What is the Mid Side Technique?
A common tactic when mastering a stereo audio file is to split the audio into what is referred to as the “mid side matrix”, “mid side Technique”, “mid side”, "Mid Side EQ", and probably a few other similar names!
To break it down to basic terms, this method gives the ability to separate the middle of the signal from the unique stereo, left-right content of the signal. Nowadays we do have a few plugins that can do this for us, but here we are going to look at the manual way to do it and what is arguably still the most flexible. It is also a lesson in routing and patience! This may seem a little convoluted and complex to put together. Just take your time and follow everything as best you can. Missing a step can make then entire thing not function properly. Please be sure you also have Delay Compensation is enabled
Step 1: Encode the signal
First, we must encode the stereo signal. This is simply the process of separating the mono mid signal and the signal that will be used for the sides.
To create the mid signal, we just combine the left and right sides in phase to a mono channel. That is as basic as it sounds!
Next we need to encode the Sides signal for stereo. To do that you just subtract the left side from the right side. So now we have:
- Mid = L+R
- Sides = L-R
So just to specify, to get the Mids (L+R), we are just summing the left and right to mono. To get the sides, flip the phase on the right side.
Lets encode this in Pro Tools
To accomplish this in Pro Tools, take the audio track that contains the stereo track and set the output to a stereo bus channel, channels (1-2, M-S, whatever you would like to use or create for a buss name)
Create 2 Stereo Aux tracks and set the inputs to match the audio track output bus. Name the First track Mid Send, Center, mono, or whatever you like. Set the output of this Mid send aux channel to a new unique Mono bus (3, M out, whatever you would like to use or create for a bus name.)
On the second stereo aux track, name it Sides for example. Set the output of the Sides Aux track to a unique mono bus. It must be able to “Stereo Couple” with the Mono Bus output for the Decoding process. For example 3-4 can stereo couple. You could set 3 to the Mid Send and 4 to the Sides. You could also create your own stereo bus with individual mono capabilities.
On the Mid Send and Sides aux tracks, insert a plugin that can invert the phase of one side of the signal (Multi-mono plug-in) EQIII 1-Band works great or even the Time Adjuster plugin. Use the same plugin for both sides to keep the possible latency consistent. On the Side aux channel, flip the phase of the right side of the plugin. You may have to “unlink” the L-R of the plugin if you use EQIII for instance. Time Adjuster does not require you to unlink it. Reduce the level by 6db on both the Mid Send-Side Aux tracks.
Step 2: Decode the signal
The decoding of the signal is the next step and is a bit more complex. This is the process to get discreet left and right signals for the stereo field. The simplest way I can think of to break this down is a way I had learned many years ago which is to just look at the formula.
- Left side = (L+R) + (L-R) or Mid+Side
- Right side = (L+R)-(L-R) or Mid-Side
Lets look at setting this up in Pro Tools.
Lets decode this in Pro Tools
Create 3 mono Aux tracks.
The first new aux will be for the middle signal. Name it as desired (Mid Mix, Middle, as example) Set the input to 3 or whatever is used for the Mid Send output. Then set the output to a new stereo bus (7-8 for this example) any desired Mid channel processing would happen on this track, such as compression, eq, FX sends, etc. Think of this channel as containing most of the bass signal, vocals, and other center content.
Then name the other 2 channels “Left” and the other “Right”. Set the inputs to match the Side aux bus output (4 in this example). Then set the outputs to a single side of a stereo pair. For this example, send “Left” to bus 5 and “Right” to bus 6.
Insert the plugin that you used previously to flip the phase on the 3 mono aux tracks you just created. You will only flip the phase on the 3rd track, which will be the “Right” channel.
Now create a stereo aux track. With this track we will combine the “Left” and “Right” back into a stereo track. You can name this track LR, LR Mix, Left-Right, or something similar. The input will be the outputs from the “Left” and “Right” channels (5-6 in this example). Now bus 5 should JUST feed the left side and bus 6 will feed JUST the right side of this aux track. Set the output to 7-8 for this example. This fader may be debatable by some, as it adds an extra layer on the stereo side, but not on the mono side. Assuming all latency compensation is working correctly, it should not be a problem. You could just group the “Left” and “Right” track to mix if no plugins will be used and skip this step.
Lets bring it back together
Now create 1 more stereo aux track. This will be the channel where all of the summing actually happens. I typically just call this channel “Summed”. To match the above examples, set the input to 7-8. Now the “Mid Mix” channel as well as the “LR Mix” will all feed into this track. You can add full stereo processing on this track or you can bus to another track or use a Master Fader.
Lets reference the original audio
One last thing to look at is creating a way to play back the original unaltered audio for reference and A/B'ing. I do this by creating a stereo aux track and sending a pre fader bus send from the original audio track. Then I can just mute / un mute the track I wish to listen to. Set the output of this reference bus to the final output as to be sure no processing happens on it.
Finally, the conclusion!
You now have the full ability to add plugins and stereo processing and even bus to reverbs from the mid and sides independently. You can also adjust the stereo width of the track easily now by adjusting volume fader on the “Mid Mix” and “LR Mix”.
This may seem like a long winded process, though it becomes quick once you get the hang of it. You can also easily setup a session to use as a template or “Save As” and pull in the stereo file you would like to work on.
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