How to Extract Vocal Samples Using Phase Cancellation

For all of you remixers out there, it can be a daunting task to find quality a cappella tracks for the songs you are planning to remix, especially if the original songs are from lesser-known artists. Thankfully, there is a way to get those sought-after vocal stems yourself without having to browse the millions of websites claiming to have them but clearly do not. If you are able to track down a master-quality instrumental, on the other hand (which is usually much easier to do), you will be in good shape. You may be wondering “how the hell is an instrumental going to help me in the vocal sample department” and I don’t blame you if you are new to this in any degree, but as you will soon see, it is not only possible but relatively easy. Now, let’s get on with it, shall we?

The first thing you want to do is to drag both the original song and the instrumental version into your session. Bring the beginning of each song right up to the 0:00 marker so that they would both start at precisely the same time. The key is to get the songs to line up so that the waveforms match, and the easiest way of doing that is by ensuring the unison of their start point, i.e. dragging them both right up against the 0:00 point all the way on the left of your screen.

You may notice that even with this seemingly fool-proof measure, the waveforms might in fact still be slightly off as far as lining up correctly goes. If this is the case, you will need to pick a track and nudge it into alignment with the other. You may have to zoom in quite a bit to ensure that all of the peaks are lining up as close as they possibly can. Use the cursor as a guide.

The next step is where the magic happens. Once you have the waveforms lined up, you will need to invert your instrumental track. Depending on how your DAW is organized, you should likely be able to locate this function in an options menu, audio effects menu or even as a pop-up option when right-clicking. As a Pro Tools user, I know that I can get to the invert function by going to the Audio Suite drop menu and looking in the “Other” sub menu. I would imagine that Logic, Cubase, FL Studio and other such DAWs use similar wording as to the names of menus in which the invert function can be found, but if you are having trouble finding it, a simple Google search should yield the answer in a matter of seconds.

When you find the invert function, apply it to your instrumental track. You may not notice any difference in appearance and when you solo it back it will sound exactly the same, but the science behind “inverting” the audio lies in the fact that the actual waveforms themselves have been flipped so that the troughs (dips) and crests (rises) are in the opposite position that they were before.

As some of you recall in my “The Correct Way to Choose Drum and Percussion Samples” article, I made mention of phase cancellation and the hassles one could run into when trying to layer samples. The way in which the troughs and crests of two different sounds line up dictates whether or not the combined sound will be more or less powerful in amplitude. For example, a kick drum sample starting with a crest will lose its snap and punch when layered with a kick that begins with a trough in its waveform. They essentially “cancel” each other out, resulting in the complete opposite effect you were intending to get with the additional layer.

On the contrary, this phenomenon works wonders for isolating vocal stems! The inverted instrumental track, with its opposing crests and troughs, will “cancel out” the majority, if not all of, the instruments in the original song, leaving you with a crisp, clean vocal stem with minimal background noise. Since the only variable between the original song and the instrumental is the vocals, the vocal frequency waves will be the only waves left over after the instruments cancel each other out.

A slight bit of work might still need to be done at this point, however. You may notice that some of the instrument noise slipped through the inversion cancellation process leaving you with some strange, muffled sounds in the background or in the parts that lack vocals in the original song. This noise can be deleted pretty easily via noise gates and proper EQing. Start off with a noise gate and set your threshold as wide as you can so that you hear no difference initially. Then slowly start decreasing it and you will begin to hear the noise become less and less. Tread carefully with gating, however, as you wouldn’t want to jeopardize the quality and nuances of the vocal lines like breathiness and softer phrasings which might accidentally get cut out with too much gating. Use an EQ to gently high and/or low pass the isolated vocal track to get rid of additional noise that your noise gate was not able to take care of properly. Again, pay special attention to the nature in which the filters might interact with the vocals themselves. These tools should be used very sparingly so as to leave the main vocals intact with all of their power and clarity. More chances than none, a little bit of leftover noise is hardly noticeable in the mix anyway, especially in a heavy dubstep track with a full range of pumped frequencies.