Audioclinic (Feb. 1985)

Vocal Elimination

Q. During a recent visit to a popular bar, I noticed that the now-extinct "mechanical bull" had been replaced by a "sing-along" machine which played hit records, but which deleted the vocal, allowing the patrons themselves to sing the track. I have seen several ads over the years for such a machine, and I have always wondered how they operate. Do they use frequency filters? Do they respond to transients? Please enlighten me so that I can know the sing-along machine as well as I knew the mechanical bull!

-Scott Hampton; De Kalb, Ill.

A. The odds are that you were not listening to hit records with deleted vocals, but to specially made records with no vocal tracks, intended for just this purpose. The Japanese call this "karaoke" (which means "empty orchestra"). It's quite popular in Japan, and many karaoke recordings already exist.

However, you may be correct in assuming that these were ordinary records whose vocals had been deleted electronically. This is usually done by wiring one channel out of phase with the other, then combining the channels. Vocals are usually centered in the mix, with equal amplitude in both channels, and so will be cancelled out. All other center-channel or monophonic information will be cancelled out as well, including most of the bass. Instruments which are recorded primarily in one channel or the other will still be heard.

The process usually results in a thin sound, monophonic and bassless. (If the vocalist is female, however, it is often possible to boost the bass in one channel, increasing bass response while still keeping the voice out of the mix.) Any reverberation added to the voice in the recording will be stereophonic, and therefore won't be cancelled by this system. Though blurred, the vocal is still audible. But in a noisy environment such as you describe, this echo of a voice is not likely to be all that annoying.

Tuner Loss of Highs in Mono

Q. After removing my tuner from its equipment cabinet in order to clean and dust around it, and, after reattaching it, I noticed something strange.

When I switch my tuner from stereo to mono, the highs seem to disappear on certain program material. When programs are broadcast in mono, the switching between stereo and mono has no effect on the highs whatsoever.

I have also noticed that some instruments disappear almost completely when I set my tuner switch to mono, and this is not just a loss of highs.

Please let me know what's happening, in case I should have to return my tuner for alignment during the warranty period.

-Larry Cook; Albany, Ga.

A. The symptoms you describe are those of phasing problems, either in your system or, more likely, at the broadcasting stations.

It's a bit puzzling why a tuner new enough to be under warranty should need to be removed for cleaning and dusting. If the tuner is new, and if your previous tuner did not show these symptoms, then perhaps you should have it checked. The same holds true if the tuner is not new, but you are certain the problem did not exist before.

However, it's more probable that you are more aware of this situation now because, having rearranged your system, you are doing much more critical listening than you normally would if you were not trying to be certain that you have your system properly reassembled. Tuners are subject to far greater mishandling during shipping than you have subjected yours to during your cleaning. You definitely did not mis align it.

So what is causing these phenomena? The loss of highs you have described is quite common and is to be expected. This phenomenon depends upon the program source the station is using. If the source is a phonograph record, you may not hear this effect at all. If, however, the source is a tape- and this is possible even when you believe it is a record-you will hear this loss of highs because of phase differences between the channels of the playback head of the machine reproducing the program with respect to the recording head of the machine on which it was recorded.

The fact that some instruments virtually disappear or lose luster is a matter of the way in which they were mixed during the original recording. If they were so placed as to represent a difference signal (vertical modulation, on a disc), these instruments will disappear when your tuner is switched to mono. This is reasonable when you recall that mono implies that only information common to both channels will be reproduced.

Phase Inversion

Q. There is an amplifier specification which describes whether or not there is phase inversion in a given piece of audio equipment. What is the sonic significance of this?

-Jeff Horowitz; Poughkeepsie, N.Y.

A. In general, whether a piece of equipment inverts phase or not makes little difference to the sound it produces. The only time the phase relationship between input and output absolutely must be taken into consideration is when bi-amplifying a speaker system. If one of the amplifiers inverts phase and the second one does not, the upper and lower frequency drivers will be out of phase with one another, even when they are "properly" connected. Even then, swapping the two connections to one of the speakers should solve the problem.

If an amplifier does not invert phase, then positive-going signals applied to its input will produce positive-going output signals; if it inverts phase, then positive-going inputs produce negative-going outputs, 180° out of phase.

(Editor's Note: In a recent demonstration of an amplifier with a phase reversal switch, I noted that sounds seemed to move forward or back depending on the switch position, and that there also seemed to be some slight, subjective frequency response effects. The same effect can be obtained-though less dramatically, because of the delay involved-through reversing both sets of speaker connections, plus to minus and vice versa. Phasing also varies from record to record, so no amplifier will be "correct" all the time; if you care about correcting it for individual records, you'll have to set up a switch to reverse the phase of both channels at once. -I.B.)

(adapted from Audio magazine, Feb. 1985; JOSEPH GIOVANELLI)

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