CA1095162A - Method and system for suppressing or identifying interference signals in sounding devices - Google Patents

Abstract

Abstract of the Disclosure Method of suppressing or identifying interference signals in sounding devices which includes receiving and summing signal pulses arriving within a given number of successive sounding periods at time inter-vals having a duration substantially equal to that of one sounding period, and feeding the voltage derived from the sum of the signal pulses to indicator means for providing an indication of the value thereof.

Description

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The invention relates to a method and a system for suppressing or at least identifying interference signals in acoustic depth finder or sounders.
It has been known heretofore to blank out interfering echoes by narrowing down the measuring interval within a sounding or repetition period (so-called "window" or gating circuit~ i.e. the receiving channel is kept open onl~v during a limited time span, within which the arrival of the useful or effective echo is to be expected, but is blocked during the remainder of the time. Since one must usually reckon with a change in the echo range in practice, even though the change may be gradual, (a change in the ~ater depth or the approach ;of a hori.zontally sounded or ranged target~, the narrowed-down measuring interval must be accommodated or matched slidingly to the time of arri~al of the next useful ech.o CGerman Patent DT-PS 2 204 3521. The technical term is "follower gate".
;~ follower gate, esp~cially, demands a considerable outlay of ci~rcuitry, as well as great ali.gnment or adjustment necessary. On the other hand, it offers only relatively little reliability against the indication of interference signals.
This is all the more the case since the selected length vf the measuring interval must obviously be great enough that the useful echo will fall reliably into this time period. Thus, these window or gate circuits always fail., if an interference signal falls into the time. span of the measuring in-terval.
It i.s an o~ject of the inventi.on -to provide a method and a system which.avoids the ~oregoing disadvantages of the heretofore kno~n methods and system of this general type. More specifically, i.t i.s an object of the invention to provide a method and system in which.the blanking or suppression or at . ~ .

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least the identification of interference signals is provided with considerably grea-ter assurance than heretofore.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a me-thod Eor suppressing or at least identifying interference signals in acoustic depth finders or sounders, in ~hich the signal pulses which arrive within a de~inite number of successive sounding periods are summed up at the time spacings or intervals of at least approximately one period, and the swm voltage is fed ko a display device (~depth recorder, cathode ra~ tube, digital display or the li.ke~ or is used as a control voltage to effect the display.
In accordance with the invention there is provided a circuit for suppressing ~nterference signals in echo sounding devices rece~v~.ng and summing signal pulses arr.iving from a recei,yer amplifler within a given number of n successive sound~
ing periods at time ~ntervals having a duration substantially equal to that of one sounding period, and for controlling an ;ndicator or reco.rder ~ith the summed signal, ~hich comprises:
2Q a~ an analog-to-d~.gital converter having an input connected to an output of said receiver amplifier and forming a square pulse of Predetermined amplitude from each echo signal ~hich. exceeds a predeterm~ned minimum level;
b~ n-l shift registers serially connected to an output of sa;d analog-to-digital converter;
c~ a summing devi,ce having n inputs respectively connected to an input of the. first of said serially-connected shift regis.ters and to outputs of all of said sh.ift registers;
d~ a threshold c~rcui`t connected to an output of said 3Q summing device for gene~ating an output pwlse ~s soon as the 5~

summed signal exceeds a predetermined threshold value; and e) an electronic switch having a control inpu-t connected to an output of sai.d threshold circuit, the switching path of said electronic switch being connected between said input o~
said analog-to-digital converter and said indicator or recorder.
By the method according to the inven-tion, the sum voltage is generated from only such signals which arrive in the rhythm of the.sounding sequence or pulse repetition frequency and which are the regular useful echoes. This holds true if, due to a gradual change in range or distance, a slight dis placement or shift of the useful echoes from one sounding to the next occurs. ~tatisti.cally occurring interference pulses, on the other hand, are not added or only very rarely added, according to the laws of possibility. They will therefore be correspondingly~small as compared to the. summed-up useful echo amplitude. But eyen periodically occurring interference signals are not added or

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summed up i~ their repetitlon lrequency devlates ~rom the sounding sequence of the echo sounder Or the invention There ls only one case wherein t;he method accordlng to the inventlon would fail i.e. if an echo sounder in the vicinity runs in exact synchronism with the echo soundin~
device of the invention. It is most unlikely that this will happen completely by chance i.e. without synchron_za-tion means. But even if this unlikely condition should occur~ it could be eliminated by a slight change of the sounding sequence or pulse repetition frequency o~ the system of the invention.

It is also conceivable that an interference pulse of such amplitude will be incident that it attains the magnitude of the summed-up useful echoes or e~ceeds it. So that also in this case the effectiveness of the method accord-ing to the invention cannot be nullified, a rectangul~r pulse of predetermined pulse amplitude is provided in accordance w~th another feature of the invention by means of an analog-to-digital converter, to each received signal which has a non-negligible signal amplitude and which, in particular, exceeds the noise level.

Since this rectangular pulse is thus independent of the original magn1tude of the incident or arriving slgnal, the magnitude of the voltage fed to an indicator or display device or to switching means ~ecting the indicator or display depends only on whether the signal was summed or on ho~ many tirnes the signal was summed. -

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~ ccording to the method o~ the invention described hereinbefore, the useful echoes would appear on the display device ~depth recorder or cathode ray tube) very strong black or bright, respectively, while the interference signals would come through so weak by comparison, that they would either not reach the thres~old for paper ~lackenLng or a strip chart, for example, of a depth recorder at all or would ~e recorded at most more-or-less weakly gray, ox would be visible on the screen of a cathode ray tube, for example, only very weakly.
The interference signals ~ould therefore be at least readily recognizable as such in comparison with the useful signals.
Ho~ever, a~ter such a strong contrast between the useful echo and the interference signal has been achieved by means o~ t~e ~ethod according to the invention, the interference signals, in accordance with a further feature of the invention, may also he ~uppressed entirely by simple means, i.e.
advantageously hy feedi.ng the sum voltage to the display device, or to the switching means ef~ecting th.e display, through a threshold circuit and by providing that for an n-fold summation of the signal pulses ci.e. for a summation over n sounding periodsl the level of the t~reshold is selected so that it lies at least between l~n times and 2/n times the pulse amplitude.
one ~ishes ~urther to take into account the case ~here on occasion, statistically occurring interference pulses could also at times be summed, then, in accordance with yet another feature of the invention, one can go beyond 2/n times with the threshold height and up to ultimately ~n-l~/n times the pulse amplitude.
Further in accordance with the inventi.on, there is provided in combination with an echo receiving device having an .

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amplifier for amplifying the received echo signal, a system for carrying out the method of the invention, wherein signal pulses of n successive sounding periods are summed, comprising an analog-to-digital converter connected to an output of the echo-receiving amplifier, n-l serially connected shift registers connected to an output of the analog-to-digi-tal converter, and a summing device having n inputs respectively connected to outputs of the n-l shift register and to the output of the analog-to-digital converter.
In accordance with another feature of the invention, the summing device has an output, and comprising a threshold stage connected to the output of the summing device, the threshold stage having an output, an electronic switch connected to the output of the threshold stage, an indicator, line means for connecting the input to the analog-to-digital converter to the indicator, the electronic switch being connected in the line means.
In accordance with a further feature of the invention, the threshold stage is a Schmitt trigger.
2Q In accordance ~ith an additional feature of the invention, the s~stem includes a timing pulse generator connected to the n-l shift registers for producing shift timing of the shift registers, and a shift timing pulse generator for producing additional shift timing of the shift registers, an O~-gate having inputs connected to an output of ~oth the timing pulse generator and the shlft timing pulse L6~

~enerator, the OR-gate having an output, and a shift timing line for connecting the output of the OR-gate to respectlve inputs of sh~ft re~istersO

In accordance with a concomitant feature of the invention, the echo-receiving device and the system are mounted in a sea-vessel, and means are included connected to an input of the shift timing pulse generator for ad-justing the timing frequency thereof so that it is a function of the speed of the sea-vessel ! ~ :
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Other features which are considered as characteristic for the invention are set ~orth in the appended claims.
Although the invention is illustrated and described herein as embodied in method and system for suppressing or identifying inter~erence signals in sounding devices, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and with~n the scope and range of equivalents of the claims.
lQ The construction and method o~ operation of the invention, however, togeth.er with additional o~jects and advantages thereof will ~e best understood from the following descripti.on of specific embodiments when read in connection with the accompanying drawings, in which:
Figure 1 is a voltage diagr~m and Figure 2 a block diagram of a ~ystem ~ccording to the invention for performing the meth.od of the inyention.
Re~erring no~ to the drawing, and first, particularly to Figure 1 t~ereof, there are shown useful echoes NEl, NE2 and NE3 from respective first, second and third soundings, and inter~erence signals Stl, St2 and St3 occurring during the respective sounding periods~ It is assumed, in the diagram o~
Figure 1, th.at the analog-to-digital conversion has already taken place. ~elow the sounding diagrams in Figure 1, the voltages at the. output o~ the summing device are shown. The sum o~ the use~ul ech.oes NEl, NE2 and NE3 is a pulse SNE, which, in the ca e at hand, has a pulse height or amplitude which is three times that o~ th.e gi.~en pulse heigh~t of the respective indivi.dual echo pulses~.
: 3Q ~s~ is evident from the diagram, a predetexmined . - .. . . . .

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displacement of the useful echoes is harmless as long as at least part of the width of each useful echo has exactly the spacing of one sounding period from a part of the o-ther useful echoes, i.e. if they overlap at least partially when viewed stroboscopically. Since the interference pulses arrive with other time intervals or spacings than that of one sounding period, they are not added.
The dash-dot line SW indicates the height of the threshold of a threshold circuit which is provided between the output of the summing device and the display device. As shown, this threshold is 1.5/n, where n = 3. This threshold provides assurance that the interference pulses Stl, St2 and St3, each of which has only the simple given pulse amplitude, become suppressed reliably.
The block dîagram of the system of the invention shown in Figure 2 matches the example of Figure 1, i.e. it is set up for a triple summation.
In the system of Figure 2, there is provided an analog-to-digital con~erter 1 of conventional construction, to which arriving and preamplified signals are fed. Also provided are two successi`ve shift registers 2 and 3, also of conventional construction wherein the signal pulses of a first and second sounding period are temporarily stored ~sequentially~, so that they appear at the Input of a summer 4 simultaneously with a signal arriving in a third sounding period, always assuming that these signals follo~ each other, respectively, at a spaclng of one sounding period.
If the output of the summer 4 were connected directly to the display device, the useful echoes would appear very strong ~n the recording, but the inter~erence signals would 3~ .

i2 appear quite weak at their highest.
In the embodiment of the system of the invention shown in Figure 2, the summer 4 is followed by a Schmitt trigger 5, of a construction well known in t:he art, which acts as a threshold stage, the height of the threshold being determined by a reference voltage Uref.

If a sum voltage appears which exceeds the threshold, the electronic switch 6 is switched into conduction and connects the display device 7 to the input of the analog-to-digital converter 1 through the line 8; this means that thejust arrived, preamplified signal, which was recognized as a useful echo by means of the aforedescribed system or circuit, is fed to the display device 7. Electronic sw;tches of this type are well known in the art and are believed therefore, not to require any further detailed description.
If a cathode ray tube is used as the display device, the electronic switch 6 could also be u~ed for unblanking the luminous spot.
A synchronizing or timing pulse generator 9 supplies shift timing pulses for the shift registers 2 and 3 through the line 10. It is started at the beg~nning of each sounding period by a trigger pulæe which is derived from the transmitted pulse or is synch.ronized ~ith the latter. After a number of cycles, which is equal to the number of bits of each of the shift registers 2 and 3, the timing pulse generator 9 sets the register inputs to "O" through AND gates 11 and 12.
In additi.on to the timing pulse generator 9, a shift timing pulse generator 13 is provided which feeds into the line la through.an OR gate 14, like the. timing puls.e generator .~ 3Q 9, and wh~ch is requi.red in case of a horizontal sounding ~ ~.3~ _9_ , ~S~2 operation. If a ship operating its sounder horizontally approaches a target at relatively high speed, the useful echoes shift so greatly from sounding to sounding that they are no longer addable or summable without difficulty. To take into account this advance or forward shift of the useful echoes, ~hich is proportional to the closing or approach speed~ the shift timing pulse generator 13 is made to deliver a number of additional shift timing pulses, which shift the stored echoes originating from the previous soundings along in the shift registers so much faster that they appear at the summing device

4 simultaneously with the echo of the instant sounding.
The shift timing pulse generator 13, which is of the same basic conventional construction as that of the timing pulse generator 9, receives its velocity information for example, from the sh~p's lo~.

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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Circuit for suppressing interference signals in echo sounding devices receiving and summing signal pulses arriving from a re-ceiver amplifier within a given number of n successive sounding periods at time intervals having a duration substantially equal to that of one sounding period, and for controlling an indicator or recorder with the summed signal, which comprises:
a) an analog-to-digital converter having an input connected to an output of said receiver amplifier and forming a square pulse of predetermined amplitude from each echo signal which exceeds a predetermined minimum level;
b) n-l shift registers serially connected to an output of said analog-to-digital converter;
c) a summing device having n inputs respectively connected to an input of the first of said serially-connected shift re-gisters and to outputs of all of said shift registers;
d) a threshold circuit connected to an output of said summing device for generating an output pulse as soon as the summed signal exceeds a predetermined threshold value; and e) an electronic switch having a control input connected to an output of said threshold circuit, the switching path of said electronic switch being connected between said input of said analog-to-digital converter and said indicator or recorder.

2. Circuit according to claim 1, wherein said threshold circuit is a Schmitt trigger having a threshold level lying between 2-times and n-times the amplitude of a single square pulse.

3. Circuit according to claim 1 or claim 2 which is mounted in a ship, including a first timing pulse generator connected to said n-l shift registers for producing shift timing of said shift registers, and a second timing pulse generator for producing correction pulses in dependence on the speed of a ship, an OR-gate having inputs connected to an output of both of said timing pulse generators, said OR-gate having an output, and a shift timing line connecting the output of said OR-gate to re-spective inputs of said shift registers.