Signal synchronization

Abstract

The present invention relates to a aystem for synchronizing signal transmission between two devices communicating through acoustic signals The first device including an acoustic transmitter transmitting in a chosen time sequence and frequency range and the second including an acoustic receiver. The first device is configured to initialize the communication transmitting an initial signal representing the time sequence, the time sequence being constituted by signals including information packages being separated by time windows varying according to a predetermined rule. The second device includes an analyzing unit being configured to apply a filter on the receiver corresponding to the time sequence within the predetermined time window variation.

Description

[0001]Signal synchronization. 145557NO The present invention is related to synchronization of devices using acoustic communication in an area possibly including other devices possibly transmitting interfering signals. More specifically, this invention is related to a method and system configured to synchronize transmission and receive processing of output and input signals respectively in acoustic systems consisting of input and output transducers where the input and output transducers are embedded in different electronic devices, or the input or output transducers are not being controlled by the same processing entity in the same electronic device. The present invention is primarily intended for use in systems where either input processing or output processing or both need to be duty-cycled due to limited resources (e.g. MIPS, MCPS, memory size, bandwidth, speaker protection, etc) or power saving reasons. However, it also applies to systems (i.e. systems without resource limitations or power concerns) where the input and output processing is done at regular or irregular intervals based on the needs of the input and the output processing of the specific use-case. Different types of Human Presence Detection solutions that transmits an ultrasound output signal and does corresponding receive processing in different computing blocks for input and output processing are well known, e.g. in WO2021 / 045628, WO2022 / 189141 and WO2023 / 079022. The solution shown in WO2023 / 079022 describes the use of a separate device transmitting an acoustic signal which may be received directly at another device as well as via a reflection from an object or user, while WO2024 / 107057 describes a solution where a movement triggers a distance measurement between two devices, preferably using an acoustic signal. If similar devices are in the vicinity using the same frequency range this solution may give erroneous responses because of interference, especially if the acoustic signals are transmitted simultaneously, which may handled by varying the time of the transmissions. The object of the present invention is to solve the abovementioned problems, especially for reducing or avoiding interference between two devices communicating using acoustic signals. This is obtained as defined in the accompanying claims. Thus, the present invention provides a solution where the transmitter and receiver are synchronized to avoid or reduce interference from other devices in the vicinity. In this document, we use the terms microphone and speaker, but it should be understood to include other devices that can receive and transmit acoustic signals too such as transducers, transducer arrays, microphone arrays, MEMS microphones, CMUT, PMUT, etc. The input and output devices discussed in this invention also include devices that can do input and output concurrently or alternating between these two modes (e.g MEMS devices). The invention is also relevant for electronic devices that include at least one accelerometer with a high sampling rate as an input device for acoustic signals provided the sensitivity of the accelerometers provides an acceptable SNR in the frequency range of the acoustic signal for the specific use-case. The present invention will be described below with reference to the accompanying drawings, illustrating the invention by way of examples. Figure 1a,b illustrates two different setups of the invention. Figure 2 illustrates an alternative configuration with two transmitter modules. Figure 3a illustrates the generation or a pseudo-random sequence. Figure 3b illustrates the time sequence. Figures 4-7 illustrates different communication sequences according to the invention. In most cases, the output signal is streamed from a digital signal processor (DSP) out to an amplifier modulating the signal into the speakers. In other situations, controlled streaming of the output signal is not possible due to audio system limitation or even required for power saving reasons. The alternative is to allow the amplifier or a companion processing unit to synthesis the signal itself and let the amplifier modulate the output signal into the speakers. Figure 1 illustrates the difference between these solutions. Figure 1 a) illustrates a system where Receive Module 1 and the Transmit Module 2 are collocated in the same processing entity with a connection 8 between them usually a wired connection. The processing modules are logical modules which may be part of the same software module 7 or not. The output signal is streamed from the Transmit Module 2 through the Amplifier 6, transmitting the acoustic signal into the transmitter, e.g. a loudspeaker 4. In this setup, the Receive Module 1 and Transmit Module 3 use the same clock, e.g. in the processor 7 and are tightly synchronized through the connection 8. The Receiver Module 1 includes a receiver such as a microphone 3. Figure 1 b) illustrates a system where the Receive Module 1 and the Transmit Module 3 are not collocated in the same processing entity. The Transmit Module 3 includes the Amplifier 6 and will do synthesis of the output signal on its own processor 9, transmitting the signal through the transmitter 4. In this setup, the Receive Module and Transmit Module may or may not use the same clock and may or may not be synchronized, through a preferably wireless connection 10 In both 1 a) and 1 b), the output signals may be controlled so that they may be recognized by the Receive Module 1 and in a way that it avoids or reduces interference with signals from other devices. This time sequence may be pre- stored or transmitted at pre-determined times, so as to synchronize the transmission and the reception at the transmitter 4 and receiver 2. The system may be configured to detect loss of signal or interference, and as a response trigger a new time sequence to be initiated. In 1 b), the transmission of the output signal sequence may start when the device is powered on or at a specific time in the future or when signaled by an external processing module or the Receive Module using coding, messaging or GPIO schemes. Both Figure 1 a) and b) includes a Post-Processing Module 9 for the output signal regardless of its origin, i.e. streaming or synthesis. In figure 1b the signaling regarding the coding or time sequence may be transmitted through a wireless connection 10, using an acoustic electromagnetic such as WiFi or other signals. The transmission sequence is defined in two parts, the transmitted signal package including the information to be transmitted, as well as addition information such as time identification and coding, where the signal packages are within a known time frame. The transmission sequence also define the time windows between the signal package transmissions where the duration of the time windows may vary within chosen limits. Which may be interpreted as differentiating between the signals, which have duration as such, and the time windows between the signals which may be defined by the seed. This provides a clear separation between the transmitted signal and the time windows, reducing the chances for overlapping or conflicts in the time sequence, which may occur use of a seed or other time sequences defining the signal transmission, where the time between the signals may vary from almost zero to a longer time. The duration of the signal package transmission may be fixed based in the content, but depending on the situation the duration of the time window may be adjusted for example to allow for propagation distance, and thus delay. If, for example, it is detected that the received acoustic signal arrives later than intended, a longer reception time window is required and the limits of the time window variation may be adjusted accordingly. Such a delay is easily detected if the signal packages includes a time stamp or the transmission time is otherwise known, for example synchronized through a wireless communication channel. Another reason for adjusting the limits of the time window may be a large number of detected interferences, which requires a larger range of time variations. If this duration of the signal package transmission is increased the sequence of time windows may be shifted accordingly, or possibly the following time window is reduced so that the next transmission follows according to plan. The output signal can be subjected to additional post-processing steps prior to modulation out on the speakers. The list below includes some of the useful Post-Processing modules: 1. Gain changing module that will change the gain of the output signal for speaker protection or allowing enough headroom for other audible use- cases 2. Posture control module that may reorder the channel order of a multi- channel output signal to allow the use-case to use the ideal output speakers for a specific use-case based on current device orientation. How the device orientation is updated on a regular basis is not in the scope of this invention. 3. Beamforming module that may create a multi-channel version of the output signal with phase differences between the channels to do use- case specific transmit beamforming on a multi-channel transducer array. 4. Modulation module that may add modulation to the output signal as discussed for beam forming purposes in Norwegian patent application NO20240456. One or more components with an embedded processing unit 5 execute the Receive Module 1 processing to identify relevant signals (e.g. direct path acoustic signals, object reflections or echos, multi-path echos, interfering entities, etc) in the received microphone data using signal processing potentially including a machine learning model for classifications or identifications, while one or more Amplifiers execute the transmit processing and transmit a self- generated ultrasound signal at regular intervals. Each Receive Module and Amplifier component incorporates at least one processing unit (e.g. Digital Signal Processor, NN Inference engine, microcontroller, etc). The Receive Module 1 may have or be connected to a processing block 7 with microphones 3 directly attached or via an audio codec. The Receiver Amplifier 5 will generate the output signal by itself using either a stored signal in a playout buffer (i.e. low-latency SRAM) or on-the-fly ultrasound signal generation in a low-power processing entity (e.g. DSP, microcontroller, etc). It is also possible with solutions where parts of the signal are stored in a buffer and the processing entity does on-the-fly synthesis of the final output signal by combining the stored signal parts with generated samples into the desired signal. Since the Transmit Module 2 and Receive Module 1 are not part of the same processing block or in some cases not even part of the same electronic device as in WO2023 / 079022, synchronizing transmit and receive processing is an issue especially if the transmit and receive blocks are duty-cycled for power saving reasons. Signal processing in the receive module needs to know when the signal was transmitted and when it is expected to reach the microphone via a direct acoustic path and other multi-path reflections. Knowing the timing difference and subsequently the exact receive processing time window, preferably at sub-sample accuracy, between the transmit and receive modules is important for performance reasons and for smart duty-cycling of transmit and receive processing. Therefore the modules may be syncronized through a wired or wireless connection 8,10. Even though it is possible for the Receive Module to process the incoming signal to find the direct acoustic path of the signal from the transmitter in the same device or another device, the complexity, the processing requirements, the resource usage, and the power consumption will all increase. Equally important, the receive processing and therefore the use-case may be subjected to performance issues due to inaccurate estimations of the receive processing window due to interference from other devices or other signals and various echos from the reflections of the original or reverberated output signal. Therefore, the transmit and receive processing should use the same time- sequence for the duty-cycling to make sure the receive processing is active when the relevant signal created by the transmitted output signal becomes available in the microphone data. If the system consists of multiple independent components containing separate Receive Modules, the window for receive processing may not overlap due to the distance between the input and output devices. The configuration of processing windows for the different components may have to be done individually. This may happen if there are different use-cases in use at the same time. In some systems, there will be a plurality of independent Receive Modules (e.g. Smart Microphones with embedded DSP) that will operate either independently or split the receive processing between them. These Receive Modules can operate independently and control their own Transmit Module if they handle different use-cases and use non-interfering output signals. They can also share the Transmit Module if the output signal is the same and the use-cases can use the same time-sequence. If they need to support different output signals for different use-cases, they must use different Transmit Modules with time- sequences ensuring that interference issues are minimized or non-existent. If a use-case is demanding resource-wise, a plurality of Receive Modules may do load-sharing to meet the use-case requirements regarding performance. In all these cases, the Receive Modules must be tightly coupled. If they use the same Transmit Module, one of the Receive Modules would preferably be the primary device to control the Transmit Module on behalf of all the Receive Modules. If the Receive Modules are using two different Transmit Modules, they need to synchronize the time-sequence of these Transmit Modules to minimize interference between the use-cases with respect to receive processing and use- case performance. In some systems, there will be a plurality of transmit modules. This is illustrated in Figure 2 which shows a system with two transmit modules 2a,2b , each with an Amplifier 6a,6b. If the Transmit Modules 2a,2b needs to transmit synchronously, all the Amplifiers need to get all the necessary information about the time-sequence and react synchronously or with a use-case specific offset to the same initiation signal. If the Transmit Modules should transmit out-of-sync, the information about the individual time-sequence and the corresponding initiation signals will not be synchronized but controlled closely and independently by each Receive Module. In some use-cases, it is required that the output signal or even different output signals are transmitted from different speakers in different ways (e.g. alternating output to left and right speaker) as discussed in WO2022189141. In other scenarios where multiple use-cases use different speakers, the corresponding Receive Modules need to control the Transmit Modules independently to make sure the potentially different time-sequences or encodings are known by the different Transmit modules and that the initiation signal for every Transmit Module is recognized and respected. In other embodiments with multiple use-cases, the Amplifiers need to handle a plurality of Receive Modules, time-sequence information, initiation signals to make sure that the output signals are generated independently, If the output signal in a specific Amplifier needs to be a mix of independent output signals from different Transmit Modules, the output signals should be combined in an acoustic mixer module (i.e. hardware or software processing entity) before being modulated out on one or more speakers. The acoustic mixer could alter the amplitude of the different output signals during the mixing process to make sure that the mixed signal has required properties (e.g. no clipping, output signal weighting, overall headroom for other concurrent signals, speaker protection, etc). Receive Modules could potentially optimize the operation of all the concurrent use-cases based on a plurality of parameters such as power consumption, use-case interference, use-case optimizations, etc. If power consumption of either transmit or receive processing is key, the Receive Modules could try to do concurrent processing with other use-case that can be run concurrently. If some of the use-cases will interfere with each other, the Receive Modules could make sure the time-sequence of these interfering modules would not overlap or at least minimize the overlap. Even though a plurality of concurrent use-cases introduces more complexity, there are scenarios where these use-cases could take advantage of each other´s information and receive processing to improve the performance of their own use-case. One example would be a use-case that detects a user tapping their Smartphone on the laptop screen in a system that includes a Human Presence Detection use-case, as described in WO2025 / 023843. If the HPD use-case is not detecting a user close to the laptop, the tapping use-case could be suspended to save resources and power consumption. The present invention provides a solution for avoiding or reducing such interference in an environment including several transmitter units using the same frequency range. As discussed in the present invention this is preferably performed by transmitting the signals in a time sequence of signals with varying time intervals between each signal, where the receiver is synchronized and informed about the expected time sequence, where the time sequence takes into account the length of the signal package and also preferably maintains the time windows in the sequence within predetermined limits, the limits possibly taking into account on the expected propagation time for the acoustic signals and the registered number of interferences in previously received signals. In one embodiment where the Transmit Module is in the same electronic device as the Receive Module, the Receive Module may use one or more GPIOs to enable transmission of the output signal from the Transmit module. These GPIOs can enable transmission for a limited time-period (e.g. one chirp, two chirps, 512 ms, etc) or enable a continuous output signal until triggering of one or more GPIO lines signaling that the transmission should stop. It is of course possible for the transmit module to use a random time-interval or a hardcoded time-interval for the transmit sequence when it to starts transmitting after it has been enabled. Similarly, it can use one or more GPIOs to let the receive module know (i.e. GPIO triggered) when the receive processing should be enabled and when it can be disabled. These GPIOs can enable receive processing in the receive module for a limited time-period or enable a continuous receive processing until triggering of one or more GPIO lines signaling that the receive processing should stop. If there are communication lines between the receiver and transmitter module allowing protocol messages or other electromagnetic communication signals to be transmitted between them, either of them can either send a message or use electromagnetic signals to let the other module know that it has been enabled and that the other module should enable itself too. One scheme to synchronize the duty-cycling is to use a fixed time-period, known apriori by both the transfer and receiver modules, between each transmission of the output signal. The time-period could be the time between when the last output signal ends until the new output signal begins. Alternatively, it could be the time between the start of two consecutive output signal duty-cycles. Time could be measured in nanoseconds or sample counts relative to the sample rate provided by the audio system for the audio path in question. The advantage of this scheme is that it is simple to implement and deterministic. Both transmitter and receiver can use a hard-coded time-period or a configurable time-period. The fixed time-period could also be transferred from one module to another prior to initializing the duty-cycling in both modules. Outside of the hard-coded time-period, the transmitter and receiver can be idle to save power. Using a well-defined time-period known by both transmitter and receiver makes it possible to control the receive processing regardless of the duty-cycle. However, it requires that the clock source in both transmitter and receiver are either the same or that there is no relative drift between the clock sources. If there is clock drift between the transmitter and receiver paths due to for example use of different clock sources, a clock synchronization mechanism to adjust the clocks accordingly during operation needs to be part of the acoustic system. There are different ways to support this (e.g. GPIO trigger at regular intervals from transmitter to receiver, etc) but it is not part of the scope for this invention. Another solution is to let the Transmit and Receive Modules use the same stored duty-cycling time-sequence if there is memory available in both modules. The size of the stored sequence can be adapted to the available memory in the modules. The stored sequence can be pre-configured for either the devices or the use-case in question. It can also be generated by one module and transmitted to the other if there are communication paths between the modules (e.g. shared memory segments, module mailbox, etc). Another signal synchronization scheme is to synchronize the duty-cycling with a deterministic sequence (e.g. Fibonacci sequence) of numbers starting at a given time known by both entities. Another signal synchronization scheme is to synchronize the duty-cycling with a randomized time-period using pseudo random numbers. Pseudo-random numbers (PRNs) are numbers that appear random but are generated using a deterministic process. Unlike true random numbers, which are derived from physical random processes, PRNs are generated by algorithms and, therefore, can be reproduced if the initial conditions (or seed) of the algorithm are known. This characteristic of PRNs is particularly valuable in various fields of technology, including cryptography, simulations, and digital communication systems, where reproducibility and predictability are crucial. In the embodiments described here, both the transmitter and receiver should generate the pseudo- randomized time-period by using an identical sequence of pseudo-random numbers that multiplied by a frame size is the actual time-period. Multiplying the random number by a factor (i.e. frame size) will make sure that the time-period aligns on the actual frame size. Figure 3a illustrates the PRN process where the next pseudo-random number is calculated deterministically from the previous number in the PRN sequence. Figure 3b illustrates a time sequence constituted by two different type of parts, the signal packages and the time windows. A first time window 10 is followed by a signal 11 including the signal package, followed by a new time window 12 having a different duration before the next signal package 13. Typically the signal packages have the same duration while the time windows may vary according to specified rules, such as the abovementioned pseudo-random seed. The time windows 10,12 may be adjusted depending on for example propagation time and / or processing time while the signal packages 11,13 may vary with the content of the signal. The initialization signal should specify these variables and, if changes occur, a new initialization signal may be transmitted. If the random seed is the same whenever the transmitter and receiver starts their processing, the sequence of pseudo-random number will be the same. The random seed could be preconfigured in the modules or sent from one module to the other using a messaging scheme before starting the duty-cycling in both modules. The random seed could also be changed using the same or a different PRN algorithm for every time the transmitter / receiver pair is started if required. Using pseudo number generators with a hard-coded or configurable random seed allows the transmitter and receiver to be synchronized without requiring a fixed time-period. One advantage of a scheme with randomized time-period, is for interference handling when several transmitting devices are nearby each other. Even though their output signals sometimes will collide, the randomness of the transmit period ensure that the collisions will be temporal and not constant recurrence which a fixed time-period would ensure. The receive processing needs to take collisions into account and use the information about collisions in the processing, such as by extending the duration limits of the time windows. It may reduce performance by delaying detections or classifications, but it makes it a lot easier to handle scenarios with collisions and interference from nearby devices. All transmitting devices may either use the same preconfigured random seed or generate a device-specific seed based on some random properties in its own device. There is of course a probability that nearby devices start transmitting the output signal simultaneously and that their output signal using the same time- sequence will continue to collide but the probability of that happening is very low for the embodiments discussed above. If a collision is detected through a deviation in the received signal when compared to the expected signal a new seed or sequence may be initiated, possibly also in a different frequency range. Another embodiment is a system for measuring the distance between a primary electronic device and a secondary electronic device based on transponding where the primary device initiates the transponding process as described in WO2024 / 107057. Basically, the primary device sends out an acoustic signal triggering the response from another electronic device where the time-sequence of acoustic signals sent from the secondary device must be known by both devices for the primary device to measure the distance to the secondary device. Once the transponding phase is completed, the distance measurements can continue if the secondary device keeps transmitting acoustic signals at regular intervals or preconfigured intervals allowing the primary device to continue measuring. The intervals do not have to be regular as long as both devices know the precise time-sequence of the transmission of these acoustic signals from the secondary device. The timing sequence can also be controlled by a shared PRN-based time-sequence generation of the consecutive duty-cycled signals used to measure the current distance between the devices. As discussed above, this scheme allows the device and other nearby devices to handle interference issues better. In figures 4-7 the blocks on the left side represent the receiver module and the right side represent the transmitter module. In Figure 4 the Receiver Module RX transmits a trigger signal 150, e.g. through Wifi or similar communication channels to the transmitter module 200 requesting a signal transmission, such as a chirp, every time a signal is required. This way the receiver module controls the sequence using the PRN, Illustrated by the variable time between the blocks. The Receive Module will trigger signal transmission mandated by the PRN-based timing. In the drawing the Left side is RX processing duty-cycles 100. The right side is TX processing 200, i.e. signal transmission. The RX module sends a signal 150 whenever it need the TX module to send a chirp. The time sequence is controlled by the RX module. Once the RX module is stopping / suspending, it will send a terminate TX module signal. In Figure 5 the receiver module 100 transmits a trigger signal 300, e.g. through WiFi or similar requesting the transmitter to start a prestored sequence 400. As both the initialization time and the time sequence 400 is known to both they can operate independently until the receiver module transmits a stop signal. Thus the left side is RX processing duty-cycles. The right side is TX processing, i.e. signal transmission. The RX module sends a initiation signal 300 at the start to make sure the TX module starts and that it transmits immediately. The TX and RX modules have now agreed on the time sequence and will operate independently. Once the RX module is stopping / suspending, it will send a terminate signal 301 to the TX module. If the RX module receives incomplete or deviations form the agreed signal it may also initiate a new secuence with a new initiation signal 300. Figure 6 illustrates a situation similar to figure 4, where the transmitter module TX transmits a trigger signal 151 to the receiver module RX every time an acoustic signal is to be transmitted. As the Transmit Module triggers signal transmission based on the PRN-based timing agreed between the parties. Thus, in this case again the left side is RX is the receiver processing duty- cycles while the right side is TX processing, i.e. signal transmission. The TX module sends a signal whenever it has transmitted the signal and the RX module should start RX processing the received acoustic signal based on the initiation or trigger signal 151. The time sequence is controlled by the TX module. Once either the RX or TX module is stopping / suspending, it will send a terminate signal to the other module. Figure 7 illustrates a situation similar to figure 5, but where the transmitter module TX initiates a prestored sequence transmitting a trigger signal 302 to the receiver RX, where the sequence continues until the receiver or transmitter transmits a stop signal. Thus in figure 7 the Left side is RX processing duty-cycles. The right side is TX processing, i.e. signal transmission.The TX module sends a initiation signal 302 at the start to make sure the RX module starts and that it starts RX module processing 500 immediately. The TX and RX modules have now agreed on the time sequence and will operate independently. The time sequence is controlled by the TX module. Once either the RX or TX module is stopping / suspending, it will send a terminate signal 303 to the other module and may restart the process, e.g. when registering errors in the transmission, using agreeing on another seed or signal sequence. Synchronizing the start-up of the generating sequence in the transmitter and receiver can be done with a simple low latency protocol messaging scheme or using one or more GPIO lines to trigger the sequence in both modules simultaneously. In another embodiment, the Receive Module can by using an incoming signal arriving via the direct acoustic path from the speaker and knowledge of the generated time sequence used by the transmitting module resynchronize with the transmit module if a deviation over a certain threshold is detected. This can be done by estimating the location of multiple consecutive output signal pulses and compare them to the known generating sequence based on their estimated differences in time or samples. Thus, even if the start sequence of the Receive and Transmit Module is not synchronized, the Receive Module could resynchronize the modules through signal processing of the incoming microphone data. To summarize the present invention relates to a system for synchronizing signal transmission between two devices communicating through acoustic signals. The first of said devices includes an acoustic transmitter such as a speaker or other sound generator, preferably suitable for transmitting signals in the ultrasound range. The transmitter transmits a signal in a chosen time sequence and frequency range, and the second including an acoustic receiver suitable for receiving signals in the transmitted range. The first device is configured to initialize the communication transmitting an initial signal representing the time sequence where the time sequence is constituted by signals including information packages having known durations being separated by time windows varying according to a predetermined rule. The second device includes an analyzing unit being configured to apply a filter on the receiver corresponding to the time sequence within the predetermined time window variation, thus being configured to receive signals in the transmitting time windows so as to receive the signal during the periods between the time windows. This way the system will be less sensitive to interference from other devices transmitting in the same frequency range but using other time windows. The analyzing unit in the second device may be configured to detect directly transmitted signals and reflected signals and to calculate the proximity of an object reflecting the transmitted signal. The length of the time windows and / or receiving periods may be adjusted based on the proximity and to adapt to the propagation time of the signals. The first device may be configured to initialize the communication transmitting an initial signal, for example being an acoustic signal with predefined characteristics so that the second device activates the analyzer at the receipt of the initial signal. As an alternative the devices may incorporate an electromagnetic communication system, where the initial signal is an electromagnetic signal. The time sequence may be a predefined sequence, preferably a pseudo random time sequence, where the initial signal includes a seed identifying the sequence and thus synchronizes the receiver to the transmitted sequence so that the receiver applies a filter corresponding to the sequence specified by the seed.. The transmitted signals may also include including information packages defining processing time windows having chosen lengths allowing for signal processing. The second device may be configured to, based on the analysis of the received acoustic signal received during the processing time window, to adjust the adjust the length of said processing time window to allow for receiving the complete signal, such compensating for distance between the devices and propagation time. One of the devices may also include an acoustic receiver and analyzing unit being configured to identify and communicated any time sequences and / or frequencies without interfering signals, and to communicate this to the other device by acoustic or electromagnetic communication. If this is performed by the first device it may choose the signal characteristics, such as time sequence, frequency, amplitude, etc, transmitted in the initial signal. In addition, the second device may be configured to register a deviation in the received signal from the agreed sequence and to send a request signal to the first device to initiate a new time sequence. The transmitted acoustic signal includes a predetermined redundancy, enabling the receiver to receive the transmitted information when the received time sequence includes errors, e.g. from overlapping signals from other devices. This may also provide a method for detecting errors and for requesting a new time sequence. The transmitter and the receiver may have initially synchronized clocks in order to maintain the transmitted time sequence. The information packages may include a time signature and the receiver is configured to resynchronize with the transmitter at the receipt is said time signature.

Claims

Claims 1. System for synchronizing signal transmission between two devices communicating through acoustic signals, the first of said devices including an acoustic transmitter transmitting in a chosen time sequence and frequency range, and the second including an acoustic receiver, wherein the first device is configured to initialize the communication transmitting an initial signal representing the time sequence, the time sequence being constituted by signals including information packages having known durations and being separated by time windows varying according to a predetermined rule, the second device including an analyzing unit being configured to apply a filter on the receiver corresponding to the time sequence within the predetermined time window variation, so as to receive the signal during the periods between the time windows.

2. System according to claim 1, wherein the analyzing unit in the second device is configured to detect directly transmitted signals and reflected signals and to calculate the proximity of an object reflecting the transmitted signal.

3. System according to claim 1, wherein the initial signal is an acoustic signal.

4. System according to claim 1, wherein the devices also incorporate an electromagnetic communication system, the initial signal being an electromagnetic signal.

5. System according to claim 1, wherein the time sequence is a pseudo random time sequence, and the initial signal includes said seed identifying the sequence of said time windows.

6. System according to claim 1, wherein the signals including information packages are defined as processing time windows having chosen lengths.

7. System according to claim 1, wherein at least one of the devices include an acoustic receiver and analyzing unit being configured to identify and communicated any time sequences and / or frequencies without interfering signals, and to communicate this to the other device.

8. System according to claim 1, wherein the second device is configured to register a deviation in the received signal from the agreed sequence and to send a request signal to the first device to initiate a new time sequence.

9. System according to claim 1, wherein the transmitted acoustic signal includes a predetermined redundancy, the receiver thus being capable of receiving the transmitted information when the received time sequence includes errors, e.g. from overlapping signals from other devices.

10. System according to claim 6, wherein the second device is configured to, based on the analysis of the received acoustic signal received during the processing time window, to adjust the adjust the length of said processing time windom to allow for receiving the complete signal, e.g. compensating for distance between the devices and propagation time.

11. System according to claim 1, wherein the transmitter and the receiver have initially synchronized clocks.

12. System according to claim 1, wherein the information packages include a time signature and the receiver is configured to resynchronize with the transmitter at the receipt is said time signature.

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