EP2117221A2

EP2117221A2 - Device and method for generating a side tone

Info

Publication number: EP2117221A2
Application number: EP09004814A
Authority: EP
Inventors: Ingolf Beiten; Bernhard Kratzer
Original assignee: Eurocopter Deutschland GmbH
Current assignee: Airbus Helicopters Deutschland GmbH
Priority date: 2008-04-11
Filing date: 2009-04-01
Publication date: 2009-11-11
Also published as: DE102008018615B3; EP2117221A3

Abstract

The invention relates to a device and a method for generating a side tone (84), in particular for application in aviation and space technology. The device comprises a headset (50) with a microphone and earphones for the acquisition of an original signal (80) and for the reproduction of a side tone (84), a communication unit (43, 40) for connecting and controlling the headset (50), and a transmit/receive module (20). The device for generating the side tone (84) is characterised in that it furthermore comprises a comparator (60) which receives both the signal (81) that enters the transmit/receive module (20) and the signal (82) that is in fact being transmitted by the transmit/receive module (20), and in that it determines that the original signal (80) is actually being transmitted. In this case the comparator (60) instructs the communication unit (43, 40) to output the original signal (80) as a side tone (84) by way of the earphone of the headset (50).

Description

The invention relates to a device and a method for generating a side tone, in particular for application in aviation and space technology.
In aircraft, for example in helicopters or fixed-wing aircraft, it is common that speech signals which are to be transmitted are fed back to users by way of their headphones. On the one hand this has ergonomic reasons, because, due to the considerable noise level, users often cannot hear their own words, which generally leads to lack of certainty in speech communication. On the other hand this also involves safety-relevant aspects, for this method is in particular used in aviation in order to indicate to the crew that the speech signal to be transmitted is in fact being transmitted. This type of feedback is usually also required for aeronautical certification. Feedback is achieved in that in the final stage of the transmitter the signal to be transmitted is demodulated and is reflected to the earpiece of the headset. This reflected signal is generally referred to as a "side tone".
The fact that the signal to be transmitted is only tapped and reflected at the final stage of the transmitter of the transmit/receive module ensures that a side tone is only generated if the signal is in fact being transmitted. Consequently the crew is immediately informed about any possible problems in transmission.
However, this late reflection also results in delays being experienced between pickup of the speech signal and the reflected side tone, which delays are annoying to the crew, in particular to the person speaking, because the delays are perceived as a disagreeable "echo" that greatly impedes the flow of speech. Investigations in a language laboratory have shown that delays in the side tone exceeding 38 ms are perceived to be disagreeable.
This problem is further exacerbated if the picked-up speech signal is encrypted, for reasons of security, prior to transmission. In such cases upstream of the transmit/receive module an encryption module, also referred to as an encryption device, which encrypts the speech signal and due to the mathematical algorithms applied still further delays the signal flow. In special cases such delays can amount to approximately 85 ms in one signal direction. Since the side tone is only reflected at the final stage of the transmitter, said side tone passes through the encryption module twice, once on the way from the headset to the transmitter, and once more on the way back from the transmitter to the headphones. However, the resulting delays of additionally approximately 170 ms between the spoken signal and the side tone are untenable.
Even in non-encrypted operation, delays of the magnitude stated above have to be expected because for security reasons, in particular in military applications, it is undesirable to bridge the encryption module, because such bridging paths have a negative influence on the system's general safety from interception.
The present invention discloses a device and a method that make it possible to generate a side tone signal that has a very short delay time when compared to the original signal. In this arrangement delays in the order of less than 10 ms are made possible. Furthermore, it is ensured that, as is the case with the system from the state of the art, a side tone is generated only if the signal to be transmitted is in fact being transmitted by the transmitter. Finally, aspects of safety from interception are taken into account in that it is made impossible for the fed-back side tone to be able to be decoupled between the transmitter and the headphones and to be intercepted.
According to one aspect of the invention, a device for generating a side tone in aircraft is disclosed, which device comprises a headset with microphone and earphones for recording a signal that hereinafter is referred to as the "original signal". Furthermore, the device comprises an interface module, also referred to as the "operator device", for connecting and controlling the headset, and a transmit/receive module, also referred to as the "radio", for transmission of the original signal. Apart from a headset, other input/output devices are also imaginable, for example a loudspeaker by way of which other members in the cabin can monitor the pilot's conversation. The interface module usually forms part of an internal communication unit, also referred to as the "interface control device", within the aircraft, to which device the transmit/receive modules can also be connected. Hereinafter the interface module is considered to be the functional part of the communication unit. In contrast to this, the transmit/receive modules are considered to be units that are functionally separate from the communication unit, even if technically they can be implemented as plug-in modules of the communication unit. The communication unit can be an analog or a digital unit. Moreover, IP-based communication units are also imaginable. Depending on a particular application, a multitude of transmit/receive modules can be used, among others in the high-frequency (HF) range and/or in the very high frequency (VHF) or ultra high frequency (UHF) range. The signal flow in transmit operation is such that by means of the microphone of a headset an original signal, e.g. a speech signal, is recorded which, by way of the interface module and the communication unit, is forwarded to a transmit/receive module. In the transmit/receive module the signal is modulated and transmitted. In receive operation, by way of the transmit/receive module, a signal is received, demodulated and subsequently forwarded, by way of the communication unit and the interface module, to the headset, where said signal is output by way of the headphones/earphones/loudspeaker.
The device for generating the side tone is characterised in that in addition it comprises a comparator which receives not only the signal entering the transmit/receive module, which signal is hereinafter referred to as "the input signal", but also the signal in fact being transmitted by the transmit/receive module, which signal is hereinafter referred to as the "modulated transmit signal", and determines that the signal recorded by way of the microphone of the at least one headset is actually being transmitted. In this case the comparator instructs the communication unit to output the original signal as a side tone by way of the earphones of the headset. As an alternative, the comparator could also instruct the interface module, which is integrated in the communication unit, to which interface module the headset is connected, to output the original signal as a side tone by way of the earphones of the headset. Below, the general case is discussed in which the side tone is generated by the communication unit, which general case also includes the special case in which the side tone is generated directly by an interface module, in other words by a functional part of the communication unit.
In transmit operation the signal arriving at the transmit/receive module, in other words the input signal, generally is the acquired original signal, while the signal that is in fact being transmitted by the transmit/receive module, in other words the modulated transmit signal, is a modulated version of the recorded original signal. For processing within the comparator this modulated version of the original signal needs to be demodulated at the input of the comparator. As an alternative it would also be possible to use the transmit/receive module for demodulation of the signal that is in fact being transmitted by the transmit/receive module. Below, this modulation aspect is not given any further consideration; instead, reference is exclusively made to the (demodulated) transmit signal.
By means of the device described above it is possible to eliminate the problem of considerable delays between the original signal and the side tone because the side tone is generated directly in the communication unit from the original signal. At the same time the use of the comparator ensures that the side tone is generated only if the original signal is in fact being transmitted by the transmit/receive module.
According to a further aspect of the invention, the comparator comprises a multitude of signal processing components for processing the input signal and the transmit signal. By means of a filter stage a relevant frequency range can be isolated. If the original signals are speech signals, usually the speech frequency range of from 300 Hz to 3.4 kHz is of interest. With the use of filters, for example Bessel filters, signal components outside this frequency range can be cut out. By means of an amplifier stage, the signal levels of the input signal and of the transmit signal can be assimilated. In a preferred embodiment this is of importance, in particular, in the case of analog signals that are digitised in a subsequent comparator stage, e.g. in an analog/digital converter. If the signals are already digital signals, then in particular the comparator stage could be left out.
According to another aspect of the invention, the comparator comprises a correlator that determines the correlation between the input signal and the transmit signal. In this arrangement the two signals can previously have passed through the signal processing components described above. One way of determining the correlation between the two signals consists of deducting both signals from each other, and to use the differential signal itself or characteristic variables of the differential signal, e.g. its energy, as a measure of the correlation. The differential signal can be determined either block by block, i.e. in relation to signal sequences of a particular length, or continuously. For example, it would be possible to occupy two memory locations with blocks of scanning values of the input signal and of the transmit signal, and then to compare these, successively, scanning value by scanning value. The measured quantity of the difference of the two memory locations would then represent a correlation degree. This process could then be carried out sequentially for subsequent blocks of scanning values of the input signal and of the transmit signal.
In this arrangement it is important that any delays that the transmit signal has experienced relative to the receive signal due to passing through the transmit/receive module are taken into account. This can take place in that the delay caused by the transmit/receive module is determined in advance, and are then in the determination of the correlation taken into account by corresponding delay in the receive signal. Typical delay times of transmit/receive modules are approximately 5 ms. In a further embodiment the delay time could be determined during an initialisation process or continuously, on an automatic basis, by the correlator. For example, the correlator could shift the input signal and the transmit signal over a period of a few milliseconds, e.g. 10 ms, towards each other, and determine the delay time which returns the maximum correlation between the two signals.
Other methods to determine the correlation between the input signal and the transmit signal are also imaginable, in particular with a view to the robustness in relation to detecting that an original signal is in fact being transmitted. Thus, distortions in the transmit signal vis-à-vis the input signal could lead to challenges in determining the correlation. Since such distortions hardly have any influence on the signal envelope, it may be advantageous, as shown in Figure 1, to determine the envelopes 10 of the input- and transmit signal 11, and to determine the correlation of these two signals, e.g. according to the method shown above. As an alternative, it would also be possible to modulate the input signal and the transmit signal, e.g. with the use of a predictive speech model or a model based on frequency analysis, and then to determine the correlation of the signals by way of the correlation of the respective model parameters.
After the correlation between the input signal and the transmit signal has been established, it is possible, by way of a previously established or dynamic, i.e. adaptive, threshold value, to determine whether the input signal and the transmit signal are sufficiently similar, and whether it is thus ensured that the original signal is being transmitted. If this is the case, according to the invention the comparator instructs the communication unit to output the original signal to the earphones of the headset as a side tone. This instruction can take place by way of a binary control signal between the comparator and the communication unit, which control signal assumes, for example, the logical value "0" when the input signal and the transmit signal agree, i.e. when the original signal is in fact being transmitted, and assumes the logical value "1" when the two signals do not agree.
According to a further aspect of the invention, the device comprises various measures in order to ensure the device's fail-safe operation and safety from interception. In order to ensure that in the case of a failure of the comparator a side tone continues to be generated, the device can be operated in such a way that basically a side tone is generated by feeding back the transmit signal. Only in the case of a control signal with the logical value "0" is the side tone directly output by the communication unit to the earphones of the headset. In the case of a control signal with the logical value "1", feedback of the side tone takes place by way of the transmit/receive module. If the control signal on the comparator is forced to a logical value "1", by a pull-up resistor, in the idle state, it is ensured that the side tone is only generated in transmit operation, namely by the transmit/receive module in the case of a malfunction of the comparator, or by the communication unit in the case of proper functioning of the comparator.
This approach furthermore ensures that even in the initialisation phase, when, due to the signal transit times and due to possible build-up transients, the comparator has not yet been able to detect whether the transmit signal and the input signal agree, a side tone is nevertheless generated.
The device's safety from interception can be improved in that the comparator including the feedback is electrically separated from the other speech circuits in order to prevent bridging of other speech circuits. This is necessary, in particular, when on the other speech circuits signals that are classified as secret are transmitted.
According to a further aspect of the invention, the device further comprises, between the communication unit and the transmit/receive module, an encryption module for encryption of the signals exchanged by way of the headset. In this arrangement it is quite possible that the encryption module, as well as the transmit/receive module, forms part of the communication unit, and is, for example, implemented as a plug-in module. However, hereinafter, irrespective of their technical implementation the encryption module as well as the transmit/receive module is considered to be a functional unit that is separate from the communication unit. Encryption ensures that by means of an encryption method, a so-called cryptographic system, a clear signal is converted to an unintelligible signal that is resistant to interception. In this arrangement one or several keys is/are used as decisive parameters of encryption. A distinction is made between symmetric and asymmetric key methods, for example the Diffie-Hellman or the RSA method.
Apart from encryption modules, other modules, among others for example analog/digital converters, VoIP gateways, etc., can be connected between the communication unit and the transmit/receive module, which other modules can also have a negative influence on the delay time.
According to a further aspect of the invention, additional information relating to the operational state of the device is provided to the comparator. Among other things the communication unit or the interface module could provide information to the comparator, by way of a binary interface, to the effect that the headset is presently in transmit operation, i.e. that an original signal is being acquired and transmitted to the transmit/receive module. This is, in particular, advantageous in the case of push-to-talk (PTT) units, in which the person speaking occupies the speech channel by pushing a button, which speech channel is frequently operated in the half-duplex mode, with the person speaking thus ensuring that their speech signal is being transmitted. The act of pushing the PTT key can be communicated to the comparator, by way of a binary interface, as a sign of transmit operation. The comparator can use this information, and in particular when the comparator comprises a correlator can limit correlation between the input signal and the transmit signal to transmit operation. Prior signalling, by the communication unit or by the interface module, to the comparator, of transmit operation could be a prerequisite for the presence of a positive correlation and thus a logical value "0" of the control signal to the communication unit.
Furthermore, an encryption module could provide information to the comparator as to whether it operates in the encryption mode or in the clear mode, i.e. in the non-encrypted mode. As already discussed above, encryption modules experience considerable delays even in the clear mode because bridging the encryption mode is often not permitted for reasons of safety from interception. Consequently the function of the comparator could be limited to the clear mode so that in the case of a correlator being present the clear mode could be a prerequisite for the presence of a positive correlation, i.e. for a logical value "0" of the control signal to the communication unit. In another embodiment the information relating to the encryption mode could have an influence on the method used for determining correlation.
According to a further aspect of the invention, the comparator, and in particular the correlator, can be implemented as a separate hardware- and software component. A microprocessor can calculate the correlation between the input signal and the transmit signal. Moreover, the signals to be compared can be written into memory components and can be placed in intermediate storage. As an alternative, the comparator could be integrated, as an additional module, in already existing components. Since the comparator is advantageous, in particular with the use of encryption modules, due to the additional delay, it may be sensible to implement the comparator within the encryption module. This can be advantageous in particular in order to facilitate correlation of encrypted input- and transmit signals.
Below, the invention is described in more detail with reference to an exemplary embodiment. The following are shown:

Figure 1 an already mentioned illustration of a possible original-, input-, transmit-signal with envelopes;
Figure 2 a block diagram of a communication device with generation of a side tone from the state of the art;
Figure 3 a block diagram of a communication device with a comparator; and
Figure 4 a section of the block diagram of Figure 3.

Figure 2 shows a block diagram of a communication device with generation of a side tone from the state of the art. The headsets 50-54 comprise microphones and earphones. The headsets 50-54 are connected to the communication unit 40 by way of interface modules 41-43. As already stated above, the interface modules 41-43 are considered to be a functional part of the communication unit 40. On the transmit side, transmit/receive modules 20-24 are connected to the communication unit 40, optionally by way of encryption modules 30-32. As already stated above, the transmit/receive modules 20-24 and the encryption modules 30-32 can technically be implemented as plug-in modules of the communication unit 40; however, in this document they are considered to be units that are functionally separate from the communication unit 40.
As described above, an original signal is acquired by the microphone of a headset 50 and on the way to the transmit/receive module 20 passes through the interface module 43, the communication unit 40 and the encryption module 32. In the transmit/receive module 20 the side tone is generated in transmit operation, which side tone is then conveyed in the reverse direction to the earphones of the headset 50. Transit along this path results in delays of approximately 170 ms between the side tone and the original signal, which delays are perceived to be annoying by the user of the headset 50, and generally speaking considerably slow down the flow of speech.
Figure 3 shows a communication device with the comparators 60-62 according to the invention.
Figure 4 shows a section from Figure 3 with a headset 50, an interface module 43, a communication unit 40, an encryption module 32, a transmit/receive module 20 and a comparator 60.
An original signal 80, picked up by the microphone of the headset 50, passes through the interface module 43 and the communication unit 40. In the encryption module 32 the original signal is optionally encrypted. The control signal 91 indicates whether the encryption module 32 is in clear mode or in encryption mode. After passing through the encryption module 32, the original signal 80 becomes the input signal 81, which is then forwarded to the transmit/receive module 20. In the transmit/receive module 20 the input signal 81 is modulated and transmitted by way of the antenna 70. Furthermore, this modulated signal is reflected, subsequently demodulated and as a transmit signal 82 output at an interface of the transmit/receive signal 20. After the signal passes through the encryption module 32, in which the transmit signal 82 is optionally decrypted, the side tone 83 arises in this way, which side tone 83 is then output, by way of the communication unit 40 and the interface module 43, as a side tone 84, to the earphones of the headset 50.
Depending on whether the communication unit 40 is an analog or a digital unit, the signals 80, 81, 82, 83, 84 are analog or digital signals.
Figure 4 also shows an exemplary embodiment of the comparator 60, which in the present example exclusively comprises a correlator, which picks up the input signal 81 and the transmit signal 82, as well as the control signal 90 that indicates transmit operation, and the control signal 91 that indicates the encryption mode. In this arrangement the input signal 81 and the transmit signal 82 can pass through a number of signal processing components, e.g. a filter stage, an amplifier stage and a comparator stage.
By means of the methods previously presented, based on these four signals, it is determined whether the original signal 80 is in fact being transmitted by the transmit/receive module 20. In one embodiment this takes place in that it is checked whether the headset 50 is in transmit operation, i.e. whether the control signal 90 has a determined predefined binary value, and whether the decryption module 32 is in clear operation, i.e. whether the control signal 91 has a particular predetermined binary value.
Only if both conditions apply, is, in the present example, the correlation between the input signal 81 and the transmit signal 82 determined. The correlation is, for example, determined in that, gradually, defined blocks of scan values of the input signal 81 and of the transmit signal 82 are deducted from each other, and checks are made that the energy of the differential signal does not exceed a predefined threshold value. In this arrangement the delay between the input signal 81 and the transmit signal 82, which delay is caused by the transmit/receive module 20, is compensated for in advance. Of course, other methods, for example the methods described earlier in this document, can also be used to determine the correlation.
If in the above-mentioned case a correlation between the two signals is detected, then the control signal 92 is forced to assume the logical value "0" and is forwarded to the communication unit 40 or directly to the interface module 43 (this case is not shown). This control signal 92 then instructs the communication unit 40 or the interface module 43, as a functional part of the communication unit 40, to reflect the original signal 80 directly in the communication unit 40 as a side tone 84 to the earphones of the headset 50. In this case the side tone 83 generated by the transmit/receive module 20 is not taken into account.
In cases where, for example as a result of a malfunction of the comparator or of the correlator, no correlation between the input signal 81 and the transmit signal 82 has been detected, although the control signal 90 shows that the headset 50 is in transmit operation, then the control signal 92 retains the logical value "1", which is, for example, predetermined by a pull-up resistor. In these cases the communication unit 40 will forward the side tone 83, which is generated by the transmit/receive module 20, as a side tone 84 to the earphones of the headset 50.
In the present invention a device and a method for generating a side tone in aircraft has been disclosed, which device and method make it possible to prevent the occurrence of annoying delays between an original signal, in particular a speech signal, and the associated side tone. Even if encryption modules and other intermediate modules are used, it is thus possible to achieve delay times that are clearly below the audible threshold of approximately 38 ms, and furthermore to ensure that a side tone is generated only in the case of successful transmit operation. This is made possible by the introduction of a comparator according to the invention, which comparator determines whether the original signal is in fact being transmitted by the transmit/receive module, and in this case instructs the communication unit to generate the side tone directly from the original signal.

LIST OF REFERENCE CHARACTERS

10: Envelope of an exemplary embodiment of an original-, input-, transmit signal
11: Exemplary embodiment of an original-, input-, transmit signal
20, 21, 22, 23, 24: Transmit/receive modules
30, 31, 32: Encryption modules
40: Communication unit
41, 42, 43: Interface modules
50, 51, 52, 53, 54: Headsets
60, 61, 62: Comparators
70: Antenna
80: Origin signal
81: Input signal
82: Transmit signal
83: Side tone (between transmit/receive module and communication unit)
84: Side tone (between communication unit and headset)
90: Control signal (transmit operation)
91: Control signal (encryption mode)
92: Control signal (successful transmit operation/correlation)

Claims

A device for generating a side tone (84) in aircraft, comprising :
- a headset (50) with a microphone and earphones,

- a communication unit (43, 40) for connecting and controlling the headset (50); and

- a transmit/receive module (20);
characterised in that the device further comprises a comparator (60) which

- receives the signal (81) that enters the transmit/receive module (20);

- receives the signal (82) that is in fact being transmitted by the transmit/receive module (20);

- on the basis of the signals received determines that an original signal (80) acquired by way of the microphone of the at least one headset (50) is actually being transmitted; and

- in this case instructs the communication unit (43, 40) to output the original signal (80) as a side tone (84) by way of the earphones of the headset (50).
The device according to claim 1,
which, furthermore, between the communication unit (43, 40) and the transmit/receive module (20) comprises an encryption module (32) for encryption of the original signal (80).
The device according to claim 1 or 2,
wherein the comparator (60) further
- receives status information about the device, for example transmit operation (90) and/or encrypted/non-encrypted operation (91); and

- in addition uses this information in order to determine that the original signal (80) is actually being transmitted.
The device according to any one of claims 1 to 3,
wherein the communication unit (43, 40) is designed such that
- when it is determined that the original signal (80) is not being transmitted, or

- when a malfunction of the comparator (60) is determined,
the side tone (84) is fed by way of a back channel from the transmit/receive module (20) by way of the communication unit (43, 40) to the earphones of the headset (50).
The device according to any one of claims 1 to 4,
wherein the feedback (92) between the comparator (60) and the communication unit (43, 40), by way of which the comparator (60) instructs the communication unit (43, 40) to generate the side tone (84), is electrically separated.
A comparator (60) for use in a device according to any one of claims 1 to 5.
The comparator (60) according to claim 6,
wherein the frequency band of the signal (81) entering the transmit/receive module (20) and of the signal (82) that is in fact being transmitted by the transmit/receive module (20) is limited by a filter stage.
The comparator (60) according to claim 6 or 7,
wherein the signal (81) entering the transmit/receive module (20), and the signal (82) that is in fact being transmitted by the transmit/receive module (20) are standardised by an amplifier stage.
The comparator (60) according to any one of claims 6 to 8,
wherein the signal (81) entering the transmit/receive module (20), and the signal (82) that is in fact being transmitted by the transmit/receive module (20) are scanned by a comparator stage.
The comparator (60) according to any one of claims 6 to 9,
which comparator comprises a correlator that compares the signal (81) entering the transmit/receive module (20) with the signal (82) that is in fact being transmitted by the transmit/receive module (20), wherein the difference between the two signals represents a measure of the degree of correlation between the two signals.
The comparator (60) according to claim 10,
wherein the delay between the signal (81) entering the transmit/receive module (20) and the signal (82) that is in fact being transmitted by the transmit/receive module (20), which delay is caused by the transmit/receive module (20), is taken into account when the degree of correlation is determined.
A method for generating a side tone (84) in aircraft, with the method comprising the steps of:
- tapping the signal (81) entering a transmit/receive module (20);

- tapping the signal (82) that is being transmitted by the transmit/receive module (20);

- determining, on the basis of the tapped signals, that an original signal (80) picked up by way of a microphone is being transmitted by the transmit/receive module (20); and

- in this case instructing a communication unit (43, 40), which is used to connect and control the microphone and earphones, to output the original signal (80) as a side tone (84) to the earphones.