Modulator processing for a parametric speaker system

• US 6,584,205 B1
• Filed: 08/26/1999
• Issued: 06/24/2003
• Est. Priority Date: 08/26/1999
• Status: Expired due to Term

• Current Assignee: Turtle Beach Corporation
• Original Assignee: American Technology Incorporated (Emerson Electric Company)
• Inventors: Norris, Joseph O., Spencer, Michael E., Croft, James J. III
• Primary Examiner(s): Banks-Harold, Marsha D.
• Assistant Examiner(s): ARMSTRONG, ANGELA A
• Application Number:
  US09/384,084
• Time in Patent Office: 1,398 Days
• Field of Search: 381/111, 381/98, 381/3, 381/6, 381/14, 381/15, 381/16, 381/55, 381/59, 381/77, 381/96, 381/103, 704/200, 704/201, 367/118
• US Class Current: 381/98
• CPC Class Codes: H04R 2217/03 Parametric transducers where sound is generated or captured by the acoustic demodulation of amplitude modulated ultrasonic waves
• CPC Class Codes: H04R 3/00 Circuits for transducers , loudspeakers or microphones

Abstract

A parametric loudspeaker system using improved modulators to compensate for the non-linearity of the parametric process in air when driving the air at saturation levels and below saturation levels. The parametric loudspeaker uses a pre-processed single sideband modulator that offers ideal linearity as characterized by square root pre-processed double sideband modulators but with a lower carrier frequency and without the wide bandwidth requirements. By eliminating some or all of the lower sideband the carrier frequency can be reduced without producing sideband frequencies in the audible range. Lower operational frequencies result in greater translation efficiency and greater output capability before reaching the saturation limit of air. A pre-processor minimizes the effects of saturation limits for double sideband, truncated double sideband or single sideband processing to achieve superior output.

Claims

1. A signal processor for a parametric loudspeaker system, comprising:

at least one carrier frequency generator to produce a carrier frequency;

a modulator which receives at least one audio signal and modulates the at least one audio signal onto the carrier frequency to produce a modulated signal, wherein the at least one audio signal is converted to sideband frequencies which are divergent from the carrier frequency by the frequency value of the at least one audio signal;

an error correction compensator coupled to the modulator to compensate for inherent parametric demodulation distortion by modifying, substantially only within the modulated signal'"'"'s bandwidth, the modulated signal to approximate an ideal signal which should be output by the system;

wherein the error correction compensator adjusts for the inherent parametric demodulation distortion by comparing the modulated signal with a reference signal which models parametric demodulation distortion, and thereby generates an inverted error difference to add back into the modulated signal substantially within the modulated signal'"'"'s bandwidth to correct for distortion.

2. The signal processor as in claim 1 wherein the error correction compensator further comprises:

a non-linear demodulator to simulate demodulation of an ultrasonic signal;

a transducer model coupled to the non-linear demodulator to simulate a system transducer;

a difference processor coupled to the transducer model to calculate the distortion difference between an original audio signal and a simulated distorted audio signal generated by the non-linear demodulator and the transducer model; and

a summing node to add the distortion difference received from the difference processor into the original audio signal.

3. The signal processor as in claim 1 wherein the error correction compensator further comprises a plurality of error correction compensators recursively chained together to apply iterative distortion correction to the modulated signal.

4. The signal processor as in claim 3 wherein the plurality of error correction compensators recursively chained together further comprises recursively chaining error correction compensators less than 8 times.

5. The signal processor as in claim 1 wherein the error correction compensator further comprises a non-linear demodulator to generate a distorted signal which simulates the conversion of ultrasonic modulation input to an acoustic audio output.

6. The signal processor as in claim 5 wherein the non-linear demodulator further comprises:

an AM demodulator to remove the carrier frequency from an ultrasonic acoustic input;

a squaring function processor coupled to the AM demodulator to model secondary resultant output from a parametric loudspeaker which is proportional to the square of the modulation envelope;

a high pass filter coupled to the squaring function to remove a direct current (DC) output component from the squaring function processor; and

a gain module coupled to the high pass filter to scale a simulated acoustic audio output.

7. The signal processor as in claim 6 wherein the AM demodulator further comprises:

a Hilbert transformer to shift input tone phases; and

a magnitude processor coupled to the Hilbert transformer to compute an instantaneous signal amplitude.

8. The signal processor as in claim 5 wherein the error correction compensator further comprises a single sideband channel module.

9. The signal processor as in claim 8 wherein the single sideband channel module further comprises:

a single sideband modulator to receive the audio signal and modulate the audio signal with a carrier signal;

a transducer response to receive a modulated signal from the single sideband modulator, wherein the transducer response models an uncompensated parametric transducer; and

a nonlinear demodulator coupled to the transducer response wherein the demodulator receives modulated signals and models a secondary resultant output from a parametric loudspeaker which is proportional to a square of a modulation envelope.

10. A signal processor for a parametric loudspeaker system used in air, comprising:

a single sideband (SSB) modulator to receive at least one audio signal and modulate a single sideband carrier signal with the audio signal to create a modulated signal having a signal envelope and a bandwidth;

an error correction compensator coupled to receive the modulated signal from the SSB modulator and to substantially match the signal envelope of the single sideband (SSB) modulated signal with an ideal signal which has been pre-processed to correct parametric demodulation distortion, wherein the audio signal contains corrective frequencies which are added substantially only within the audio signal'"'"'s bandwidth;

wherein the error correction compensator further comprises a non-linear demodulator wherein the demodulator simulates a medium'"'"'s non-linear distortion.

11. The signal processor as in claim 10 wherein the non-linear demodulator further comprises:

an AM demodulator to remove the ultrasonic carrier signal from an ultrasonic acoustic input;

a squaring function processor coupled to receive the output from the AM demodulator and to model secondary resultant output from a parametric loudspeaker which is proportional to the square of the modulation envelope;

a high pass filter to remove the direct current (DC) component of output of the squaring function processor; and

a gain module connected to the high pass filter to scale the acoustic audio output.

12. A method for producing a reduced distortion audio signal for use with a parametric loudspeaker system, comprising the steps of:

(a) receiving at least one audio signal;

(b) producing a carrier frequency which is modulated with the at least one audio signal to produce a modulated signal with sideband frequencies;

(c) compensating for an inherent parametric demodulation distortion in parametric loudspeaker demodulation by modifying the modulated signal with added frequencies substantially only within the modulated signal'"'"'s bandwidth to closely approximate an ideal modulation envelope;

wherein step (c) further comprises the steps of;

comparing the modulated signal with the ideal signal to which parametric demodulation distortion has been applied, to thereby produce an inverted error signal;

adding the inverted error signal back into the modulated signal to produce a compensated modulated signal which is provided to a transducer for audio reproduction.

13. The method of claim 12 wherein the steps of comparing the modulated signal with the ideal signal and then adding the inverted error signal back into the modulated signal further comprises the step of repeating the comparing and adding steps recursively at least two times.

14. The method of claim 13 wherein the step of recursively repeating the comparing and adding steps further comprises recursively repeating the comparing and adding steps until the error correction is within a selected error amount.

15. The method of claim 13 wherein the step of recursively repeating the comparing and adding steps further comprises recursively repeating the comparing and adding steps less than 8 times.

16. The method of claim 13 wherein the step of recursively repeating the comparing and adding steps further comprises recursively repeating the comparing and adding steps until distortion terms are at a lowest possible value.

17. A signal processor for a parametric loudspeaker system, comprising:

at least one carrier frequency generator to produce a carrier frequency;

a modulator which receives at least one audio signal and modulates the at least one audio signal onto the carrier frequency to produce a modulated signal, wherein the at least one audio signal is converted to sideband frequencies which are divergent from the carrier frequency by the frequency value of the at least one audio signal;

an error correction compensator coupled to the modulator to compensate for transducer distortion by modifying, substantially only within the modulated signal'"'"'s bandwidth, the modulated signal to approximate an ideal signal which should be output by the system;

wherein the error correction compensator adjusts for the transducer distortion by comparing the modulated signal with a reference signal which models parametric demodulation distortion, and thereby generates an inverted error difference to add back into the modulated signal substantially within the modulated signal'"'"'s bandwidth to correct for distortion.

18. The signal processor as in claim 17 wherein the error correction compensator further corrects for inherent parametric demodulation distortion by modifying, substantially within the modulated signal'"'"'s bandwidth, the modulated signal to approximate the ideal signal which should be output by the system;

and

wherein the error correction compensator adjusts for the inherent parametric demodulation distortion by comparing the modulated signal with a reference signal which models parametric demodulation distortion, and thereby generates an inverted error difference to add back into the modulated signal substantially within the modulated signal'"'"'s bandwidth to correct for distortion.