Mid/Side Processing

What Is Mid/Side Processing?

Mid/Side processing is a special way to process stereo signals. This stereo signal consists of 2 mono signals, one for the left and one for the right channel. However, when we create this stereo signal, it is likely that some sounds will occur on the left or on the right side. Nevertheless, we can pan some signals to the center. Mid/Side processing processes the center or mid sounds separately from the sounds on the sides. For example, we might want to apply an equalizer to the sounds in the center of the mix and a compressor to the sounds positioned on the sides of our mix.

Use It With Other Processing Tools

Mid/Side processing does not change the signal on its own. I recommend using it with another tool like equalization or compression. Mid/Side processing separates a conventional stereo signal with left and right channels into a mid and side channel. This allows you to equalize or compress the sounds in the center of a mix independently of the sounds on the sides.

Inside Mid/Side Processing

To understand the Mid/Side processing process, we first must look at the three principal ways a mastering engineer can select a given track. The first and the most immediate deselection tools are, of course, the left and right channels of a conventional audio signal. Mastering equalization, for example, might differentiate between the two channels of an L/R stereo mix. They were perhaps applying slightly more highs to the left side than the right. Another dissection tool is multi-band processing. Here, a two-track stereo mix is divided up into three or four different frequency bands. With multi-band processing, an audio engineer can apply more compression to one frequency band than another. He can shape both the timbre and dynamics of the end master. Dissecting a track through Mid/Side processing is essentially a "third dimension" of signal processing. However, to understand the Mid/Side, we must understand a new way of looking at the stereo. A stereo playback is a neat audio trick, using two speakers to create a 180-degree soundstage. What's particularly interesting about this two-way speaker setup is that the soundstage center is created by the two speakers having a degree of "shared" information. In short, a signal with equal amplitude to the left and right speaker creates a "phantom center" image. It's almost as if there's a third speaker positioned equidistant between the two speakers. The mid signal is the sum of the left and right channels.

Controlling the Master with M/S

Let's look at the middle of the soundstage as the sum of the left and the right channels. It logically follows that the difference between the left and right channels forms the two "extreme" sides. Now, we are looking at the stereo image differently from the sum—the difference between the two channels with discrete controls for the mid and side of the master. By controlling the master using mid and side controls (rather than the left and right), we yield new flexibility. We can differentiate between EQ and compression applied to the instruments in the center of the mix (such as vocals) and signals on the sides (such as cymbals, overheads, reverbs, and so on).