Modulator processing for a parametric speaker system
View Patent ↗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.
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 within the modulated signal's bandwidth, the modulated signal to approximate the ideal audio signal which should be output by the system.
2. The signal processor as in claim 1 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.
3. The signal processor as in claim 2 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.
4. The signal processor as in claim 2 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.
5. The signal processor as in claim 4 wherein the plurality of error correction compensators recursively chained together further comprises recursively chaining error correction compensators less than 8 times.
6. The signal processor as in claim 2 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.
7. The signal processor as in claim 6 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.
8. The signal processor as in claim 7 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.
9. The signal processor as in claim 6 wherein the error correction compensator further comprises a single sideband channel module.
10. The signal processor as in claim 9 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.
11. The signal processor as in claim 1 wherein the error correction compensator further includes at least partial modulated signal correction for the second time derivative function of a parametric loudspeaker demodulation.
12. The signal processor as in claim 1 wherein the parametric loudspeaker system further comprises a high frequency parametric transducer to emit the modulated signal, wherein the transducer has a high pass filter characteristic to minimize sideband output of the parametric transducer at frequencies in and slightly above an audible range.
13. The signal processor as in claim 1 wherein the error correction compensator further includes a high pass filter to minimize sideband frequencies of the parametric loudspeaker system in or near an audible range.
14. The signal processor as in claim 1 wherein the modulator produces sideband frequencies only above the carrier frequency to allow the carrier frequency to be at a lower frequency while avoiding audible distortion in the carrier frequency and sideband frequencies.
15. The signal processor as in claim 1 wherein the ideal audio signal is created by applying a square root function to the ideal audio signal, and wherein the ideal signal is used as a reference to modify the modulated signal and correct for the inherent parametric demodulation distortion.
16. The signal processor as in claim 1 wherein the error correction compensator compensates for the inherent parametric demodulation distortion in parametric loudspeakers using a demodulation exponent of ½ to determine a modulated signal distortion which is then used to correct the signal, wherein the demodulation exponent is increased and approaches one as the modulated signal power increases.
17. The signal processor as in claim 16 wherein demodulation exponent is increased and approaches one as the modulated signal approaches saturation.
18. A signal processor for a parametric loudspeaker system, comprising:
at least one carrier frequency generator to produce a carrier frequency, wherein the carrier frequency is included in a single sideband (SSB) signal that includes a distortion compensator to correct for parametric demodulation distortion;
a modulator for (i) receiving audio signals within an audible range and modulating the audio signals onto the carrier frequency to produce a modulated signal, and (ii) for reducing the carrier frequency of the modulated signal to a value close to an upper limit of the audible range, wherein the audio signals are converted to sideband frequencies which are divergent from the carrier frequency by the frequency value of the audio signal; wherein
the distortion compensator uses an ideal audio signal created by applying a square root function to the ideal audio signal, wherein the ideal signal is used as a reference to modify the modulated signal and correct for an inherent parametric demodulation distortion.