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Drum Replacement Primer

November 1, 2006
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FIG. 1: In the Pro Tools AudioSuite plug-in Sound Replacer, you can see that three different snare drum samples are assigned to corresponding threshold levels. Notice that a rim shot sample is assigned to the highest threshold.

Imagine that you have finished tracking your latest project, and the time has come to mix. You've got the vocals sounding clear and lively, the guitar and bass tones are just right, but no amount of processing makes the drums sound like you want them to. Although the drummer's performance was fine, you might be wishing you had captured it differently. Wouldn't it be great if you could change the drum sounds without rerecording the track?

Drum replacement, a technique that Roger Nichols pioneered in the studio with Steely Dan in the late '70s, has long helped engineers and producers fix problem drum tracks in pro studios (see the sidebar “Meet Wendel”). Drum replacement is invaluable when your original choice of drums and miking techniques leaves you with sounds that don't quite fit the final mix.

The idea of drum replacement is to double, or sometimes replace, a subpar drum track using a second track of high-quality samples. In this introduction, I'll show you how to replace kick and snare tracks manually, semiautomatically with MIDI, and with an automatic replacer plug-in. Although the kick and snare aren't the only replaceable drums in the kit, their sounds are often the most prominent in mixes: replacing these drums is often enough to bring the rest of a drum mix in line.

Like any fix-it-in-the-mix technique, drum replacement is more time-consuming than recording the instruments right the first time, and some problems are just too big for it. Nonetheless, the following tools and techniques are worth learning to use, particularly if the physical limitations of your personal studio make drums tough to record well.

Replacement Done Right

A natural sound is the hallmark of successful drum replacement. No one who listens to your song should be able to tell you've replaced the drums. That means you must retain a part's groove and the dynamics of the drummer's performance. The simpler the drum part is dynamically and rhythmically, the easier drum replacement is to perform. However, even somewhat intricate parts may be replaced using the right tools and a little patience.

Your replacement track must blend seamlessly with the other tracks in your drum mix: each drum sample should sound like it was part of the drummer's original kit. The best way to do this is to combine the original and replacement drum audio in the final mix. This reinforcement approach preserves some of the leakage that is characteristic of live drum recording and often helps replacement tracks sound natural.

For the purpose of drum replacement, all drum sounds have two basic elements. First, there is the attack transient produced by the drummer's stick or bass drum beater striking the head. Second, a resonant tone with sustain and decay characteristics follows the transient. The volume peak can coincide with the transient or it can be part of the tone, depending on the type of drum, hit, and recording method.

The first step in drum replacement is choosing the right replacement sounds. These may come from your own sessions or a drum sample library. Whatever the source, your replacement kick and snare should complement the tracks you want to replace, particularly if you adopt the reinforcement approach. Solo the kick or snare track, listen to it along with the rest of the drum mix, and then listen to it in the context of the main mix.

Focus on what you got right when tracking the drums. Do individual hits come through sharply? If so, select a replacement sample that enhances the drum tone and doesn't have too sharp a transient. Does your kick or snare sound good when you solo it but fail to cut through the main mix? In this case, the best replacement sample will have a sharp attack but will decay rapidly. If neither the attack nor the decay of your drum track sounds good to you, select a replacement sample that has it all, but try to use some of your original track to keep things sounding natural.

Kick and Snare Characteristics

Kick drums and the sounds they produce vary widely between different musical genres. A jazz drummer's kick may be tuned as high as a rock drummer's rack tom and sustain as long; a heavy-metal drummer's kick is sometimes heavily dampened and tuned so low that the sound is not much more than a transient with zero sustain.

Typically, the attack transient of a kick drum has a frequency range between 2.5 and 6 kHz, while the tone may range anywhere from 50 to 100 Hz. The tone's decay time can range from 15 ms to several seconds.

Two aspects of kick drums merit special attention in drum replacement. First, hard kick hits are slightly brighter than soft ones. If the drummer's kick dynamics vary, you'll need two or more replacement samples that reflect the timbral differences that harder hits produce. Second, the kick drum in a song almost always has a special relationship with the bass instrument. Whether the kick and bass lock into a single groove or are relatively syncopated, the interaction of these instruments' frequencies is critical. Choose replacement kicks that complement the song's bass parts and tone.

In pop and rock songs, the snare drum regularly forms the centerpiece of the drum part. Whether the drummer plays a simple two-four backbeat or a more complex pattern, the sound of the snare may be the most prominent and distinctive part of your drum mix. While snare sounds vary as widely as kick sounds, the frequency range of snare transients is narrower, tending to fall between 4.5 and 5.5 kHz. The snare's fundamental tone can be anywhere from 100 to 300 Hz, but most of the snap and sizzle is above 6 kHz.

Drummers hit the snare drum in several different ways. Each hit has a unique sound, which your replacements should mirror. If the drummer plays a rim shot, for example, your replacement sound should be a rim shot as well (see Fig. 1). A snare's timbre may become brighter as it's hit harder, but not as much as a kick drum's.

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Give Yourself a Hand

Drummers hit the snare drum in several different ways. Each hit has a unique sound, which your replacements should mirror. If the drummer plays a rim shot, for example, your replacement sound should be a rim shot as well (see Fig. 1). A snare's timbre may become brighter as it's hit harder, but not as much as a kick drum's.

There are several ways to add replacement drums to your songs. The part you're replacing will often dictate the best approach. You can replace simple kick and snare parts in any digital audio sequencer using nothing more than its built-in editing tools. For instance, a rock song with a basic backbeat, even dynamics, and few fills or fast-repeating hits on the kick and snare is a strong candidate for replacement by hand. If you want to replace a drum sound in just one section of a song, you can also do that manually.

Create a new audio track for your replacement samples beneath the original drum track. If your sequencer doesn't automatically compensate for plug-in — induced latency, turn off any plug-ins that may cause playback timing problems on the original track.

Your goal is to align one replacement sample with each transient on the original track. Transients should be easy to spot in the original audio waveform, although an original track with excessive leakage can make this a little tricky. Audition the original track while looking at its waveform and get a sense for what the hits you're replacing look like.

In Digidesign Pro Tools, you can use the Tab to Transient feature. Steinberg Nuendo offers hit-point detection functions for locating transients. With sequencers that lack a comparable feature, use the Strip Silence feature on a copy of the original track to separate the transients from surrounding leakage. This method works in most sequencers, such as Steinberg Cubase, MOTU Digital Performer, Apple Logic (but be sure to strip silence within the Arrange window), and Cakewalk Sonar.

Loop the first bar of the song where the drum you want to replace is hit. Next, paste a replacement sample at the first transient, solo the original and replacement tracks, and click on Play. Panning the original and replacement tracks apart will make it easier to hear whether the hits sync up. With your program's window resolution high, nudge the replacement until you hear it double the original hit. Note that the original and replacement hits may sound best a few milliseconds apart. If that's the case, maintain this offset when positioning the rest of your replacement samples.

Listen carefully for phase artifacts between the original and replacement samples. To correct phase problems, begin by inverting the phase of your replacements. If that doesn't help, insert a sample delay on the replacement track and adjust it one sample at a time until both tracks are in phase.

If you're replacing a drum part in which the drummer plays a roll or a quick succession of beats, you'll need to build small crossfades between your replacements. If you let the replacement samples cut each other off, the effect will often sound unnatural and drum machine — like. Building dozens of crossfades may be tedious, but the extra effort will pay off in the final mix.

MIDI Me

Back when automatic drum replacement was a luxury available only in elite studios, MIDI provided a relatively economical way to replace drums without rerecording them. Today, MIDI drum replacement remains a viable option for fixing your drums before mixing, even if you didn't have the foresight to put piezo triggers on the kit you tracked.

Writing MIDI data from audio drum tracks works the same way as recording MIDI simultaneously with audio. All you need is a trigger-to-MIDI converter such as the Roland TMC-6 or a drum brain that accepts trigger inputs. Send the audio drum track you want to replace out of your DAW and into the trigger-to-MIDI converter. The device will have sensitivity settings — chiefly, a triggering threshold and several Velocity curves — but your trigger-to-MIDI converter will need extra help in order to respond accurately to your audio tracks.

To improve tracking, gate the original drum track, setting the gate's threshold just below the quietest transient and use as short an attack time as possible; you'll arrive at properly short sustain and release times after a bit of experimenting to find your trigger-to-MIDI converter's sweet spot, where each hit on the audio track yields a single MIDI note. Once you've created a complete MIDI track, you can use it to drive the drum sample playback software or hardware device of your choice, having saved yourself hundreds of mouse-clicks.

Automatic for the People

For complex kick and snare parts, manual drum replacement may be too time-consuming or impracticable. An automatic drum replacer plug-in can speed up your work tremendously because it listens to an audio track for transients and uses this data to trigger replacement samples.

FIG. 2: Drumagog''s main screen offers a variety of controls to let you fine tune the plug-in''s response to your audio track.

To optimize the plug-in's performance, you will want to prepare special duplicate versions of your original tracks that emphasize the transients (see the sidebar “Planning for Replacement”). For example, you can use sharp EQ cuts to get rid of as much drum tone as possible. A gate with a fast attack and short hold and decay times may be inserted after the EQ as well. It doesn't matter how these tracks sound at this point, because you'll be replacing their contents completely.

In the following example, I will use Wavemachine Labs Drumagog for automatic drum replacement (see Fig. 2). It is available as an AU, RTAS, and VST plug-in, and it supports WAV, AIFF, SDII, and GIG audio formats, as well as samples in its own GOG format. However, the basic principles are the same with other automatic replacer plug-ins, such as Digidesign TL Drum Rehab, Digidesign Sound Replacer, and apulSoft aptrigga2.

Speedy Snare Surgery

In almost every style of music, the snare drum is the most challenging drum to replace. No other drum in the kit is capable of producing a greater number of distinct sounds depending on where, how hard, and how frequently it is hit. A snare replacement track may easily require a dozen discrete samples to sound realistic if the drummer's part is even moderately complex. An auto replacer plug-in is perfect for such a big job.

To get started, insert Drumagog on your replacement track, select a multisampled snare drum from the sample bank, set the blend knob to 50 percent to hear an equal amount of the original and replacement audio, and click on Play. If things sound a little off, adjust the Sensitivity control (or the threshold parameter in other plug-ins). Every hit that exceeds this threshold triggers a replacement sample.

If drums other than the snare are louder on the snare track than the quietest snare hit, you probably haven't done enough prereplacement preparation. However, you may still be in luck if the nonsnare hits are not too close to the snare hits: by adjusting the Resolution (or attack) control, you can delay replacement retriggering. A 100 ms attack time is a good starting point.

Drumagog also has a pair of neat features for dealing specifically with bleed. Let's say you didn't mic the drummer's hi-hats separately, and you like the sound of the hi-hats bleeding into the snare mic, but not the snare sound itself. Select Stealth mode and listen as Drumagog passes the original hi-hats through, crossfades in with a replacement snare, and crossfades back to the hi-hats.

What if you hear too much of the old snare sound in your overheads? Put Drumagog on the overhead tracks and set it to autoduck when the snare hits. This lets you use the plug-in like you would a sidechain on a compressor.

No Substitute

Drum replacement can be a lifesaver in your personal studio, but it will always behoove you to plan and record your projects in such a way that you don't have to use it. For one thing, there are drum parts that even the most sophisticated replacers can't handle. An elaborate cross-sticking pattern, for example, is something you need to get right the first time.

Indeed, there are no true replacements for a skillful drummer, a well-tuned and great-sounding kit, and good recording chops. That said, drum replacement can certainly help you out of a jam as well as open up myriad creative possibilities.

Alec Tabak produces records on the Lower East Side of Manhattan with his team Thunder, Lightning & Lightning. An email sent to alec@tllaudio.com cues Traveling Wilburys, vol. 1, on his phone as of this writing.


SIDEBAR
MEET WENDEL
On January 17, 1978, Roger Nichols used a COMPAL-80 computer as a 12-bit, 125 kHz sampling sequencer to build the rhythm track for Steely Dan's “Hey Nineteen,” using samples from the kits of Jeff Porcaro, Rick Marotta, Steve Gadd, and Bernard Purdie. He called his invention Wendel (see Fig. A). “The drum pattern for the song was based on an eighth-note click track,” recalls Nichols. “The audio from the click track was fed to Wendel, where it was converted to digital for triggering. The computer would count the clicks and figure out where each of the sampled drums should be played.” Later, the session drummers whose kits Nichols recorded returned to the studio to record fills for “Hey Nineteen” and the rest of Gaucho.

FIG. A: Roger Nichols and the Wendel drum replacement computer. (Photo courtesy of Roger Nichols)

Nichols developed a second-generation Wendel while working on Donald Fagen's Nightfly three years later. By 1985 computer technology had advanced to the point where he was able to build a 16-bit, 50 kHz automatic drum replacer into a single-rackspace unit. Giving it the name Wendeljr, Nichols built 700 boxes, selling every one of them except the three that are still in his rack today. “MIDI was the new rage then,” he explains. “Everybody wanted Wendeljr to be MIDI triggered. I said, ‘Wendeljr is 1,000 times faster than MIDI. It will never have MIDI.'' The pressure to add MIDI grew greater, so I quit building them. Once in a while I see one on eBay for triple what they sold for new.”

However, by early 2007 Wendel drum-replacement technology will be easier to come by. Nichols and his company, Roger Nichols Digital, are set to unveil the Wendel-izer plug-in, which will support VST, AU, RTAS, and TDM formats. “There are a few things not taken into account in current drum-replacement software,” says Nichols. “I have been fine-tuning the process for 28 years. I'm not giving up my secrets, but everyone will get to use the Wendel-izer when it comes out.”


SIDEBAR
PLANNING FOR REPLACEMENT
If you get into the habit of replacing your kick and snare tracks, it makes sense to anticipate the drum replacement you may eventually do when tracking. By starting out with drum tracks that are as replaceable as possible, any replacement method you adopt can be less time-consuming when you prepare for mixing.

Start by close-miking the snare bottom to create a transient-heavy signal with minimal bleed from the rest of the kit. Any kick drum leakage this mic picks up can be removed easily with a highpass filter. For the kick, try placing a mic inside the drum a few inches off the batter head where the beater strikes. In my experience, condensers are usually better for this application than dynamic mics, but experiment with different types and placement techniques to find the combination that yields the strongest, fastest transient response with the least amount of leakage.

An unorthodox but effective way of recording replacement-ready drum tracks is to record the output of drum triggers rather than converting the signal to MIDI data. Use these audio files to line up replacement samples by hand or feed them to an auto replacer. Triggers are a boon to any personal studio: they take only seconds to set up, are small enough not to interfere with the rest of your drum miking scheme, and cost less than a set of decent dynamic mics.

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