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The Truth
About Vocal Eliminators

by Ethan Winer

(This article first appeared in
August, 1999 in ProRec, the
online audio magazine.)

NOTE: Please do not send email
asking if you can hire me to remove the vocals from a track. I'm too busy to
take on new projects. If you don't have a suitable audio editor program, or the
techniques described in this article are beyond your skills, I suggest you buy
Vogonetm available from MTU. This program is priced very fairly, and the latest
version (October 2008) performs even better than the manual editing described in
this article. Vogone's new Hold-My-Handtm feature makes it
very simple to adjust the various parameters for minimal vocal
leak-through. They also offer an even more affordable Easy
version.

Another fine product is the Thomson Vocal
Eliminator, which is a hardware device and thus more expensive than
Vogonetm. You can learn all about the TVE, and download demo
MP3 files, at the LT Sound web
site.

Another option, if you're adventurous, is Center Cut which is a public domain utility that claims to
remove (or isolate) vocals in the center while maintainting the left and right
stereo channels. The link above is to a page that describes the algorithm, and
you can download the program in the pane at the left.

Yet another possible method is to use a stereo
receiver that can decode Dolby ProLogic surround. Even inexpensive receivers
have this feature, which also can remove or keep the center while maintaining
the left and right channels. To use a Dolby-capable receiver you'll set it to
ProLogic while playing the music, then record from the left and right line
outputs to keep the music only. Or record the center channel to keep the vocal
and discard the left and right sides.

Finally, the best way to obtain sing-along tracks
is to buy them from a Karaoke provider. These are not the original performances,
but many companies offer very high quality tracks for thousands of popular
songs. Here are a few Karaoke type product sites, and Google will find you many
more:

PocketSongs
Magic Microphones
Karaoke.com

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 just as often the
results are disappointing. 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. In this article I will explain what vocal removal is
all about and how it works. I'll also describe the procedure and show how to do
it yourself using common audio editing tools.

HOW VOCAL REMOVAL
WORKS

You can reduce the level of a vocal (or other lead
instrument) 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 probably 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. However, there are ways to synthesize a stereo effect afterward, and
that will be described later.

Important note added
November 21, 2002: You cannot remove vocals effectively if
your source is an MP3 file. In order to remove vocals, the vocals in the left
and right channels must be exactly identical. Then when the polarity is
reversed in one channel and the channels are combined, anything common to both
channels - what's panned in the center - is cancelled. But MP3 encoding
processes the two channels separately, so they are not identical enough to
cancel.

It is impossible to completely remove a vocal or 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. If you plan to record yourself singing over the resultant track,
the new vocal can have its own reverb added, and you may be able to mix your
voice loud enough to mask the ghost reverb from the original vocal track.
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! However, you can minimize
this problem by rolling off the lowest bass frequencies on one channel before
combining it with the other. Since one channel now has less low end than the
other, the low frequency instruments will not completely cancel. In fact, 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.

I developed the following procedures using two different
types of music. One is a tune from a friend's self-produced country music CD;
the other is a cello concerto I wrote and recorded in my home studio using live
classical musicians from a local orchestra. I created excerpts of these pieces
in the popular MP3 format and they are available here for downloading. This way
you can compare the original recordings with the processed result, to see for
yourself how well vocal elimination works in practice.

STEPS FOR REMOVING
VOCALS

The most basic procedure is to load a stereo Wave file of
the original song into an audio editor program, flip the polarity of one channel
and lower the bass level somewhat, and then combine the left and right channels
into a new, mono track. I use Sound Forge 4.5 from Sonic Foundry, which includes
all the tools needed to manipulate audio files this way. Most other 2-track
audio editors have similar capabilities, and this technique will apply to those
programs as well. Sound Forge lets you load a single stereo file, manipulate the
left and right channels separately, and then combine them to mono all within one
edit window. But for these instructions, I split the channels into separate
files to make each step easier to follow.

Load the original stereo file.

Copy just the left channel to a new edit
window.

Copy just the right channel to another new edit
window.

* Reverse the polarity of the new left
channel.

* Apply a low end shelf cut starting at 200 Hz (at
least 12 dB/octave) to the new left channel.

Paste the processed left channel into the new right
channel in Mix mode (not Overwrite).

Audition the result and, if it's acceptable, save it to
a new Wave file.

* See the notes added at the end of this
article.

It is possible that combining the two channels will exceed
0 dB, and you will need to reduce the level of both channels a few dB.
If you lower only one channel, the two channels will not combine equally, and
the vocal level won't be reduced as much as possible. To roll off the bass
frequencies, I used Sound Forge's Parametric EQ in the high-pass mode set for 20
dB of cut starting at 200 Hz. (This filter setting affects the lows, so why does
Sonic Foundry call it high-pass rather than low-cut?!) If you use Sound Forge,
be sure to select the highest accuracy filter mode, since how quickly the EQ is
written to the file is less important than having the filter perform exactly as
you ask it to. Besides cutting the extreme low end on one channel, you can
optionally reduce some of the highs too. This lets you retain strings and
cymbals and other instruments that have treble content and are centered in the
mix. In general, you can cut those frequencies that are outside the vocal range
- for male singers you need to start the roll-off at a lower frequency than for
females. Remember, the frequencies you cut from one channel are the ones that
will not be canceled when you reverse the polarity and merge it with
the other channel.

A BETTER WAY

Rather than use a typical stereo audio editor program, a
much better approach is to separate the left and right channels into separate
files and load them into a multi-track audio recording program. The main
advantage is that you can more easily adjust the channel levels to fine tune the
process for the most complete vocal cancellation. This also lets you experiment
with different high and low frequency turnover points, assuming your multi-track
software offers EQ for the tracks. Start with just the very lowest and highest
frequencies removed, and then slide the cut-off frequencies closer to the middle
until the vocal starts to leak through. Again, you are combining the two mono
tracks at approximately equal levels - but with the polarity reversed, and the
extreme highs and lows rolled off on only one channel. I use SAW Plus, which
has EQ and polarity reverse effects built in. These effects are non-destructive
and can be adjusted in real time while the left and right channel Wave files are
playing. So all I had to do was extract the Left and Right files from the
original stereo Wave file, load those into separate tracks in SAW, and add
polarity reverse and low-end shelf cut at 200 Hz to the left channel. Once you
are satisfied that you have removed as much of the vocal as possible and with
minimum damage to the rest of the track, save the mix to a new Wave
file.

One useful tip is to reduce the number of playback buffers
if your multi-track recorder software allows that. Normally, the more buffers
you have the better because that avoids "stuttering" when playing back many
tracks at once. But the trade-off is that more buffers yields a longer time lag
between when you change a volume level or EQ setting and when you hear that
change. So when working with only two mono tracks for removing vocals, I set SAW
to use the minimum number of buffers, thus making my mix changes audible
immediately.

Earlier I mentioned that removing vocals always yields a
mono sound file because the left and right channels are combined as part of the
process. There are several ways you can synthesize a stereo effect to recreate
some of the lost ambience. I used the BlueLine series of plug-ins by digilogue,
available in a fully functional shareware version ($35 to purchase) from the
author's web site at http://www.digilogue.de/. These plug-ins are provided in the
universal DirectX format and also as VST versions for use with Steinberg's
Cubase. I used the BlueLine Stereo plug-in, which did a great job of recreating
a stereo effect on the mono result files.

You can also create a fake stereo image using
equalization. Split a mono track into two identical left and right channels, and
then equalize each side differently. One method is to apply a 10-band graphic
equalizer to each channel, and then boost and cut alternate bands on each
channel. That is, on the left channel you apply 6 dB of boost at 62 Hz, the same
amount of cut at 125 Hz, boost at 250 Hz, and so forth. The right channel is
then cut and boosted by the same amounts, but at the frequencies opposite the
left channel: Where the left channel is boosted the right is cut, and vice
versa.

Two final items are worth mentioning. First, if your
multi-track software requires DirectX plug-ins for EQ and polarity reversal, the
inherent delay will prevent the desired cancellation and all you'll get is a
phased sound with the vocal still present. In that case you should reverse the
polarity and roll off the low end in a stereo editor that writes directly to the
file, and load the result back into your multi-track recorder. I'll also mention
that it is possible to cancel a vocal from a stereo file while keeping the
original stereo image. If you create a mono Wave file that is a simple mix of
both the left and right channels, you can reverse its polarity and mix it with
the original stereo recording. This cancels the vocal and other centered
instruments, and reverses the left and right channels as a side effect. Although
this should be superior to my method of reducing the mix to mono, in practice it
did not work as well. More of the vocal leaked through, and the non-centered
instruments were partially canceled.

THE BOTTOM
LINE

Does vocal removal really work? Is it worth the effort to
even try? I'll leave that for you to decide. Following are two pairs of MP3
clips containing Before and After versions of my attempts. The first piece (265
KB for each MP3 file) is Rollin' from the CD 20 Years Late by
Tom Schulz. Click here to
download a 34-second MP3 clip of the original recording, and click here
for the result after removing the lead vocal track. The second selection is
from my Concerto for Cello and Orchestra in A minor (313 KB per file).
Click here to download a
38-second MP3 fragment of the original, and here
for the version with the solo cello removed from the track.

Both of the After tracks were processed in SAW Plus as
described previously, and then a stereo effect was synthesized using the
BlueLine Stereo plug-in. I rolled off the lows starting at 200 Hz, but didn't
bother experimenting with the highs. As you can tell I was quite successful
removing Tom's lead vocal, mostly because so little reverb was added to his
voice. In fact, before I rolled off the low end on one channel to bring back the
bass and kick, the vocal was practically inaudible. All that remains now is a
muffled hint of his voice. Of course, the bass and kick have lost definition in
the process, since all but the deepest components were canceled along with the
vocal. With the cello recording you can clearly hear the ghost reverb, and the
beginning passage also leaks through because those notes are lower than the 200
Hz cut-off point. I could have lowered the EQ frequency, but that would have
removed more bass content from the rest of the track.

* Added November 14,
2004: I've been getting a lot of emails asking how to
reverse the polarity and roll off the low end of one channel in Sound Forge.
Here are the specific steps using Sound Forge version 6:

Double-click in the upper portion of the Wave file view to
highlight the entire length of just the left channel. If both channels turn dark
you didn't have the cursor low enough when you double-clicked. Then from the
Process menu select Invert/Flip. Next, apply a low frequency shelf to roll off
below 200 Hz. From the Process menu select EQ, and choose Paragraphic from the
sub-menu. Check the box at the bottom labeled Enable Low Shelf, then either
adjust the slider at the right until the display reads 200, or simply type 200
in that field. Finally, move the smaller slider all the way to the left until
the display reads -Inf.

Entire contents of this web site
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by Ethan Winer. All rights reserved.