Tips for Better Mixes with Virtual Instruments

October 8, 2014

Virtual instruments (VIs for short) are great. I don’t get involved in discussions like “does a virtual analog synth really sound like an analog synth?” because a VI has its own sound, and should be judged on its own merits. But, it often has a different sound compared to a physical instrument, so VIs don’t always sit in a mix the same way as physical ones—especially when combined with real electric or acoustic instruments. There are also certain practical considerations involved in working with VIs that aren’t a factor with conventional audio tracks, so let’s address all these issues in our quest for better mixes.

Bounce, freeze, or increase latency? VIs can load your CPU, so many include different modes (e.g., an “eco” mode for tracking with low latency, and a CPU-hungry mode for mixing; see Fig. 1 above.). While some people increase latency (i.e., their DAW buffer setting) during mixing, significant latency can be problematic with complex mix automation moves. Rather than freezing tracks or increasing latency, I prefer bouncing with the instrument’s highest-resolution setting and then archiving the instrument track to disconnect it from the CPU. This provides the highest-quality sound and leaves you with an audio file you can open in the future—even if the plug-in is no longer compatible with your host or operating system.

Construct an acoustic environment. Combining a VI with miked acoustic instruments can sound “fake” because the acoustic instruments incorporate the sound of the room where they were miked. One solution is to construct a virtual room for your VI with four (or more) delays. Insert a multi-tap delay (see Fig. 2 at left ), or bus your track to some delay effects set to short, prime number delays (e.g., 13, 17, 19, and 23ms or for a bigger room, use 29 instead of 13ms). Set feedback for a single repeat, although you sometimes a tiny bit of feedback will give a denser ambience. Mix the delays in the background, and your virtual instrument will inhabit a virtual room.

Trim the highs. Synths can produce energy in frequency ranges that no real-world instrument can touch, which is another factor that gives VIs a “one of these things is not like the other” quality. Applying a steep lowpass filter at a high frequency (e.g., 10 to 15kHz) can sometimes be a magic fix to help a VI sit well in a track.

Carve out frequencies that “step on” other instruments. Much of mixing is about creating a unique space for each instrument, but not just with stereo placement. A classic example is cutting the vocal range in an instrument when accompanying a singer, but given a keyboard’s wide frequency range, trimming the response for the lower left hand notes may be needed to accommodate bass (a shelving EQ works well for this).

I double on guitar and keys, and sometimes the guitar gets in the way because the tonic note of a key matches open strings that sustain longer and have more level than fretted notes. Compression can even this out, but then you have a compressed signal. An alternate approach is to create EQ notches at the tonic and a few octaves above (see Fig. 3 at left). This “scales back” the guitar notes and leaves more space for the keyboards. Conversely, applying this EQ to the keys can make the guitar more prominent.

How to deal with bass frequencies. Low frequencies have it tough: Transducers have a hard time reproducing them, human ears don’t hear them as well, and room acoustics sabotage the bass response with cancellations because a bass note’s wavelength can be longer than the room itself. Mix the bass to center (which you have to do anyway if you plan a vinyl release), but also consider using a multi-band compressor as you would a graphic equalizer where you just happen to be able to compress the lowest band. This evens out the bass notes so they aren’t as affected by other variables—without having to compress the entire signal.

Limit or compress? In a word, limit—especially with synthesizers. Hit a note that matches up with a filter’s resonant frequency, and you’ll hear a major volume boost. The same issue occurs if oscillators beat against each other and cause peaks. In both these situations, a limiter controls the peaks while leaving the rest of the dynamics intact. Compression is more about evening out low- and high-level signals.

Narrow the stereo spread. Stereo keyboard outputs can give a wide stereo image in a mix, but that’s not always best. Again using the example of guitar and keyboards, I’ll often have each instrument take up half the stereo field—i.e., the guitar’s right channel pans to center and the left channel pans full left, while the keyboard’s left channel pans to center and its right channel pans full right. You can’t always do this with pan pots on stereo channels, which usually act as balance controls. Pro Tools is an exception; with Sonar and Studio One Pro, plug-ins can split a stereo signal into two paths and pan them independently (see Fig. 4 at left ).

Pseudo-oversampling. If an older VI doesn’t offer oversampling and you’re running a project at 44.1 or 48kHz, you can increase fidelity with sounds having lots of high-frequency content. Open a project with an 88.2 or 96kHz sample rate, load the MIDI file driving the instrument and the instrument itself, bounce the track at the higher sample rate, then sample-rate-convert the audio back down to 44.1kHz and bring this file into your original project. For more details, see my column “Should You Record at 96kHz?” in the August 2014 issue.

The joy of amp sim cabinets. Guitar amp speaker cabinets are complex signal processors that emphasize the midrange and introduce numerous response anomalies; if you seek a more old-school keyboard sound, remember that Wurlitzer and Fender Rhodes electric pianos often went through guitar amps in classic recordings. Many amp sims let you place virtual mics in different positions on the cabinets, or place the cabinets in a virtual room. These options sometimes let a keyboard “speak” with more authority in a mix, because it can “punch through” the midrange instead of dissipating its energy over a wide frequency range.
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