Simple Spatial Audio Systems
The simplest spatial audio systems are limited to localizing in azimuth only. There are three basic classes of such systems:
Two-channel (stereo)
Multichannel
Binaural recordings
To go beyond the limited capabilities of these approaches, we need to know more about Head-Related Transfer Functions (
HRTF's
).
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On to HRTF's
Up to 3-D Audio
Back
to Psychoacoustics 
On
to HRTF's 
Up
to 3-D Audio