Head Tracking
When headphones are used, if the listener moves his or her head and if the
signals sent to the ears are not modified, the configuration of sources
appears to move also. This is intolerable for virtual reality applications.
In addition, some of the spatial effects can be weakened or even destroyed.
This seems to be particularly troublesome for sources that are supposed
to be directly ahead or directly behind, since the rate of change of binaural
cues is greatest in those directions. A typical result is that sources that
are supposed to be directly ahead seem to be much too close, even appearing
to originate inside the head.
A standard solution is to use a device called a head tracker
to measure the location and orientation of the head, and periodically to
recalculate the relative position of each source, modifying the HRIR's accordingly.
In addition to the usual concerns for cost, reliability, and accuracy, two
other engineering concerns arise:
- Allowable latency. Latency is the time between when
a motion is made and the corrected HRIR is used. Experience shows that it
should definitely be less than 50 ms or the lag will be perceived.
- Unwanted transients. If one merely switches between
one HRIR and another, audible clicks may result. Some kind of "crossfading"
between the the two states is usually desirable.
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