Mid-Side and Stereo Recording

Stereo Recording Techniques

When you place two microphones to capture a stereo image, the spacing and angle between them determines the character of the stereo field. There are several standard configurations, each with its own tradeoffs:

XY (Coincident Pair)

Two cardioid microphones placed with their capsules as close together as possible, angled typically at 90-135 degrees. The stereo image comes purely from the level difference between the two mics — the left mic is more sensitive to sounds from the left, the right mic to sounds from the right.

Pros: Excellent mono compatibility (no phase issues because the capsules are in nearly the same position). Tight, focused stereo image. Cons: Narrower stereo width than spaced techniques. Can sound somewhat “flat” compared to techniques that use time differences.

ORTF

Two cardioid microphones spaced 17 cm apart, angled at 110 degrees. Named after the French broadcasting organization that developed it. ORTF creates stereo from both level differences and time differences — the spacing introduces a small arrival-time offset.

Pros: More natural-sounding width than XY. Still very good mono compatibility. Widely used for classical recording, drum overheads, and acoustic ensembles. Cons: Slightly more complex setup than XY.

Spaced Pair (A/B)

Two microphones placed some distance apart (anywhere from a few inches to several feet), both facing the source. The stereo image comes primarily from time differences — sounds arrive at each microphone at different times depending on their position in the room.

Pros: Wide, spacious stereo image. Natural-feeling depth. Cons: Potential phase issues when summed to mono — the time differences between the two mics mean some frequencies cancel when combined. The wider the spacing, the greater the risk.

Mid-Side: The Concept

Mid-side (M/S) is both a recording technique and a processing technique. It’s based on a different way of thinking about stereo — instead of left and right, think center and sides.

Mid (M): Everything that’s the same in both channels — the center of the stereo image. Vocals, bass, kick, snare — anything panned to the middle lives in the mid signal.

Side (S): Everything that’s different between the two channels — the width of the stereo image. Panned instruments, reverb tails, stereo effects — anything that creates the sense of width lives in the side signal.

The math is simple:

• Mid = Left + Right (sum the channels — what’s common to both stays, what’s different cancels)
• Side = Left - Right (subtract one channel from the other — what’s common cancels, what’s different stays)

To get back to left and right:

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

This encoding and decoding is lossless — no information is created or destroyed. It’s just a different way of looking at the same stereo signal.

Mid-Side Recording

In M/S recording, you use two microphones:

1. A cardioid microphone pointed at the source — this captures the mid signal (the center image)
2. A figure-eight microphone placed perpendicular to the source — this captures the side signal (the ambient, off-axis sound)

The figure-eight mic picks up equally from its front and back (left and right of the stereo field) but rejects sound from the sides (the direction the cardioid is pointed). The two signals are decoded in the DAW using the sum-and-difference formula above.

The advantage of M/S recording: you can adjust the stereo width after the recording by changing the balance between mid and side signals. More side = wider image. Less side = narrower, more focused image. All side = pure ambient sound with no center. All mid = mono. This flexibility makes M/S particularly useful for live recording, broadcast, and situations where you can’t predict the ideal width at the time of capture.

Mid-Side Processing for Mixing

M/S isn’t just for recording. Many mixing plugins (EQ, compression, stereo wideners) offer M/S processing modes. Instead of processing left and right independently, you process the mid and side signals independently.

Practical applications:

• Widen the mix. Boost the side signal relative to the mid. The stereo image expands. Useful for making a chorus feel wider than a verse.
• Narrow the mix. Reduce the side signal. The stereo image collapses toward the center. Useful for tightening the low end (bass and kick should be mono — you can high-pass the side signal to remove low-frequency width).
• EQ the sides differently from the center. Add brightness to the side signal for “air” without affecting the center vocal. Cut the low end of the side signal without thinning the bass (which lives in the mid).
• Compress the mid and side differently. A multiband compressor in M/S mode can control the center dynamics without affecting the width, or vice versa.

Stereo Width Control

The combination of panning decisions (Chapter 22) and M/S processing gives you comprehensive control over stereo width:

1. Pan for placement — where things are in the left-right field
2. M/S balance for overall width — how wide the stereo image feels
3. M/S EQ for spectral width — which frequencies are wide and which are narrow
4. Mono check for safety — ensure nothing collapses destructively

A common mastering technique: widen the high frequencies (adding shimmer and air to the sides) while keeping the low frequencies mono (preventing muddiness and phase issues in the bass). This is done with M/S EQ — boost the side signal above 5 kHz, cut it below 200 Hz.

What to Practice

1. Decode a stereo signal into M/S. Take a stereo track — a full mix or a stereo recording. Use a utility plugin to solo the mid signal (L+R sum), then solo the side signal (L-R difference). Listen to each. The mid should contain the centered elements. The side should contain the width and ambience.
2. Adjust width with M/S balance. On a stereo mix, boost the side signal by 3 dB and listen to the width increase. Then cut it by 3 dB and hear it narrow. Check in mono after each change — over-widening creates phase problems that become audible in mono.
3. Try M/S EQ. Using an EQ in M/S mode, cut the side signal below 200 Hz (mono-ifying the bass) and boost the side signal above 5 kHz (adding width to the top end). This is a standard mastering move — listen to how it cleans up the low end while opening up the top.
4. Record an XY pair (if you have two mics). Point two cardioid mics at an angle and record an acoustic source. Pan one hard left, the other hard right. Listen to the stereo image. Then sum to mono — it should collapse cleanly without phase cancellation. That’s the advantage of coincident pair recording.