Stereo and Panning
Stereo is the foundation of all spatial audio techniques.
Stereo Principles
Two-Channel System
Stereo uses two speakers (L and R) positioned symmetrically relative to the listener, typically at ±30°.
Phantom Source
When both speakers emit the same signal at equal levels, a "phantom" source is perceived at the center.
Panning Types
Amplitude Panning
The most common technique: the source is distributed between L and R based on desired position.
| Position | L Level | R Level |
|---|---|---|
| Full Left | 100% | 0% |
| Center | 70.7% | 70.7% |
| Full Right | 0% | 100% |
Panning Laws
Linear Law:
L = 1 - pan
R = pan
Problem: level drop at center
Sine/Cosine Law (constant power):
L = cos(pan × π/2)
R = sin(pan × π/2)
Maintains constant energy
Square Root Law:
L = sqrt(1 - pan)
R = sqrt(pan)
Good compromise
Time-Based Panning
Uses delay differences between channels. Less common but useful for specific effects.
Recording Techniques
Spaced Pair (AB)
Two omni microphones spaced apart:
- Captures mainly ITD
- Natural, spacious sound
- Can lack focus
Coincident Pair (XY)
Two directional mics at same point:
- Captures mainly ILD
- Good mono compatibility
- Tighter image
ORTF
Cardioid mics at 17cm, 110° angle:
- Compromise AB/XY
- Natural and focused
Mid-Side (MS)
One cardioid (M) + one figure-8 (S):
- Adjustable width in post
- Perfect mono compatibility
Stereo Width
Natural Width
Recording captures ambient spatial information:
- Room reflections
- Source size
- Natural ambience
Artificial Width
Widening techniques:
- Haas delay (1-30 ms offset)
- M/S processing (boost S)
- Stereo enhancers
Width Issues
Too narrow:
- Mono-like sound
- Lacks immersion
Too wide:
- Unstable image
- Phase issues in mono
- "Hole in the middle"
Link to HSR
The HSR algorithm extends stereo panning concepts to multichannel:
- Position: Equivalent of pan control
- Width: Controls stereo spread across speakers
/bus/A/hsr/position 0.0 # Centered
/bus/A/hsr/width 1.0 # Natural stereo width
Spacelite preserves original stereo imaging while distributing to multiple outputs.