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Mid-Side Processing

Learn about Mid-Side processing techniques and how they can transform your stereo imaging and mixing approach

8 min
By Fracture Audio

Mid-Side Processing: Advanced Stereo Control

Mid-Side (M/S) processing is a powerful technique that allows you to process the center and sides of your stereo image independently. This method gives you unprecedented control over your mix's stereo field and can solve many common mixing problems.

What is Mid-Side Processing?

Mid-Side processing works by converting a stereo signal into two components:

  • Mid (M): The sum of left and right channels - everything that appears in the center
  • Side (S): The difference between left and right channels - everything that appears on the sides

This conversion allows you to apply different processing to each component before converting back to stereo.

The Mathematics Behind M/S

The conversion formulas are surprisingly simple:

Encoding (L/R to M/S):

  • Mid = (Left + Right) / 2
  • Side = (Left - Right) / 2

Decoding (M/S to L/R):

  • Left = Mid + Side
  • Right = Mid - Side

Common M/S Applications

1. EQ Processing

Mid Channel EQ:

  • Remove muddiness from the center without affecting stereo instruments
  • Enhance vocal clarity
  • Control bass frequencies that should be centered

Side Channel EQ:

  • Add sparkle to stereo elements without making vocals harsh
  • Remove unwanted high-frequency noise from the sides
  • Create more focused low-end by high-passing the sides

2. Compression

Mid Compression:

  • Control vocal dynamics without squashing the stereo field
  • Tighten up the center image
  • Manage bass and kick drum consistency

Side Compression:

  • Control the width of reverbs and delays
  • Manage stereo instrument dynamics
  • Create pumping effects on ambient elements

3. Stereo Width Control

M/S processing is excellent for adjusting stereo width:

  • Narrow the sides: Reduce side level for a more focused mix
  • Widen the image: Boost sides for a more spacious feel
  • Mono compatibility: Check how your mix translates by monitoring the mid channel

Practical M/S Techniques

Vocal Enhancement

  1. High-frequency clarity: Boost 3-8kHz on the mid channel to enhance vocal presence
  2. De-essing: Apply de-essing to the mid channel to control sibilance without affecting stereo elements
  3. Warmth: Add subtle low-mid warmth to the mid channel for vocal body

Mix Bus Processing

M/S EQ on the mix bus:

  • High-pass the sides around 80-120Hz to keep bass centered
  • Add subtle high-frequency air to the sides
  • Address harshness in the mids without dulling the sides

M/S Compression on the mix bus:

  • Light compression on the mid for glue
  • Different attack/release settings on sides for ambience control

Problem Solving

Common issues M/S can solve:

  • Muddy low-end: High-pass the sides
  • Harsh vocals in dense mixes: EQ only the mid channel
  • Lack of stereo interest: Process sides differently from the center
  • Phase issues: Monitor and correct in M/S domain

M/S Tools and Plugins

Hardware Options

  • SSL consoles with built-in M/S processing
  • Maselec MLA-2 mastering EQ
  • Dangerous Music 2-Bus+

Software Plugins

  • FabFilter Pro-Q 3 (built-in M/S mode)
  • Waves S1 Stereo Imager
  • iZotope Ozone (M/S processing modules)
  • Plugin Alliance Brainworx plugins

Advanced M/S Techniques

Parallel Processing

Process a copy of your mix in M/S and blend it back:

  1. Send your mix to an auxiliary track
  2. Apply M/S processing (heavy compression, saturation, etc.)
  3. Blend the processed signal back with the original

Frequency-Dependent Width

Use multiband M/S processing to control width at different frequencies:

  • Keep bass frequencies centered
  • Allow mid-frequencies moderate width
  • Open up high frequencies for air and space

Creative Effects

M/S Reverb:

  • Send only the sides to reverb for width without center muddiness
  • Use different reverbs for mid and side channels

M/S Saturation:

  • Saturate the mid channel for warmth and presence
  • Add different saturation to sides for character

Monitoring and Checking Your Work

Mono Compatibility

Always check how your M/S processing affects mono compatibility:

  1. Monitor the mid channel alone
  2. Check the full mix in mono
  3. Ensure important elements remain audible

Reference Checking

  • Compare your M/S processing with professional references
  • Use correlation meters to monitor stereo spread
  • Check on different speaker systems

Common M/S Mistakes

  1. Over-processing the sides: Can lead to unstable stereo image
  2. Ignoring mono compatibility: Essential for broadcast and streaming
  3. Excessive width: Can make mixes sound disconnected
  4. Not considering the listening environment: M/S effects are more noticeable on good speakers

Conclusion

Mid-Side processing is a powerful tool that offers surgical control over your stereo image. When used thoughtfully, it can solve mixing problems, enhance clarity, and create more engaging stereo experiences. Start with subtle adjustments and always check your work in mono and on different speaker systems.

Remember: the goal is to serve the music. M/S processing should enhance your mix's emotional impact, not distract from it with technical wizardry.

Next Steps

  • Experiment with M/S EQ on individual tracks
  • Try M/S compression on your mix bus
  • Practice identifying when M/S processing can solve specific mix problems
  • Study how professional mixes use stereo imaging