The View from 2005: This is an interesting way of looking at things. I now call this “Sum and Difference” Listening or Mixing, but the concept is exactly the same. I use it a lot in my work, and it is fundamental to studying recordings and to problem-solving in mixes. It also remains a viable and widely used aesthetic approach to most multitrack recording genres.

In recorded music, there has been an informal, generally held notion that the stereo image is a re-creation of a stage, an imaginary space stretching between the two speakers, on which the recorded artists appear, reproducing the reality of their live performances. In classical music recording, this notion has some validity, because the recording techniques used to create the recording include the use of a concert hall and a stage, and the producers make the evocation of that concert hall ambience a central part of their vision.

However, such an approach is only one of many ways to use stereo to evoke the sense of space and presence that makes it such an effective entertainment medium. In popular music (i.e. music created via multi-track recordings, with overdubs, effects and extended production and remix processes), the “stage" is a much more artificial and highly stylized idea. In fact, it is possible to view popular multitrack recording as a fairly well-developed musical style itself, a style that is characterized by use of the Center phantom image which interacts with sounds panned Left and Right in a musical counterpoint that is unique to stereo loudspeaker playback. It has no direct analog in the world of live or acoustical musical performance.

Many of the elements of this style are driven by both the limits and the predisposition of the recording technology in use during the years this style has been developing. Placement of the sound of the kick drum and electric bass in the Center of the stereo image was originally necessitated by analog disc mastering requirements. In fact, all significant low-frequency energy was placed in the center of the stereo image for the same reason. Similarly, placement of the lead vocal (often the loudest element in such recordings) in the Center was also desirable. Electric guitar solos, horn leads, and other primary musical highlights followed suit. The reason for this was that the stylus motion on an analog disc that represents any signal other than A+B has a vertical motion component— and if it’s too loud the stylus bounces out of the groove, which used to be called a “skip.” Consumers not willing to stack dimes (quarters for particularly loud A-B moments) on their phono cartridges would return the records that “skipped”. Such events were not lost on the mastering engineers of the day, whose livelihood depended on happy customers with “skipless” records. As a result, mastering engineers developed a passionate interest in scrutinizing A, B and particularly A-B signals, in addition to the normal stereo recording. (If you’re a little cloudy on this A+B/A-B business, you might want to check back to my article in the September, 1993 issue of H&SR.)

At the same time, to enhance the sense of space and to prevent lead parts from being psychologically “cluttered" or obscured, supporting parts came to be panned to Left and Right. A spatial interplay between lead and supporting parts, between the solo vocal and the harmony vocals, etc., has developed, becoming a stylistic signature of stereo pop/rock recordings. The musical relevance and force of each of these elements is reinforced by this counterpoint. The presence everywhere of inexpensive amplitude-based pan-pots and the comparative lack of time-delay based pan-pots (which are quite expensive) have effectively limited the effect of panning to generalized zones called Left, Center and Right except in particular cases where the engineer or producer was willing and able to devote the resources necessary to create something a little fancier. As you may know (or if you read my article on early delays in the September, 1992 H&SR), conventional pan-pots cannot be used to reliabiy position a phantom image at a specific point between Left and Center or Right and Center, thanks to our hearing localization system’s basic insensitivity to small amplitude differences.

I started thinking about this some years ago after I had the good fortune to serve as host for mastering engineer Bob Ludwig when he came to talk to my students at SUNY/Fredonia. In the course of some fairly extended conversation, he told me that he always listened to the A-B version of recordings that he mastered and that he’d come to find them very interesting from a production standpoint as well as a technical one. He found various producers’ characteristic practices and tricks were much more audible in A-B, and that many elements of the original tracks were more clearly revealed in A-B. He suggested that my students might find it useful to listen to recordings this way in order to quickly identify and comprehend various producers’ styles.

I spent some time after Bob’s visit listening in A-B and found that he was, as he usually is, right. I found something else, too. Once I began to study recordings in this way, I began to recognize stereo multitrack recording as a style in its own right — a style in which the recording can be thought of as having two primary elements: A+B and A-B, as opposed to the more traditional view in which stereo is thought of as a recreation of a concert or club stage. This new point of view, it turns out, leads to powerful and effective stereo recordings, recordings that also work really well in mono and that are particularly well-suited to a wide variety of playback situations.

Further, it became obvious to me that many producers have picked up this same sensibility, and are intuitively utilizing these elements very effectively as a central organizing approach to their multitrack recording production efforts. This has been a very useful insight for me, and it has led me to rethink my own approach to recorded music. I pass it on to you here, for your own use and edification.

There are two parts to this. First, you can use the A-B component of the signal as a tool for the study of recordings, including your own. Such study will reveal many interesting, useful and sometimes startling things about recordings that are simply not audible in stereo or mono (A+B). Second, A+B/A-B can be treated as an aesthetic approach to recording, and it is being used with great success by many producers today. We’ll spend some time talking about that aspect of it as well.

Briefly, A+B is the audio sum, or (mono) mix, of left and right channels, and A-B is the audio difference (also a mix, but with the polarity of the right channel reversed). To generate the A-B signal, you have to mix Left and Right together, but first you have to reverse the polarity of the right channel (it doesn’t have to be the right channel but traditionally it is). Many consoles make this easy, by including polarity reversal as a feature on the input modules.

See SIDE-BAR 1: The "POL-REV" Button