioned slightly to the right of its center position. That will typically produce the best stereo effect. Also experiment with toggling the Phase button on and off, as that can make fairly dramatic changes to the processed track’s tone.
PSP PseudoStereo is only available as part of the PSP StereoPack bundle, but don’t let that deter you from buying: The bundle—which includes three other plug-ins that process and analyze stereo signals—costs a mere $49, a terrific value for tight budgets.
Michael Cooper is a recording, mix, mastering and post-production engineer and a contributing editor for Mix magazine. You can reach Michael at firstname.lastname@example.org and hear some of his mixes at soundcloud.com/michael-cooper-recording.