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RE/P Files: Mixing Stereo Monophonically

Exploring the additive effect of audio information of equal intensity on both stereo channels

From the archives of the late, great Recording Engineer/Producer (RE/P) magazine, this feature provides an interesting snapshot of recording methods and techniques circa May 1970. (Volume 1, Number 1). The text is presented unaltered.

A short while ago a mild furor was generated when AM radio stations began to receive stereophonic promotion records and discovered to their horror that the soloists were 3 dB or more too loud when the stereo discs were played monophonically.

It was quickly determined by these broadcasters that an instrument or voice which was recorded with equal intensity on both of the channels would encounter the effects of simple addition of the two portions and become at least 50 percent too loud.

To overcome this “oversight,” Howard Holzer, audio engineer, developed a device which is inserted into the disc mastering system and detects information appearing with equal intensity on both of the incoming sources, automatically suppressing it without affecting the other program material.

While this approach is certainly worthwhile, if not mandatory, the fact that the recordists made its advent necessary is inexcusable.

Ten years ago this writer produced, without even the benefit of today’s “mix down,” a two-channel stereo master of a vocalist backed by a 30-piece orchestra. The tape was transferred to stereo disc, but no mention was made of the disc being stereo.

It was sent to many AM radio stations and was bought by the public for play on home stereo and monophonic record players… for it was assumed by the users that the album was available only in monophonic format.

Not a single report was ever received from any user to the effect that the vocalist was too loud under mono listening conditions. The disc played perfectly and no one noticed anything unusual. Why?

Because there was nothing unusual… except in the method of splitting the vocalist between the two channels.

Some, although unfortunately not enough, sound mixers already were aware, way back then, that the additive effect of audio information of equal intensity on both stereo channels could be avoided by splitting the source to both channels in unequal amounts.

To be precise – it was discovered that if you split the soloist in such a way that he is 3 dB (or more) ” hotter” on one channel than on the other, the cumulative effect when the stereo recording is played monophonically is minimized to the point of being unnoticeable.

In fact, with exactly 3 dB difference in levels, and with the level of the louder channel set for proper balance between orchestra and soloist (letting the weaker portion of the split source fall where it may on the other channel) not only is there no noticeable additive effect when listening monophonically to the stereo recording, but when listening in stereo it is impossible for the consumer, and for most experts, to tell that the soloist is not split equally between the channels.

A unique console specified by this writer and designed and built by Charles S. Broneer, provides splitting of any source to any pair of output lines in any ratio except 50-50. The console purposely will not allow the latter; the closest to this that it permits is a split with the 3 dB intensity difference between channels.

It has thus been positively established that with a console capable of providing other than 50-50 splitting, a two-track recording will reproduce perfectly in a stereo or monophonic system, and without the requirement of expensive and critical supplementary devices which should never have been necessary in the first place.

Editor’s Note: This is another in a growing a series of articles from Recording Engineer/Producer (RE/P) magazine, which began publishing in 1970 under the direction of Publisher/Editor Martin Gallay. After a great run, RE/P ceased publishing in the early 1990s, yet its content is still much revered in the professional audio community. RE/P also published the first issues of Live Sound International magazine as a quarterly supplement, beginning in the late 1980s, and LSI has grown to a monthly publication that continues to thrive to this day.

Our sincere thanks to Mark Gander of JBL Professional for his considerable support on this archive project.

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