Acon Digital Remix

There are now some impressive offline source separation tools — but this one works in real time!

Source separation sometimes seems like a technology in search of an application. It’s remarkable that we can now unbake the audio cake, extracting individual sounds from within a mixed recording — but how useful is this in a real‑world music‑production context? The results can be impressive, but they’re never completely free of artefacts so, where they’re available, it’s always better to go back to the original multitracks. And where the multitracks aren’t available, that’s often for a very good reason!

Such applications as there are have also perhaps been restricted by existing implementations of the technology. Apart from AudioSourceRe’s RePan, none of the tools I’ve tried has operated in real time, and many require audio to be uploaded to a server before it can be separated. Moreover, user interface design hasn’t always made it easy to fully exploit the possibilities. So I was intrigued by Acon Digital’s new Remix, which operates as a real‑time plug‑in in AAX Native, VST and AU formats, and which promises an extremely simple, streamlined interface.

Both promises are undeniably fulfilled, and once you instantiate Remix on a track or bus, you’ll be greeted with a freely resizable mixer window containing five channels labelled Vocals, Piano, Bass, Drums and Other. Each has the usual fader, solo and mute controls, plus a Sensitivity dial that runs from ‑100 to 100 percent. This, in essence, controls the ‘weighting’ of the detection process; the more you turn up the Sensitivity of the Vocals, say, the more likely it is that audio within the track will be interpreted as belonging to that source. If you leave all the controls in their default positions, the separated sources sum perfectly to recreate the original audio file. And in hosts that permit it, Remix can address multiple outputs, so you can split out the individual stems to different channels in the DAW mixer.

Stem Power

As luck would have it, the opportunity to review Remix arrived just as I finally ran across not one but two real‑world uses for source separation. A friend sent me an old soundboard recording of a gig where the balance between drums, vocals and guitars was badly askew; and, having recorded a jazz trio live in a studio with no isolation booths, I found myself with uncomfortable levels of drum spill on the double bass mic. In both cases, I compared Remix with Hit’n’Mix’s RipX, which probably achieves the most natural results of any source separation software I’ve tested to date.

I had expected Remix to be very CPU‑intensive, but surprisingly, this doesn’t appear to be the case.

I had expected Remix to be very CPU‑intensive, but surprisingly, this doesn’t appear to be the case. In fact, when running on an older Intel Mac Mini (not a particularly powerful computer) I could barely detect any system load at all in the Pro Tools System Usage window. However, it does introduce a processing delay of 11,264 samples, which is a quarter of a second at 44.1kHz, so it’s not an option for live use.

It works, though. Everything in my test tracks was correctly identified, with the Other channel effectively becoming a guitar track, and Remix didn’t falsely detect a piano where there was none. If you solo a channel, it’s easy to hear the effect of the Sensitivity control; as you turn it up, the wanted source sounds clearer and brighter, but spill from the other sources begins to be introduced too. Remix was able to make a positive difference to both my test sources — but, perhaps inevitably, RipX delivered more natural results, with fewer obvious artefacts. I could go significantly further with rebalancing the live take using the RipX stems before warbling or watery noises began to be apparent, and on the double bass, RipX retained more upper‑mid detail and articulation. This is hardly surprising when you consider that the offline ripping process took several minutes for a three‑minute bass part: an offline splitter is simply able to throw more CPU cycles at the problem, and has no restriction on look‑ahead time.

I think the real value of Remix, then, is that you can use it in situations where you couldn’t or wouldn’t use an offline alternative. There are plenty of times during sessions when you don’t want to interrupt the flow in order to leave the DAW, set up an offline split and then reimport the results. Remix is highly affordable, delivers useful results and — uniquely — does so instantly.