Audioclinic (Q and A) (Feb. 1973)

by Joseph Giovanelli

Making Live, Remote Recordings

Q. Circumstances have made it necessary for me to make a number of live, remote recordings. Can you outline briefly some procedures which might make this work easier?

-Donn Petrak, Lethbridge, Alberta, Canada

A. Briefly, if you want to make an acceptable recording, take two cardioid mikes, place them perhaps two feet apart on a bar. This bar, in turn, can be mounted on a single mike stand. Face the mikes 90 degrees from one another and aim this combination at the sound source to be recorded.

The sound source will strike each mike at 45 degrees. The result is really excellent stereo, with little fuss. Because of the rather close spacing of the mikes, the low frequencies are well centered, eliminating problems in disc mastering.

In addition, you may have to resort to "solo" or "accent" mikes here and there, which use would then require a mixer. This miking technique avoids the clutter of cables and mike stands on the stage, which otherwise can ruin the visual aspects of a performance.

Naturally, you must avoid overloading the tape. If you can attend a dress rehearsal or have control over the actual performance of a group, you can take all the time you need to set the recording level. This, however, is not always possible. Therefore, some kind of limiting is helpful. The limiting should be placed between the mixer and the tape machine.

This simple set of suggestions will help solve 90 percent of your remote recording problems.

Noise Reduction

Q. Would you be kind enough to explain how a compressor/expander noise reduction system affects an audio signal in order to achieve a reduction in noise and an extension of dynamic range?

- Lawrence Bobrowski, Chicago, Illinois

A. If we take the soft portions of a recording and compress them by increasing their signal level, their recorded levels on the tape will be higher than they would be if left uncompressed. These portions of the tape will have a better signal-to-noise ratio than they otherwise would have.

This is achieved because the noise content, or background noise, of a tape has remained unaltered, but more signal is recorded on the tape. When this tape is played back, many listeners will not know that compression has taken place. Noise, however, will be audibly lower.

We are striving for realism in sound reproduction. Hence, the next logical step in this arrangement would be to expand the tape's dynamic during playback. This expansion would be exactly equal to the amount of compression which was introduced during the recording process. The result of this is that the full dynamic range of the program source is once again present. Because the signal-to-noise ratio, however, was greater on the soft passages than it would have been if no compression was used, the result is reflected in better overall signal-to-noise ratio during the playback process, despite the re-expansion.

Because of this improved performance, it would be possible to record with a greater dynamic range than would have been possible without this form of noise reduction. If the performance was improved by 10dB, we could increase the total dynamic range by 10dB and have no more tape hiss than would be present if no noise reduction had occurred, and no 10dB increase in dynamic range had been introduced.

What I have described are general principles of noise reduction. They are embodied in various forms. The DBX works pretty much along the lines I have described: the entire band of frequencies is compressed on recording and then re-expanded on playback. The Dolby B system takes the higher frequencies and compresses and re-expands them. The higher frequencies are the ones which are related to tape hiss in home systems. The A Dolby noise reduction system divides the audio spectrum into several bands and compresses and expands each one independently. These are just a few approaches based on this arrangement.

(Audio magazine, Feb. 1973)

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