Mid/Side Stereo Part 2: Using processing and effects
By Rob Stewart - JustMastering.com - Last updated August 7, 2017
Last updated July 1, 2017
Many people use Mid/Side processing at the end of their mix or during mastering to adjust stereo width or create interesting and musical effects. Mid/Side processing will influence the sound quality of your production, and this article will discuss some common processing options along with the related risks.
Probably the most popular use for Mid/Side processing is to adjust the stereo width of a sound or a whole mix. Very briefly, stereo width is created when you have two speakers, and there are differences in what you hear between the left and the right speaker. The differences can created by combinations of variation in gain (e.g. louder on the left than the right), in tone (brighter sounding on the right than the left), in pitch, time (heard first on the left, then the right) or polarity. These differences are essential to stereo sound reproduction.
How do you achieve wide stereo width in your mix?
Width can be created at the recording stage. The perceived width of a recording depends upon how wide the subject sounds, where you place microphones and the amount of natural reverberation in the environment. You can play with all of these factors to record the desired amount of width. For example, recording an acoustic guitar “Nashville style” will allow you to capture a very wide guitar sound. Place the mics closer together further from the guitar and you will record a much narrower sounding guitar sound, instead. The correct amount of width depends upon your vision for the mix, but it is important to know that the width you capture at the recording stage can play a role in the width you hear in the final production.
Width continues to develop during the mixing and production stage. The primary goal of a mix engineer is to create a compelling listening experience. Stereo width, at the right time and in the right proportions, plays a huge role in making a mix more enjoyable for listeners. Many mix elements of a multi-track recording will be monaural because they were captured that way (i.e. with one microphone) at the start. There may be some exceptions such as string sections, piano, or acoustic guitar, but a lot of the other elements will be mono. The mix engineer creates width from this collection of tracks by using panning, delay, reverb and other processing. The topic of achieving width in a mix is warrants a separate article, so I won’t spend more time on it here. The key message is that a great mix has stereo width built in into it from the ground up, in a musically compelling way.
I explain this in detail in my article Enhancing and maximizing stereo width of your mix.
Using Mid/Side processing to adjust stereo width (and limitations)
The easiest way to adjust width using Mid/Side processing is to either give the Side channel a boost, or lower the Mid channel. This will exaggerate the amount of “Difference” information that you hear (recall in Part 1 that Mid/Side is really “Sum and Difference”), which can widen the stereo field, depending upon how much stereo information exists within it. Conversely, if you boost the mid channel relative the side, you can narrow the stereo width. You can adjust the width somewhat selectively as well, using equalization. You may have heard of “Stereo Shuffling” which combines this approach with EQ so that you are giving more of a boost to the Side channel in certain frequency ranges (usually below 650Hz).
The main limitation with using Mid/Side to adjust width is that you cannot control which mix elements you adjust. If your mix did not contain enough stereo width information to begin with, using Mid/Side will only diffuse whatever image you captured or created in the mix. If your mix is relatively narrow but contains stereo reverb, widening it with Mid/Side processing will only cause your mix to sound washed out, losing impact. Again, it's because you won’t have control over what gets widened. Our brains are always analyzing contrast (bright vs dark, soft vs loud, left vs right, up vs down). If you diffuse the stereo field using Mid/Side, you are not increasing width/positional contrast with as much control as you could if you went back to the mix to selectively widen aspects of your mix while leaving others anchored close to the center.
This is why I consider Mid/Side processing to be a last resort “mix rescue” option that should only be used when you have no time to explore other options. You will achieve much better results by improving stereo width inside your mix. You can do this by incorporating more of what creates stereo width to begin with (differences in gain, tone or time between the left and right channels). I discuss this in more detail in this article, but here are some basic ideas to help you improve stereo width within you mix:
- Pan wider: Place certain elements further to the left or right, and only keep elements in the center that need to be there (for example, the lead vocal, the bass and the kick). If you have any tracks in the mix that were recorded in stereo, pan them hard left and right.
- Apply different EQ settings between the left and the right: This can help pull apart similar sounding electric guitar tracks. For example, boost the left guitar track a bit in one area, and apply the reverse of that EQ adjustment on the right guitar track, and then adjust the gain levels to balance them.
- Use stereo delay or reverb: Applying stereo delay or reverb on selected elements will widen them up relative to the rest of the mix.
Other Mid/Side processing options
I touched upon Mid/Side EQ above when I mentioned Stereo Shuffling, above. Mid/Side EQ allows you to equalize the Mid channel separately from the Side channel. Whenever you use Mid/Side EQ, you are adjusting the ratio of the Sum and Difference information in your mix, in the area you are changing. For example, boosting the upper midrange in the Mid Channel can help clarify the vocal region increasing the sum information relative to the difference information in that range. This reduces the stereo width in that range as well, which may noticeable depending on the mix.
When you change the ratio of Sum and Difference information in different frequency ranges with Mid/Side EQ, you can inadvertently create audible changes in width over the scale and it often won’t sound musical. To avoid this, I recommend using small (0.25 to 1.5dB) broad EQ changes (wide bandwidth, low filter slope).
Many EQ’s change the phase of the signal you are adjusting. This is important because with Mid/Side EQ if you alter the phase of the Mid channel differently from the Side channel, you will impact the integrity of the original stereo image. This is less critical for multi-track mixes, but can be a real concern for stereo and binaural recordings. To minimize problems caused by phase changes, I again recommend using very small and broad EQ adjustments. Using a Linear Phase EQ is also a great option because it will not shift the phase.
Mid/Side Compression and Limiting
This is my least favorite form of Mid/Side processing because now we are getting into an area where it is possible to do a great deal of damage to your mix. When you compress the Mid or the Side channel you are changing the ratio of Sum and Difference information, but it is dependent upon how you configured the compressor or limiter. For example, if you use a fast compressor to compress the Mid channel, and key the compressor off the kick, you can set the compressor to widen the mix with every kick hit which can be an interesting and musical effect.
If Mid/Side compression is used without care, it can easily destroy your mix because you may accidentally create a narrower mix by compressing too much of the Side channel too often, or you may be reducing impact and punch by compressing the mid channel too much or too often. Worse still, there is little that a mastering engineer can do to fix this issue after it has happened.
Rather than using this effect on a whole mix, I recommend using it on select elements in your mix. For example, apply it to a synth pad, and then key the Mid/Side compressor off the kick so that you’re widening the pad with every kick. This allows you to only impact the width of the synth pad while preserving the rest of the mix (including the kick) so that you can preserve impact.
Again, this effect will do a lot of damage on a mix, so it is best used inside a mix on select elements, you can use a gate to flip between mono and stereo for creative effect. It is well worth playing with this effect. As with Mid/Side compression, consider keying the gate off of a rhythmic element while using the gate on something else in the mix.
Mid/Side Reverb, Delay, Distortion and other effects processing
The stereo image of a mix is completely dependent upon the individual mid and side components remaining "connected". If you apply a process that *breaks* that connection, you will compromise the stereo image. It may not be obvious at first, but you may notice that the panning is no longer as precise or as wide as it was in your original mix.
Reverb, Delay, Distortion and other effects dramatically alter sound by distorting it, changing its timing and tone, and even adding sound. Using these effects only on the Mid or Side can be beneficial as a creative effect, but they are risky. I recommend using them in the mix rather than at the end of a mix. The last thing you want is to completely destroy the stereo image you built so carefully during the mix stage.
Mid/Side processing can be an effective tool to improve your mix, but it is also capable of damaging your mix. I recommend using Mid/Side processing as a last resort “mix rescue” tool when you have no other option, or as a creative tool within your mix on selected elements.