Audio distortion correction for a parametric reproduction system
View Patent ↗A system and methods for distortion reduction in a non-linear audio system are disclosed. A method includes the operation of defining a non-linear system transfer function which relates a system electronic input signal with a system acoustic output over a range in which the system acoustic output is in a transition between non-saturated and saturated conditions in a medium in which the system acoustic output of the non-linear audio system is broadcast. A preprocessing transfer function is created that is the inverse of the non-linear system transfer function. The preprocessing transfer function is applied to the system electronic input signal to reduce the level of harmonic distortion at the system acoustic output.
1. A method of distortion reduction in a non-linear audio system, comprising the steps of:
defining a non-linear system transfer function which relates a system electronic input signal with a system acoustic output and is valid over a range of input amplitude levels corresponding with at least a portion of a range of acoustic output sound pressure levels in which the system acoustic output is in a transition between non-saturated and saturated conditions in a medium in which the system acoustic output of the non-linear audio system is broadcast;
creating a preprocessing transfer function which is the solved inverse of the non-linear system transfer function; and
applying the preprocessing transfer function to the system electronic input signal to form a pre-processed electronic input signal, wherein the pre-processed electronic input signal is configured to provide a modified system acoustic output having a reduced level of harmonic distortion relative to the system acoustic output;
wherein the preprocessing transfer function is of the form:
y
(
t
)
=
[
E
(
t
)
+
u
2
3
E
(
t
)
2
]
1
/
2
wherein y(t) is the preprocessed electronic audio signal, E(t) is the input electronic audio signal, and u is an ultrasound pressure index value.
2. A method as in claim 1 , wherein applying the preprocessing transfer function to the system electronic input signal further comprises altering the system electronic input signal using at least one of the operations selected from the group consisting of multiplying the input signal by a scalar value at each time instant and adding a scalar value to the input signal at each time instant.
3. A method as in claim 1 , further comprising modulating a first ultrasonic signal having a first frequency with a second ultrasonic signal having a second frequency to form the system electronic input signal.
4. A method as in claim 1 , further comprising modulating the first and second ultrasonic signals using dual sideband modulation when the system electronic audio signal has an amplitude less than a predetermined value and amplitude modulation when the system electronic audio signal has an amplitude greater than the predetermined value.
5. A signal preprocessor system for reducing distortion of an input audio signal in the output of a non-linear audio system, including:
an input line configured to receive an input electronic audio signal;
an output line configured to output a preprocessed electronic audio signal to the non-linear audio system;
a processor disposed between and connected to the input line and output line, and programmed to implement a preprocessing transfer function that is the solved inverse of a non-linear system transfer function defining distortion of the input electronic audio signal in an output acoustic of the system, enabling reduced harmonic distortion in the output acoustic of the non-linear audio system; wherein:
the preprocessing transfer function is of the form:
y
(
t
)
=
[
E
(
t
)
+
u
2
3
E
(
t
)
2
]
1
/
2
wherein y(t) is the preprocessed electronic audio signal, E(t) is the input electronic audio signal, and u is an ultrasound pressure index value.
6. A system as in claim 5 , wherein the output acoustic of the non-linear audio system is a parametric acoustic signal comprising two ultrasonic acoustic signals operable to demodulate in air and form a difference acoustic signal.
7. A system as in claim 5 , wherein the input electronic audio signal is comprised of a first ultrasonic audio signal modulated with a second ultrasonic audio signal.
8. A system as in claim 7 , wherein the first and second ultrasonic audio signals are modulated using dual sideband modulation when the input electronic audio signal has an amplitude less than a predetermined value and amplitude modulation when the input electronic audio signal has an amplitude greater than the predetermined value.