转载：The Truth About Vocal Eliminators
2024-02-14：当年转载此文，确实是因为歌曲消人声这件事难以有好的解决方法（算法），但现在已经有了基于神经网络算法的 Ultimate Vocal Remover，几乎是完美地解决了问题。时隔多年之后重读此文，深感技术进步的力量。
by Ethan Winer
(This article first appeared in August, 1999 in ProRec, the online audio magazine.)
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.
1. Load the original stereo file. 2. Copy just the left channel to a new edit window. 3. Copy just the right channel to another new edit window. 4. * Reverse the polarity of the new left channel. 5. * Apply a low end shelf cut starting at 200 Hz. (at least 12 dB./octave) to the new left channel. 6. Paste the processed left channel into the new right channel in Mix mode (not Overwrite). 7. 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 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 Copyright 1999 Ethan Winer. All rights reserved.
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