Mid/Side processing is a clever technique which, once learned, will probably find its way into all your mixes. Its strength lies in making subtle and creative changes to the stereo image of your audio tracks, and it has the potential to make you feel like a 'real' audio nerd. It's one of those tricks that never fails to impress, and, thankfully for us, Ableton Live makes it easy to achieve.
So What Is It?
Mid/Side processing works by decoding a stereo signal into two components. The 'Mid' channel contains just the information that appears in both the left and right channels, and the 'Side' channel contains all the information that differs between the left and right channels. Once encoded into M/S, these two signals can be processed completely separately, before being matrixed back into conventional L/R stereo.
The easiest way to demonstrate Mid/Side processing is by using Live's own EQ8 device. You might use EQ8 a lot already, and so you should. It's a great EQ device with eight bands, six filter types, and a scale function that allows you to adjusts the gain in all bands at once. It also sounds good and is easy on the CPU. The cherry on the cake, however, is the often overlooked Mode parameter.
EQ8 can work in three modes. The first mode is the default Stereo. The second is 'L/R' mode, in which the left and right channels can each have their own EQ settings. In this mode, you can try boosting the high frequencies in one channel while reducing the same frequencies in the other. This will have the effect of panning only the high frequencies of a stereo signal while leaving the low frequencies centred, which is much more subtle than simply panning the entire signal left or right.
The third mode, and the focus of this month's Live technique column, is the 'M/S' mode.
How Do I Use It?
There are some great uses for a Mid/Side equaliser. Remember, what EQ8 is doing is allowing you to equalise the sum and difference portions of your signal completely separately. To start with, let's demonstrate exactly what Mid/Side processing is by listening to the two elements in isolation. Load up a song (either your own or something commercial). Now insert EQ8 on the master channel and switch the mode to 'M/S'.
Firstly, we'll listen to only the Mid portion of the song (the summed mono left and right signals). In the Edit field, make sure that 'S' is showing, indicating that we are editing the Sides signal. Ensure Filter 1 is enabled and in Low Cut mode and bring the Frequency all the way up to 22kHz. In doing this we effectively cut all of the Sides, leaving only the Mid. What you hear now will be only that component of the signal that is present equally in the left and right channels.
Now let's reverse things and listen only to the Sides signal. Disable Filter 1 on the Sides signal so you hear it again, switch the Edit mode to M and repeat what we just did, cutting the entire Mid signal using a Low Cut filter set to 22kHz. This will leave us listening to the Sides signal, matrixed back to L/R stereo. Interesting isn't it? (To hear the true Sides signal, which should be mono, you'll also need to reverse the polarity of the right channel.) Listening to some of your favourite artists in this way can be very revealing. Try cutting the Mid signal in and out to compare only the Sides signal with the full mix. You will get an entirely different view of how your favourite records were mixed!
Let's move on and look at some practical tips for mixing your tracks.
Mono The Bass
It's accepted as standard practice that low‑frequency instruments such as kick drums and bass should be kept in the centre of the stereo field. There are a couple of reasons for this. Firstly, the human brain finds it very difficult to locate the source of low frequencies, so it's fairly pointless to pan them anyway. The second reason is linked to the production of vinyl records. If bass frequencies are heavily mismatched in the left and right channels, the needle can potentially bounce right out of the groove, causing skipping.
Let's say we have a huge synth bass that has not only a lot of sub-bass energy, but also a lot of additional harmonics created by a stereo distortion effect. Our mix will probably be more successful if we can restrict the stereo component of this effect to higher frequencies. Apply EQ8 and enable M/S mode. In the Edit field, make sure that 'S' is showing, telling us that we are editing the Sides signal. Now ensure Filter 1 is enabled and in Low Cut mode and bring the Frequency up to about 700Hz. By doing this, we have effectively filtered out any Sides signal below 700Hz, leaving only the Mid signal. This will, in effect, make the bass mono below 700Hz, while retaining the nice stereo effect on the top end.
Adding Air & Space
On either your master channel or your reverb return channel, try inserting an EQ8 and gently boosting the low-mid and high frequencies in only the Sides channel. This will enhance the stereo space of your track without muddying up the Mid channel. Another way to achieve this is to scoop some of the mid-range frequencies from the Mid channel on your reverb return.
Further Stereo Adventures
EQ8 has built-in Mid/Side functions, but with a little ingenuity, we can isolate sum and difference signals for processing with any plug‑in. Grab a Utility device from the Live Library and apply it to the track you wish to process. Now right‑click on the Utility's header and select Group. This will put the Utility device inside an Audio Effect Rack. Click the Show/hide Chain button to show the Rack's chains, and then duplicate the existing chain by right‑clicking on its name and selecting Duplicate (or Ctrl-D/Cmd-D). Name one chain 'Mid' and the other chain 'Sides'. Now select the Mid chain and set the Utility device's Width to zero percent, then set the same parameter on the Sides chain to 200 percent.
What you have now are two chains working in parallel, one providing the 'sum' and one providing the 'difference'. Note that this is not M/S as such, as both are still stereo chains — as before, what you have are the Mid and Sides channels separately decoded to L/R stereo, which means that the Sides chain contains the same signal in the left and right channels, but in opposite polarity.
Before we get creative, save your new stereo processing rack to your Ableton Library by clicking the save icon in the audio effect rack header. Now you'll be able to quickly apply the rack to any audio track. At this point, no real processing is happening. If you bypass the whole audio effect rack you won't hear any difference. What's more, because the 'L' and 'R' signals in the Sides chain have opposite polarity, any plug-in that sums its input to mono won't 'see' any signal at all, so a typical stereo compressor or mono-in/stereo-out reverb will do nothing. However, processors that take a stereo input will produce extremely 'wide' effects. For example, a true stereo reverb on the Sides chain alone can add space and width without the main signal (the Mid part) being muddied up with reverb.
From here on, experimentation is the key. Try applying just about anything to the Sides signal: delays, phasers, even glitchy processors such as Beat Repeat. The effects of processing the Sides signal can be very subtle, so it's often best to work on buses or groups of tracks. Try grouping your drums and applying EQ only to the Sides signal, or group all your synth tracks together and apply some delay to the Sides signal to add depth without clouding the main content.
Mid/Side processing can change the width and depth of your mixes in subtle and effective ways, many of which would otherwise be impossible. Once mastered, these techniques could start appearing in all your mixes. Go forth and widen!
Here's a quick general EQ8 tip: right‑click on its title bar and make sure that Hi‑Quality mode is enabled (it should be ticked). Now, without making any other adjustments to the device, select Save As Default Preset from the same menu. This will ensure that EQ8 always works in the high-quality mode. It uses slightly more CPU power, but with modern computers this is negligible, and it does sound better.