Mastering offers the last chance to get all elements of a production perfected before it’s released to the world. Of all the tasks entailed therein, fixing problems with the underlying music mixes—spectral, amplitude and channel imbalances; issues with dynamic range, ambience, azimuth, noise and so on—is paramount.

To fix those problems, you need to be able to hear them in the first place. Mastering requires a flat room, accurate monitoring chain and great pair of ears. Less tangible but equally important is the mastering engineer’s ability to objectively assess what needs to be done—keeping the artist’s vision foremost in mind—and make the changes while leaving minimal trace of their hand on the project. First, do no harm.

This month, Mix takes a look at some of the most intractable problems with stereo music mixes and their mastering fixes. For clarity and facility’s sake, we’ll detail solutions using leading mastering plug-ins. While many of the same principles and techniques can be applied using the high-end, custom-built analog gear exclusive to marquee mastering houses, for this piece it’s more illustrative to show GUIs for tools readily available to all mastering engineers.

Of course, the big challenge with mastering is how to unbake the cake. When all the ingredients have already been mixed together and cooked into a composite whole, how do you reduce the salt and add more sugar? In many cases, the solution lies in mid-side processing. That’s exactly what we’ll use to address our first challenge: fixing a mix’s weak foundation.

NO CLICK TO THE KICK

Problem: You’re mastering an EDM track that sounds great, with one exception: The kick drum’s beater slaps sound too dull. The drum is plenty loud, and it has the perfect amount of bottom end. But its mushy attack is keeping it from popping the way it should on this uptempo track. How do you make the kick hits sound brighter without hyping other elements of the mix?

Clearly the solution lies in processing the mid channel, where the kick drum lives. But simply goosing high frequencies in the mid channel won’t work; while it would brighten the kick’s beater slaps, it would also make the lead vocal, snare drum and bass guitar sound more present—something this mix definitely does not need.

Solution: The fix is to apply high-frequency EQ boost to the mid channel only during the attack portion of each kick drum hit, quickly nulling the filter at all other times. For this you need a multiband upward expander with highly flexible sidechain filtering, like the FabFilter Pro-MB plug-in provides (see Fig. 1).

Click in Pro-MB’s display to create a bell-curve filter at roughly 3.6 kHz. Move the lower crossover frequency to roughly 2.1 kHz and the upper crossover to around 6.2 kHz. Steepen the filter’s slope at both crossover points to diminish its influence outside the crossover points. Select Pro-MB’s Dynamic Phase or Linear Phase mode to minimize phase distortion at the crossover frequencies (a potential liability when fashioning steep slopes). Select the sidechain’s internal filter for the sidechain input.

Make the 3.6kHz filter act on and be triggered solely by the mid channel: Click on Pro-MB’s Stereo Link Mode button, select Mid in the dropdown menu, and drag the Stereo Link slider all the way to the right.

Next, put Pro-MB into Expand mode and set its range knob to a positive value to enable upward expansion. (A negative value would create downward expansion.) Depending on where you position the threshold knob, you may need to set the range knob as high as +10 dB and the ratio to 100:1 in order to get as much as 3 dB of boost on kick hits.

You want the high-frequency boost to happen as fast as possible and quickly return to 0 dB. To wit, set the attack and release controls fully counter-clockwise (their fastest settings). A look-ahead setting of 1 ms should suffice to make the filter act on the leading edge of each kick drum hit.

With the threshold set low enough, Pro-MB’s 3.6kHz filter should provide high-frequency boost on every kick drum hit. Problem is, it will also boost when the vocal, bass guitar and snare drum exceed the expander’s threshold. We’re going to fix that.

Setting the sidechain filtering mode for your 3.6kHz filter to Free, a slider appears in Pro-MB’s frequency display. Drag the slider’s left handle all the way to the left, to 30 Hz, and the right handle to 80 Hz. The filter’s detector will be triggered only by signals in the 30-80Hz range: the bass guitar and the low-frequency thump of each kick drum hit. The vocals and snare drum will be virtually completely removed from the sidechain signal (enough so that their signal levels will be far below threshold). Set the filter’s threshold control to be lower than the level of kick hits but higher than the bass guitar’s. The kick drum should now pop nicely, while leaving the rest of the mix untouched. Mission accomplished.

DRY MOUTH

Problem: You’re mastering a country album for an indie label. The A&R rep doesn’t like how the lead vocal was mixed on one particular song; it’s bone-dry, and he’d like it to have a little ambience. The project is over budget and has no money for remixing the cut, so it’s up to you to add ambience to the lead vocal track without audibly affecting the rest of the mix.

Simply goosing the side channel won’t work, for two reasons. First, raising the side channel would decrease the contribution of the mid channel to the mix, lowering the relative levels of the lead vocal, kick, snare and bass—all of which sound balanced as is and should not be messed with. Second, there’s no side information on the lead vocal to begin with, so raising the side channel would only increase the ambience for cymbals, keys and double-tracked, hard-panned guitars. There’s no getting around it, you’re going to have to add reverb to the track. But how can you add reverb to the vocal without affecting everything else?

Fig. 2: iZotope Ozone 5 Advanced’s Mastering Reverb adds subtle ambience to the lead vocal in a mix’s mid channel.

Solution: Instantiate iZotope Ozone 5 Advanced Mastering Reverb plug-in (see Fig. 2) on the track and select the impulse response that sounds best for the vocal. (Note: The Mastering Reverb plug-in is not available in the new Ozone 6, so hang on to your legacy Version 5 if and when you upgrade!) The darker-sounding Hall reverb is least intrusive and often a good choice. A 0.32-second decay time will be just long enough to sprinkle some fairy dust on the vocal without radically changing the mix.

Place the plug-in in mid-side mode and send it to the mid channel. The cymbals and other side-channel content will not be processed, but we’re still going to have to weed out the mid channel’s kick, bass and snare from the reverb’s output. Click on the bandpass-filter icon in the upper-left corner of the plug-in’s GUI. Drag the two nodes in the Mini-Spectrum Window (top-center of the GUI) toward each other to create a bandpass filter with corner frequencies at roughly 1 and 5.2 kHz. Virtually all of the kick and bass will be filtered out, leaving only the snare drum’s high frequencies to contend with. Plunge the Early Reflections slider to subdue ERs and preclude snare hits from producing audible discrete echoes. Also drag the High Decay slider to its lowest setting to apply heavy high-frequency damping to the ‘verb. Unless the snare drum was extremely loud and bright in the mix to begin with, it should now be almost or entirely inaudible in the reverb’s output, especially when masked by the vocal.

The final tweak is to adjust Mastering Reverb’s wet-mix fader to taste: A setting of 85 to 95 percent usually does the trick when using a short, highly damped ‘verb. The vocal will be blessed with subtle, short-lived ambience, while the rest of the track will sound virtually untouched.

STAR BRIGHT

Problem: The singer sounds great on the rock track you’re mastering, until the chorus hits and she soars into the top of her vocal range. During the hook, she sounds like a shrill banshee—considerably louder and brighter than the electric guitars and drums. How do you tame the ear-splitting vocals without dulling the entire mix?

Solution: We’re going to compress select high frequencies at the top of the singer’s range, which means another trip to the mid channel. A mid-side dynamic equalizer such as Brainworx bx_dynEQ (see Fig. 3) will get the job done nicely. Key to our approach will be to use a filter in the sidechain and carefully chosen attack and release times that will leave all other elements of the mix untouched.

Fig. 3: Brainworx bx_dynEQ takes the edge off lead vocals during choruses. The section of the GUI for the side channel (which is bypassed) is omitted here for clarity’s sake.

Set dynEQ to mid-side mode, and bypass the side channel. From here on, all dynEQ controls I mention will refer only to the mid channel’s control set.

Toggle dynEQ’s boost/cut switch to the cut position to enable downward compression, and select the internal sidechain filter for the sidechain input. Solo the sidechain while adjusting its built-in filter’s shape and bandwidth to home in on the most offending vocal frequencies and weed out unwanted triggers such as the kick drum and bass guitar; a narrow bandpass filter centered somewhere between 1 and 3 kHz often does the trick. Exit the sidechain’s solo mode. Next, solo the audio path’s filter while adjusting its filter type and bandwidth as well as the plug-in’s threshold control; you’ll know you’ve got those controls set properly when you’re hearing audio only during the vocal’s offending peaks.

With the audio path’s filter still soloed, increase the attack time to weed out snare hits; you want dynEQ to preserve transients by reacting only after they’ve already passed. Increase the release time if the filter’s action lasts too briefly to quell sustained vocals (long-held vowels) that offend. Just be aware that if the release time is set too long, any instruments overlapping the vocal’s “banshee band”—for example, snare hits and any guitars in the mid channel—will also be softened before the compressor releases its action. Once you’ve fine-tuned the attack, release, threshold and filter parameters, exit solo mode.

Adjust dynEQ’s max gain (range) and factor (gain multiplier) controls to attain the proper amount of gain reduction during vocal peaks—in most cases, 2 dB of gain reduction will get the job done without causing audible modulation in the processed band. (The faster your attack and release times, the less gain reduction you can get away with without calling attention to the processing.) With all controls set properly, the vocal will sound much smoother during the song’s choruses, and all other elements of the mix will sound virtually untouched. Bye-bye banshee.

ALL ABOUT THE BASS

Problem: A track you’re mastering sounds terribly thin on account of the bass guitar being mixed too low. The soprano female vocal doesn’t help. You can’t simply boost bass EQ on the master’s mid channel, however, because the kick drum already has a huge bottom. How do you beef up the bass guitar without blowing up the kick drum?

Solution: Once again, we need to use a mid-side multiband compressor with sidechain filtering. Equally important, our compressor must offer look-ahead detection and a range control. The most capable plug-in for the job is FabFilter Pro-MB (see Fig. 4).

Create a filter centered at roughly 80 Hz to boost the bottom end, and adjust the crossover frequencies and filter slopes to extend the bandwidth from roughly 40 to 250 Hz. Because we’ll be processing using steep slopes in the bass band, we’ll want to use Pro-MB’s Dynamic Phase filter to avoid any pre-ringing that a linear-phase filter would otherwise produce. Assign the 80Hz filter to—and key its sidechain with—the mid channel. Select the internal sidechain filter for sidechain input, and activate Free mode so you can restrict the sidechain filter’s bandwidth to 30-80 Hz; the narrow bandwidth will remove everything but the kick drum and bass guitar from the sidechain.

Fig. 4: FabFilter Pro-MB boosts the bottom end on bass guitar without affecting the kick drum.

Boost the output of the bass filter in the audio path 6 dB. Drag the filter’s pan ring counter-clockwise to the “Mid 0 dB, Side -6 dB” position to prevent the filter boosting 6 dB in the side channel. (Only the mid channel will be boosted.) Set Pro-MB to Compress mode and the range control to -6 dB. Set the attack and release controls for their fastest response and the ratio to 100:1, with a hard knee. Use at least 1 ms of look-ahead detection. Then lower the threshold below the level of kick drum hits.

With this setup, bass frequencies will be boosted 6 dB in the mid channel at all times—except when kick drum hits occur, at which times the sky-high compression ratio will plunge the filter’s response to exactly 0 dB (nulled) on account of your -6dB range-control setting. Fine-tune the filter’s output control—and adjust the range and pan-ring controls in inversely proportional measure to it—as needed to attain the perfect amount of boost for the bass guitar. If the kick drum’s sustain also sounds boosted, increase the filter’s release time. And if the female vocal sounds too bass-y, drag the filter’s high crossover point lower or steepen the slope to limit the filter’s extension into her lower range. With all controls set properly, the bass guitar’s bottom end will be greatly enhanced and the rest of the mix virtually untouched. It’s a wrap.

Michael Cooper is a recording, mix, mastering and post-production engineer and a contributing editor for Mix magazine.