DE102018117032B4 - Device and method for determining a signal propagation time, as well as a system for monitoring a signal propagation time - Google Patents

Abstract

Device (10) for determining a signal propagation time (T) of an image signal (16) from an image source (12) to a display (14), the device having a signal combiner (18) which is designed to receive the signal from the image source (12 ) to combine the transmitted image signal (16) with a stimulus signal (20) which is designed to cause a change in a luminance value (L) of the image signal (16) so as to generate a modified image signal (22) which is sent to the display (14) is sent, and a measuring device (24) which is designed to measure a course of an electrical physical measured variable which is caused by a luminance of an image representation of the modified image signal (22) on the display (14), wherein the device is designed to detect a period of time between a first point in time (T1) at which the modified image signal (22) is sent and a second point in time (T2) at which the measuring device (24) receives a The change in the luminance of the image display caused by the stimulus signal (20) is detected, and the signal propagation time (T) of the image signal (16) is determined based on the first point in time (T1) and the second point in time (T2), and the measured variable is a performance variable of the display (14). and the performance variable is a power consumption of the display (14) or a current supplied to the display (14).

Description

The present invention relates to a device for determining a signal transit time of an image signal from an image source to a display, as well as a corresponding method. The invention also relates to a system for monitoring a signal propagation time of an image signal from an image source to a display.

In an operating room, many processes are only indirectly operated and controlled. This means, among other things, that the user, e.g. the surgeon, selects and coordinates his work on the basis of visual information that is shown to him on a display or display device. The surgeon no longer perceives information on a direct optical path, but sees, for example, an image on a display that was recorded by a video camera at the treatment site.

Since the light information from the treatment site no longer reaches the surgeon's eye via the direct optical path, direct perception of the actual situation is no longer guaranteed in the case of indirect operation. However, many operations, including actions, activities and actions that the surgeon performs during an operation, rely on reliable hand-eye coordination. Therefore, even with indirect operation, the delay between the actual situation and how it is perceived by the operator must be very short.

Even with an advanced system, which aims to achieve the best possible synchronicity between the actual and the perceived situation, it cannot be ruled out that in unfavorable constellations or in the event of an error, delays occur that make the operator's work more difficult or even impossible pose a risk to the patient.

For example, the following possible problems have been identified within the scope of the invention. For example, the image representation on the display could fail completely. One cause could be that image information is no longer being transmitted. On the other hand, the cause could also be that the image information reaches the display, but the display no longer shows it. There is also the possibility that the image will freeze. A frozen image is shown on the display, but continuously shows obsolete image information. It is also possible that the image representation on the display is considerably delayed. The delay can go so far that hand-eye coordination is difficult or no longer possible.

In addition, the inventor has recognized that the increasingly complex networks in an operating theater, in a network of operating theaters, in a hospital and in a network of hospitals lead to additional challenges. Today it is no longer sufficient to send an image signal from a video endoscope to a screen in the same operating room. Rather, today there is also a need to transmit an image signal from one operating theater to another, to transmit it to another area of the hospital, or even to transmit it to another hospital. The inventor has recognized that due to the large range and the heterogeneous network structures involved in it, an image signal can be routed via various routing systems and run through very different signal path combinations, which can also change.

For example, a camera sends its image signal via DVI to a network adapter, from there via an Ethernet-based route to a network player, which then forwards the signal via a display port to a display in a group of monitors. Several discrete transmission mechanisms and communication buses are connected in series, all of which would have to be monitored in terms of runtime in order to determine and monitor a signal runtime from the image source to the display.

This effort is very high in practice. Therefore, for practical reasons, it has so far been considered sufficient to measure and store the runtime for networks only once. Since the individual parts are serviced regularly, this should be sufficient to ensure that no noteworthy delays occur. However, the inventor has recognized that this is not always the case in practice, especially not when the system is running. In particular, wired and wireless networks were identified as possibly problematic because, due to their rather stochastic communication volumes or transmission capacities, they do not allow any reliable statements about the effective transmission speed and thus the runtime. For reliable monitoring it is therefore necessary to monitor such networks from node to node with regard to their latency. This requires a considerable technical effort.

US 2017 / 0 142 295A shows a display device including a signal detector configured to detect variations in an input video signal input to it; a video delay measuring part configured to measure a video delay time related to a video display operation based on the input video signal when the signal detector detects variations in the input video signal; a sound delay measuring part configured to measure a sound delay time related to processing a sound signal inputted thereto when the signal detector detects variations in the input video signal; and an audio delay processor configured to delay the audio signal based on the video delay and the audio delay time.

US 2007 / 0 091 207A shows a system including means for providing a video test signal to a display system; means for supplying an audio test signal to an audio system; and means for determining a delay between a video generated by the display system using the video test signal and an audio generated by the audio system using the audio test signal.

EP 1 758 387 A1 Figure 12 shows an amplifier that measures a display delay time caused until video is actually displayed after a video signal is generated and that automatically adjusts a delay time of an audio signal so that audio is synchronized with that display. Test video data is stored in a memory. This test video data is video data that produces a significant change in luminance in such a way that a screen suddenly changes from deep black to pure white. This test video is sent to a video display device, e.g. B. PDP, and a delay time is measured that is generated until the test video is actually displayed after that test video has been output. Since the luminance of the screen suddenly changes when the test video is actually displayed, this change can be detected by an optical sensor. The audio signal is delayed by setting this delay time in an audio signal processing part so that the video can be synchronized with the audio.

US 2014 / 0 307 107 A1 Fig. 13 shows an audio-video synchronization detection apparatus that detects an object-to-be-checked capable of generating an image signal and an audio signal. The audio-video synchronization detection apparatus includes a delay circuit, an optical sensor and a signal processor. The delay circuit delays the audio signal by a predetermined time and accordingly generates an audio correction signal. The optical sensor detects a light emitted from a display panel when the display panel displays the image signal and generates a corresponding image sensing signal. The signal processor calculates a delay time between the audio correction signal and the image pickup signal to obtain a synchronization state between the image signal and the audio signal.

It is an object of the present invention to provide an improved device and an improved method for determining the signal propagation time, as well as an improved system for monitoring the signal propagation time, with which effective monitoring is possible even in heterogeneous network structures.

According to one aspect, the object is achieved by a device according to claim 1.

One consideration in the present invention is to integrate the stimulus signal required for determining or monitoring the signal propagation time into the image signal, that is to say the useful signal that is already present, and thus to transport it along on the actual signal path. The stimulus signal is a change in a luminance value of the image signal. The modified image signal causes a recognizable reaction on the display, which can be recognized by measuring an electrical physical measured variable.

The electrical physical measured variable is related to the energy consumption of the display. This is because this depends on the luminance of the image shown on the display. While a high luminance, in which many pixels emit with a high intensity, requires a higher energy consumption, a low luminance, in which many pixels emit with a low intensity, has a lower energy consumption.

In this way, the stimulus signal fed in on the side of the image source can be detected within the modified image signal by measuring an electrical physical measured variable which depends on a luminance of an image representation on the display.

It is thus possible to determine and monitor the transit time from the image source to the display, even if the signal path leads through a large number of internal and external components. The determination and monitoring of the signal propagation time is possible even if individual components of the signal path can only transmit image information and not, for example, additional digital information such as a time stamp.

The luminance value of the image signal can be changed in various ways. If, for example, it turns out on the basis of practical tests that even small changes to the Luminance value on the side of the image source can be recognized on the display, it is preferred to change the luminance value of the image signal only in a part of a frame of the image signal or, if necessary, to change the luminance value of exactly one frame of the image signal. If one has to assume or determine on the basis of experiments that such a stimulus is not sufficient, for example due to attenuation along the signal path, the change in the luminance value is preferably carried out in two, three, four or more frames. Preference is given to subtracting already known delays along the signal path. In this way, a generally valid reference can be created in a simple manner, according to which a signal propagation time of zero means that no further delay has been added to the delay inherently present in the signal path anyway.

It should be noted that attenuation is to be assumed along the signal path. The attenuation can, however, be viewed as essentially constant regardless of the information content of the modified image signal. It is thus possible to take into account and subtract the attenuation along the signal path. Taking the attenuation into account then makes it possible to determine the actual signal propagation time even more precisely. A PID controller is preferably used here.

In particular, the jump response time for the route from the image source to the display is first calculated or determined experimentally. This serves as the basis for the actual signal propagation time. The proportional component can take into account constant delays, such as damping due to component capacities or power supply elements. The integral component can take into account slowly changing or short-term constant effects, such as damping that depends on changes in impedance. The differential component is then preferably used to determine the signal transit time, i.e. to determine the effect of the stimulus signal on the display.

The measured variable can also be understood as an analog measured variable. It is therefore not about digital content in an analog signal, but about the analog signal itself, which is recorded by means of the measured variable.

The power consumption of the display is measured directly or indirectly. With direct measurement, the power consumed is determined taking voltage and current into account. The current can also be measured using the magnetic field it generates. In the case of indirect measurement, the current is measured under the assumption that the operating voltage is essentially constant. In this case, the current is measured directly in some configurations and indirectly measured in other configurations, for example by means of a clamp meter. This refinement is based on the consideration that a change in the luminance of the image signal is reflected in a corresponding change in the power consumed.

In an advantageous embodiment, the power consumption is a primary-side power requirement of a power supply unit of the display.

The measurement can in particular take place directly via the power or indirectly via the current. Even if the attenuation by the power supply unit is likely to have a noticeable effect in this embodiment, but which can be determined and taken into account as described above, it is assumed that this solution is particularly easy to implement.

In a further advantageous embodiment, the power consumption is a secondary-side power requirement of a power supply unit of the display.

Here, too, the measurement can take place directly via the power or indirectly via the current. Since the secondary side of a power supply unit generally generates a direct voltage, the current flow can be detected here in particular by means of a Hall sensor. This is particularly advantageous when retrofitting, since the secondary-side power line can simply be enclosed by the Hall sensor without having to intervene directly in the electronics or the current path of the display. It is currently assumed that the stimulus signal can be easily recognized on the secondary side of the power supply unit, since fewer capacitive components or capacitive components with lower capacitance are involved here.

In a further advantageous embodiment, the power consumption is a power requirement of a display panel of the display.

In this embodiment, the stimulus signal is detected as close as possible, but still practicable, in front of the pixels of the display. Since the display supply of the display panel only contains a few components relevant to attenuation, it is assumed that the stimulus signal can be recognized particularly well.

In a further advantageous embodiment, the stimulus signal causes a significant change in the luminance value, preferably a change to a minimum or maximum luminance value, in particular a maximum change in relation to the luminance of the image signal.

In this embodiment, there is a significant change in the luminance value as a stimulus signal selected, ie the change is so great that the stimulus signal can be recognized with a predeterminable probability, for example 90%, 95%, 98%, 99%, 99.5% or more. This reliability can be determined and checked both mathematically and experimentally. Specifically, a change to a minimum or a maximum luminance value can also be selected in order to produce a reaction on the display that is as easily recognizable as possible. In particular, the minimum or the maximum luminance value can be selected as a function of which of the two values causes a maximum change with respect to the current luminance value of the image signal. The type of stimulus signal can be known in advance for recognition on the side of the display. The modified image signal arriving at the display can then be analyzed specifically with regard to an upward or downward deflection. Such knowledge is not required for other configurations. A general check is then made as to whether the modified image signal has a sudden change with regard to the measured electrical physical measured variable. In particular, in these refinements, the change in the electrical physical measured variable caused by the stimulus signal is detected without taking into account or without knowing the type of stimulus signal.

In a further advantageous embodiment, the device is designed to detect the change in the luminance of the image display caused by the stimulus signal, taking into account the stimulus signal.

If information about the stimulus signal is present on the display side, even if this information is not necessarily complete, the effect of the stimulus signal in the electrical physical measured variable can be determined more reliably. There is then also the possibility of designing the stimulus signal as a pattern, for example as a defined sequence of several similar intensity peaks or as a defined sequence of maximum and minimum intensity peaks, with the stimulus signal only being detected on the display side if the predetermined pattern is detected becomes.

In a further advantageous embodiment, the change in the dominance of the image display caused by the stimulus signal is barely or not perceptible from a human physiological point of view.

This refinement can in particular be achieved by keeping the stimulus signal so short that it is hardly or not perceived by the human eye. For example, it can be assumed that if the stimulus signal is less than 1/10 s, better less than 1/15 s or 1/20 s, a briefly inserted light or dark image is barely perceptible. However, it cannot be ruled out with certainty that sensitive users may perceive a flickering on the display. It is therefore considered advantageous to keep the duration of the stimulus signal less than 1/25 s, better less than 1/30 s or 1/40 s. The fact that a user does not perceive the stimulus signal or, if possible, not perceive it, can also be achieved or supported by keeping the distances between individual stimulus signals large enough. It is considered advantageous if a stimulus signal is used only at most every 20 frames, better still at most every 30, 40 or 60 frames. It is currently assumed that it is advantageous to keep the stimulus signal short if the distances between the individual signals are to be kept small. If, on the other hand, it is sufficient to work with greater distances between individual stimulus signals, then longer stimulus signals may also be barely or not perceptible at a greater distance.

In a further advantageous embodiment, the stimulus signal has a sequence of several changes in the luminance value of the image signal.

Such a refinement increases the reliability in the detection of the stimulus signal on the basis of the physical electrical measured variable.

In a further advantageous embodiment, the primary-side power requirement is measured by means of a plug-in module which is connected between a mains plug of the display and a socket for the mains plug of the display.

This configuration enables a particularly simple retrofitting for already existing displays.

In a further advantageous embodiment, the device evaluates a plurality of determined signal propagation times in order to determine an average signal propagation time, which is used as a target value for checking a determined signal propagation time.

Since the signal path can be very complex and possibly also change, e.g. in the case of connections via the Internet, a typical or average signal propagation time can be determined in practice by means of this configuration. On the basis of this average time, it can then be determined how far a currently determined signal propagation time deviates from the typical signal propagation time.

In a further advantageous embodiment, the image source is a video camera and / or the display receives the modified image signal via an HDMI or DisplayPort connection.

In this configuration, commercially available transmission options are used. The costs are therefore kept within reasonable limits. In particular, the image transmission does not have to be adapted to the displays, even if the image format as such does not transmit any additional information to the image data.

According to a further aspect, the object is achieved by a method according to claim 12.

According to a third aspect, the object is achieved by a system according to claim 13. The target value or the target range can either be specified on the basis of extrinsic specifications or can be determined by practical tests.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination, but also in other combinations or on their own, without departing from the scope of the present invention.

• 1 einen Verbund mehrerer Operationssäle;
• 2 ein System zur Überwachung einer Signallaufzeit;
• 3 einen ersten beispielhaften zeitlichen Verlauf von Luminanz und Leistung;
• 4 einen zweiten beispielhaften zeitlichen Verlauf von Luminanz und Leistung;
• 5 eine Ausführungsform eines Verfahrens zur Ermittlung einer Signallaufzeit;
• 6 eine erste Ausführungsform einer Messvorrichtung außerhalb eines Displays;
• 7 eine zweite Ausführungsform einer Messvorrichtung an einer Primärseite eines Netzteils des Displays;
• 8 eine dritte Ausführungsform einer Messvorrichtung an einer Sekundärseite eines Netzteils des Displays; und
• 9 eine vierte Ausführungsform einer Messvorrichtung in einer Zuführung zu einem Displaypanel des Displays.

Exemplary embodiments of the invention are shown in more detail in the drawing and are explained in more detail in the description below. Show it:

• 1 a network of several operating theaters;
• 2 a system for monitoring a signal propagation time;
• 3 a first exemplary temporal course of luminance and power;
• 4th a second exemplary time course of luminance and power;
• 5 an embodiment of a method for determining a signal transit time;
• 6th a first embodiment of a measuring device outside of a display;
• 7th a second embodiment of a measuring device on a primary side of a power supply unit of the display;
• 8th a third embodiment of a measuring device on a secondary side of a power supply unit of the display; and
• 9 a fourth embodiment of a measuring device in a feed to a display panel of the display.

1 shows a composite 100 of four operating theaters OR1 , OR2 , OR3 and OR4 . The association 100 is with a gateway 102 connected, which can be connected, for example, to the Internet, other parts of a hospital or to facilities outside the hospital. This is indicated by means of the three points.

In the example shown here, each is an operating room OR1 , OR2 , OR3 , OR4 with an appropriate router 104 , 106 , 108 , 110 equipped with the gateway 102 connected is. Inside the operating room OR1 , OR2 , OR3 , OR4 can connect to both wired networks 112 such as a LAN using Ethernet, and wireless networks 114 such as WLAN. The operating room OR2 assigns a gateway 116 , in particular a Piccard gateway, from which VNC connections originate. In the operating room OR3 is an example of a smartphone or tablet 124 over the wireless network 114 with the router 108 connected. The operating room OR4 has an adapter 120 on, from the DisplayPort connections 122 go out.

The composite shown 100 allows for example the image signal from an image source 12th , here a video camera, to a display 14th to send. Alternatively, it is possible to send the image signal to one of the other displays 15th to send. Based on the exemplary structure shown, it can be seen that reliable runtime monitoring is difficult due to the complexity of some signal phases.

2 shows an apparatus 10 for determining a signal propagation time of an image signal 16 from an image source 12th to a display 14th .

The device has a signal combiner 18th that is designed to receive that from the image source 12th generated image signal 16 with a stimulus signal 20th which is adapted to a change in a luminance value L of the image signal 16 so as to effect a modified image signal 22nd to generate that to the display 14th is sent.

The device 10 furthermore has a measuring device 24 which is designed to measure a profile of an electrical physical measured variable derived from a luminance of an image representation of the modified image signal 22nd on the display 14th is caused.

The device 10 is designed for a period of time between a first point in time T1 at which the modified image signal 22nd is sent, and a second time T2 to detect on which the measuring device 24 one by the stimulus signal 20th Caused change in the luminance of the image display is detected and based on the first point in time T1 and the second point in time T2 the signal propagation time T of the image signal 16 to determine.

The device 10 forms together with the image source 12th , the display 14th and a monitoring device 26th a system 28 for monitoring the signal transit time T. The monitoring device 26th is designed to be used by the device 10 to compare the determined signal transit time T with a target value or a target range and a warning signal when the target value is exceeded or the target range is left 30th to create.

3 shows a temporal course of the luminance L and one on the display 14th measured power P, once without a stimulus signal 20th and once with a stimulus signal 20th . The time t is plotted along the abscissa.

The first graph shows the time course of the luminance L, identified by the reference symbol 30th , as well as the envelope 32 this course. It is not a stimulus signal 20th available.

The second graph again shows the course over time from the first graph, but now for the points in time T1 and T1 ' one stimulus signal each 20th will be added. At the time T1 the luminance is reduced, in particular set to zero. At the time T1 ' the luminance is increased, in particular made maximal.

The third graph shows the power P over time on the display 14th , which results from the course of the luminance L, as shown in the first graph.

In comparison to this, the fourth graph now shows the course of the power P when the respective stimulus signal 20th from the second graph is used. It can be seen that in the course of the power P now at the points in time T2 and T2 ' clear rashes can be seen.

The difference between T2 and T1 or. T2 ' and T1 ' results in the signal propagation time T.

4th shows one opposite 3 modified design. All versions of the 3 also apply here. The respective stimulus signal 20th is now designed here as a pattern in which a reduced luminance is followed by an increased luminance, which in turn is followed by a reduced luminance. It can be seen that this pattern is also reflected in the time profile of the power P on the display 14th shows. The pattern enables more reliable detection of the stimulus signal 20th . In the example of the signal curve shown here, it can be seen that the signal propagation time T2 - T1 is less than in the example according to 3 .

5 shows a procedure 50 for determining a signal propagation time of an image signal 16 from an image source 12th to a display 14th . In step 52 becomes one of the image source 12th generated image signal 16 with a stimulus signal 20th combined, which is adapted to change a luminance value L of the image signal 16 so as to effect a modified image signal 22nd to generate that to the display 14th is sent. In step 54 the course of an electrical physical measured variable is measured, which is derived from a luminance of an image representation of the modified image signal 22nd on the display 14th is caused.

In step 56 will be a first point in time T1 detected at which the modified image signal 22nd is sent, and a second point in time T2 , on which one by the stimulus signal 20th caused change in the luminance of the image display is detected. Finally, in step 58 the signal propagation time T of the image signal 16 based on the first point in time T1 and the second point in time T2 determined, in some configurations as T = T2 - T1.

6th symbolically shows a display 14th with a display panel 60 , a power supply 62 , which is a primary 64 and a secondary side 66 has, as well as a cable 68 with a plug 70 for plugging into a socket 72 . In this embodiment, the primary-side power requirement is achieved by means of a plug-in module 74 measured that between the power plug 70 of the display 14th and the socket 72 for the power plug 70 of the display 14th is switched.

In 7th is a second embodiment of a display 14th shown, with the reference numerals from 6th continue to apply. The measuring device 24 is here on or in the primary side 64 of the power supply 62 arranged.

In 8th is a third embodiment of a display 14th shown, with the reference numerals from 6th continue to apply. The measuring device 24 is here on or in the secondary side 66 of the power supply 62 arranged.

In 9 is a fourth embodiment of a display 14th shown, with the reference numerals from 6th continue to apply. The measuring device 24 is here on or in an electrical feed to the display panel 60 arranged. Furthermore, the possibility is shown, additionally or alternatively, the output signal of a sensor 68 to tap the luminance of the image on the display 14th measures.

Claims (13)

Device (10) for determining a signal transit time (T) of an image signal (16) from an image source (12) to a display (14), having the device - a signal combiner (18) which is designed to combine the image signal (16) sent by the image source (12) with a stimulus signal (20) which is designed to change a luminance value (L) of the image signal (16) cause in order to generate a modified image signal (22), which is sent to the display (14), and - a measuring device (24) which is designed to measure a course of an electrical physical measured variable that is derived from a luminance of a Image representation of the modified image signal (22) is caused on the display (14), the device being designed to measure a period of time between a first point in time (T1) at which the modified image signal (22) is sent and a second point in time (T2 ) at which the measuring device (24) detects a change in the luminance of the image display caused by the stimulus signal (20), and on the basis of the first point in time (T1) and the second point in time (T2) to determine the signal transit time (T) of the image signal (16), and wherein the measured variable is a performance variable of the display (14) and the performance variable is a power consumption of the display (14) or a current supplied to the display (14). Device according to Claim 1 , wherein the power variable is a primary-side power requirement of a power supply unit (62) of the display (14). Device according to Claim 1 , wherein the power variable is a secondary-side power requirement of a power supply unit (62) of the display (14). Device according to Claim 1 , wherein the power quantity is a power requirement of a display panel (60) of the display (14). Device according to one of the preceding claims, wherein the stimulus signal (20) causes a significant change in the luminance value, preferably a change to a minimum or maximum luminance value, in particular a maximum change compared to the luminance value of the image signal (16). Device according to one of the preceding claims, wherein the device (10) is designed to detect the change in the luminance of the image display caused by the stimulus signal (20), taking into account the stimulus signal (20). Device according to one of the preceding claims, wherein the change in the luminance of the image display caused by the stimulus signal (20) is hardly or not perceptible from a human physiological point of view. Device according to one of the preceding claims, wherein the stimulus signal (20) has a sequence of several changes in the luminance value (L) of the image signal (16). Device according to Claim 2 , the primary-side power requirement being measured by means of a plug-in module (74) which is connected between a mains plug (70) of the display (14) and a socket (72) for the mains plug (70) of the display (14). Device according to one of the preceding claims, wherein the device (10) evaluates a plurality of determined signal propagation times (T) in order to determine an average signal propagation time which is used as a nominal value for checking a determined signal propagation time (T). Device according to one of the preceding claims, wherein the image source (12) is a video camera and / or the display (14) receives the modified image signal via an HDMI or a DisplayPort connection. Method (50) for determining a signal transit time (T) of an image signal (16) from an image source (12) to a display (14), the method with the steps - Combining (52) an image signal (16) sent by the image source (12) with a stimulus signal (20) which is designed to cause a change in a luminance value (L) of the image signal (16) in order to produce a modified image signal ( 22) that is sent to the display (14), - measuring (54) a course of an electrical physical measured variable which is caused by a luminance of an image representation of the modified image signal (22) on the display (14), - Detecting (56) a first point in time (T1) at which the modified image signal (22) is sent, and a second point in time (T2) at which a change in the luminance of the image display caused by the stimulus signal (20) is detected, and - Determination (58) of the signal transit time (T) of the image signal (16) on the basis of the first point in time (T1) and the second point in time (T2), the measured variable being a performance variable of the display (14) and the performance variable being a power consumption of the display (14) ) or a current supplied to the display (14). System (28) for monitoring a signal transit time (T) of an image signal (16) from an image source (12) to a display (14), the system (28) having a device (10) according to one of the Claims 1 to 11 , the image source (12), the display (14) and a monitoring device (26) which compares the signal transit time (T) determined by the device (10) with a target value or a target range, and when the target value is exceeded or the target range is left, a warning signal (30) is generated.

Images