Audioclinic (Q and A) (Jan. 1971)

Four Channel Sound

Q. My question is about the new quadraphonic, or four channel, system every one is talking about these days. I have read many articles on the subject. If I understand all of them correctly, the rear right and left channels are about the same as the front right and left channels except that the rear ones have more reverberation. Now, why don't the manufacturers of four channel recorded tapes make them so that each channel is different from the other? What I am trying to say is that it would be the same as a four-track recording in a studio, except that it would not be mixed down to one or two tracks as is now done.

That way every listener would balance his system the way he wishes. For instance, if I wanted the guitar to be louder, all I would have to do is to turn up the guitar track. If I wanted the lead singer to have less bass in his voice, all I would have to do is adjust the bass of this channel without disturbing the instrumental backup.

Perhaps I think this way because I attend more and more recording sessions and enjoy every minute. What do you think about this topic? Am I the only one who would like to see (or hear) it that way?

-Louis Hone, Montreal, Canada.

A. What you proposed is already offered on some "pop" music tapes.

Here the audience is surrounded--really in the middle of the performance. You therefore can do anything you want with any of the channels, just as you propose.

However, with concert music you probably would not want to do your own mixing. The rear channels generally contain only reverberation, recorded at the rear of the hall. The idea is to attempt to get the concert hall into the living room. The hall brought into your living room might not really sound like the hall in which the tapes were made.

I'm not sure such an accurate re-creation is possible in a small room. However, the listener will still get a heightened feeling of liveness which was heretofore unavailable. Possibly binaural techniques can also work out well, too.

Experiments in four channel recording are in the infancy. Reproducing systems have not been standardized. Much research is needed on all fronts before we really have a feel for making such recordings. Some speaker placement schemes which have been proposed are designed so that some of the sound is produced a short distance from the front of the hall rather than at the rear. Until enough work has been done, we can't know what ultimate recording techniques will be used. Even with standardization of reproducing equipment, I think we can look forward to and enjoy lots of interesting experiments with recording techniques.

In the meantime, even though most of us don't own four-channel tape players and despite the fact that there are as yet no four channel discs, we can still get some idea of four channel sound merely by experimenting on our own with such speaker arrangements as proposed by Dynaco. It involves no major purchases, except possibly for speakers. I believe Dynaco is supplying all interested persons with the proper circuitry to allow them to connect up speakers in such a way that very interesting sonic results are produced.

High- and Low-pass Filters

Q. Will you explain in plain and clear language what is meant by 6 db or 12 db of rolloff per octave, as applied to high and low-pass filters? What is the best cut-off point for suppressing scratches on a record?

-F. Guerrier, Brooklyn, New York.

A. If you feed a signal of constant amplitude into a piece of equipment and if the equipment into which this frequency is fed is flat, the signal in the output of the amplifier will also be flat.

However, if we introduce a 6 db per octave rolloff below a certain frequency (which is called the cut-off frequency), it means that if we decrease the frequency by half, the power produced in the output of the amplifier will be one quarter of what it was at a point above the cut-off frequency. (This frequency reduction corresponds to a one octave decrease in frequency.) If we decrease the frequency again by one half, making it two octaves below the cut-off frequency, our power will be reduced to 1/16 of what it was at the cut-off frequency.

Actually, at the cutoff frequency, the filter has just started to work. Hence, the power produced at the cutoff frequency will be about 0.7 of that produced at some midband point.

I have assumed in this discussion that the rolloff is 6 db per octave. However, if it is 12 db per octave, each octave below the cutoff frequency will result in a decrease of power to 1/16 that of the octave above.

What we have been discussing is a high-pass filter. It allows all frequencies higher than the so-called cut-off frequency to pass unobstructed to the output of the amplifier or any other piece of equipment which might be equipped with such a filter. The low-pass filter, however, produces the reverse situation.

Such filters become effective above, rather than below, the cut-off frequency.

I would not say that there is a best frequency above which point the signal should be rolled off in the interest of suppressing record scratch. This frequency will depend on the amount of record scratch, the kind of pickup and speakers, and upon the user's willingness to sacrifice some high frequencies along with the improvement in background noise. Remember that these filters do interfere with the sound produced by the high fidelity system. Some people would prefer to put up with the record scratch so that they can still hear the entire frequency range originally recorded on their discs.

Magnetic and Electrostatic Headphones

Q. What is the difference between electrostatic headphones and "normal" headphones?

-Sgt. Dennis O. Mueller, APO San Francisco, California.

A. The conventional headset operates in the same way that loudspeakers do. In fact, you could say that the headset is a miniature dynamic loudspeaker, having a moving coil centered in a magnetic field. The coil is fixed to a diaphragm. When signal is applied to the coil, the coil moves in and out, depending on whether it is repelled or attracted by the magnet.

The electrostatic headphone is like an electrostatic speaker. It operates by having a high voltage alternately repel and attract a thin diaphragm. This is the same principle as operates when a comb picks up bits of paper when rubbed through the hair. The static voltage built up on the comb attracts the paper. In both the electrostatic headphone and loudspeaker this attracting voltage is made to vary in accordance with the signal supplied to it by the amplifier.

(Audio magazine, Jan. 1971, Joseph Giovanelli)

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