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What Is Mid/Side Processing?

Mid/side processing seems to be everywhere these days. The vast majority of Ozone modules support it, and Soundwide partner Brainworx builds it into a great many of their plug-ins. But if I asked you, “What is mid/side processing, how does it actually work, and if I make an EQ boost on the mid channel, will it affect hard-panned sounds,” could you answer with confidence and accuracy?

To start, mid/side processing is the practice of applying processing—EQ, compression, saturation, etc.—to the mid and side channels individually.  

In this article we’ll get into all that and more, discovering how mid/side processing is an undeniably powerful technique. It gives mixing and mastering engineers a wide range of sonic sculpting tools not available with traditional stereo processing, however, as we all learned from Spiderman’s Uncle Ben, “with great power comes great responsibility.”

What is mid/side?

Mid/side (sometimes called sum/difference) is an alternative way of using two channels of audio to represent stereo information. In that sense, it’s actually got a bit in common with left/right stereo.

In fact, the connection between mid/side and left/right goes even further. Mid/side was originally developed as a mic’ing technique by Alan Blumlein, perhaps better known for his “Blumlein Pair”—a left/right stereo mic’ing technique. In mid/side mic’ing, a cardioid mic is pointed at the sound source, while a figure-eight—or bi-directional—mic is set up with its axis offset by 90 degrees.

It’s worth pointing out now that the two sides of a figure-eight mic have opposite polarity to one another. This is very important, as we’ll see later, but back to the matter at hand. In left/right stereo recording, a sound’s position is determined by its level balance between the left and right channels. If you’ve ever used a pan knob this probably feels pretty intuitive, but to state the obvious:

  • Signal in the left channel only = hard-panned left
  • Equal signal in the left and right channels = center-panned
  • Signal in the right channel only = hard-panned right
  • And of course different balances of left to right can give us positions anywhere in between

In mid/side recording, things are a little different. Here, a sound’s stereo positioning is determined by both the level and polarity relationship between mid and side. Here’s how it breaks down:

  • Signal in the mid channel only = center-panned
  • Equal level and polarity signal in the mid and side channels = hard-panned left
  • Equal level but opposite polarity signal in the mid and side channels = hard-panned right
  • Signal in the side channel only = wonky (in practice this doesn’t happen)
  • As level drops in the side channel compared to the mid, the stereo position moves toward the center (polarity still determines left or right)
  • If level in the side channel becomes greater than in the mid channel, the stereo position can appear to move “outside the speakers”—left of hard left, or right of hard right. A little of this can sound exciting, but too much can quickly sound very weird, and has implications in mono.

Another way to think of this is that the mid mic captures everything, and the side mic encodes the direction of each sound. The polarity relationship between mid and side determines whether a sound is to the left or right, and the level balance determines how far in that direction it is.

To answer the titular question then, mid/side processing is the practice of applying processing—EQ, compression, saturation, etc.—to the mid and side channels individually. But how do you do this, and why would you want to? Glad you asked.

How does mid/side processing work?

I’m willing to bet that when you create or receive stereo tracks they’re in left/right format rather than mid/side. So if you want to apply mid/side processing, how do you convert from left/right to mid/side, and back? Luckily, this isn’t something you really have to worry about since pretty much any plug-in that offers mid/side processing will include the conversion—also known as a mid/side matrix. ………

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