What Is Mid-Side Equalization/EQ (Audio) & How Does It Work?

Mid-side (M/S) processing may not be the most popular style of processing when it comes to stereophonic audio, but it's certainly effective in some situations. Combine this fact with the power of equalization, and it makes perfect sense that mid-side EQs would exist in the realm of audio technology.

In this article, we'll describe mid-side equalization with a focus on mid-side processing and general EQ. We'll consider a few mid-side EQ unit examples, along with the situations and applications that call for mid-side equalization.

What Is Mid-Side Processing?

Let's begin by discussing the idea of mid-side and mid-side processing.

Mid-side audio is similar to stereo audio in the fact that it represents a stereo image with 2 channels. However, unlike stereo (which has a left channel and a right channel), the mid-side has a mid-channel and a side-channel.

Note that, though mid-side is a powerful processing technique, it is rarely used in signal transfer. Devices (hardware or software) that have mid-side processing capabilities will generally not have “mid-side” inputs or “mid-side” outputs.

Rather, these mid-side processors will have stereo inputs and outputs and mid-side encoders and decoders to change the input stereo signal to mid-side effectively, process it in mid-side, and change it back to stereo for the output.

As the name would suggest, the mid-channel represents the information in the centre or middle of the audio's stereo field. In other words, the mid-channel holds common information between the encoded left and right stereo channels.

As the name would suggest, the side channel represents the information to the sides (left and right) of the audio's stereo field. Put another way; the side channel holds information that is different between the encoded left and right channels.

The routing required to change stereo audio into mid-side audio can be visualized in the following signal flow chart:

Similarly, the routing required to change from mid-side back to stereo can be visualized with the next signal flow chart:

It's important to note that, when decoding a mid-side-processed audio file, we must reduce each channel by 3 decibels (dB) to compensate for the fact that the stereo-to-M/S transitioning causes a doubling of level in both the mid and side channels.

Otherwise, we'd have the following:

L + R - (-L + R) = 2L

L + R - (L - R) = 2R

Note that, by dropping the levels by 3 dB, we half the signal power.

Processing mid-side audio, then, allows us to affect the centre information independently from the side information. This can be extremely useful in terms of affecting the perceived width of the mix.

What Is Audio Equalization?

Now that we know what mid-side processing is in audio, let's turn our attention to the second portion of the term “mid-side EQ”.

Equalization is an audio process that can alter the relative balance between frequencies within an audio signal. EQ works by increasing or decreasing the relative amplitudes of some frequency bands compared to other frequency bands. From mixing, tone shaping, crossovers, feedback control and more, EQ is an invaluable tool for audio.

Cutting and boosting with EQ refers to decreasing and increasing the relative amplitude of defined frequency bands, respectively.

Filters, generally speaking, eliminate frequency content below, above or between two set frequency points. However, the term “filter” can also be attributed to “bell-curve” or “peak” filters that are used to boost or cut.

What Is Mid-Side Equalization?

Now that we understand mid-side processing and the basics of EQ, we can fully understand M/S EQ.

Mid-side EQ is simple equalization done via mid-side processing. A mid-side EQ will have independent EQ paths with control parameters that will act upon the mid-channel and the side-channel.

As we discussed previously, these units will typically have a stereo input and stereo output. They will rely on “encoders” and “decoders” to change the stereo input to mid-side channels and then change the EQed mid-side channels back to stereo for the output.

To function properly, these mid-side EQs would need a stereo signal.

Most M/S EQs are actually stereo equalizers with optional mid-side functionality. This design makes sense as a mid-side EQ requires essentially the same signal flow as a stereo EQ with the addition of the aforementioned encoder/decoder.

By EQing the mid and side channels differently, the width of the mix can be fine-tuned to the mixer's liking. Another common use of mid-side EQ is to high-pass filter the side information to ensure a solid and tight mono bass response (stereo information in the sub-bass range can be detrimental to the mix's clarity).

Examples Of Mid-Side Equalizers

Before we wrap things up, it’s always a great idea to consider some examples. Let’s look at a couple of different mid-side equalizers to help solidify our understanding of this EQ type.

In this section, we’ll discuss:

• 500 Series mid-side EQ unit: TK Audio TK-lizer 500
• Mid-side EQ plugin: FabFilter Pro-Q 3

TK Audio TK-lizer 500

The TK Audio TK-lizer 500 is a 500 Series 3-band Baxandall-style EQ (with an included high-pass filter) with mid-side capabilities.

This powerful yet simple unit can be used in stereo (L/R) or mid-side mode (M-S). The M/S toggle switch will toggle between the two modes.

Each of the bands has ±8 dB of boosting/cutting in 41-step controls. The low and high bands can be switched between peak filter and shelving filter modes. The 12 dB/octave high-pass filter can be set at 25, 30, 35, 40, 45 or 100 Hz.

FabFilter Pro-Q 3

The FabFilter Pro-Q 3 is a powerful equalization plugin that offers everything we could want in an EQ, including dynamic, linear phase, parametric and, of course, mid-side EQ functionality.

Up to 24 bands can be easily added or removed from the Pro-Q 3 plugin. In addition to the wide variety of filter types (including brickwall), the Pro-Q 3 also offers customization of the order (dB/octave slope), centre or cutoff frequency, gain (±30 dB), and Q factor (0.025 to 40) of each band.

Most importantly, in the context of this article, is that each of these bands can be set to work on the side or mid channels of the audio signal. Note that left channel, right channel and stereo link are also options for each band.

Related Questions

What are the different types of EQ? When it comes to audio equalization, there are several types of EQ to be aware of. They are as follows:

• Graphic EQ
• Parametric EQ
• Semi-Parametric EQ
• Dynamic EQ
• Linear Phase EQ
• Passive EQ
• Shelving EQ
• Stereo EQ
• Mid-Side EQ

What is the mid-side miking technique? Mid-Side is a coincident stereo miking technique with a cardioid “mid” mic pointing at a source and a bidirectional “side” mic placed above or below facing left/right. The mid mic is mono/centre. The “side” mic signal is duplicated: one channel panned left, the other panned right and phase flipped.