I have added this page to my website to help clarify the fact that it is impossible to acceptably remove vocals or any other single element from an audio recording.

The Bottom Line:  You can't take the eggs out of a cake after it has been baked.

For many years the back pages of audio and recording magazines have featured ads for hardware devices that claim to remove vocal tracks from a stereo recording. Lately, several audio editing programs have also claimed to offer a vocal remover feature. Is this possible? Is there really a magical way to remove the lead vocal entirely from a commercial recording to create your own instant Karaoke backing tracks?

The short answer is No. Sometimes a vocal can be removed almost completely, but usually the results are unacceptable. In most cases you'll be able to reduce the vocal level, but some audible remnant of the original performance will probably remain. Further, any process that changes the vocal track is sure to affect the other instruments as well.

How Vocal Removal Works

You can reduce the level of a vocal in a stereo recording by taking advantage of how vocals are generally recorded: in mono and placed centered in the mix. Since the vocal track is present in both the left and right channels equally, you can, in theory, remove it or at least reduce its level by subtracting one channel from the other. Instruments panned away from center will not be removed, although the tone of those instruments will be affected.

The basic procedure is to reverse the polarity of one channel, and then combine that with the other channel. Any content that is common to both channels will thus be canceled, leaving only those parts of the stereo mix that are different in the two channels. Reversing the polarity of an audio signal means that the parts of the waveform having a positive voltage are made negative, and vice versa. This is often incorrectly called reversing the phase.

One important drawback inherent in vocal removal is that, by definition, it reduces a stereo mix to mono. Since you are combining the two channels to cancel the vocal, you end up with only one channel.

It is impossible to completely remove a vocal or even reduce its level without affecting other instruments in the mix. First, even though most vocals are placed equally in the left and right channels, stereo reverb is usually added to vocal tracks. So even if you could completely remove the raw vocal itself, some or all of the reverb is sure to remain, leaving an eerie "ghost" image.

Another limitation arises because vocals are not the only thing panned to the center of the mix. Usually, the bass and kick drum are also smack in the middle, and those get canceled along with the vocal.   Unfortunately, of the software programs I've seen that offer a vocal removal feature, none alter the low end on one channel before combining, so the bass and kick are eliminated along with the vocal.

This page is an excerpt from an article written by Ethan Winer, www.prorec.com