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.