Abstract:
Methods and apparatuses process signals. The method according to one aspect of the present invention receives a first signal; obtains a second signal and a third signal from the first signal, wherein a gain of the second signal is smaller than a gain of the third signal; detects saturation in the third signal; and generates a composite signal from the second signal and the third signal, the step of generating a composite signal including selecting a part of the second signal for the composite signal, when the detecting step detects saturation in the third signal, and selecting a part of the third signal for the composite signal, when the detecting step does not detect saturation in the third signal.

Description:
[0001]    The present application claims priority under 35 USC §119(e) to U.S. Provisional Application No. 60/743,128 filed Jan. 13, 2006, which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to an underwater sounding technique, and more particularly to a method and apparatus with wide receiving dynamic range for processing signals from various underwater targets. 
         [0004]    2. Description of the Related Art 
         [0005]    Acoustic sounding apparatuses, such as echo sounders and scanning sonar, are typically used to detect underwater objects such as fish, seabed, etc., and to evaluate properties of underwater objects, such as length of fish, size of seabed rocks, etc. Such sounding apparatuses are typically installed on a ship, from where they transmit acoustic signals beneath the ship and into the water, to search for underwater objects and receive echo signals from them. The echo signals received from underwater objects are then processed to determine properties of the underwater objects that generated the echo signals. 
         [0006]    Typical conventional underwater sounding apparatuses include two or more receiving-amplifiers with different gains, that amplify echo signals received from underwater targets. For a given echo signal received from a target, a proper receiving-amplifier is selected from among, for example, a low-gain amplifier and a high-gain amplifier. The receiving-amplifier is selected based on the distance to the target, or on the time interval of travel of an echo signal from the target to a source such as a ship. The distance to a target is determined based on the time it takes for a sound pulse from a source such as a ship or a submarine, to bounce off a target and return as an echo signal to the source. Typically, a low-gain amplifier is selected for strong echo signals, such as signals originating at close-by targets, while a high-gain amplifier is selected for weak echo signals, such as signals originating at far away targets. Typical/conventional sounding technology uses Time Variable Gain Control (TVG) to compensate for underwater propagation loss of echo signals. Typically, to detect a given underwater target, a TVG curve suitable for the target is selected. 
         [0007]    While the typical/conventional underwater sounding systems can work when the TVG curve selected for a target corresponds to the strength of the echo signal reflected by that target, the typical/conventional underwater sounding systems encounter challenges and are ineffective for mixed echo signals that are reflected by two or more targets of different target strengths, such as, for example, a small fish together with a big fish and a seabed area, as the TVG curves selected for one target may not be suitable for the other targets. 
         [0008]    For example, if an echo signal reflected by a target with a small target strength, such as a small fish, is amplified by a low-gain receiving amplifier and coupled with a TVG curve appropriate for larger objects, the signal-to-noise ratio of the signal becomes unacceptably low. And if an echo signal reflected by a target with a large target strength, such as the seabed, is amplified by a high-gain receiving amplifier and coupled with a TVG curve suitable for small targets, the signal level saturates. Hence, the typical/ conventional technology is not effective for target detection for a plurality of targets of different strengths, because the typical/conventional technology gives inaccurate or noisy results. 
         [0009]    Disclosed embodiments of this application address these and other issues by using underwater sounding methods and apparatuses with a wide receiving dynamic range for processing underwater signals. The methods and apparatuses can receive and process signals associated with one target among two or more targets of different strengths, or with a plurality of targets of different strengths. The methods and apparatuses process signals received from underwater objects, output signals without switching errors or discontinuous points, and provide highly accurate measurements for underwater objects. The methods and apparatuses can be applied to other signals besides acoustic signals, originating in other media besides a water medium. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed to methods and apparatuses that process signals. According to a first aspect of the present invention, a method comprises: receiving a first signal; obtaining a second signal and a third signal from the first signal, wherein a gain of the second signal is smaller than a gain of the third signal; detecting saturation in the third signal; and generating a composite signal from the second signal and the third signal, the step of generating a composite signal including selecting a part of the second signal for the composite signal, when the detecting step detects saturation in the third signal, and selecting a part of the third signal for the composite signal, when the detecting step does not detect saturation in the third signal. 
         [0011]    According to a second aspect of the present invention, a method comprises: receiving a first signal; obtaining a second signal and a third signal from the first signal, wherein a gain of the second signal is smaller than a gain of the third signal; and generating a composite signal, the generating step including performing a gain compensation for the third signal, to obtain a gain compensated third signal, selecting a part of the second signal for the composite signal, when a saturation of the third signal is detected, and selecting a part of the gain compensated third signal for the composite signal, when no saturation is detected in the third signal. 
         [0012]    According to a third aspect of the present invention, an apparatus comprises: a signal input unit for providing a first signal; a signal processing unit for obtaining a second signal and a third signal from the first signal, wherein a gain of the second signal is smaller than a gain of the third signal; and a signal selection unit for generating a composite signal from the second signal and the third signal, the signal selection unit generating a composite signal by detecting saturation in the third signal, selecting a part of the second signal for the composite signal, when saturation is detected in the third signal, and selecting a part of the third signal for the composite signal, when no saturation is detected in the third signal. 
         [0013]    According to a fourth aspect of the present invention, an apparatus comprises: a signal input unit for providing a first signal; a signal processing unit for obtaining a second signal and a third signal from the first signal, wherein a gain of the second signal is smaller than a gain of the third signal; and a signal selection unit for generating a composite signal, the signal selection unit generating a composite signal by performing a gain compensation for the third signal, to obtain a gain compensated third signal, selecting a part of the second signal for the composite signal, when a saturation of the third signal is detected, and selecting a part of the gain compensated third signal for the composite signal, when no saturation is detected in the third signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Further aspects and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a general block diagram of a system including an underwater sounding apparatus according to an embodiment of the present invention; 
           [0016]      FIG. 2  is a block diagram illustrating in more detail aspects of an underwater sounding apparatus according to an embodiment of the present invention; 
           [0017]      FIG. 3A  is a block diagram illustrating conventional technology for processing of signals from underwater targets; 
           [0018]      FIG. 3B  is a diagram illustrating operations for the conventional technology for processing of signals from underwater targets illustrated in  FIG. 3A ; 
           [0019]      FIG. 4  is a block diagram illustrating an underwater sounding apparatus according to an embodiment of the present invention illustrated in  FIG. 2 ; 
           [0020]      FIG. 5A  is a flow diagram illustrating operations performed by an underwater sounding apparatus according to an embodiment of the present invention illustrated in  FIG. 4 ; 
           [0021]      FIG. 5B  illustrates exemplary aspects of operations performed by an underwater sounding apparatus according to the operations illustrated in the flow diagram of  FIG. 5A ; 
           [0022]      FIG. 6  is a block diagram illustrating an underwater sounding apparatus according to a different embodiment of the present invention; 
           [0023]      FIG. 7A  is a flow diagram illustrating operations performed by an underwater sounding apparatus according to an embodiment of the present invention illustrated in  FIG. 6 ; and 
           [0024]      FIG. 7B  illustrates exemplary aspects of operations performed by an underwater sounding apparatus according to the operations illustrated in the flow diagram of  FIG. 7A . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Aspects of the invention are more specifically set forth in the accompanying description with reference to the appended figures.  FIG. 1  is a general block diagram of a system including an underwater sounding apparatus according to an embodiment of the present invention. The system  100  illustrated in  FIG. 1  includes the following components: a signal input unit  20 ; an underwater sounding apparatus  40 ; a display  70 ; a user input unit  80 ; a signal output unit  60 ; and a printing unit  50 . Operation of the system  100  in  FIG. 1  will become apparent from the following discussion. 
         [0026]    The signal input unit  20  provides signals to underwater sounding apparatus  40 . Signals can be acoustic signals, electromagnetic signals, etc. Examples of signals that can be provided by signal input unit  20  are acoustic echo signals reflected from fish, seabed, underwater rocks, etc. Signal input unit  20  may be one or more of any number of devices providing signal data. Signal input unit  20  may be, for example: a sensor; an electromechanical energy-converting device; an electro-acoustic energy-converting device; a transducer; a receiver; etc. The signal input unit  20  may be installed in a location where signals can be received. For example, the signal input unit  20  may be installed on the hull bottom of a ship, in a “look” forward position along a watercraft propulsion axis, etc. 
         [0027]    The underwater sounding apparatus  40  receives signal data from the signal input unit  20 , and processes signals in a manner discussed in detail below. The underwater sounding apparatus  40  processes signals and determines characteristics of underwater objects associated with the signals. A user may view outputs of underwater sounding apparatus  40 , including intermediate processing results of signals, via display  70 , and may input commands to the underwater sounding apparatus  40  via the user input unit  80 . In the embodiment illustrated in  FIG. 1 , the user input unit  80  includes a keyboard  83  and a mouse  86 , but other conventional input devices could also be used. 
         [0028]    In addition to performing processing of signals in accordance with embodiments of the present invention, the underwater sounding apparatus  40  may perform additional signal processing and preparation operations, in accordance with commands received from the user input unit  80 . Such signal preprocessing and preparation operations may include known operations for signal amplification, quantization, compression, frequency correction, etc. 
         [0029]    The printing unit  50  receives the output of underwater sounding apparatus  40  in various forms, such as in the forms of graphs of signal amplitudes, graphs of frequencies, geometric data related to underwater objects, etc., and generates a hard copy of the processed signal data. The printing unit  50  may be, for example, a conventional color laser printer, a black-and-white printer, etc. In addition or as an alternative to generating a hard copy of the output of the underwater sounding apparatus  40 , the processed signal data may be returned as a file, e.g., via a portable recording medium, a CD-R, a floppy disk, a USB drive, or via a network (not shown). The display  70  receives the output of the underwater sounding apparatus  40  and displays signal data such as, for example, frequency graphs, waveforms, etc. The output of underwater sounding apparatus  40  may also be sent to signal output unit  60 . Signal output unit  60  may be, for example, a database that stores signal processing results received from underwater sounding apparatus  40 ; an application that uses signal processing results from underwater sounding apparatus  40  to determine characteristics of underwater objects, such as length and size of fish, quality of seabed, distance to seabed or fish, etc. 
         [0030]      FIG. 2  is a block diagram illustrating in more detail aspects of an underwater sounding apparatus  40  according to an embodiment of the present invention. As shown in  FIG. 2 , an underwater sounding apparatus  40  according to this embodiment includes: a signal amplification module  110 ; a signal conversion module  120 ; a signal selection module  130 ; and a filtering and outprocessing module  140 . Although the various components of  FIG. 2  are illustrated as discrete elements, such an illustration is for ease of explanation and it should be recognized that certain operations of the various components may be performed by the same physical device, e.g., by one or more circuit boards or microprocessors. 
         [0031]    Generally, the arrangement of elements for the underwater sounding apparatus  40  illustrated in  FIG. 2  receives signals from signal input unit  20 , performs amplification, conversion, and selection of signals, filters signals, and outputs a signal. Signals received from signal input unit  20  may be, for example, echo signals reflected by underwater targets. Such underwater targets include fish, seabed, rocks, etc. Signal data output by underwater sounding apparatus  40  may be, for example, waveforms obtained from echo signals that were received by underwater sounding apparatus  40 , digital data, etc. Underwater sounding apparatus  40  may output signal data to printing unit  50 , display  70  and/or signal output unit  60 . 
         [0032]    Signal amplification module  110 , signal conversion module  120 , signal selection module  130 , and filtering and outprocessing module  140  can be electronic systems and circuits, hardware systems, purpose built hardware such as FPGA, ASIC, etc., in exemplary implementations. Signal amplification module  110 , signal conversion module  120 , signal selection module  130 , and filtering and outprocessing module  140  may also be software systems/applications, or a combination of software and hardware systems. In one exemplary implementation, signals are digitized at signal conversion module  120 , and the succeeding units, that include signal selection module  130  and filtering and outprocessing module  140  are implemented as a software application. Operation of the components included in underwater sounding apparatus  40  illustrated in  FIG. 2  will be next described with reference to  FIGS. 4-7B . 
         [0033]      FIG. 3A  is a block diagram illustrating conventional technology for processing of signals from underwater targets. A conventional technology apparatus  201  for processing of signals from underwater targets includes: a sensor  211 ; a low-gain amplifier  213 ; a high-gain amplifier  215 ; a selector  217 ; a discontinuous compensation unit  219 ; a bandpass filter (BPF)  221 ; a gain compensation unit  227 ; a keying pulse unit  223 ; a timer  225 ; and a Time Variable Gain (TVG) control unit  229 . 
         [0034]    The dynamic range of a conventional receiving amplifier or of an A/D conversion device intended for general use is not wide enough for use in underwater target observation. As a result, it is hard to obtain a wide receiving dynamic range for underwater target observation, using a receiving amplifier or an A/D conversion device. 
         [0035]    The echo signal reflected by a target decreases gradually in intensity according to the distance traveled by the echo signal from the target, or according to the time interval needed for the echo signal to travel back from the target. To improve the dynamic range for echo signals, two receiving-amplifiers with different gains may be used, instead of a single receiving-amplifier. As shown in  FIG. 3A , a conventional technology uses two such receiving-amplifiers. The gain of the low-gain amplifier  213  is lower than the gain of the high-gain amplifier  215 . 
         [0036]    For a given echo signal received from a target, a receiving-amplifier is selected from among the low-gain amplifier  213  and the high-gain amplifier  215 , based on the distance to the target or the travel time of the echo signal from the target. The distance to a target is determined based on the time it takes for a sound pulse from a source, such as a ship, to bounce off a target and return as an echo signal to the source. Typically, the low-gain amplifier  213  is selected for strong echo signals, and the high-gain amplifier  215  is selected for weak echo signals. Hence, the low-gain amplifier  213  is generally selected for echo signals from near field (close-by targets), because such echo signals are strong. On the other hand, the high-gain amplifier  215  is generally selected for echo signals from far field (far away targets), because such echo signals are weak. The selection of a receiving-amplifier from among the low-gain amplifier  213  and the high-gain amplifier  215  is performed using a signal output from timer  225 . Timer  225  counts a time interval from a time reference of a keying pulse from keying pulse unit  223 , to compensate for a propagation loss to a target. 
         [0037]      FIG. 3B  is a diagram illustrating operations for the conventional technology for processing of signals from underwater targets illustrated in  FIG. 3A . In  FIG. 3B , the X-axis is distance or time, and the Y-axis represents the input level of receiving amplifiers  213  and  215 . The dynamic range of low-gain amplifier  213  extends between points Y 2  and Y 3  on the Y-axis. Low-gain amplifier  213  is generally selected for strong echo signals. Strong echo signals are typically from near field (close-by targets). The range of near field extends between points X 1  and X 2  on the X-axis. 
         [0038]    The dynamic range of high-gain amplifier  215  extends between points Y 1  and Y 4  on the Y-axis. High-gain amplifier  215  is generally selected for weak echo signals. Weak echo signals are typically from far field (far away targets). The range of far field extends from point X 2 , to point X 3 , and further out along the X-axis. Points from X 2  further out on the X-axis correspond to larger distances and larger signal return times. 
         [0039]    Conventional sounding technology uses Time Variable Gain Control (TVG) to compensate for underwater propagation loss. Typically, for detection of an underwater target, a suitable TVG curve is selected. Next, a gain control is adjusted, for target detection. A TVG curve is set based on the response from a detection target, and a table may be used to set the TVG curve based on a propagation loss from the target. Since propagation loss depends on transmission frequency and on the detection target, a TVG curve can be obtained as a table on which each curve value is determined by transmission frequencies and a size of the detection target. For example, one TVG curve may correspond to 50 KHz-big fish, while another TVG curve may correspond to 200 KHz-small fish. In  FIG. 3B , a TVG curve, suitable for the propagation loss associated with signals from a big fish is selected. 
         [0040]    The conventional time selecting system illustrated in  FIG. 3A  can be effective when the TVG curve selected for a target corresponds to the strength of the echo signal reflected by that target. However, the conventional system illustrated in  FIG. 3A  encounters challenges and becomes ineffective for mixed echo signals that are reflected by two or more targets of different strengths, such as a small fish, a big fish, and a seabed, because one TVG curve selected for multiple targets may not be suitable for multiple echo signals received from multiple targets.  FIG. 3B  illustrates cases in which the conventional sounding technology is not effective in target detection. 
         [0041]    In  FIG. 3B , a TVG curve is set for a big fish. The switching timing (a change at point X 2 ) of amplifiers is determined according to this TVG curve. However, an echo signal reflected by a different target, such as a small fish, will have a small target strength, even if the small fish is spatially close (at near field) to the underwater apparatus  201 . Hence, the level of a received echo signal from a small fish located in the vicinity of the underwater apparatus  201  is below the dynamic range of the low-gain amplifier  213  before a switching timing of amplifiers occurs from X 1  to X 2 . Hence, the level of a received echo signal from a small fish located in the vicinity of the underwater apparatus  201  lies between points Y 1  and Y 2 . When the received echo signal level is below the dynamic range of the low-gain amplifier  213 , the signal-to-noise (S/N) ratio of the signal from the small fish becomes unacceptably low. 
         [0042]    Moreover, an echo signal reflected by a target such as a seabed, has a large target strength even if the seabed is spatially far away (at far field) from the underwater apparatus  201 . Hence, the level of a received echo signal from a seabed located far away from the underwater apparatus  201  is above the dynamic range of high-gain amplifier  215 , so the signal lies between points Y 3  and Y 4  after the switching timing of amplifiers occurs from X 2  to X 3 . If the received echo signal level is above the dynamic range of the high-gain amplifier  215 , the echo signal becomes saturated. 
         [0043]    Therefore, with the underwater apparatus  201 , only if an echo signal is reflected by a target with a medium target strength, such as a big fish, and coupled with a TVG curve, a receiving-amplifier (selected as either the low-gain amplifier  213  or the high-gain amplifier  215 ) will produce an output signal without a low S/N ratio and without saturation regions. 
         [0044]      FIG. 4  is a block diagram illustrating an underwater sounding apparatus  40 A according to an embodiment of the present invention illustrated in  FIG. 2 . As shown in  FIG. 4 , an underwater sounding apparatus  40 A according to this embodiment includes: a low-gain amplifier  413 ; a high-gain amplifier  415 ; A/D converters  417  and  419 ; a demodulator  421 ; a saturation detector  425 ; a selector  423 ; a band-pass filter (BPF)  427 ; a gain compensation unit  429 ; and a Time Variable Gain (TVG) control  431 . Low-gain amplifier  413  and high-gain amplifier  415  are included in a signal amplification module  110 A. A/D converters  417  and  419  are included in a signal conversion module  120 A. Demodulator  421 , saturation detector  425 , selector  423  and gain compensation unit  429  are included in a signal selection module  130 A. Band-pass filter (BPF)  427  and TVG control  431  are included in a filtering and outprocessing module  140 A. 
         [0045]    Sensor  411  is included in signal input unit  20 , and receives signals such as, for example, acoustic echo signals reflected from fish, seabed, underwater objects, etc. Sensor  411  may be, for example, a conventional electromechanical energy-converting device, an electro-acoustic energy-converting device, a transducer, a receiver, etc. Sensor  411  transmits received signals to high-gain amplifier  415  and low-gain amplifier  413 . 
         [0046]    High-gain amplifier  415  and low-gain amplifier  413  perform amplification of received signals. High-gain amplifier  415  and low-gain amplifier  413  may be electric or electronic circuits used for amplification. The gain of high-gain amplifier  415  is higher than the gain of low-gain amplifier  413 . High-gain amplifier  415  and low-gain amplifier  413  send amplified signals to A/D converters  419  and  417 . 
         [0047]    A/D converters  417  and  419  are analog-to-digital converter electronic or electric circuits that convert continuous (analog) signals to digital signals. A/D converter  417  sends a low-gain digital signal to selector  423 . A/D converter  419  sends a high-gain digital signal both to selector  423  and demodulator  421 . Selector  423  may be, for example, one of the functions programmed in an FPGA. Selector  423  can operate like a multiplexer, to select one of the output signals from A/D converter  417  or A/D converter  419 , based on instruction from saturation detector  425 . 
         [0048]    Demodulator  421  is an electronic or electric circuit used to recover information from the carrier wave of a signal. Demodulator  421  may be, for example, an envelope detector, a diode detector, a rectifier, a digital signal processor, etc. Demodulator  421  demodulates the high-gain digital signal and extracts a demodulated signal, such as, for example, an envelope signal, from the high-gain digital signal. 
         [0049]    The demodulated signal is sent to saturation detector  425 , which detects saturation regions in the demodulated signal. The saturation detector  425  detects a saturation part of the demodulated signal of the high-gain digital signal. Saturation detector  425  outputs a Yes/No flag to selector  423  and to gain compensation unit  429 , to indicate whether the input signal was saturated or not. The selector  423  typically selects the high-gain digital signal from the A/D converter  419 , unless the saturation detector  425  detects a saturation part in the demodulated signal. If the saturation detector  425  detects a saturation part in the demodulated signal, the selector  423  selects the low-gain digital signal from the A/D converter  417 . 
         [0050]    Saturation detector  425  may be, for example, an electric or electronic circuit including comparators, diodes, a comparator in an analog system, a function programmed in an FPGA in a digital system, etc. Saturation detector  425  sends the results of saturation detection to selector  423  and to gain compensation unit  429 . High-gain and low-gain selected signals from selector  423  are filtered by band-pass filter BPF  427  to remove system noise, and then sent to gain compensation unit  429 . Gain compensation unit  429  uses the filtered high-gain and low-gain digital signals together with saturation information from saturation detector  425 , to compensate for differences in high-gain and low-gain signal levels and obtain a smooth signal. Gain compensation unit  429  may include electric and electronic components such as resistors, potentiometers, amplifiers, adders, etc. Gain compensation unit  429  may be a register, a potentiometer, or the like in an analog system, one of the functions programmed in an FPGA in a digital system, etc. The output of gain compensation unit  429  is sent to TVG control  431 . TVG control  431  corrects for propagation loss. TVG control  431  may perform time variable gain control for the signal output from gain compensation unit  429  by, for example, providing a higher gain for signals that originated from objects at long ranges underwater, and a lower gain for signals that originated from objects at short ranges underwater. TVG control  431  thus compensates for larger acoustic propagation losses for signals arriving from longer ranges. TVG control  431  produces an output signal. 
         [0051]    The underwater sounding apparatus  40 A in  FIG. 4  outputs an unsaturated and fully magnified signal for an input signal. Although the underwater sounding apparatus  40 A is illustrated with two receiving-amplifiers (units  413  and  415 ), A/D converters ( 417  and  419 ) and a detector circuit (including the demodulator  421  and the saturation detector  425 ), the underwater sounding apparatus  40 A may include more than two receiving-amplifiers, A/D converters and more detector circuits associated with the receiving-amplifiers. 
         [0052]    The underwater sounding apparatus  40 A outputs a fully magnified and unsaturated signal regardless of the strength of the targets that produced the input signal at sensor  411 . Hence, underwater sounding apparatus  40 A has the effect of a wide dynamic range amplifier. 
         [0053]    The A/D converters  417  and  419  are optional. Filtering and outprocessing module  140 A is also optional. When A/D converters are used, converted digital signals are processed. In one exemplary embodiment, A/D converters are not used, analog signals are used instead, and signal selection module  130 A is implemented with hardware, instead of an FPGA. 
         [0054]    Sensor  411 , low-gain amplifier  413 , high-gain amplifier  415 , A/D converters  417  and  419 , demodulator  421 , saturation detector  425 , selector  423 , BPF  427 , gain compensation unit  429 , and TVG control  431  may be implemented using hardware and/or software. 
         [0055]      FIG. 5A  is a flow diagram illustrating operations performed by an underwater sounding apparatus  40 A according to an embodiment of the present invention illustrated in  FIG. 4 . Low-gain amplifier  413  and high-gain amplifier  415  receive (S 451 ) a signal from sensor  411 . The signal may be an acoustic signal, an echo signal received from an underwater object, etc. The low-gain amplifier  413  outputs (S 453 ) a signal S 1  amplified with low gain and sends it to A/D converter  417 . A/D converter  417  coverts (S 455 ) the signal S 1  amplified with low gain from analog to digital. High-gain amplifier  415  outputs (S 457 ) a signal S 2  amplified with high gain and sends it to A/D converter  419 . A/D converter  419  then coverts (S 459 ) the signal S 2  amplified with high gain from analog to digital. 
         [0056]    A/D converter  417  sends (S 461 ) the digital low gain signal S 1  to selector  423 . A/D converter  419  sends (S 463 ) the digital high gain signal S 2  to selector  423 . A/D converter  419  also sends (S 465 ) the digital high gain signal S 2  to demodulator  421 . Demodulator  421  demodulates (S 467 ) the signal S 2  from the A/D converter  419  and sends (S 467 ) a demodulated waveform S 3 , such as an envelope waveform, to saturation detector  425 . 
         [0057]    Saturation detector  425  detects (S 468 ) whether the demodulated waveform S 3  includes saturated regions, and outputs (S 469 , S 470 ) a Yes/No flag to indicate whether the signal S 3  has been saturated or not. Saturation detector  425  outputs the Yes/No flag to selector  423  and to gain compensation unit  429 . 
         [0058]    Selector  423  performs a test (S 471 ) to determine if there is saturation in signal S 3  at a time t 1 . If no saturation occurs at time t 1 , the selector  423  selects (S 473 ) the digital high gain S 2  signal at time t 1  for a selector signal S 5 . If saturation occurs at time t 1 , the selector  423  selects (S 475 ) the digital low gain signal S 1  at time t 1  for the selector signal S 5 . Selector  423  generates (S 479 ) selector signal S 5  for a time interval, by performing steps S 471 , S 473  and S 475  at multiple times in the time interval. The selector  423  next sends the selector signal S 5  to BPF  427 , which performs (S 482 ) bandpass filtering for the selector signal S 5 . The bandpass filtered selector signal S 5  is then sent to gain compensation unit  429 , which performs (S 484 ) gain compensation for the bandpass filtered selector signal S 5 . Gain compensation may, for example, change the gain of the high gain signal S 2  portions in the selector signal S 5  by a fixed factor, while keeping the low gain signal S 1  portions in the selector signal S 5  unchanged, so that the transitions between regions of the high gain signal S 2  and regions of the low gain signal S 1  in the selector signal S 5  are smooth. Gain compensation unit  429  outputs (S 488 ) a smooth gain compensated signal S 6  to TVG control  431 . TVG control  431  performs time variable gain control (S 490 ) for the gain compensated signal S 6 . TVG control  431  may perform time variable gain control by, for example, providing a high gain for signals that originated from objects at long ranges underwater, and a lower gain for signals that originated from objects at short ranges underwater. TVG control  431  thus compensates for larger acoustic propagation losses for signals arriving from longer ranges. To compensate for propagation losses, TVG control  431  may use a TVG curve selected beforehand by the user, for that signal. 
         [0059]      FIG. 5B  illustrates exemplary aspects of operations performed by an underwater sounding apparatus  40 A according to the operations illustrated in the flow diagram of  FIG. 5A .  FIG. 5B  illustrates exemplary aspects of operations for obtaining a gain compensated signal S 6 . 
         [0060]    In  FIG. 5B , the low-gain signal S 1  is the signal obtained from the low-gain amplifier  413 . The high-gain signal S 2  is the signal obtained from the high-gain amplifier  415 . The high-gain signal S 2  exhibits saturation between points N 501  and P 505 , and between points N 507  and P 509 . Demodulated signal S 3  is the signal obtained after demodulation of high-gain signal S 2  in demodulator  421 . Selector data S 4  illustrates status for a Yes/No flag that indicates whether the signal S 3  was saturated or not. The selector data S 4  is obtained by saturation detector  425  from demodulated signal S 3 , and tracks saturation of high-gain signal S 2 . The selector data  54  detects saturation between points S 503  and S 505 , which correspond to points P 503  and P 505  of high-gain signal S 2 . Consequently, selector  423  selects the low-gain signal S 1  between corresponding points M 503  and M 505  of the low-gain signal S 1 . Selector  423  selects the high-gain signal S 2  everywhere else. 
         [0061]    Gain compensation unit  429  also receives the selector data S 4 . In one exemplary embodiment, gain compensation unit  429  compensates the gain of the signal from the high-gain amplifier to match the signal from the low-gain amplifier. For example, gain compensation unit  429  performs gain compensation of the high-gain signal S 2  so that the transition points T 503  and T 505  between high-gain signal S 2  and low-gain signal S 1  occur at the same signal value. Gain compensation unit  429  outputs the gain compensated signal S 6 . 
         [0062]      FIG. 6  is a block diagram illustrating an underwater sounding apparatus  40 B according to a different embodiment of the present invention. As shown in  FIG. 6 , an underwater sounding apparatus  40 B according to this embodiment includes: a low-gain amplifier  413 ; a high-gain amplifier  415 ; A/D converters  417  and  419 ; a saturation detector  625 ; a gain compensation unit  629 ; a selector  623 ; a band-pass filter (BPF)  627 ; and a Time Variable Gain (TVG) control  631 . Low-gain amplifier  413  and high-gain amplifier  415  are included in a signal amplification module  110 B. A/D converters  417  and  419  are included in a signal conversion module  120 B. Saturation detector  625 , selector  623  and gain compensation unit  629  are included in a signal selection module  130 B. Band-pass filter (BPF)  627  and TVG control  631  are included in a filtering and outprocessing module  140 B. Filtering and outprocessing module  140 B is optional. 
         [0063]    Sensor  411  is included in signal input unit  20 , and receives signals such as, for example, acoustic echo signals reflected from fish, seabed, underwater objects, etc. Sensor  411  may be, for example, a conventional electromechanical energy-converting device, an electro-acoustic energy-converting device, a transducer, a receiver, etc. Sensor  411  transmits the received signals to high-gain amplifier  415  and low-gain amplifier  413 . 
         [0064]    High-gain amplifier  415  and low-gain amplifier  413  perform amplification of received signals. High-gain amplifier  415  and low-gain amplifier  413  may be electric or electronic circuits used for amplification. The gain of high-gain amplifier  415  is higher than the gain of low-gain amplifier  413 . High-gain amplifier  415  and low-gain amplifier  413  send amplified signals to A/D converters  419  and  417 . 
         [0065]    A/D converters  417  and  419  are analog-to-digital converter electronic or electric circuits that convert continuous (analog) signals to digital signals. A/D converter  417  sends a low-gain digital signal to selector  623 . A/D converter  419  sends a high-gain digital signal to both gain compensation unit  629  and saturation detector  625 . Saturation detector  625  detects saturated points in the high-gain digital sampling for the signal received from A/D converter  419 , and sends detection information to selector  623 . Saturation detector  625  may be an electric or electronic circuit including comparators, diodes, etc., may be a function programmed in an FPGA in a digital system, etc. 
         [0066]    Gain compensation unit  629  changes the gain of the high-gain digital signal received from A/D converter  419  to obtain a gain compensated digital signal which is compatible in magnitude with the low-gain digital signal from A/D converter  417 . Gain compensation unit  629  may include electric and electronic components such as resistors, potentiometers, amplifiers, adders, etc., may be a function programmed in an FPGA in a digital system, etc. 
         [0067]    Selector  623  receives the low-gain digital signal from A/D converter  417 , the gain compensated digital signal from gain compensation unit  629 , and the saturation detection information from saturation detector  625 . Selector  623  selects, for each sampling of a composite signal, data points from a sampling of the low-gain digital signal or from a sampling of the gain compensated digital signal. Instead of using a demodulator, selector  623  uses the detection information from saturation detector  625  to detect saturation parts in the gain compensated digital signal that resulted from the high-gain digital signal from A/D converter  419 . When the signal level outputted from the high-gain side is smaller than a saturation level, the selector  623  outputs the gain compensated digital signal from gain compensation unit  629 . When the signal level outputted from the high-gain side is larger than the saturation level, the selector  623  outputs the low-gain digital signal from A/D converter  417 . Selector  623  may, for example, operate like a multiplexer, may be one of the functions programmed in an FPGA, etc. 
         [0068]    In an exemplary embodiment, the phases of the receiving-amplifiers  413  and  415  are preferably the same, and the A/D converters  417  and  419  preferably perform a simultaneous sampling, so that the low-gain digital signal from A/D converter  417  and the gain compensated digital signal from gain compensation unit  629  are comparable at each sampling time. In this manner, underwater sounding apparatus  40 B avoids errors associated with saturated signal data points. 
         [0069]    The composite signal from selector  623  is next filtered by band-pass filter BPF  627 , and then sent to TVG control  631 . TVG control  631  corrects for propagation loss. TVG control  631  may perform time variable gain control by, for example, providing a high gain for signals that originated from an object at a long range, and a lower gain for signals that originated from an object at a short range. TVG control  631  thus compensates for larger acoustic propagation losses for signals arriving from longer ranges. 
         [0070]    Although the underwater sounding apparatus  40 B is illustrated with two receiving-amplifiers (units  413  and  415 ), A/D converters ( 417  and  419 ) and a detector circuit (including the gain compensation unit  629 , the saturation detector  625 , and the selector  623 ), the underwater sounding apparatus  40 B can comprise more than two receiving-amplifiers, A/D converters, and more detector circuits or detector circuit elements. The underwater sounding apparatus  40 B outputs a fully magnified and unsaturated signal regardless of the strength of the targets that produced the input signal at sensor  411 . 
         [0071]    Sensor  411 , low-gain amplifier  413 , high-gain amplifier  415 , A/D converters  417  and  419 , saturation detector  625 , gain compensation unit  629 , selector  623 , BPF  627 , and TVG control  631  may be implemented using hardware and/or software. 
         [0072]      FIG. 7A  is a flow diagram illustrating operations performed by an underwater sounding apparatus  40 B according to an embodiment of the present invention illustrated in  FIG. 6 . Low-gain amplifier  413  and high-gain amplifier  415  receive (S 651 ) a signal from sensor  411 . The signal may be an acoustic signal, an echo signal received from an underwater object, etc. The low-gain amplifier  413  outputs (S 653 ) a signal amplified with low gain and sends it to A/D converter  417 . A/D converter  417  coverts (S 655 ) the signal amplified with low gain from analog to digital to obtain a low-gain digital signal S 11 . High-gain amplifier  415  outputs (S 657 ) a signal amplified with high gain and sends it to A/D converter  419 . A/D converter  419  coverts (S 659 ) the signal amplified with high gain from analog to digital to obtain a high-gain digital signal S 12 . 
         [0073]    A/D converter  417  sends (S 661 ) the low-gain digital signal S 11  to selector  623 . A/D converter  419  sends the high-gain digital signal S 12  to gain compensation unit  629  and to saturation detector  625 . Saturation detector  625  detects (S 664 ) saturated data points in the high-gain digital signal S 12 , and sends (S 664 ) detection information to selector  623 . Gain compensation unit  629  changes (S 667 ) the gain of the high-gain digital signal S 12  received from A/D converter  419 , to obtain a gain compensated digital signal S 13  that is compatible with the low-gain digital signal S 11 . 
         [0074]    Selector  623  receives the low-gain digital signal S 11  from A/D converter  417 , the gain compensated digital signal S 13  from gain compensation unit  629 , and the saturation detection information from saturation detector  625 . Using the saturation detection information, selector  623  performs a test (S 671 ) to determine if a data point of the gain compensated digital signal S 13  is saturated. If the signal level of the gain compensated digital signal S 13  is smaller than a saturation level, the selector  623  selects (S 673 ) data of the gain compensated digital signal S 13  for a composite signal S 15 . If the signal level of the gain compensated digital signal S 13  is larger than or equal to a saturation level, the selector  623  selects (S 675 ) data of the low-gain digital signal S 11  for a composite signal S 15 . Selector  623  generates (S 677 ) composite signal S 15  for a time interval. 
         [0075]    The selector  623  then outputs the composite signal S 15  to BPF  627 , which performs bandpass filtering (S 682 ) for the composite signal S 15 . The bandpass filtered composite signal is then sent to TVG control  631 . TVG control  631  performs time variable gain control (S 690 ) for the bandpass filtered composite signal. TVG control  631  may perform time variable gain control by, for example, providing a high gain for signals that originated from an object at a long range underwater, and a lower gain for signals that originated from an object at a short range underwater. TVG control  631  thus compensates for larger acoustic propagation losses for signals arriving from longer ranges. 
         [0076]      FIG. 7B  illustrates exemplary aspects of operations performed by an underwater sounding apparatus  40 B according to the operations illustrated in the flow diagram of  FIG. 7A .  FIG. 7B  illustrates exemplary aspects of operations for obtaining a composite signal S 15 . 
         [0077]    In  FIG. 7B , the low-gain digital signal S 11  from A/D converter  417 , the high-gain digital signal S 12  from A/D converter  419 , and the selection for composite signal S 15  performed by selector  623  are shown. Gain compensation unit  629  inputs the high-gain digital signal S 12  and compensates the gain of the output signal to match the gain of the low-gain digital signal S 11 . 
         [0078]    When a data point of the high-gain digital signal S 12  is lower than a saturation level, that data point is selected for composite signal S 15 . Hence, points H 701 , H 703 , H 705 , and H 707  are selected from signal S 12  for composite signal S 15 . When a data point of the high-gain digital signal S 12  is equal to or above the saturation level, a corresponding data point from low-gain digital signal S 11  is selected for composite signal S 15 . Hence, points L 702 , L 704 , L 706 , and L 708  are selected from low-gain signal S 11  for composite signal S 15 , because H 702 , H 704 , H 706 , and H 708  from the high-gain signal S 12  are above the saturation level. 
         [0079]    The current application describes methods and apparatuses for processing underwater signals. The methods and apparatuses described in the current application provide a wide receiving dynamic range for underwater acoustic instruments. The methods and apparatuses described in the current application can be adapted to receive and process signals from one or more targets among targets of different strengths. 
         [0080]    The methods and apparatuses of the present invention can receive and process simultaneous signals from a plurality of targets of different target strengths. The methods and apparatuses of the present invention obtain signals without saturation and with good S/N ratio, by changing amplifiers according to the target strength. Hence, methods and apparatuses of the present invention obtain good output signals even when signals of different target strengths are received in one transmission. Hence, while in the operation of the conventional technology the switching timing (a change at point X 2  in  FIG. 3B , for example) of amplifiers is set to match the characteristics of a TVG curve, in the present invention the switching timing (a change at point X 2 ) of amplifiers varies automatically to match the input level of signals at the receiving amplifiers. 
         [0081]    The methods and apparatuses of the present invention provide highly accurate results for measurement of underwater object properties such as fish length, quality of seabed, etc. The circuit diagrams implementing the methods and apparatuses in the current application are more efficient and easier to implement than conventional systems, because measurements of distance or time intervals are not needed to obtain non-saturated signals for acoustic signals reflected from underwater objects. The methods and apparatuses described in the current application process signals received from underwater objects and output signals without switching errors or discontinuous points. 
         [0082]    Although the detailed embodiments described in the present application relate to processing of underwater signals, principles of the present invention may also be applied to other signals different from acoustic signals, originating in other media different from a water medium. 
         [0083]    Although detailed embodiments and implementations of the present invention have been described above, it should be apparent that various modifications are possible without departing from the spirit and scope of the present invention.