Middle-Side Processing... We've Come A Long Way(?)

In fact mid side processing is as “new” as stereo. FM stereo is transmitted in Mid - Side... The central carrier signal is the mid (mono) signal and the sub carrier is the difference / Side / (stereo) signal. To prove this.., note what happens when you go nearly out of range on an FM station in your car. The program reverts to mono, because that signal is broadcast “stronger” at higher power than the sub carrier…

The very first stereo gain control devices were mid / side as opposed to Left and Right. One of the reasons for this was, you didn’t need to have the two channels perfectly matched. You can even use two entirely different processes (as long as they are analog.

MS processing can be very misleading. To make it “simple” many people explain it in very simple terms… but you have to listen very carefully in an excellent stereo environment to be sure you are not doing more harm than good.

There are often “hidden” side effects to processing M-S. One important tool you should have is the ability to monitor the mid signal separately from the side. Here is a simple way to do this in our audio workstation:

  1. Take a stereo track or mix.
  2. Setup up a stereo plugin that allows you to invert the phase of the left channel (only)
  3. Flip the phase of the left channel only.
  4. Set the left and right pan to Center (0). Sum to mono.

What you now hear is the “Difference” portion of the track… This is the “Side” in the mid side. It’s also the Vertical motion when cutting vinyl… but that’s another big topic. … So if what you are trying to change is isolated in this “side” signal then you can probably be successful. Note there are not two “sides”, only one.. And for most engineers that don’t have a minor in AC electronics… it’s “Magic” how the left and right channels re-appear when converted back to Left and right.

This process of flipping the phase and summing to mono is so important it’s built into almost any serious mastering monitor section. You can do this at the monitor for a quick listen. Again – you simply reverse the phase of the left channel then sum to mono.

Now the MID signal is easier. This you can here by just summing to mono… but you will quickly notice that there is usually a lot more energy in the mid signal than the side.

So if you are processing Gain via a compressor.. You may not have enough gain or threshold range to properly compress both signals. – but that is often ok… it’s often nice to leave the side alone – this prevents too much pumping.

What I find I use the M-S eqs and level controls for.

This all takes practice and careful listening. I don’t like to say – leave this to the pros… but seriously, if you don’t use this tool with care, you will do more harm than good.

-Scott Hull