Anyone who mixes for hire knows the scenario: A client sends a song to be mixed without individual drum tracks (snare, kick, etc.) that are instead embedded in a single track. I'll send an email, "Hey, can I get the actual tracks for the drums so I can really mix your song?" They then email back an excuse as to why this is the only version of the drums they have. WaveMachine Labs, known for the venerable drum replacement app Drumagog [Tape Op #84], have created ReStem, a standalone app and plug-in (both non-real time) that will de-mix a stereo or mono drum submix into seven parts: Kick, Snare, Hi-Hat, Toms, Ride, Crash, and Other. The app looks like a simple stereo mixer, with the seven sources each having level knobs, mute and solo buttons, Gate and Shaper controls, plus the Kick and Snare channels even have Drumagog installed if there's a need to replace or blend in a new sound. But does this AI-trained software work? Yup!
I took a stereo drum submix stem from a session I mixed on my console last year. The drummer's parts were not simple, with intricate but subtle hi-hats and intense tom patterns in some sections. It took about two minutes for the standalone app to process the track after I loaded it in. Note that this is all being done on your computer, and unlike some de-mixers it is not handled remotely – no need for an internet connection. I was amazed to hear the hi-hat extracted from the submix, and all the other sources as well. It was easy to increase/decrease the level of any track in the new mix via the app, which could be the lifesaver we sometimes need, like when clients ask me for revisions to an analog console mix.
As with any de-mixing program, playing the separated tracks together often sounds artifact-free, but when soloing an isolated source, I would sometimes hear unwanted residual pumping noise (especially on the Crash track) that can create a tremolo sound from the parallel compression and bleed in the original submix. It was funny to solo ReStem’s Other track, revealing scratch vocal and guitar bleed from the live backing track take! Mute that, and there’s no more weird stuff in the background. As I examined the separated tracks further, I was gradually more impressed by the processing quality, as there were only a few times that I heard much of anything that was not the intended sound. WaveMachine Labs trained AI to understand and discern between the actual components of a drum kit, and it seems to have worked. It can also separate drum machine stems, which I tried it out on a previously mixed project while wishing for a time machine. Next time, I'll be using ReStem. Though the GUI’s cleanly laid-out mixer interface is convenient and versatile, it can also de-mix and export the seven new files. Handy – especially if there's a need to edit, cut parts out, or apply new processing – compression, EQ, reverb, and such. If sampled drums were added or transients and Gates applied, it can export either the processed or raw stems, plus it can include the full new audio submix from the app. There’s also a ReStem Core version for only $99 that is basically just the standalone app with no plug-ins, Drumagog, Gate, or Shape features.
The plug-in version of ReStem is inserted on a track. The user hits the red dot (record) button while playing back the full drum part, and it gets captured. Processing begins in the app when playback is stopped and can take a few minutes. When monitoring the entire session post-processing, the drums are active through ReStem and stay in time with the song. The seven separated drum tracks can then be fed out to auxes for further processing, similar to most virtual drum plug-in instruments. This makes for a fairly versatile workflow and would allow for backward "undo" changes or "breadcrumb trails," as I call them. I enjoyed (and preferred) using the standalone app instead of mucking around in sessions with the plug-in. However, the plug-in did allow me to adjust and mix the new drum stems against the music to hear what I needed. It's a tradeoff either way, so pick the best path for your needs.
I threw The Beatles' "Back in the U.S.S.R." in the app to see if it could pull individual drum parts out of a full band mix. It did. It may not be quite what Giles Martin [Tape Op #103] gets Peter Jackson's team to do for him, but us mortals are getting closer! It did mess up the sound of the whole song when all the tracks were played back, so be warned. This is not what ReStem is designed for!
WaveMachine Labs has recently added MIDI output for the Kick and Snare sections, but I didn't see an obvious way to export these MIDI files (though we all know of other ways to generate this data). The MIDI from ReStem can be sent to trigger sounds elsewhere in a session – or even out to hardware. I would have loved to have seen a pitch change control (similar to other existing drum replacement tools) for most of the tracks, something WaveMachine Labs admits is a good idea. Many times, I find bass guitars out of tune with tom tracks, or even the kick drum. Let's fix that! A way to further de-mix the Toms track would be awesome, but I'll wait a couple of years for that one. Or how about drum reverb separation? A mixer can dream! However, I am incredibly impressed with this drum de-mixer. I've been waiting for software that does this for decades, and I'm glad WaveMachine Labs thought to build it. ReStem does what it claims and will save me some grief in the future.