Stereophony

🎯 Learning Objectives

By the end of this topic, you should be able to:

Understand the principles of XY, AB, and equivalence stereophony.
Compute phantom source angles using level differences and time differences.
Analyze and select microphone configurations for different stereo and multi-channel setups.
Understand the effect of microphone placement on the spatial impression, coloration, and stability of phantom sound sources.
Apply formulas for angle mapping, level differences, and time differences in practical recording setups.

Introduction:

Stereophony is the art of creating a spatial image of a sound field using multiple microphones and playback channels. Its goal is to reproduce the direction, distance, and spatial impression of a live source through loudspeakers or headphones.
In stereophony, microphones capture sound information from different locations and angles. The listener perceives a phantom sound source, which is a virtual source created by combining the signals of multiple channels.

Key factors in stereophony:

Angle mapping between primary source and phantom sound source.

Stability of phantom sources across the listening area (sweet spot).

Level differences, time differences, and directional characteristics of microphones.

Effects on timbre and coloration.

XY Stereophony (Coincident Microphones):

XY stereophony uses two directional microphones placed at the same location but with different angles (coincident), capturing level differences to create a phantom image.

Angle Mapping

Let $α$ be the perceived angle of the phantom source and $β$ be the actual source angle. The mapping function is:

\alpha = \alpha(\beta)

Maximum angular range during recording: $β_{max}$
Mapping the distance of the primary source and spatial impression of the recording room is essential.
Stability of the phantom source outside the "sweet spot" affects localization.
Coloration is the effect of microphone choice and angles on timbre.

Level Differences

The level differences between channels are generated by directional microphones:

\Delta L = 20\lg(A_X) - 20\lg(A_Y)

Where:

A_X = a + b \cos(\epsilon + \beta)

A_Y = a + b \cos(\epsilon - \beta)

$\epsilon$ is the angle of microphone orientation
Typical configurations

Microphone Pair	Orientation (ε)
Cardioid / Cardioid	45°
Figure-eight / Figure-eight	45°
Cardioid + Figure-eight (MS Stereophony)	90°

Signal domain illustration — **Figure.** XY Stereophony

AB Stereophony (Spaced Microphones):

AB stereophony creates a phantom source using time differences between spatially separated microphones.

Time Differences

For two microphones spaced distance $d$ apart:

\Delta t = \frac{d}{c} \cdot \sin(\beta)

Where:

Δt is the time difference between channels
c is the speed of sound.
β is the angle of the source relative to the microphone axis.

Angular mapping:

\alpha = \alpha (\Delta t) = \alpha (\beta)

Maximum time for localization in loudspeakers: $∣Δt∣≤2–3$ ms

Typical AB Configurations

Microphone Pair	Distance (d)
Omnidirectional / Omnidirectional	60 cm
Cardioid / Cardioid	30 cm
Hypercardioid / Hypercardioid	20 cm

Equivalence Stereophony (ORTF, NOS, EBS)

Equivalence stereophony combines level and time differences using spatially distributed directional microphones.

Level and time differences have almost additive effects on phantom source localization.
Typical configurations:

System	Microphone Type	Orientation (ε)	Distance (d)
ORTF	Cardioid / Cardioid	65°	17 cm
NOS	Cardioid / Cardioid	45°	30 cm
EBS	Hypercardioid / Hypercardioid	45°	25 cm

Recording for Multi-Channel Stereo Playback:

Stereo recording techniques can be extended to 5.1 and surround setups:

Main microphone captures the entire ensemble for a larger spatial impression.
Surround microphones may be placed to capture ambient or rear reflections.
Example: Williams 5.1 setup:

Main microphone array for front channels.
Auxiliary microphones for surround channels.
Ensures a coherent spatial image when replayed in a multi-channel system.

Summary Table: Microphone Stereophony Techniques

Technique	Principle	Key Parameters	Typical Use
XY	Coincident mics, level differences	ε, ΔL, α(β)	Studio stereo, minimal coloration
AB	Spaced mics, time differences	d, Δt, α(β)	Orchestras, room ambience
Equivalence (ORTF, NOS, EBS)	Combine level & time differences	ε, d, ΔL, Δt	Natural stereo images
Multi-channel	Extended stereo techniques	Mic arrays, ambient capture	5.1 / 7.1 surround

🎨 Interactive Demo

Multi-Technique Stereo Microphone Studio

Config:Source Angle: 0°ε: 45 Wavefronts Mic Glow AB Markers

Current Phantom Angle: 0.00°

Key Takeaways

XY stereophony uses coincident microphones and level differences to create phantom sources.
AB stereophony uses spaced microphones and time differences.
Equivalence stereophony (ORTF/NOS/EBS) combines level and time differences for accurate localization.
Maximum angular range, sweet spot stability, and coloration depend on microphone type and placement.
Multi-channel extensions allow recording for surround sound playback.

🧠 Quick Quiz

1) Which stereophony technique uses time differences for phantom source creation?

A) XY
B) AB
C) ORTF
D) NOS

2) In XY stereophony, what does ε represent?

A) Microphone orientation angle
B) Distance between microphones
C) Phantom source angle
D) Sound pressure level

3) For AB stereophony, what is the maximum Δt allowed for proper localization in loudspeakers?

A) 0.5 ms
B) 1 ms
C) 2–3 ms
D) 5 ms

4) Which configuration is an example of equivalence stereophony?

A) Cardioid/Cardioid, ε = 45°, d = 30 cm (NOS)
B) Omni/Omni, d = 60 cm
C) Figure-eight/Omni, ε = 90°
D) Cardioid/Figure-eight, ε = 90° (MS)

5) What is the purpose of microphone arrays in multi-channel recording?

A) To create phantom sources only
B) Capture ambient information and surround channels
C) Increase signal amplitude
D) Reduce electrical noise

Chapter - 1

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