Mid/Side Stereo

• A way of dealing with the stereo image as center and sides, instead of left and right.
• A truly stereo coincident mic setup (not 'fake' stereo)
• Different set of advantages/disadvantages from L/R stereo– extremely flexible!
• Can also be used in post-production (unrelated to miking)

Two Mics:

1 Bi-directional

1 Cardioid (variations are possible)

Usually condensers, like most stereo pairs  

Unlike other stereo pairs, mics needn't be the same

What You Need

• 2 Mics:  Cardioid & Bi-directional
• 3 Mixer channels
• The ability to reverse polarity
  (i.e. 'flip the phase')
• The ability to split the same mic to 2 channels
• A sturdy mic stand!

What You Need

Set up Mics at 90°

Cardioid facing 0° (forward)

Bi-directional positive side (manufacturer emblem) facing 90° left

Cardioid = Mid Mic (picks up center of the stereo image)

Bi-directional = Side Mic (picks up width of the stereo image)

• Split, for identical signal in 2 different channels
• Those channels panned L & R
• Polarity reversed in the R channel only

Routing ​

Cardioid:  Panned C

Bi-Directional:

Split & Polarity

Analog

Split & Polarity

Sony

1. Touchscreen: Audio > Input Routing
2. Select the physical input of bidirectional mic
3. Select the channel to use for the R side of the bidirectional split

Split & Polarity

Sony

3. ACCESS R side channel, activate its polarity reverse (to the left of BUS ASSIGN)
4. Gang the L & R side mic faders

Split & Polarity

Pro Tools

1. Set the 2 side tracks to the same input & pan L/R
2. On the R track, insert the Trim plug-in with a polarity reverse on

• Clever application of basic old-school analog principles (devised by Blumlein in 1933!)
• Image comes from electrical interaction of the signals, not 2 stand-alone mics
• Requires the entire setup to work correctly

How Does this Wacky Thing Work?

Record the mid and side tracks separately to the multitrack.  Decode them to L/R only for monitoring (listening in M/S is usually not very useful).  This makes it much easier to adjust the mid/side balance when mixing later.

This happens automatically with the Sony (because of pages 1–24), and with Pro Tools (because it records the track before the mixer).

Recording​

1. Listen to just the mid (cardioid) channel. This will be in mono, the center of the stereo image.
2. While lowering the level of the mid, bring up the two side (bidirectional) channels together. The image will get wider, and the amount of room sound will probably increase.
3. Adjust the faders of the mid vs. side channels to balance width and reflections.
4. When listening to the sides only, you should hear an out-of-phase signal with more reflections.  The 2 side channels should always move together at the same level.

Balancing​

Variations

• Remotely change direct sound vs. room balance
• Different mics for mid and side
• Better width control of stereo pickup = easy to make the image narrower/wider
• Better sound, since source is on-axis (unlike X/Y or Blumlein)
• Easy adjustment after recording, in the mix!

Advantages

• Can be awkward physical setup
• More complex routing = opportunity for error
• Requires an extra channel
• Can sound 'phasey' if side is too far up in the mix 

Disadvantages

• Want to be close to a large source
  (piano, drum set, big acoustic guitar sound)
• Might need to make significant changes to the width or room sound balance
• Want the sound of a coincident stereo pair:
  More precise imaging, less blend, less space/depth
• Are recording SFX, for film/TV/web, or anything else that might be heard in mono 

Consider M/S When You...

Recording guitar + vocals is notoriously difficult.  M/S can help here.

The Double-M/S monstrosity records surround in a stereo-friendly way.