Drum Mixing Techniques

By David Franz

Keeping with the drum theme from the previous two columns, this month's installment of Groundwork moves forward from beat creation and editing to mixing drum tracks. Specifically, I'll discuss phase coherence and creating a compressed copy of the drum tracks, and demonstrate adding effects and using SoundReplacer.

Download this Pro Tools session to see these techniques in action PC | Mac (downloads approx. 8 MB). These drum tracks were taken from an album I mixed recently, by a band called Grey Star Morning (www.greystarmorning.com).

The Evil Twin: A Compressed Copy of the Drum Tracks
A popular mixing technique is to add an effected copy of a track in with the original track to alter or thicken the sound. It's a common technique used on vocals, guitars, and almost anything else you can think of. Particularly within the Rock, Pop, and Hip-hop genres, adding an effected (or at least compressed) copy of the drum tracks is a cool way to beef up the drum sound. When I start mixing a song in one of these musical genres, I often set up this double track right away, at the beginning of the mix, if I think the song would benefit from it.

The most important thing to consider when trying to add an effected double track is phase coherence between the original and the copy. In this case, phase coherence means that two identical waveforms (the original and copied double) are exactly in phase with each other. In other words, the waveforms line up at exactly the same time so no phase cancellation occurs.

When doubling drums, even a few milliseconds of delay between the two tracks can really cause problems. Phasing can cancel out the impact of this technique, or even drastically reduce the power of the drum sound that you are trying to beef up. So getting the proper signal routing in and out of Pro Tools is key. Here's a good way to do it that will make sure the two signals are in phase with each other.

A Phase Coherent Setup
First, choose "No Output" as the output on all your drum tracks. Then create two sends on each drum track (for example, send "d" and send "e") both with stereo bus outputs (Bus 1-2 and Bus 3-4). Create two stereo auxiliary (aux) tracks, choose Bus 1-2 and Bus 3-4 as inputs, select Outputs 1-2 as outputs on both aux tracks, and label the tracks something like "Drums Submix" and "Drums Compress." see Figure 1.

NOTE: In the Pro Tools session example, I use post-fader sends for the "Drums Submix" bus so that I can use the faders to ride the volume for those individual tracks. In contrast, I use pre-fader sends for the "Drums Compress" bus, and set different send volume levels than the track faders. By tweaking these send levels separately, I can make an entirely different drum mix and get creative with how these tracks hit the compressor and are EQed. FUN!

Unclear on the differences between pre-fader and post-fader sends? Check out the archived Groundwork column in the June issue of DigiZine.

Adding the Effects
Once you've got the routing set up, add plug-ins as inserts on the "Drums Compress" aux track to create the compressed and otherwise altered sound of the drums. As you add plug-ins to the "Drums Compress" track, copy the same plug-ins to the "Drums Submix" track, but make sure they're bypassed. Doing this ensures that the same amount of processing delay happens for each submix, keeping the two signals phase coherent. Bypassed plug-ins still add delay to the track — however, deactivated plug-ins do not. Check out Figure 1 for an example of this setup.

To check the amount of delay on a track, Control + click (Win) or Command + click (Mac) the Track Level indicator to toggle between Volume (which appears on screen as "vol"), Peak ("pk"), and Channel Delay ("dly"). View the delay values (measured in samples) and make sure both aux tracks have the same amount of delay. You can see this right under the I/O section of the "Drums Submix" and "Drums Compress" aux tracks in Figure 1.

Geek Stuff
With a sampling rate of 44.1 kHz, one millisecond is approximately equal to 44 samples. Thus, when you see that some of your track delays are off by just a few samples, it's not really going to cause any major phasing issues. However, if you instantiate some plug-ins on a track that seriously delays a specific track, you might need to nudge that track ahead in time to compensate for plug-in processing delay. Or you can get tricky, use math, and run the other tracks through a Short Delay DigiRack plug-in to ensure phase coherence.

NOTE: Pro Tools 6.x has an updated version of the Mod Delay plug-in. When opening sessions created in Pro Tools 5.x that use Mod Delay I plug-ins, Pro Tools 6.x makes them inactive. To avoid this, copy the Mod Delay I plug-in on your Pro Tools 6.x software CD into your Plug-ins folder. Doing this will ensure that the Mod Delay I plug-in works in both Pro Tools 5.x and 6.x sessions.

If you look in our example session (or in Figure 1), you can see that some of the tracks have six samples of delay while others have either two or four. Using our geek calculations, I've inserted Short Delay plug-ins so that all tracks have the same amount of delay (in samples). Although the delay from within the plug-in does not register in the Track Level indicator, all of the drum tracks have six samples of delay each.

Also note that instead of instantiating the same plug-ins on both the compressed and uncompressed aux tracks as mentioned above, you can use this Short Delay plug-in technique for the uncompressed aux track to reduce the number of plug-in instantiations.

What About Outboard Gear?
If you're using analog outboard gear (such as a hardware compressor) on the aux tracks instead of plug-ins, route the uncompressed signal out of Pro Tools too so that both signals incur the same amount of delay. If this isn't an option, add a DigiRack Short Delay plug-in on the "Drums Submix" that matches the delay incurred by routing the "Drums Compress" track to a hardware insert. The delay time is based on D/A and then A/D conversion (if using analog gear) and your current buffer setting.

Geek Stuff Part 2: On my system, this delay time is approximately 6ms at a buffer setting of 128 samples. (3ms for D/A/D conversion, 3ms for the buffer at a sampling rate of 44.1 kHz.) As buffer settings increase, delay time increases too, but higher sampling rates decrease the buffer and delay amounts. Check out the following chart — you can extrapolate the numbers when using higher buffer settings.

What Else is Going On In This Mix?
Even in this short soloed drum section of a song that also includes guitars, bass, and vocals, there's a lot going on from a mix perspective. Besides the compressed copy of the drums, there's EQ, gating, compression, reverb, volume rides, and sound replacement. All of these things contribute to a more exciting and emotional sound. This is simply one mixing interpretation that seemed to fit the song, where I've really pushed the levels, brought the cymbals to the forefront, and tweaked/tightened up the kick, snare, and toms.

In the Pro Tools session, you'll notice that the first four bars are just the raw drum tracks, which sound good by themselves. I added automation on the plug-ins so that they kick in on the second four measures to show the difference between the raw and mixed tracks.

Without getting into the other details of this mix in this column, look deeper into the drum tracks in this session and check out what I did to build this sound. Think about which techniques you could use in your own mixes, and what you might do differently.

Don't have any recorded drum tracks handy but want to try out these techniques? Download a demo multi-tracked Pro Tools drum session from Naked Drums (www.nakeddrums.com) and go nuts!

Like what you see in this column? Check out my book Producing in the Home Studio with Pro Tools (2nd Edition). You can buy it online right here through Digidesign's website, or visit www.protoolsbook.com. Interested in personal instruction on Pro Tools from yours truly? Visit www.berkleemusic.com and learn about several amazing Pro Tools learning experiences available online though Berklee College of Music. Interested in hearing my band? Visit www.lipfloater.com. Wanna see my studio and hear some samples of my work? Visit www.undergroundsun.com and/or www.davidfranz.com.

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