Modulation Beamforming Technology for Acoustic Applications

Abstract

A method for generating a spatial distribution of acoustic transmit frequencies by a time modulation array (TMA) of transducers includes the steps of generating a plurality of pulse sampling signals (each pulse sampling signal including a series of harmonic frequencies, with successive signals of the plurality of pulse sampling signals separated by a predetermined delay time) by a tapped delay line, mixing each of the plurality of pulse sampling signals with a time-limited information signal to generate a plurality of mixed output signals, bandpass filtering each of the plurality of mixed output signals, and generating a first plurality of synchronized TMA beams from the filtered and weighted plurality of output signals by driving a plurality of acoustic transducers in the spatial array of acoustic transducers, wherein each synchronized TMA beam of the first plurality of synchronized TMA beams is associated with one of a plurality of transmit carrier frequencies.

Description

[Technical field]

[0001] An embodiment of the present disclosure is to use a Time Modulated Array (TMA) to obtain spatial frequency diversity beamforming to significantly enhance target interrogation under noise-limited and clutter-limited conditions. Targets can be biological or non-biological entities. [Background technology]

[0002] The TMA system is characterized by time modulation of each array element of the transducer array to produce synchronous beam steering, with each beam associated with a different carrier frequency. Although the physics involved are pertinent to either electromagnetic or acoustic transmission (Non-Patent Document 1), the remainder of this disclosure is primarily directed to acoustic applications.

[0003] After the effectiveness of sonar and radar was demonstrated in World War II, the next 30 years saw the development of the temporal and spatial characteristics of acoustic and electromagnetic waves in their respective propagation media, the ocean and the atmosphere. From 1945 to 1975, considerable progress and innovations were made in space-time radar signal processing by applying time-frequency domain techniques to electronic scanning of antenna systems. During that time, the US Navy's research efforts were focused on characterizing the multimodal acoustic characteristics of ocean behavior and applying radar and astronomy processing techniques to sonar (Non-Patent Documents 1-3).

[0004] Below is a brief review of key research papers on TMA-related technologies, both in the radar and sonar fields.

[0005] One of the early prominent researchers in this field was Shelkunoff (Non-Patent Document 4), who in 1941 published a general method of antenna analysis based on Maxwell's equations and the principles of circuit theory. Shelkunoff's method improved the understanding of the transmission process and facilitated the calculation of the input impedance of antennas of arbitrary shape. In 1943, Shelkunoff (Non-Patent Document 5) published a mathematical method that influenced the space-time diversity methods proposed in the 1960s: Shnitkin (Non-Patent Document 6), Shanks (Non-Patent Document 7), Kummer et al. (Non-Patent Document 8), Davies and McCartney (Non-Patent Document 9) and Johnson (Non-Patent Document 10). These advances were particularly related to the use of TMA, which applied modulation and coding techniques to beam steering of the transmit and receive beams and was characterized by obtaining very low side lobes to minimize off-axis interference.

[0006] Shnitkin (1960-1961) published an investigation into electronically scanned antennas consisting of beam switching, phase shifting and frequency scanning. Electronic scanning replaced mechanical repositioning of large and heavy antennas, which required large moments of inertia and structural stresses that would lead to antenna distortion and degradation of the radiation pattern. Focusing on receiving systems, technological approaches were introduced to avoid the need for high speed and high precision RF phase shifters that allowed for frequency conversion phasing schemes.

[0007] Shanks (1959-1961) enhanced the concept of a time-modulated antenna by realizing that amplitude modulating a transmit carrier plane wave with a Fourier space-time sequence results in a time-varying radiation pattern. Time multiplexing the element signals produces a frequency-multiplexed beam signal that can be used for beam steering. This is the result of a Fourier transform between the array aperture and the far-field pattern space. Controlling the modulation frequency, duty cycle, pulse repetition rate, and other modulation parameters on the array geometry supports the possibility of the TMA concept achieving specific operational targets in real environments.

[0008] This makes the inter-element phasing control proposed by Shnitkin unnecessary for sidelobe control. Instead, simple active on-off element switching can be used for both transmitting and receiving arrays. Kummer et al. (1962) applied this switching technique to a receiving antenna array, producing a Chebyshev sidelobe level of -50 dB from the usual uniform static distribution of -13 dB for a linear array.

[0009] Electron beam scanning arrays were initially applied almost exclusively to linear or rectangular planar arrays. Davies and McCartney (1965) showed that these principles could also be applied to convex geometries such as circular and cylindrical arrays. Finally, following Shanks, Johnson (1968) applied time and frequency multiplexing to linear and ring receiving arrays.

[0010] In the 1960s, considerable research was carried out at the US Department of Defense, especially at the now defunct US Naval Underwater Acoustics Laboratory in New London, Connecticut. System architectures became more complex and performance evaluation required new metrics in the calculation of reverberation masking levels. In 1968, Cole and Hanrahan (11) extended the reverberation index (RI) introduced in 1948 by Eyring et al. (12) as a measure of discrimination against reverberation (clutter) associated with the sound power intensity pattern of a single transmitter / receiver. RI was used to optimize multiplexing and high search rates in operation in multiple acoustic modes, such as shallow water, surface ducts, convergence zones, and bottom-bounce propagation in deep water (15). During this period, there was also a focus on the field of nonlinear acoustics, and in particular Westerfelt (13; 1963) studied the range of interacting primary frequencies generated by arrays called active parametric arrays.

[0011] Sonar experimental work in the 1970s focused on understanding the limitations of reverberation in multimodal operation and evaluating the application of coding principles to beamforming. Among the many outstanding contributors during this period were Vogliss (14), Winder (15), and Haykin (16).

[0012] The basic principles of receive beam scanning using matrix formulation were developed by Vogliss, who described limitations on scanning speed, modulation frequency, etc. in Part 1 of Non-Patent Document 14 (1971) and design flexibility in Part 2 of Non-Patent Document 14 (1972).

[0013] Winder (1975) provided a state-of-the-art overview of sonar systems, examining the various acoustic transmission and signal processing techniques commonly employed and the major operational considerations involved, such as cross-beam reverberation build-up and resulting dead-zone rescan times.

[0014] Haykin (1976) developed Johnson's multi-beam sampler architecture, employing a structure similar to the fast Fourier transform to reduce the hardware required, and succeeded in reducing the circuit complexity as the number of elements increased.

[0015] For the past 60 years, the concept of TMA has focused primarily on the architecture of the receive array, primarily due to advances in FFT processing and ultra-miniaturization. The real power of TMA is in transmit, where it can steer and optimize the beam with multimode transmit to maximize the signal-to-masking noise ratio at receive, utilize frequency to spatially decouple clutter and reverberation at receive, and employ high-resolution FFT spectrum algorithms to greatly simplify the receiver architecture, in both radar and sonar applications. The concept of spatial decoupling is to transform coherent summation into non-coherent summation, i.e., instead of being linear in amplitude, it is linear in energy like white noise.

[0016] It is therefore desirable to provide efficient and functional TMA designs within the specific operational transmission constraints of practical application examples utilizing arrays of various geometries, including linear, convex, planar, and parametric.

[0017] It would also be desirable to provide a real-time operational TMA system that takes into account transmit design constraints such as inter-beam interference within the transmit bandwidth, azimuth Doppler ambiguity, and spectral leakage interference from outside the passband.

[0018] It would further be desirable to provide a TMA system capable of real-time operation that takes into account design constraints such as power amplifier source levels and peak power ratings on transmit, and cross-beam reverberation and clutter masking levels on receive. Summary of the Invention [Problem to be solved by the invention]

[0019] The embodiments of the present disclosure are directed to using time-modulated arrays (TMAs) to obtain spatial-frequency diversity beamforming for significantly enhanced target interrogation under noise-limited and clutter-limited conditions. Targets may be biological or non-biological components. Depending on the application, real-time operational TMA systems consider design constraints such as inter-beam interference within the transmit bandwidth, azimuth Doppler ambiguity, spectral leakage interference from outside the passband, transmit source level of the power amplifier, peak power rating on transmit, and cross-beam reverberation and clutter masking levels on receive. These system design requirements have not been considered in previous studies on multiple beamforming systems. [Means for solving the problem]

[0020] According to one embodiment of the present disclosure, a method for generating a spatial distribution of acoustic transmit frequencies by a time modulated array (TMA) of transducers enabling simultaneous multiple beam steering includes the steps of generating a plurality of pulsed sampling signals by a tapped delay line, each pulsed sampling signal including a series of harmonic frequencies, successive signals of the plurality of pulsed sampling signals being separated by a predetermined delay time; mixing each of the plurality of pulsed sampling signals with a time-limited information signal to generate a plurality of mixed output signals; band-pass filtering each of the plurality of mixed output signals; and extracting a plurality of mixed output signals from the filtered and weighted output signals by driving a plurality of acoustic transducers in the spatial array of acoustic transducers. a first plurality of synchronized TMA beams from a first plurality of synchronized beams, each beam of the first plurality of synchronized beams associated with one of a plurality of transmit carrier frequencies, the first plurality of synchronized beams being subject to design constraints controlling azimuth-target Doppler ambiguity, uniform response in azimuth, out-of-band spectral leakage interference, peak power level, and cross-beam reverberation; a second plurality of beams including a plurality of different primary carrier frequencies that intersect with the first plurality of synchronized TMA beams in the far field are formed based on nonlinear characteristics of a propagation medium in which the parametric sonar array is generated; a plurality of echo frequencies are received and processed by a spectrum analyzer, and each spectral component of the plurality of echo frequencies corresponds to a unique spatial orientation.

[0021] According to a further embodiment of the present disclosure, the method includes the step of time modulating each array element of the spatial array to generate a spectral component having a phase tilt corresponding to a particular beam steering angle.

[0022] According to a further embodiment of the present disclosure, the spatial beamformer comprises two sub-beamformers, one or both of which are TMA beamformers, the outputs of which intersect one or more common maximum response axes (MRAs) in a particular region of the far field.

[0023] According to a further embodiment of the present disclosure, the spatial array is an N-point element linear array with constant element spacing d, and the far-field pressure of the time-varying monochromatic signal is given as:

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[0024] According to a further embodiment of the present disclosure, the method further comprises:

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[0025] According to a further embodiment of the present disclosure, the design constraint is at least f s =f D +6 / T, including the sampling frequency fs, D is the maximum Doppler frequency where T is the signal pulse width.

[0026] According to a further embodiment of the present disclosure, the design constraints include:

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[0027] According to a further embodiment of the present disclosure, the pointing angle

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[0028] According to a further embodiment of the present disclosure, if the information signal is s(t) with complex Fourier spectrum s(f'), the time-varying far-field pressure can be expressed as:

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[0029] According to a further embodiment of the present disclosure, TMASL for a linear array with constant input power TMA The transmit source level of the linear array SL is reduced by the number of frequency-coded beams according to LIN The transmit source level is equal to SL TMA =SL LIN -10logM,SL TMA = 20 log rms pressure of the MRA.

[0030] According to a further embodiment of the present disclosure, the spatial array is an N-point element circular array with constant angular spacing ζ between elements, and the time-varying far-field pressure for any signal with complex Fourier spectrum s(f′) is given as:

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[0031] According to a further embodiment of the present disclosure, the method has an additional phase weighting step in which each element is phased back into a line or arc to maximize the echo-to-noise ratio of the signal, enhancing spatial filtering.

[0032] According to a further embodiment of the present disclosure, the method further comprises the step of selecting a frequency that transmits a main lobe that reduces cross-beam masking clutter and / or reverberation levels and reduces alignment of spatially interacting beams.

[0033] According to a further embodiment of the present disclosure, the spatial arrangement has an element spacing d 1 A point N with 1 The element spacing in the second direction is d 2 A point N with 2A planar array of elements having a carrier frequency w o A complex signal s(t)=2u(t)cosω representing the low-frequency amplitude modulation u(t) of o The complex Fourier spectrum of the complex signal excited by tu(t) is S(ω')=U(ω'+ω o )+U(ω'-ω o ), and the time-varying far-field pressure for any signal with complex Fourier spectrum S(f') is given by:

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[0034] According to a further embodiment of the present disclosure, the spatial array further comprises a dual beamformer generating a set of collinear first primary difference frequencies, the first beamformer including a set of beams of the multi-element TMA array, and the second beamformer generating a wide beam encompassing all major TMA transmit lobes for deep sea underwater applications, the source level of the wide beam being equal to the peak source level of the TMA array.

[0035] According to a further embodiment of the present disclosure, a set of collinear first primary difference frequencies {Df i} is defined as follows:

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[0036] According to a further embodiment of the present disclosure, the collinear first difference frequency comprises a transmit spectrum, and an echo of the transmit spectrum is input to a spectrum analyzer.

[0037] According to a further embodiment of the present disclosure, the collinear first difference frequency of the parametric array maintains high efficiency in the demodulation process, with a frequency down ratio of 3:10.

[0038] According to a further embodiment of the present disclosure, the method further comprises a step of filtering each of the plurality of mixing output signals, where one of the sum frequency sidebands or the difference frequency sidebands is filtered from each of the plurality of mixing output signals.

[0039] According to a further embodiment of the present disclosure, the method further comprises the step of complex weighting each of the plurality of mixed output signals.

[0040] According to a further embodiment of the present disclosure, the bandpass filtering is one of sum-frequency bandpass filtering or difference-frequency bandpass filtering.

[0041] According to a further embodiment of the present disclosure, the different primary carrier frequency of the plurality of different primary carrier frequencies is one of a lower primary carrier frequency or a higher primary carrier frequency.

[0042] According to another embodiment of the present disclosure, there is provided an acoustic spatial frequency diversity system comprising: a tapped delay line including a plurality of taps for generating a plurality of pulse sampling signals, each tap generating one of the plurality of pulse sampling signals, each pulse sampling signal including a series of harmonic frequencies, consecutive signals of the plurality of pulse sampling signals being separated by a predetermined delay time; a plurality of mixers, each mixer of the plurality of mixers combining one of the plurality of pulse sampling signals with a time-limited information signal to generate one of a plurality of mixed output signals; and a spatial array of transducer elements, each transducer element of the spatial array of transducer elements receiving one of the plurality of mixed signals, the spatial array of transducer elements generating a plurality of synchronized beams from the plurality of mixed output signals, each beam of the plurality of synchronized beams being associated with one of a plurality of carrier frequencies, the spatial array of transducer elements, the plurality of synchronized beams being subject to design constraints on receive that control azimuth-target Doppler ambiguity, in-azimuth uniform response, out-of-band spectral leakage interference, peak power level, and cross-beam reverberation.

[0043] According to a further embodiment of the present disclosure, the spatial array of transducer elements includes three rectangular elements that synchronously generate endosteal and periosteal bone fusion beam patterns, each rectangular element having a size of 25 mm x 3.4 mm and a spacing of 3.5 mm between each rectangular element.

[0044] According to a further embodiment of the present disclosure, the system further comprises a set of bandpass filters, each bandpass filter of the plurality of bandpass filters receiving one of the plurality of mixing output signals and filtering one of a sum frequency sidelobe or a difference frequency sidelobe from the one of the plurality of mixing output signals. [Brief description of the drawings]

[0045] [Figure 1]1A and 1B are diagrams illustrating the TMA principle in reducing cross-beam reverberation for multiple beam transmission, according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 illustrates an exemplary TMA configuration comprising a linear array of N equally spaced elements, according to an embodiment of the present disclosure. [Diagram 3] FIG. 2 illustrates an exemplary TMA configuration comprising a circular array of N equally spaced elements, in accordance with an embodiment of the present disclosure. [Figure 4] FIG. 2 illustrates an exemplary TMA configuration comprising a planar array of N1N2-equally spaced elements, in accordance with an embodiment of the present disclosure. [Diagram 5] FIG. 1 is a functional block diagram of a receiver for a TMA processor analyzer having an omni receive beam output driving a multi-spectral line FFT receiver according to an embodiment of the present disclosure. [Figure 6] 1 illustrates an exemplary BGS TMA transmission, according to an embodiment of the present disclosure. [Figure 7] 1A and 1B are diagrams illustrating a configuration of a BGS according to an embodiment of the present disclosure ((A): normal operation, and (B): TMA operation). [Figure 8] FIG. 1 illustrates the application of TMA principles for underwater sonar operation to generate parametric arrays, according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Embodiments of the present disclosure are directed to a unique acoustic spatial frequency diversity system that includes a time modulation arrangement that creates a spatial distribution of transmit frequencies that allows for synchronous multiple beam steering, with each beam associated with a different carrier frequency. Each transmit beam can be steered to a different spatial direction in the vertical or horizontal plane for maximum acoustic range and signal-to-masking ratio. Multiple transmit frequencies can be received by a single omnidirectional piezoelectric hydrophone whose output is processed by a high-resolution Fast Fourier Transform (FFT) receiver, with each spectral component corresponding to a unique spatial orientation. In this way, the spatial acoustic field can be encoded in frequency to minimize the complexity of the receiver architecture and significantly reduce most of the cross-beam clutter and reverberation masking levels at the receiver display output.

[0047] Figures 1A and 1B show the basic principle of TMA transmission to reduce reverberation coherence in multibeam echoes using a common scanning method called rotational directional transmission (RDT). RDT transmits beams sequentially in a circular pattern at a fixed frequency and narrow bandwidth, producing coherent summation (i.e., linear summation based on amplitude) of reverberation in any one of the receive beams due to (1) the product of transmit and receive in the major lobe and (2) side lobe interference from adjacent beams in the major lobe. This is shown in Figure 1A with left, center, and right beams of the same frequency and phase. In Figures 1A and 1B, the vertical axis represents signal strength or amplitude, the horizontal axis represents range, the diagonal axis represents frequency, and the brackets represent signal bandwidth. TMA operation tends to make this non-coherent summation (i.e., linear summation based on energy) by employing separate frequencies for each beam and spreading the set of transmit frequencies over a larger bandwidth, often referred to as spatial decoupling. Furthermore, the spatial distribution of transmit frequencies also tends to reduce the spatial variance of background fluctuations, thereby improving the false alarm rate. This is shown in Figure 1B, where the left, center, and right beams now have different frequencies.

[0048] The embodiments of the present disclosure address the above challenges by improving the frequency encoding procedure in the TMA beamforming process of an operational system.

[0049] First, we present a set of well-defined signal representations for linear, convex and planar array geometries, which, when operated on, yield a general expression for the instantaneous space-time pressure field that generates a synchronous beam as a function of frequency.

[0050] Second, in practical operation of TMAs, the spatial distribution of the beam is constrained by finite pulse width, finite array bandwidth, element directivity patterns, and target Doppler, which will affect engineering and operational considerations such as:

[0051] 1. Inter-beam interference within the transmission bandwidth 2. Orientation Doppler Ambiguity 3. Spectral leakage interference from outside the passband Inter-beam interference results in non-uniform signal response on the maximum response axes (MRA's) of a preformed beam (PFB) receiver. To obtain efficient detection capability in all orientations, the spatial distribution of the maximum response axes must be (almost) constant, i.e., the separation frequencies (f s ) must be sufficiently large, and so on.

[0052] Azimuth-Doppler ambiguity is a consideration in applications characterized by target and / or platform motion. Since normal TMA operation encodes the spatial distribution of the beams in frequency, if the frequency separation between the beams is insufficient, a target moving in one azimuth may generate Doppler-shifted echo frequencies that lie within the frequency bands assigned to adjacent beams. This does not significantly affect the sonar detection capabilities, but the target will appear on the display in the wrong azimuth. This is called "azimuth-Doppler ambiguity" and must be eliminated to allow accurate target localization and tracking. Thus, in practice, the harmonic separation must be much larger than that shown above to avoid azimuth-Doppler ambiguity due to the combined effect of target and sonar platform motion.

[0053] For a moving source, target, and receiver, the frequency of the echo pulse (f e ) is given by (Horton), f e =f p [(cv t )(cv r )] / [(cv s )(cv t’ )] where f p is the transmitted frequency at the source, (c) is the speed of the sound wave moving away from the source, and (cv s ) is the relative velocity of the source, and (cv t ) is the relative velocity of the transmitted pulse at the target, and (cv t’ ) is the relative velocity of the reflected sound wave from the moving target, and (cv r ) is the relative speed of the sound wave at the moving receiver.

[0054] Maximum Doppler generated frequency (f D ) over the spread of the sending and receiving speeds and the opening and closing target speeds, (f e -f p ) is determined by calculating the range of v t =vt’ and

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[0055] Combining the results so far, we can determine the separation frequency f needed to eliminate azimuth Doppler ambiguity and provide a reasonably uniform response in azimuth. s f s =f D +6 / T'=0.7f p |v s -v t | max +6 / (pulse width).

[0056] Spectral (out-of-band) interference leakage can be reduced to insignificant levels (below 50 dB for MRA) by appropriate selection of the separation or sampling frequency fs, the information frequency fo, and the number of beams as follows:

[0057]

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[0058] By increasing the sampling sufficiently to cover the entire Doppler spread, including both the platform and the target, inter-beam interference and azimuth Doppler ambiguity can be eliminated. However, for a given piezoelectric design, increasing the sampling frequency reduces the number of frequency diversity beams allowed. Also, to avoid out-of-band leakage interference, the information frequency fo must be increased according to the above equation.

[0059] Third, the source level (SL) on the maximum response axis (MRA) of a linear array with N elements with constant input power is LIN ) is reduced by the number of frequency-encoded beams, so the TMA source level becomes

[0060] SL TMA =SL LIN -10logM=[71.6+10logP e N+10logE ff +DI]-10logM where the reference (for sonar operations) is 1 microbar at a distance of 1 yard from the face of the transducer element, (P e ) is the input power per element, (E ff ) is the transmit frequency, (DI) is the directivity index of the transducer array, and (M) is the number of beams formed with each element simultaneously contributing to the source level. In an embodiment, SL=20 log rms pressure on the MRA.

[0061] How the number of beams affects the rated peak power of a power amplifier is important. For M harmonics of the peak amplitude (A), when the tones add coherently in phase, the peak amplitude increases over the fundamental period (1 / f s ) to (MA). This means that the effective power per tone required to give a specified source level is P rms Then the total peak power required for the (M) tone is (2M 2 P rms The complexity and cost of the power amplifier design increases with the tone and number of TMA beams, i.e., narrowband or wideband systems.

[0062] (Linear TMA formulation) According to an embodiment, synchronous multiple beam frequency steering can be obtained by generating a controlled frequency varying phase shift between adjacent transducer elements. A basic TMA acoustic system, which represents the technology for a general array, broadly comprises a signal generator which is an information signal, a pulsed sampling signal, a tapped delay line, a mixer, a bandpass filter, a TMA beamformer with complex weights, a power amplifier normalized to 1 watt, and a transducer array.

[0063] For a linear array, the formulation of the instantaneous pressure field p(θ,t) is based on FIG. 2, which shows an exemplary TMA configuration with a tapped delay line 202 receiving a pulsed sampling signal 201, a mixer 204 mixing the output of the tapped delay line with an information signal 203, a sum-frequency bandpass filter 205 and complex weighting 206 acting on the output signal from the mixer 204, and a linear array of N equally spaced transducer elements 207 with inter-element spacing d. Each element of this array 207 is fed from a tap on the delay line 202 excited by a set of harmonic frequencies 201. According to an embodiment, the map of transducer elements 207 includes a power amplifier. The requirement of synchronization for multiple beam frequency steering can be reduced to the requirement of generating a controlled frequency-varying phase shift between adjacent transducer elements. The resulting pattern generated by this array consists of a set of different beams of different frequencies, pointed in different directions. The transmit acoustic frequency and beam steering angle of each beam are uniquely determined by the harmonic frequency, with the zeroth harmonic producing a beam whose maximum response axis (MRA) is perpendicular to the array. The linear TMA is analyzed as follows: According to an embodiment, the tapped delay line 202, mixer 204, bandpass filter 205 and complex weighting 206, together with the pulsed sampling signal 201 and the information signal 203, constitute a beamforming device.

[0064] Referring to FIG. 2, according to an embodiment, a harmonic generating function or pulse sampling signal E s(t) 201 is a finite Fourier series input to tapped delay line 202 and is expressed as follows:

[0065]

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[0066]

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[0067]

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[0068]

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[0069]

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[0070]

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[0071]

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[0072] After filtering by a bandpass filter 205 that filters one of the lobes of the curly bracket term in Equation 6, the resulting signal waveform may be weighted by a weighting factor Wm 206 before driving the mth array element, given by Equation 7:

[0073]

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[0074]

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[0075] More realistic filter characteristics would result in less attenuation. In reality, the signal would not abruptly terminate at t=2T, but would continue to attenuate, eventually reaching a steady state of zero amplitude. By exploiting the properties of the interaction between conjugate Fourier domains, wideband filters can produce more rapid attenuation than narrowband filters. Finally, comparing Equation 1 with Equation 8, the main difference is clearly shown: each spectral component has a phase slope defined by the harmonic number. Each spectral component has a phase slope defined by the harmonic number. m It will be shown later that when (t) is applied, every harmonic number, i.e., each spectral component, corresponds to a particular beam steering angle.

[0076] Each signal is input to the m-th transducer element of the linear array 207. When a signal with a time-varying propagation delay is applied, the time-varying sound pressure at any point in the far field is expressed as a signal JPEG0007689575000028.jpg9170 is the vector sum of the far-field TMA sound pressure p(θ,t) in a direction θ, where θ is the angle of the line 208 to the far-field observation point with respect to the array normal 209, and can be expressed as the following Equation 9.

[0077]

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[0078] When summation is performed using Equation 8, the output of the linear multi-element transducer array 207 is expressed as Equation 10 below.

[0079]

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[0080] For better visualization and understanding of the TMA process, the above derivation of the beam pattern distribution expressed in Eq. (10) assumes a long signal pulse width to approximate the monochromatic case and neglects spectral leakage.

[0081] For an information signal s(t) having a complex Fourier spectrum s(f'), the far-field time-varying pressure of a monochromatic signal given by Eq. (10) is spectrally weighted and given by Eq. (11) below.

[0082]

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[0083] In a linear array variation of the embodiment, the TMA beamformer for a linear array of N equally spaced transducer elements shown in Figure 2 can exploit the nonlinear properties of the acoustic medium by transmitting at a power high enough to generate finite amplitude waves in the far field, as described further below in the "Underwater Mining and Communications" section.

[0084] (TMA beam steering constraints) The sum of Equation 10 is the frequency that points in the direction of Equation 12. It may be recognized as a linear array beam pattern of JPEG0007689575000033.jpg9170.

[0085]

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[0089] Equation 15 shows that the parameters of the acoustic (or E / M) system dictate the direction in which the beams are synchronized. As the number of elements and element spacing increase, the MRAs of the harmonic beams move closer together. For a fixed element spacing, increasing the center frequency (zeroth harmonic) narrows the 3 dB beamwidth. This means that as the separation frequency increases, the beamwidth becomes narrower for the positive harmonics and wider for the negative harmonics, the latter with a steering angle that diverges more from the zeroth harmonic. For separation frequencies much smaller than the center frequency, the harmonic wavenumber becomes essentially constant and has a symmetrical distribution.

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[0090]

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[0091]

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[0092] According to one embodiment, an additional phase weighting can be introduced to phase each element back to a straight line or into a circular arc. This acts as a spatial filter to maximize the signal to noise ratio. Let this phase adjustment circle 312 pass through the vertex A 313 of the circular transducer array 307 and be perpendicular to the axis of symmetry. The phase weighting is as follows:

[0093]

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[0094] (Formulation of TMA Planar Array) FIG. 4 is a block diagram of a N 1 ×N 2 4 shows an exemplary TMA configuration consisting of a planar array of elements spaced d apart in a first direction D1. In Fig. 4, like elements in Fig. 2 are numbered likewise. Fig. 4 shows a first tapped delay line 402 receiving a first pulse sampling signal 401, a second tapped delay line 412 receiving a second pulse sampling signal 411, a mixer 404 mixing the outputs of the first and second tapped delay lines with an information signal 403, and a first tapped delay line 412 spaced d apart in a first direction D1. 1 N 1 Element and distance d in the second direction D2 2 N 2A planar array of elements 407 is shown. For ease of illustration, the sum frequency bandpass filters and complex weights are not shown. The transmit process involves two different linear modulation excitations, each applied simultaneously along one of the directions of the array, thereby providing a three-dimensional fan of beams coded at different frequencies. A simple analysis of the TMA planar array system is as follows:

[0095] According to the linear array analysis method, the pulse sampling signal E s (t) inputs a finite harmonic sequence to a tapped delay line, as in the case of Equation 1.

[0096]

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[0097] s(t)=2u(t)cosw o For an information signal 303 of shape t, where u(t) is a low frequency amplitude modulation, the complex Fourier spectrum is S(w') = U(w' + w o )+U(w'-w o ) can be expressed as follows. Therefore, m l The output of the mixer is given by Equation 21 below.

[0098]

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[0100] Each signal JPEG0007689575000051.jpg9170 is a plane array of 407 m 1,2 When applied to the transducer elements, the time-varying sound pressure established in the far field is proportional to the relative propagation time delay (resp. m 1 , m 2 =1) to consider the signal The vector sum of JPEG0007689575000052.jpg9170 is obtained. As a result, the far-field TMA sound pressure of the TMA planar array 407 can be written as the following Equation 23.

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[0103] (TMA Receiver) In the above linear, convex, and planar TMAs, a special receiver is required to process the echo return. This is because, since the space is frequency-coded, a receiver capable of detecting and tracking frequencies with high precision is needed.

[0104] Figure 5 is a functional block diagram of a receiver TMA processor analyzer implemented as a passive detector with a detection probability (PD) of 90% or more and a false alarm rate (FAR) of 1 or less per day. The TMA echo signal, which is the RCV beam output, is processed by three consecutive modules: an input signal conditioning unit (ISCU) 510, a spectrum analyzer unit (SAU) 520, and a post-processing unit (PPU) 530. The ISCU 510 is controlled by the system clock signal SYSCLK. The SAU 520 includes a spectrum analyzer such as an FFT processor 521, a band-tuned band-pass filter or a heterodyne correlator, and an amplitude spectrum weighting unit 522 that takes the absolute amplitude of the output of the FFT processing unit 521. An exemplary post-processing unit (PPU) 530 includes a cascade of proven processing techniques. The PPU 530 includes averaging logic and data memory for an additional algorithm processing unit 531 to increase the SNR, and a unit 532 that performs automatic spectral line detection and tracking of azimuth frequencies, pattern recognition, and ambient noise removal. As shown in the figure, the output of the PPU is then sent to the BFHAI (Bone Fracture Healing Assessment Indicator).

[0105] <Application Examples of TMA Technology> There are many application examples of TMA technology that have not been considered so far (in 2020), such as medical therapeutic ultrasound, acoustic microscopes, and mining, exploration, and communication in water.

[0106] (Bone Growth Stimulation (BGS)) TMA can promote bone growth stimulation (BGS) that simultaneously treats endosteal and periosteal fractures. For this purpose, as shown in Figure 6, two types of spatial beams with different steering angles are required.

[0107] 7A and 7B show an exemplary configuration of BGS where endosteal and periosteal healing are stimulated simultaneously. In this case, no acoustic mode transducer (AMC) is required, but an array with three rectangular strips is needed to form a piston transducer. The transmitted power is adapted to the Beam Nonuniformity Ratio (BNR), Spatial-Averaged-Temporal Averaged Intensity (SATA), Effective Radiation Area (ERA) and Transmitted Frequency of the transducer proposed in the AMC.

[0108] FIG. 7A is a block diagram of normal operation, in which a signal generator 71 is used to generate a low intensity pulsed ultrasound (LIPUS) signal with a transmit signal of 1 MHZ, constant envelope, 1 kHz pulse repetition rate, 200 msecs pulse width, and 20% duty cycle, which is sent through a signal conditioner 72 to a power amplifier 73 to provide an rms output power of 314 mW to a circular piston transducer 74 with a -6 dB bandwidth of approximately 400 kHz, i.e., 0.8 to 1.2 MHz, 50 mW / cm. 2 SATA, 194mW acoustic effective power, 50% efficiency, 3.14mW / cm 2 Has an ERA of.

[0109] FIG. 7B is a block diagram for a TMA system operating for the same signal parameters as FIG. 7A, including a three-section TMA module 76 based on the general configuration shown in FIG. 7Each TMA section of the TMA module 76 in B includes a tapped delay line receiving a 50KHz pulsed sampling signal 77, a double balanced modulator suppressed carrier (DBM-SC) that combines the output of the delay line with the information signal 78, a bandpass filter (BPF), a linear power amplifier (LPA) with an output of approximately 90-100mWrms and a transducer element 79. The delay of each step is 6.66667μsecs. The information signal is 1MHZ with a repetition rate of 1KHz and a pulse width of 200μs. The information signal contains the zeroth and fourth harmonics, but the first, second and third harmonics are removed. The output of each delay step drives the RF input of the DBM-SC, and the intermediate frequency (IF) output is bandpass filtered and sent to the LPA. As shown in Figure 6, each of the three LPA outputs, assuming 50% efficiency, drives a specific rectangular transducer slat to synchronously generate two unique acoustic beams (one at 0 degree orientation for endosteal healing and the other at 31.5 degrees for periosteal healing). Each beam generates approximately 52 mW / cm 2 SATA, approximately 135mW acoustic effective power, and ERA 2.58cm 2 Note that when TMA is employed, no absorbable AMC is required to achieve periosteal healing as in Figure 7A. The dimensions of each transducer element are approximately 25 mm x 3.4 mm, with a spacing d of 3.5 mm.

[0110] (acoustic microscope) The TMA approach according to the embodiments can also be used with ultrasound imaging (SAM). SAM devices can provide unique information about the absorption, elastic properties, and density of tissue cells, improving the visualization and clinical assessment of consistent patterns of cellular malignancies.

[0111] In acoustic biophysics, the maximum frequency of the SAM is estimated to be about 4-5 GHz, resulting in a wavelength λ of 347 nm. However, a problem when operating at this frequency is that typical aluminum motor-driven space-position-stepping systems are fabricated by an aluminum extrusion process that produces a position error of about 1 μm (≈1 GHz). The TMA according to the embodiment is able to eliminate this effect by electronically generating the acoustic beam to cover the desired sector.

[0112] (Subsea resource exploration and communications) The TMA approach according to the embodiment can also be used for underwater communication and marine mining and exploration. Accurate spatial communication in some specific orientation requires minimal side lobe interference, while mining and exploration requires special consideration of orientation, which correlates to beams propagating to deposits at different ocean depths and frequency dependent attenuation. In these applications, the term orientation refers to either azimuth angle or D / E (depth / altitude).

[0113] Two transmit beam configurations are shown in Figure 8. The first is a nine element linear array as shown in Figure 2, using the TMA modulation beamforming technique. Each element is 2.292 inches square with 2.4 inch element spacing, producing a complex signal at four harmonics (-1,0,1,2) and transmitting four beams simultaneously covering a 45 degree sector (-20 degrees to +25 degrees) at -3 dB. It may be hull mounted on the D / E plane of a surface ship. The transmit frequency at the zeroth harmonic is 13 kHz.

[0114] The same nine-element linear array may include a second beamformer, which utilizes complex weights to generate a wide beam, shown in Figure 8, that covers the four narrow TMA beams and matches their peak source levels. If the transmit power is high enough, the nonlinear properties of the acoustic medium can be exploited to reduce the f omni From the frequency of the wide directional beam such as f-1, f o , f 1 , f 2A set of first-difference frequencies obtained by subtracting each of the frequencies can be generated.

[0115] According to an embodiment, in order for the parametric array of FIG. 8 to maintain high efficiency in the demodulation process, it is desirable that the frequency down-conversion ratio be about 3 to 10. In an exemplary embodiment, f omni is about 15 to 19.0 kHz. Thus, very low-frequency propagation signals such as about 4.83 kHz, about 4.00 kHz, about 3.17 kHz, and about 2.34 kHz can be generated with a relatively small high-frequency array {TMA: 13.00 kHz; OMNI: 17.00 kHz}. The advantages of an active parametric sonar are its broadband function, high angular resolution with reduced side lobes, and almost constant beam width over a very wide frequency band. The operating performance of a parametric sonar is usually superior to that of a conventional linear system due to reduced propagation loss, reduced reverberation, reduced multipath, and improved data rate. Therefore, a multidimensional array can be employed to trade off between improved parametric performance and design complexity.

[0116] <Unique Features of TMA Technology> (For Medical Applications) The TMA approach according to an embodiment can be characterized by at least six unique features for obtaining a spatial frequency diversity beamformer for various medical applications, namely, six features.

[0117] a. Induce periosteal healing and endosteal healing of long bone fractures simultaneously.

[0118] b. Promote the generation of shear waves to maximize the integrin response of the extracellular matrix (ECM) of bone tissue instead of an acoustic mode converter.

[0119] c. Efficiently improve overall healing by slightly angularly adjusting the ultrasonic beam within the fracture channel and along the bone tissue.

[0120] d. Minimizing clutter interference in tissue parameter estimation.

[0121] e. Tissue space is frequency encoded to enhance parameter estimation with FFT processing on receive.

[0122] f. Scanning tissue regions of interest in the high MHz (25-200 MHz) and low GHz (1-5 GHz) frequency ranges for dermatology and scanning ultrasound microscopy applications, respectively.

[0123] From the foregoing, it will be appreciated by those skilled in the art that embodiments of the present disclosure may provide effective methods and apparatus that overcome many of the limitations associated with mechanical stimulation of biological materials.

[0124] (for underwater sonar applications) The TMA approach according to the embodiments can be characterized by at least eight unique features, namely eight features, for obtaining spatial-frequency diversity beamforming for underwater sonar applications.

[0125] a. Achieve wider near-field coverage by eliminating blind zones.

[0126] b. It is easier to detect because its transmission time is shorter.

[0127] c. The cross-beam reverberation per receive beam is reduced.

[0128] d. Synchronization of heading finding information.

[0129] e. Essential reduction in sonar mutual interference.

[0130] f. Low input power requirements for a given OMNI source level.

[0131] g. Underwater volume is frequency encoded and parameter estimation is enhanced by FFT processing on reception.

[0132] h. Reducing hardware complexity for multiple beamforming.

[0133] Those skilled in the art will also readily appreciate the use of methods and apparatus according to embodiments of the present disclosure in a variety of sonar applications, such as multi-mode echo range, deep-moored mine detection, and submarine active sonar, and can also provide multiple beamforming capabilities for circular and planar arrays.

[0134] While certain exemplary embodiments of the present disclosure have been specifically described herein, it will be apparent to those skilled in the art to which the embodiments of the present disclosure pertain, that variations and modifications of the various embodiments shown and described herein may be made without departing from the spirit and scope of the present disclosure. [Prior art documents] [Patent documents]

[0135] [Patent Document 1] U.S. Patent No. 2,426,460 [Patent Document 2] U.S. Patent No. 2,852,772 [Patent Document 3] U.S. Patent No. 3,012,244 [Patent Document 4] International Publication No. 2020 / 154633 [Non-patent literature]

[0136] [Non-Patent Document 1] Winder AA and Loda CJ, "Introduction to Acoustical Space-Time Information Processing", Office of Naval Research, Washington, DC / ONR Report ACR-63, January 1963. [Non-Patent Document 2] Horton J.W, "Fundamentals of Sonar", U.S. Naval Institute, Annapolis, MD, 1957 [Non-Patent Document 3] Urick R.J, "Principles of Underwater Sound for Engineers", McGraw-Hill Book Co, New York, 1967 [Non-Patent Document 4] Shelkunoff S.A, "Theory of Antennas of Arbitrary Size and Shape", Proc. IRE, Vol.29, September 1941, pp. 493-521 [Non-Patent Document 5] Shelkunoff S.A, "A Mathematical Theory of Linear Arrays", BSTJ, Vol.22, January 1943, pp. 80-107 [Non-Patent Document 6] Shnitkin H, "Survey of Electronically Scanned Antennas", The Microwave Journal, Part1: December 1960, pp. 67-72, Part2: January 1961, pp. 57-64 [Non-Patent Document 7] Shanks H.E, "A New Technique for Electronic Scanning", IRE Transactions on Antennas and Propagation, March 1961, pp. 162-166 [Non-Patent Document 8] Kummer W.H, Villeneuve A.T, Fong T.S and Terrio F.G, "Ultra-Low Sidelobes from Time-Modulated Arrays", IEEE Trans on Antennas and Propagation, November 1963, pp. 633-639 [Non-Patent Document 9] Davies, D.E.N and McCartney, B.S, "Cylindrical Arrays with Electronic Beam Steering", Proc. IEEE, Vol.112, No.3, March 1965 [Non-Patent Document 10] Johnson, M.A, "Phased-Array Beam Steering by Multiplex Sampling", Proc. IEEE, Vol.56, No.11, November 1968, pp.1802 - 1811 [Non-Patent Document 11] Cole, B.F and Hanrahan, J.J, "Influence of Beamwidth and Multiple Transmissions on Reverberation-Limited Sonars", JASA, Vol.43, No.6, 1968, pp.1373 - 1377 [Non-Patent Document 12] Eyring, C, Christensen, R and Raitt, R, "Reverberation in the Sea", JASA, Vol.20, 1948, pp.462 - 475 [Non-Patent Document 13] Westerfelt, P.J, "Parametric Acoustic Array", J. Acoust. Soc. Am, 35, 1963, pp.535 - 537 [Non-Patent Document 14] Voglis, G.M, "A General Treatment of Modulation Scanning as Applied to Acoustic Linear Arrays", Ultrasonics, Part1: Vol.9, No.3, July 1971, pp.142 - 153, Part2: Vol.10, No.3, May 1972, pp.142 - 153 [Non-Patent Document 15] A.A. Winder, "Sonar System Technology", IEEE Trans Sonics & Ultrasonics, SU-22, No.5, September 1975, pp. 291-332

Non-Patent Document 16

Non-Patent Document 17

Claims

1. 1. A method for generating a spatial distribution of acoustic transmit frequencies by a time modulated array (TMA) of transducers with synchronous multiple beam steering, comprising: receiving a plurality of pulse sampling signals including a range of harmonic frequencies through a tapped delay line; successive ones of the plurality of pulse sampling signals are separated by a predetermined delay time; mixing each of the plurality of pulsed sampling signals with an information signal to generate a plurality of mixed signal outputs; filtering one of a sum frequency sideband or a difference frequency sideband from each of the plurality of mixed output signals; generating a first plurality of synchronized TMA beams from the filtered mixed output signals; each synchronous TMA beam of the first plurality of synchronous TMA beams is associated with one of a plurality of transmit carrier frequencies; the first plurality of synchronous TMA beams are subject to design constraints controlling azimuth-target Doppler ambiguity, uniform response in azimuth, out-of-band spectral leakage interference, peak power level, and cross-beam reverberation; a second plurality of beams including a plurality of different primary carrier frequencies that intersect the first plurality of synchronous TMA beams in the far field are formed based on nonlinear characteristics of a propagation medium in which the parametric sonar array is generated; The multiple echo frequencies are received and processed by a spectrum analyzer; each spectral component of the plurality of echo frequencies corresponds to a unique spatial orientation; method.

2. and time modulating each array element of the spatial array to generate a spectral component having a phase slope defined by a harmonic number of the mixing output signal corresponding to a particular beam steering angle. The method of claim 1.

3. The spatial beamformer comprises two sub-beamformers; one or both of the two sub-beamformers output the first plurality of synchronized TMA beams, the outputs intersecting one or more common maximum response axes (MRAs) in a particular region of the far field; The method of claim 1.

4. The spatial array is an N-point element linear array with a constant element spacing d; The far-field pressure of a time-varying monochromatic signal is given by, [0010] θ is the pointing angle, which is the direction in which the response of a particular beam related to the harmonic number is maximum; a p is the weighting factor for each harmonic p, W m is the complex weight for each element m, f r is the reference frequency, f s is the fundamental harmonic frequency, f o is the transmission frequency, p is the first harmonic P o to the final harmonic P c The range is up to c is the speed of sound, ε is +1 or −1 depending on whether the sum or difference frequency band is retained. The method of claim 1.

5. Sampling Period [0025] selecting a total delay time τ over which the plurality of pulse sampling signals are separated equal to [0030] and The pointing angle θ is determined by the harmonic parameters (p) and (f s ), said center frequency [0045] and the sequence parameters (N) and (d), The method according to claim 4.

6. The design constraint is at least f s =f D +6 / T, the sampling frequency f s Including, f D is the maximum Doppler frequency, T is the signal pulse width; The method according to claim 5.

7. The design constraints are: [0050] and a design constraint to control out-of-band spectrum leakage interference, (f s ) is the sampling frequency including spread due to the source, target and receiver velocities, The method according to claim 5.

8. The pointing angle [006] and k p teeth, [0070] are harmonics dependent on wave numbers equal to The method according to claim 7.

9. The information signal is a complex Fourier spectrum [0080] where s(t) has a time-varying magnitude, then the time-varying far-field pressure can be expressed as: [0097] The method according to claim 4.

10. The total peak power required for M tones is (2M 2 P rms ) and P rms is the rms power per tone required for a given source level, The method according to claim 4.

11. The TMASL for the linear array with constant input power TMA The transmit source level of the linear array SL is reduced by the number of the frequency encoded beams according to the following formula: LIN is equal to the transmit source level of SL TMA =SL LIN -10logM SL TMA = 20 log rms pressure of the MRA, The method according to claim 4.

12. The spatial arrangement is an N-element circular array with a constant angular spacing ζ between elements, and the complex Fourier spectrum [0010] The time-varying far-field pressure for any signal with ##EQU00011## θ is the pointing angle, which is the direction in which the response of a particular beam related to the harmonic number is maximum; a p is the weighting factor for each harmonic p, W m are the complex weights for each element m, ##EQU00012## and f r is the reference frequency, f s is the fundamental harmonic frequency, f o is the transmission frequency, p is the first harmonic P o to the final harmonic P c The range is up to R is the radius of the circular array; c is the speed of sound, ε is +1 or −1 depending on whether the sum or difference frequency band is retained. The method of claim 1.

13. Further having an additional phase weighting step where each element is phased back into a line or arc to maximize the echo-to-noise ratio of the signal to enhance spatial filtering; The method of claim 12.

14. selecting a frequency that transmits a main lobe that reduces cross-beam masking clutter and / or reverberation levels and reduces the alignment of the spatially interacting beams; The method of claim 1.

15. The spatial arrangement has an element spacing d 1 A point N with 1 The element spacing in the second direction is d 2 A point N with 2 A planar array of elements of a carrier frequency ##EQU00013## A complex signal representing the low-frequency amplitude modulation u(t) of ##EQU14## and the complex Fourier spectrum of the complex signal is ##EQU00015## and Complex Fourier Spectrum ##EQU00016## The time-varying far-field pressure for any signal with ##EQU00017## α and β are the direction cosines of the far-field observation point for the one direction and the two directions, respectively; [0018] HAp 1 p 2 is the frequency of the harmonic, c is the speed of sound, The direction cosines of the individual beam MRA are defined as follows: [0019] The method of claim 1.

16. the spatial arrangement further comprises a dual beamformer having a first beamformer and a second beamformer to generate a set of collinear first primary difference frequencies; the first beamformer includes a set of beams of a multi-element TMA array, and the second beamformer produces a wide beam that encompasses all major TMA transmit lobes for deep sea underwater applications; the wide beam source level is equal to the peak source level of the TMA array; The method of claim 1.

17. said set of collinear first primary difference frequencies [0020] is defined as follows: ##EQU00021## 17. The method of claim 16.

18. the set of collinear first first difference frequencies comprises a transmit spectrum, and an echo of the transmit spectrum is input to a spectrum analyzer.

20. The method of claim 17.

19. The frequency down ratio is 3 to 10.

20. The method of claim 17.

20. and complex weighting each of the plurality of mixed output signals. The method of claim 1.

21. The plurality of different primary carrier frequencies include primary carrier frequencies that are different in frequency. The method of claim 1.

22. 1. An acoustic spatial frequency diversity system, comprising: a tapped delay line including a plurality of taps for receiving a plurality of pulse sampling signals, each tap receiving one of the plurality of pulse sampling signals, each pulse sampling signal including a range of harmonic frequencies, successive signals of the plurality of pulse sampling signals being separated by a predetermined delay time; a plurality of mixers, each mixer of the plurality of mixers combining one of the plurality of pulse sampling signals with an information signal to generate one of a plurality of mixed output signals; a filter operating on the plurality of mixed output signals from the plurality of mixers; a spatial array of transducer elements, each transducer element of the spatial array of transducer elements receiving one of the plurality of mixed signals output from the filter, the spatial array of transducer elements generating a plurality of synchronized beams from the plurality of mixed output signals, each beam of the plurality of synchronized beams being associated with one of a plurality of carrier frequencies, the spatial array of transducer elements, the plurality of synchronized beams being subject to design constraints on receive that control azimuth-target Doppler ambiguity, in-azimuth uniform response, out-of-band spectral leakage interference, peak power level, and cross-beam reverberation; An acoustic spatial frequency diversity system comprising:

23. The spatial arrangement of the transducer elements includes three rectangular elements that synchronously generate two types of spatial beams with different steering angles to promote bone growth stimulation to heal the endosteum and periosteum simultaneously, the size of each rectangular element is 25 mm x 3.4 mm, and the spacing between each rectangular element is 3.5 mm.

23. The system of claim 22.

24. a set of bandpass filters, each bandpass filter of the plurality of bandpass filters receiving one of the plurality of mixed output signals and filtering one of a sum frequency sidelobe or a difference frequency sidelobe from one of the plurality of mixed output signals.

23. The system of claim 22.

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