Data processing: generic control systems or specific application – Generic control system – apparatus or process – Optimization or adaptive control
Reexamination Certificate
1998-08-24
2001-04-24
Grant, William (Department: 2121)
Data processing: generic control systems or specific application
Generic control system, apparatus or process
Optimization or adaptive control
C700S280000, C381S017000
Reexamination Certificate
active
06223090
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns the field of auditory localization and more specifically the field of measuring head related transfer functions (HRTFs).
Over the past decade many researchers have investigated the role of HRTF in spatial hearing. The HRTF represents the relationship between the audio signal generated at a point source in free space and the sound-pressure generated by that source at the eardrums of a human listener, and is typically measured with microphones in the left and right ear canals of a human listener or anthropomorphic manikin. The HRTF includes the effects of sound energy diffraction by the head and torso, as well as spectral shaping by the outer ear and ear canal resonances. The HRTF is typically a function of both frequency and relative orientation between the head and the source of the soundfield. When a sound source is electronically filtered by a HRTF and presented to a listener through headphones, the listener perceives the sound at the location of the source relative to the head when the HRTF is measured. Such a system is known as a virtual audio display.
Traditionally, HRTF measurements have been made in the far field with a sound source disposed at a distance greater than 1 meter. At this distance, an error of a few centimeters in the relative location of the source and head is of no great consequence, since it amounts to no more than a few degrees error in direction. If fact, most loudspeakers used in prior research arrangements measure 7 centimeters or larger in diameter, so the actual effective location of the source is not precisely defined within a few centimeters. When the source is near the head, however, small changes in the relative position of the sound source and the head can have a dramatic impact on the HRTF. The fundamental differences between the “near field” and “far field” as used herein are described in
FIG. 1
which is a schematic diagram showing the measurement of the HRTF at close and far distances. In
FIG. 1
, a sound source is shown at
10
and the dashed lines at
13
,
14
,
15
,
16
and
17
represent the radiating sound wave from the sound source
10
. The thickness of each dashed line indicates the intensity of the radiating sound wave, which is inversely proportional to the distance from the source. In order to measure the HRTF in the far field, a manikin head
12
equipped with microphones in the left and right ears is located 1.2 m from the sound source. Note that in the far field the angle of the source relative to the head is relatively insensitive to small changes in the position of the manikin. A displacement of the manikin by 1 cm will change the direction of the source relative to the head not more than 0.5 degrees. In order to measure the HRTF in the near field, manikin head
11
is located 0.25 m from the source. At this distance, the angle of the source relative to the manikin head is much more sensitive to small changes in the position of the manikin head. A 1 cm displacement of the manikin head can change the angle of the source relative to the head by 2.3 degrees. At closer distances, a small displacement in the location of the head can also generate substantial changes in the intensity of the sound reaching the ear.
Because of the precise placement accuracy required for measuring the HRTF at very close distances, the methods that have been used for measuring HRTFs in the far field are not sufficient for making near-field HRTF measurements. It is believed that the best placement solution for near-field HRTF measurements is to place the sound source at a desired distance and elevation relative to the manikin by hand, and then use a motorized stand to rotate the manikin in azimuth. However, it has been discovered that this method could cause large errors in the near field when the center of rotation of the stand is not located directly below the center of the interaural axis of the manikin. When the two centers of rotation are not perfectly aligned, the center of the head moves in a circular pattern as the manikin rotates and the HRTF measurements are corrupted.
FIGS. 2
a
and
2
b
show a manikin incorrectly pitched. As used in describing the invention, the orientation of the manikin will be described in relation to a coordinate system with its origin at the point where the manikin
200
is attached to a motorized stand
203
. The x-axis of this coordinate system is parallel to the ground and in the direction of the front of the manikin, and the y-axis is parallel to the ground and perpendicular to the x-axis. The z-axis is perpendicular to the ground and increases with increasing elevation. The azimuth of the manikin will be defined as rotation of the motorized stand about the z-axis, increasing with clockwise rotation. The pitch will be defined as rotation around the y-axis, increasing as the manikin is tilted forward. The roll will be defined as rotation around the x-axis, increasing as the manikin is tilted to the left.
FIG. 2
a
shows the manikin
200
tilted slightly so the center of rotation
201
is behind the center of the head
202
.
FIG. 2
b
is a top view of the
FIG. 2
a
manikin and shows that as the manikin
200
having a center of rotation
201
slightly behind the head is rotated in azimuth, the position of the center of the head
202
does not remain fixed but rather traverses a circle
204
. Note that the manikin connects to the motorized stand
203
at a baseplate located at the waist of the manikin. The center
202
of the interaural axis is approximately 1 m above this connection, so the center of rotation of the head is displaced by approximately 2 cm for each degree of pitch or roll in the manikin
200
relative to the motorized stand
203
, and causes the head to translate through a circle 4 cm in diameter, shown at
204
, as the manikin rotates through 360° in azimuth.
FIGS. 2
a
and
2
b
illustrate, therefore, that even a small amount of pitch or roll in the manikin
200
is unacceptable when making measurements less than 25 cm from the center of the head. The present invention provides a method and apparatus for ensuring accurate computer controlled positioning of a manikin for near-field HRTF measuring.
SUMMARY OF THE INVENTION
The invention provides a computer controlled, three-dimensional closed-loop system for automatically positioning, relative to a stationary sound source, the head of a manikin situated on a motorized stand. The three-axis positioning is responsive to acoustic signals measured from microphones located at each ear of the manikin and is desirable for accurate near field HRTF measuring.
It is an object of the invention, therefore, to provide computer control for centering the coordinate axes of rotation of a manikin head over a motorized stand.
It is another object of the invention to rapidly and automatically position a manikin in azimuth angle relative to a sound source.
It is another object of the invention to rapidly and automatically position a manikin in roll angle relative to a sound source.
It is another object of the invention to rapidly and automatically position a manikin in pitch angle relative to a sound source.
It is another object of the invention to provide a manikin positioning method for high accuracy HRTF measuring in the near field.
These and other objects of the invention are described in the description, claims and accompanying drawings and are achieved by a computer controlled closed-loop three-dimensional iterative method for positioning an acoustic manikin for head-related transfer function measurements, said method comprising the steps of:
providing a selectively positioned audio signal from a sound source;
receiving said audio signal at left and right ears of said manikin;
transforming time domain representations of said audio signal received at left and right ears of said manikin in a manikin selected axis first position thereof to frequency domain phase values;
computing from said frequency domain phase values to a first phase difference between left ear and right ear signal representations of said
Bahta Kidest
Grant William
Hollins Gerald B.
Kundert Thomas L.
The United States of America as represented by the Secretary of
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