CMI-8738 Datasheet, PDF (24/28 Pages) C-Media Electronics – C3DX PCI-Based HRTF 3D Extension Positional Audio Chip

Ver. 1.2
CMI-8738/PCI-SX AUDIO Specification
HRTF 3D Positional Audio Technology White-Paper
lateral sounds. If one listens to an artificial head first without - and then with - shoulder
fitments, then it is clear that the shoulders do contribute to spatial effects in certain positions.
The shoulders provide a strong reflection from lateral sources, with a short path-length of
around 10 cm between direct sound and reflection. The effects are most important for
side-positioned sources, especially for "height" effects, where the shoulders tend to mask
sources which move below about 30 degree depression.
2. Local, Room Reflections
In simulations, it is clear that the incorporation of first-order simulated room reflections can
help in the creation of sound images which have a "solid" nature. However, the effects - if
accurately simulated - are relatively slight. Experience has shown that it is primarily the
quality of the HRTFs themselves which determine the quality and solidity of the sound image.
The further addition of second-order reflections does not help significantly, because in reality,
there is a great number of reflections in the average room. A method which does help to
recreate the acoustic experience of a room, however, is to use approximate simulations of
lateral reverb, using either 2 or 4 laterally placed "virtual" sources at, say, +-70 degrees and
80 degrees azimuth.
• The quality of the sound image relates to the HRTFs used.
• The quality of the room image relates to addition of reflections and reverb.
3. Psychological Cues
There are clearly psychological cues present in everyday life which work together with the
audio cues to tell us about the world around us. For example, if you hear the sound of a
helicopter flying, you expect it to be up in the air, and not downwards. If a dog is barking
nearby, you would expect it to be downwards.
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