Reduced bandwidth processing for ultrasonic image conversion

[57] ABSTRACT

Disclosed is an ultrasonic, image conversion, high definition sonar having a multitude of transducer channels employing improved signal processing in each channel to reduce video bandwidth and switching speed requirements. Signal processing is characterized in each of the multitude of channels by detecting and holding the peak return from a transmit pulse for the remainder of the pulse project period, then holding that peak value for the subsequent pulse project period, during which time the value may be multiplexed for conversion to video signals for a CRT display, or may be applied to elements of an LED array.

Claims, 2 Drawing Figures

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PATENTEDJAH29 1974

SHEET 1 OF 2

3,789,350

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PATENTEDJAN29 1974

3.789,350

SHEET 2 OF 2

ONE SIGNAL PROCESSOR CHANNEL

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FIG.

3,789,350

REDUCED BANDWIDTH PROCESSING FOR ULTRASONIC IMAGE CONVERSION

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

This invention relates to ultrasonic image conversion as used in high definition sonar systems, and more particularly, to improvements in video processing therein which permit cathode ray tube visual displays and brighter light emitting diode displays, when used.

The carrying out of various underwater tasks by divers is often hindered by turbid water conditions which render a diver effectively blind if he must rely on an unaided view through his faceplate or mask. Lights and other optical aids are ineffective in a turbid environment because of backscatter from particulate matter. Chemicals and mechanical envelopes can be effective to clarify turbid water, but are subject to various limitations which restrict their use.

Because ultrasonic energy propagation is relatively unaffected by turbid conditions of water, efforts have been directed to development of ultrasonic means of generating or enhancing images of objects within the working range of a diver to enable him to carry on tasks such as hull inspection and repair, detection of various objects, rescue, and the like.

Patent application Ser. No. 296,500, filed in the U. S. Pat. Office on Oct. 4, 1972, and entitled Sonar Image Converter, by A. L. Rolle describes in considerable detail a diver or swimmer carried ultrasonic imageconversion visual display system which goes far toward overcoming the problems of viewing objects in turbid water. In that system, pulsed untrasonic energy is transmitted and target reflected energy is focused on an array of transducer elements located at the focal plane of an ultrasonic lens. The output of each element of the transducer array is amplified, band limited, and envelope detected in a separate channel. The detected output signal of each transducer element channel is processed and used to control energization of an LED (light emitting diode) in an array of LED’s corresponding in number and arrangement to the transducer array. The transducer and LED arrays are mounted on a common shaft for synchronized rotation and scanning of a viewing zone. The resulting visual display is an image of the target of interest rather than an indication of bearing and range, as in conventional sonar.

For successful direct viewing of the LED array, it is necessary that energization of the individual LED’s be for a long enough period, say 500 p.sec or longer, to provide adequate light to stimulate the eye. The envelope-detector output in the above system is approximately equal to the width of the projected pulse and typically ranges from 50 to 300 /xsec. Accordingly, the mentioned signal processing following the envelope detector of each channel serves to detect the peak value of the envelope and hold it for a period inversely proportional to the range of the target of interest or at least for some minimum period which would assure visual stimulation by the LED.

Now in some circumstances it would be desirable to utilize a CRT (cathrode ray tube) display in an ultrasonic image conversion sonar rather than an LED array. These circumstances would include situations where power and weight factors are not as restrictive as in the case of swimmer/diver use, for example, when mounted on a submersible vessel. With a CRT display, the output of each envelope detector would be sequentially gated to a single video amplifier by an analog multiplexer. The video amplifier would drive the Z or amplitude axis of the CRT. The number N of transducer elements can vary from approximately 30, as in the mechanically scanned array of the copending application, to possibly 10,000 elements in an extensive electronically scanned array. For use with a CRT it is desirable to sample all of the envelope detector outputs at least once for each projected pulse. If done in a period corresponding to a projected pulse width PW, the sampling time per sample then would be PW/N. For even small transducer arrays, the sampling time per sample would be extremely short and would require high power, wide-band sampling gates, drive logic, and CRTs. Sampling at high speeds also introduces significant errors due to the spikes generated in the signal channel by the rapidly changing drive logic voltages.

For extensive arrays with thousands of transducer elements, signal processing as used heretofore would require long projected pulse widths to facilitate within pulse scanning. Long pulse widths are undesireable in turbid water, where the need for ultrasonic imaging is the greatest, as the contrast between the target echo and volume reverberation decreases as PW increases.

SUMMARY OF THE INVENTION

With the foregoing in mind, it is a principal object of this invention to provide an improved ultrasonic image conversion sonar which is particularly useful in turbid water conditions.

Another object of the invention is to provide an image conversion sonar, wherein a cathode ray tube can be utilized as the visual display means.

Still another object is the provision of a high definition sonar having a transducer array in which the transducer elements may number in the thousands.

Yet another object is to provide an improved image conversion sonar employing signal processing means, such that elements of an LED display will be excited for a period which is independent of the range to the target, thereby increasing the apparent brightness of the display for very short pulse width projections.

As another object, the invention aims to provide an improved sonar system having one or more of the foregoing characteristics and which utilizes improved signal processing in each transducer channel whereby weight, power, and most importantly, bandwidth requirements are substantially reduced in the generation of images from relatively short pulse width ultrasonic projections.

Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration in block form of

3,789,350 an ultrasonic, image conversion, high definition sonar system embodying the invention; and

FIG. 2 is a diagrammatic illustration, in greater detail, of the signal processing portion of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the general organization of an exemplary ultrasonic image conversion sonar system employing the invention comprises a transducer array 10, including a number of transducer elements 12. The transducers 12, which may number from approximately 30 to 10,000 or more depending upon the use to which the system is put, are connected as shown by lines 14 to transmit and receive switches 16. The latter serve in the usual manner to direct transmission pulses received via lines 18 from a transmitter 20 to the elements 12, and to direct return signals detected by the elements 12 to appropriate channels of a multi-channel receiver 22 via corresponding lines 24.

The transmitter 16 and the receiver 22 are under the control of a system function generator 28 for transmission, range gate, and gain control functions as indicated by lines 30, 32, and 34, respectively. The operation of the system as described thus far may be regarded as the same as in the above mentioned application, to which reference may be had for further details.

The outputs of the channels of the receiver 22, indicated by lines 36 are in the form of signal envelopes 38, each representative of the return from a corresponding transducer element within the reception time dictated by the range gate.

The receiver outputs on lines 36 are applied to corresponding channels of a reduced bandwidth signal processor 40, the construction, purposes and functions of which will be described in more detail with reference to FIG. 2, as this specification proceeds. Suffice it to say for the moment that the envelope detected signals 38 are processed in the processor 40, under the control of signals represented by lines 42 and 44 from the function generator, to generate for each channel a signal level representative of the peak response of the associated transducer, which signal levels are available at the outputs 46 of the processor for sufficient time to be individually sampled by multiplexer gates 48 in accordance with an order of succession determined by a multiplexer logic 50. The latter may take various forms but conveniently comprises shift register means having outputs 52 under the program control of the function generator 28, as shown by line 54.

The sampled outputs of the multiplexer gates 48 are summed on line 56 for application as the input to a video amplifier 58, the output of which on line 60 drives the Z axis (brightness) of a cathode ray tube display 62. Sweep generators 64 are provided for developing a suitable raster on the CRT display at a frame rate determined by a sync signal output 66 of the function generator 28.

Referring now to FIG. 2, the signal processor construction and operation will be described in more detail, the description being confined to the operation of one channel with the understanding that it applies as well to the numerous parallel channels which would be employed. The incoming detected envelope on line 36 is applied to an operational amplifier 70 having its reference side connected through a capacitor 72 to ground, and having its output 74 connected through a diode 76 and line 78 to the reference side, as well as to the input of a buffer amplifier 80. This amplifier 70, capacitor, diode configuration will be recognized as a peak detection and hold circuit.

A transistor 82 has its emitter-collector circuit connected across the capacitor 72, and its base connected to receive a reset pulse 84 via line 42. In this regard, the reset pulse 84 is conveniently generated within the function generator 28 by a delay means in the form of a single shot multivibrator, or one-shot, 86. The oneshot 86 is triggered by the negative going characteristic of each of a train of clock pulses 88 on line 90.

The clock pulses 88 are also applied via line 44 to a gate means, conveniently in the form of a field effect transistor 92 which, when rendered conductive by the positive clock pulses, transfers the output of the buffer amplifier 80 to the input of an operational amplifier 94 and to a capacitor 96. The gate 92, amplifier 94, and capacitor 96 will be recognized as constituting a sample and hold circuit.

The output signal level line 46 from the sample and hold circuit is gated by a transistor 100 to the video amplifier input line 56 in response to a control pulse 102 from the multiplexer logic 50. In this regard, it should be noted that transistor 100 is only one of the plurality of gating transistors corresponding to the plurality of channels in the system and that point 104 is a summing node for the plurality of channel sample and hold levels gated by these transistors.

MODE OF OPERATION

In the operation of the system, an isonifying pulse having a width of say 10 p.sec to 100 ptsec is projected every T seconds, the period T being referred to as the project pulse period. The pulse projection is conveniently under the control of clock signals 88, which clock signals are also applied as shown by lines 90 and 44 to the one-shot 86 and to the gating transistor 92. Consider the pulse project period T to be equal to twice the maximum range of interest in feed divided by the velocity of sound c in the medium concerned, usually sea water. For example, if the maximum range of interest for a particular application is 10 ft., T is approximately 4 msec.

At a time during the period T, depending upon the range of a target of interest within 10 feet, a detected envelope will arrive on line 36 and will be peak detected and held by the action of amplifier 70, capacitor 72, and diode 76. At the end of the period T, the gating transistor 92 is rendered conductive for a period sufficient to sample the voltage level of buffer amplifier 80 which, of course, represents the peak detected and held value, and store it in the hold circuit formed by capacitor 96 and amplifier 94. As the last mentioned clock pulse 88 goes negative, the one-shot 86 is fired causing it to provide the pulse 84 which resets the peak detect and hold circuit in readiness for the next return signal. The response time of the one-shot 86 conveniently introduces sufficient delay to assure that the signal stored on capacitor 72 is not discharged until after the output level of buffer amplifier 80 has been transferred to the capacitor 96.

The capacitor 96 and amplifier 94 hold the sampled value for the following period T, during which time the multiplexer gate transistor 100 will be called upon to

3,789,350 release that value to the video amplifier input line 56.

Keeping in mind that the preferred projected pulse width is on the order of 10 /xsec and that the project pulse period T is on the order of 4 msec in the example given, the described processing gives an increase in time by a factor of 400 during which N channels can be sampled by the multiplex logic for CRT presentation. Accordingly, this allows employing of microwatt logic and analog gates and, because of the relatively low switching speeds, feed through of the logic switching voltages into the signal channels is significantly reduced.

If an LED display is desired where the brightness of the display is assured for all ranges, rather than having the brightness inversely proportional to range as in the aforementioned application, the output of each channel on line 46 may be applied to the corresponding LED.

Obviously, other embodiments and modifications of the subject invention will readily come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing description and the drawing. It is, therefore, to be understood that this invention is not to be limited thereto and that said modifications and embodiments are intended to be included within the scope of the appended claims.