Abstract:
To reduce signal interference and improve the signal-to-noise ratio in the transmitted signal, the analog signals produced by an ultrasound transducer are promptly digitized. In accordance with the invention, the demodulator is already shielded from the ultrasound transducers, immediately after the preamplifier, which is itself shielded from the ultrasound transducers, by an analog to digital converter, for example.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application claims the priority of German Patent Application, Serial No. 10 2004 035 950.4, filed Jul. 23, 2004, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is/are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention pertains to a signal processing apparatus for ultrasound transducers, and to an ultrasound receiver apparatus. More particularly, the present invention directed to improving the signal received by such apparatus.  
         [0003]     Ultrasound receiver may be used, for example, for non-destructive materials testing of tubes, wires or belts, by generating an ultrasound wave in the test object, e.g. by means of a piezo-electric test head or, for non-contact applications, by means of an electromagnetic acoustic transducer (EMAT). The emitted ultrasound signal is then picked up by the ultrasound receiver and converted into an output signal. Information about flaws in the material of the test object is detected in the pattern of the output signal. However, for EMATs, the problem from the beginning has been that the amplitude of the ultrasound signal produced is very small, because an EMAT has about 10% of the effectiveness of a piezoelectric transducer. The remedy has been to use costly, more powerful transmitters.  
         [0004]     In particular, the ultrasound receivers used for non-destructive testing are beset by serious environmental signal distortion problems. Strong interference signals caused by the drives of the rolling frame in a rolling mill are generated during testing the material. These interference signals interfere also with the ultrasound signal, and with the processing of the ultrasound signal into an output signal. Such interference effects reduce the interpretability of the detection signal. In some instances, this makes meaningful analysis of the signal impossible.  
         [0005]     In the past, attempts were made to shield the analog electric equipment following the ultrasound transducers against such interference. However, such shields are costly. Moreover, the installation space is limited in immediate proximity to the ultrasound transducer in many areas of their application. For that reason, the ultrasound transducer is typically separated from the subsequent analog electrical detection circuits. As a result of these inalterable spatial circumstances, it was necessary to date to connect the ultrasound transducer to the electronic evaluation assembly by a cable of a length of up to 30 m. Still these cables, as well as the electronic evaluation assembly, are exposed to strong interference signals in their working environment so that it has not been possible to date to generate signals having a large separation between the signal- and interference and a large signal-to-noise ratio.  
         [0006]     It would therefore be desirable and advantageous to provide an improved ultrasound receiver to obviate prior art shortcomings and to exhibit great signal-to-noise ratio and great wanted-to-unwanted signal ratio.  
       SUMMARY OF THE INVENTION  
       [0007]     According to one aspect of the present invention, a signal processing apparatus for an ultrasound transducer, includes a first preamplifier for amplifying an analog signal produced by an ultrasound transducer to generate an amplified analog signal, and a first analog-to-digital converter directly connected to the preamplifier for producing a first digital signal from the amplified analog signal  
         [0008]     The present invention resolves prior art problems by realizing a prompt digitalization of the analog signal produced by the ultrasound transducer. In accordance with the invention, the digitalization is achieved already between the ultrasound transducer and the demodulator, suitably through an analog-to-digital converter, which is directly connected to a preamplifier connected to the ultrasound transducer.  
         [0009]     As a result of the prompt digitalization of the analog signal produced by the ultrasound transducer, the further signal processing performed by the ultrasound receiver produces a digital output signal that is less susceptible to interference signals. Furthermore, prompt digitalization permits an increase of the useful signal range. While conventional systems generate signals of about 65 dB dynamic range, it is possible for systems in accordance with the present invention to use signals having a 120 dB (signal dynamic 1,000,000/1) dynamic range. The signal&#39;s dynamic range can be increased even further in the future by speeding up the clock frequency and improving the response of digital components in the electronic evaluation assembly. The dynamic range made possible by the invention allows a simplification of the system by the absence of a controllable preamplifier and the use of only a constant preamplifier, because only rough adjustment of the analog signal produced by the ultrasound transducer to the dynamic range of the analog-to-digital converter is sufficient. All the rest of the signal processing is digital. The use of a constant preamplifier allows also simplification or elimination of interference-prone components, in particular analog components, so that signal disturbances can be reduced even prior to the analog-to-digital converter.  
         [0010]     The predominantly digital configuration of signal processing permits signal processing to be carried out with only a single power pack. The use of a single power pack is easy to shield without incurring high costs. Conventional analog signal processing units required various power packs to supply energy to the various components. As a consequence, the number of areas where interference can be introduced into the system increased, and the shielding for the numerous power packs incurred high costs so that oftentimes the power packs were not sufficiently shielded.  
         [0011]     The benefits of the invention are mainly realized by the configuration of the signal processing apparatus and primarily independent of the type of ultrasound transducer used. For reasons of completeness, the invention will however mainly be described with reference to an ultrasound receiver that comprises an ultrasound transducer and the signal processing apparatus according to the configuration of the invention, not limiting the disclosure of the invention to such ultrasound receivers, however.  
         [0012]     According to another feature of the present invention, the ultrasound transducer may be implemented as an electromagnetic acoustic transducer (EMAT). Of course, the use of a piezo-electric transducer or other suitable transducer may also be conceivable instead.  
         [0013]     According to another feature of the present invention, the analog-to-digital converter has at least a 14-bit resolution and at least a 50 MHz clock frequency, preferably an 80 MHz clock frequency. Selection of an output and clock frequency requires an optimal compromise between equipment cost, which increases as the resolution increases, and the resolution. Increasing the resolution by one bit doubles the resolution of the converter, whereas a doubling of the clock frequency merely realizes an enhancement of √2. An increase of the resolution allows production of precise signals, even when the clock-frequency is lower, compared, for example, to an analog-to-digital converter having a resolution of 8 bit and a clock frequency of 500 Mhz, even though such analog-to-digital converters may certainly also be used in an ultrasound receiver according to the present invention.  
         [0014]     According to another feature of the present invention, the digital demodulator executes a Hilbert transform. An example of this type of demodulation is described, i.a., in the publication  Discrete - time Signal Processing  2nd edition, by A. V. Oppenheim and R. W. Schafer, Prentice-Hall, N.J., 1999. Hilbert transforms are sometimes also referred to as quadrature modulators.  
         [0015]     A Hilbert transform, or a transform very similar to a Hilbert transform is realized, when the demodulator has a so-called “quadrature numerically-controlled oscillator” (NCO), an I/Q mixer, a “cascaded-integrator comb” (CIC) filter, a “finite impulse-response” (FIR) filter, and a cartesian-to-polar converter. An explanation of FIR filters is available in the publication Halbleiter-Schaltungstechnik ( Semiconductor Control Technology ) by Tieze and Schenk, Springer Verlag, Heidelberg, 2002, 12 th  ed., at page 1031, et seq. I/Q mixers, as used as a preferred part of the demodulator, are described by Tieze and Schrank at page 1308, et seq.  
         [0016]     According to another feature of the present invention, the ultrasound receiver may include a median-filter. The median-filter may hereby be constructed in such a manner as to produce an output signal which is determined by median filtering a sequence of digital signals.  
         [0017]     In non-destructive material testing, a time-graded sequence of signals is sent out by the ultrasound transmitter. For example, 100 to 150 signals per second may be emitted. The ultrasound receiver takes those signals that come to it within a given time window, and converts them into an electric signal. As window, oftentimes the region is selected which is passed by the ultrasound wave propagating directly from the transmitter to the receiver through the test object up to a fixed time-limited cut-off. The signals produced by the ultrasound transducer thus reflects the passage of the ultrasound wave propagating immediately from the ultrasound transducer to the receiver through the test object, and the—later—passage of likely reflections of portions of the originally-transmitted ultrasound wave that spreads out in other directions. With respect to the median filtering, the next signal in the filtered sequence of signals is considered as the portion of the electric signal, produced by the ultrasound transducer, that appears in the window beginning, for example, with the passage of the next ultrasound wave which propagates directly from the transmitter to the receiver through the test object. The windows are defined in such a way as to have a comparable starting point in time, for example when the respective ultrasound signal is sent out.  
         [0018]     In testing metal bands, examining an area of a width of 2 m, a 400 μs window can be used and the next signal is sent after about 6 milliseconds. The response signal produced by the 400 μs window is transferred during signal processing to a row of 32,000 sampling points.  
         [0019]     Following demodulation, each signal is comprised of a row of values in which each subsequent value in the row represents the value of the signal at the next sampling points. The row has “m” values, wherein “m” is the number of samples recorded during the sampling window for generating an output signal.  
         [0020]     According to another feature of the present invention, the median filter may transfer the row of sampling values for a digitalized signal to an array. Another array is provided for the row of sampling values produced by the next signal. By providing similar beginning points for the sampling windows used for generating the output signal and the same sampling, corresponding samples are placed in the same locations in their respective arrays in comparison to the time point at which the sampling window opens.  
         [0021]     The median filter provides “n” arrays, such that “n” is the number of signals that the median filter evaluates, so that the result array of the median filter also provides places for “n” result values. For example “n” can be any uneven number, in particular: 3, 5, or 7. In filtering, the median filter reads out the values for one of the sampling times from the “n” arrays, sorts these values according to size, and outputs the value in the middle, that is, the “median” value. If n=3 then the median value is the value in the second place in the sorted set of values; if n=5, the third place, and so on. This middle value is then written into a results array which either provides the output signal, or can be processed to provide the output signal. The median filter filters out stochastic interference which is asynchronous with the return frequency.  
         [0022]     The ultrasound transducer may include a switched bypass for the median filter so that the signals can be processed without using the median filter. For example, this permits the action and effectiveness of the median filter to be analyzed.  
         [0023]     According to another feature of the present invention, the ultrasound receiver may have a connection to a computer network, such as an internet. Suitably, the connection may be an Ethernet connection, preferably one using the TCP/IP protocol. Optical data transmission can be provided within the computer network, whereby the computer system is galvanically isolated from the ultrasound system, and signal interference is further reduced.  
         [0024]     According to another feature of the present invention, the ultrasound receiver may be constructed to produce ultrasound signals which pass the ultrasound transducer in a same direction. This unidirectional testing prevents the ultrasound receiver from picking up erroneous signals, such as echo returns, in the opposite direction.  
         [0025]     According to another feature of the present invention, the ultrasound receiver may have a first ultrasound transducer producing a first analog signal that is amplified by a first preamplifier, and an analog-to-digital converter which is directly connected to the preamplifier for producing a digital signal from the amplified analog signal. A second ultrasound transducer produces a second analog signal that is then amplified by a second preamplifier, and a second analog-to-digital converter that is connected directly the preamplifier for producing a second digital signal from the second amplified analog signal. Also, the ultrasound receiver may be constructed with a first ultrasound transducer for producing a first analog signal, a first analog-to-digital converter for producing a first digital signal from the first analog signal, a second ultrasound transducer for producing a second analog signal, and a second analog-to-digital converter for producing a second digital signal from the second analog signal, and a demodulator for producing a demodulated digital signal.  
         [0026]     The ultrasound transducers may be arranged in spatially offset relationship, especially at a distance from each other that is at a quarter of the wavelength (λ/4) of the returned ultrasound signal that is picked up by the transducers. Advantageously, the first and second preamplifiers are constructed together as a single component. Suitably, the preamplifiers may be constructed as differential amplifier.  
         [0027]     The embodiments described above provide a send/return separation of 25 dB and, in some cases, 40 dB.  
         [0028]     In the embodiments described above, the unidirectionality is realized in particular by a multiplier for the first digital signal, a delay element for the second digital signal, and an adder for adding the digital output signal of the multiplier and the digital output signal of the delay element. The delay element unit can shorten the second digital signal by the time which corresponds to the distance of the second ultrasound transducer from the first ultrasound transducer at the frequency of the received ultrasound signal. Depending on the signal propagation direction, the signals, received in staggered sequence, are added constructively or, if they come from the opposite direction, added destructively, so that the signals substantially cancel out in a propagation direction.  
         [0029]     According to another feature of the present invention, the multiplier may be configured with adjustable multiplication factor, and/or may include a delay element with adjustable delay. An analyzer for monitoring the first and second digital signal may determine the multiplication factor and/or the delay to be adjusted. By adjusting the multiplication factor or the delay element, a lack of precision resulting from manufacturing variations in the construction of the ultrasound transducer can be compensated, for example in the separation of the two ultrasound transducers from the surface of the tested material, or in the pickup sensitivity of the respective ultrasound transducer. In particular, the analyzer can determine the delay being adjusted based on the relative position of the zero passages of the first and second digital signals with respect to each other. Normally, the delay is t/4, when the ultrasound transmitters are offset by a quarter wavelength (λ/4).  
         [0030]     According to another aspect of the present invention, a signal processing apparatus for an ultrasound transducer, includes a first analog-to-digital converter producing a first digital signal from an analog signal produced by an ultrasound transducer and a demodulator producing a first demodulated digital signal from the digital signal produced by the analog-to-digital converter  
         [0031]     According to yet another aspect of the present invention, an ultrasound receiver includes an ultrasound transducer adapted to produce an analog signal, a preamplifier for amplifying the analog signal to provide an amplified analog signal, and an analog-to-digital converter directly connected to the preamplifier for producing a digital signal from the amplified analog signal.  
         [0032]     An ultrasound receiver, as described above, can also be used as a receiver for receiving other AM-modulated signals. In particular, an ultrasound receiver in accordance with the invention can be used as receiver for receiving radar or radio signals, whereby a transducer is used instead of an ultrasound transducer so that radar or radio signals are used, instead of ultrasound signals.  
         [0033]     An ultrasound receiver is especially applicable for non-destructive material testing, and especially for the non-destructive testing of metals (bands, tubes and wires) during manufacture or further shaping operations of these materials. Machinery is hereby used which generate strong interference signals, so that conventional ultrasound receivers could not reliably provide a great signal-to-noise ratio and an optimal wanted-to-unwanted signal ratio. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0034]     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:  
         [0035]      FIG. 1  is a block diagram of a signal receiver in accordance with the present invention; and  
         [0036]      FIG. 2   a - d  show flowcharts of the operation of the median-filter of an ultrasound receiver in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]     Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.  
         [0038]     Turning now to the drawing, and in particular to  FIG. 1 , there is shown a block diagram of a signal receiver in accordance with the present invention, depicting an ultrasound receiver having two ultrasound transducers  1 ,  2 , and a signal processing unit  3  connected to the ultrasound transducers  1 ,  2 . The output of the signal processing apparatus  3  is an Ethernet connection  4  and a display  5  in the form of an oscilloscope trace  5 .  
         [0039]     The ultrasound transducer is preferably designed as an EMAT. The coil of the EMAT of the first ultrasound receiver  1  is offset at a quarter-wavelength distance (λ/4) from the coil of the EMAT of the second ultrasound receiver  2 . The first ultrasound receiver  1  generates a first analog signal HF0° which leads the analog signal HF90° produced by the second ultrasound receiver  2  by a phase shift of λ/4.  
         [0040]     The signal processing unit  3  has an analog portion  6  and a digital portion  7 . Even though these separate elements  6 ,  7 , of the signal processor  3  are shown in close proximity of each other in  FIG. 1 , they can (also within a given component), of course, be arranged so as to be spatially separated from each other, if available spatial conditions so dictate.  
         [0041]     The analog portion  6  of the signal processing unit  3  has a preamplifier  8 . The preamplifier  8  is designed as a so-called “dual-channel” preamplifier, and provides 5 to 70 dB of amplification. The preamplifier  8  outputs the first analog signal HF0° as an amplified analog signal. At the same time, the preamplifier  8  outputs the second analog signal HF90° as a second amplified analog signal. The first and second amplified analog signals are supplied to respective analog-to-digital converters  9 ,  10 , and converted there into first and second digital signals, respectively. The analog-to-digital converters  9 ,  10  have a clock frequency of 80 MHz and a resolution of 14-bit. Thus, the first and second digital signals are 14-bit signals.  
         [0042]     The first and second digital signals are fed to a directional evaluation apparatus  11  which generates a digital signal which assumes values other than zero only when the ultrasound wave passes the ultrasound transducer in a predetermined direction. For each ultrasound wave that passes the ultrasound transducers  1 ,  2 , in opposite direction, this digital signal outputted by the directional evaluation apparatus  11  assumes a zero value. The generated digital signal is a 16-bit signal.  
         [0043]     The digital signal thus generated by the directional evaluation apparatus  11  is then supplied to a down-converter  12  where it is demodulated from signal with 16 bit and 80 Mhz to a signal of 24-bit and 1 MHz. This filters the 16-bit signal from the carrier frequency, leaving only the useful signal that contains the test result information. A converter  13  is connected to the down converter  12  to change linear data to logarithmic data, by using tables, for example. The thus-generated 8-bit signal is supplied to a median-filter  14  and a switch  15 . The signal output by the median-filter  14  is supplied to the switch  15  as well. The output signal of the switch  15  is supplied to a converter  16  which changes logarithmic data to linear data. Output signals from the switch  15  and the converter  16  are supplied to a further switch  17 . The signal output by the switch  17  is supplied to a digital-to-analog converter  18  that can provide an analog signal to the display  5 . Furthermore, the output signal of the switch  17  is further supplied to a data processing module  19  which can execute a preliminary analysis of the data and can execute a data compression. The output of the data processing module  19  is supplied to the TCP/IP Ethernet connection  4 .  
         [0044]     The direction evaluation apparatus  11  includes a multiplier  20  which multiplies the first digital signal by a given factor. The direction evaluation apparatus  11  also includes a delay element  21  that introduces a given delay into the second digital signal, and an adder  22  by which the outputs of the delay element  21  and the multiplier  20  are added.  
         [0045]     The down converter  12  executes, at least approximately, a Hilbert transform. For that purpose, the down converter  12  has a component  23  with an I/Q mixer and a subsequent component  24  having a Cartesian to polar converter. A portion of the component  23  having the I/Q mixer is provided with an NCO and an FIR filter, whereas the component  24  includes a FIR filter.  
         [0046]     The signal processing unit  3  also includes a regulator unit  25  and a controller unit  26  for controlling the individual components of the signal processing unit  3 . A transmitter control unit  27  is also provided. The regulator unit  25  analyses the first and second digital signals in response to a trigger signal received from the transmitter control element  27  and commands from the control unit  26  regarding multiplication factors for the multiplier  20 , the delay for the delay element  21  and a switch setting that selects either automatic or manual control. The regulator unit  25  sets the multiplication factor for the multiplier  20 , the delay for the delay element  21 .  
         [0047]     The control unit  26  sends to the transmitter control element  27  information about the transmission frequency produced by the transmitter, the signal return rate for the signal being emitted, so as to generate a sequence of signals to be transmitted. The control unit  26  also forwards a “burst-count” signal to the transmitter control element  27 . The “burst-count” signal indicates how many bursts the EMAT produces in the ultrasound signal.  
         [0048]     The transmitter control element  27  controls the unillustrated ultrasound transmitter. In particular, the transmitter control element  27  sets the time at which the ultrasound signal is transmitted, the frequency of the transmitted ultrasound signal and the return frequency of the ultrasound signals transmitted.  
         [0049]     The control unit  26  controls the switches  15  and  17 , which are optional elements that can vary the mode of the signal transmission. The switch  15  permits data to be output either as raw data or, data filtered by the median-filter. Through analysis of raw data compared to the data filtered by the median filter, the effectiveness of the median filter can, for example, be checked. The switch  17  provides a choice between linear data output and logarithmic data output. The control unit  26  also controls the filter in component  23  and the FIR filter in filter component  24  of the down-counter  12 .  
         [0050]     For non-destructive materials testing, a sequence of ultrasound signals is sent through a test object from an ultrasound transmitter (not shown). For example, the test object may, for example, be a belt moving underneath the ultrasound transmitter. Suitably, the ultrasound signals are transmitted across the direction traveled by the belt. The ultrasound receiver is suitably arranged in an area of the ultrasound transmitter and picks up the transmitted ultrasound signal and its reflections as its input signal. The transmission of the next ultrasound signal by the ultrasound transmitter begins when the signal is transmitted by the ultrasound transducer. The delay between the individual signal produced by the ultrasound transmitter is selected so that the excitation of the test object by a first ultrasound signal has completely died away before the next signal sequence is sent. This prevents the ultrasound receiver from picking up overlapping signals from the first and second ultrasound signals. When checking belts, 100 to 150 ultrasound signals are sent per second. Sending more ultrasound signals per second is, of course, also conceivable, depending on the shape of the test object. This is the case, especially when test objects are involved which quickly extinguish the test signal, or which severely dampen the signal.  
         [0051]     Depending on the spatial arrangement of the first and second ultrasound transducers  1  and  2 , the ultrasound signal is picked up, for example, first by the ultrasound transducer  2  and then by the ultrasound transducer  2 , as the ultrasound signal passes by. The frequency of the ultrasound signal and the spatial arrangement of the ultrasound transducers  1 ,  2  are suited to one another so that distance between the ultrasound transducers  1  and  2  is a one-quarter wavelength delay (λ/4) of the ultrasound signal.  
         [0052]     The ultrasound transducers  1 ,  2 , preferably EMATs, convert the received ultrasound signal into a first analog signal HF0° and a second analog signal HF90°, and supply these two signals to the preamplifier  8 . This preamplifier  8  is constructed as a constant, rather than a variable preamplifier, so as to amplify these signals using a constant amplification factor. The amplified signals are then supplied to the analog-to-digital converters  9 ,  10 , which sample the analog signals at an 80 MHz sampling rate to produce two 14-bit digital signals.  
         [0053]     The direction evaluation apparatus  11  delays the second digital signal by a delay of t/4. During subsequent addition of the processed signals, the signals are added constructively, when the ultrasound signal passes the ultrasound transducers  1 ,  2  in the predefined direction. The signals cancel each other out, when the ultrasound signals passes the ultrasound transducers  1 ,  2  from the opposite direction. The amplitude of the first digital signal can be adjusted by the multiplier  20  such that they are zeroed out, when the ultrasound signal passes the ultrasound transducers  1 ,  2 , from the opposite direction.  
         [0054]     The digital down converter  12  carries out a Hilbert transform. The summed digital signal input to the down-converter  12  is demodulated and processed by the FIR filter in filter component  24 . The signal to be analyzed is then supplied by the down converter  12  to the converter  13  that changes the signal to a logarithmic signal so that the 24-bit signal is compressed into an 8-bit signal. The compressed 8-bit signal is supplied to the median-filter  14  and the switch  15 . The median-filter  14 , described in a more detailed manner below, produces an output signal that appears to be asynchronous with the return signal frequency, from “n” received input signals through the use of stochastic interference.  
         [0055]     The subsequent switches  15 ,  17  enable the user to selectively output either raw data in linear or logarithmic form, or median-filtered data in linear or logarithmic form. Representation of this output is possible either on the local display screen  5  or by transmission through the Ethernet connection  4  for use by another computer.  
         [0056]     Not only can the output signals be transmitted over the Ethernet connection  4 , but, in addition, control data can also be inputted into the signal processing unit  3  over this connection. For example, remote fault analysis over the internet is possible. There is also the possibility that the test data can be sent over long distances.  
         [0057]     The mode of operation of the median filter will now be described in more detail, based on a median filter that determines a median value from five values, as shown in  FIG. 2 . Of course, the median-filter unit may determine a median value from other numbers of values, for example, three or seven.  
         [0058]     The median-value filter  14  shown in  FIG. 2  has five arrays. Each array has “m” lines, wherein the value m corresponds to the number of samples of a signal. At the beginning of the measurement, all the arrays have, preferably, the value 0. During evaluation of the first received ultrasound signal, the first array (Ascan 1 ) is filled with values. When the evaluation of the first ultrasound signal is complete, the first array is full of values, and these values are all then shifted to the second array (Ascan 2 ). At the beginning of the evaluation of the second ultrasound signal received, the first array fills with new values originating from the analysis of the second ultrasound signal. At the end of the analysis of the second ultrasound signal, the values in the second array (Ascan 2 ) are all then shifted to the third array (Ascan 3 ), whereas the values in the first array (Ascan 1 ) are all shifted to the second array (Ascan 2 ). The result of the analysis of the third ultrasound signal is recorded in the first array (Ascan 1 ). The values previously recorded in each array having been shifted to the next array as before. This shifting of values proceeds as an on-going, continuous process Thus, at each time point, the results of the analysis of the next-to-last, third-last, fourth last, and fifth last ultrasound signals are found in their respective arrays (Ascan  2 -Ascan  5 ).  
         [0059]     The first array (Ascan  1 ) of the input arrays ( FIG. 2   a ) always contains the current actual sample values. The outcome array ( FIG. 2   d ) contains the result of the median-filtering process. The outcome array has the same number of lines as each of the individual input arrays. The value provided by each line in the outcome array is the result of median filtering each value in the corresponding line in each input array (Ascan 1 -Ascan 5 ). In  FIG. 2   b , a row selected from the input arrays shows values stored in a corresponding place in each of the individual input arrays. These values, shown in  FIG. 2   b , are rearranged in ascending order to show in  FIG. 2   c  that “6” is the median value in row. The outcome array is then filled with each of the median values thus determined for each subsequent new sampling time. With a shortened transmission phase, filling only the first three input arrays, a median-filtered output can be produced in real time.  
         [0060]     While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.  
         [0061]     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: