WO2003084118A1

WO2003084118A1 - A method and system for synchronising digital data streams in redundant multiplex systems

Info

Publication number: WO2003084118A1
Application number: PCT/SE2003/000532
Authority: WO
Inventors: Roger Johansson; Håkan EKSTRÖM
Original assignee: Factum Electronics Ab
Priority date: 2002-04-03
Filing date: 2003-04-02
Publication date: 2003-10-09
Also published as: AU2003225448A1; SE0201008D0

Abstract

The invention relates to a method and an arrangement to synchronise a primary system (4 (4*) or 24 (24*)) and at least one redundant system (5 (5*) or 25 (25*)), and to identify CLOCK-pulses in a time synchronisation line using data frames having a predetermined repetition time, for example 24 ms. Calculating is made of how many CLOCK-pulse cycles are needed until the pattern of frames and CLOCK-pulse cycle repeat itself. A timing reference cycle is set comprising the number n of calculated CLOCK-pulse cycles. The two systems are synchronised by:• Letting the redundant system (5 (5*) or 25 (25*)) ask the primary system (4 (4*) or 24 (24*)) to give information of when one of its frames enters a determined CLOCK-pulse cycle in its timing reference cycle; and after being provided with the asked information, the redundant system (5 (5*) or 25 (25*)) calculates the number of frames, which should pass before the next timing reference cycle appears, and calculates for the redundant system (5 (5*) or 25 (25*)) a frame counting that a 'synch frame' should have in order to be the same as in the primary system.

Description

A METHOD AND SYSTEM FOR SYNCHRONISING DIGITAL DATA STREAMS IN REDUNDANT MULTIPLEX SYSTEMS

This invention relates to a method to synchronise data streams in redundant multiplex systems of the kind disclosed in the preamble of claim 1, and a system for synchronising.

BACKGROUND

The European project Eureka 147, which defines a technique for digital radio (DAB = Digital Audio Broadcasting, defined in ETS 300 401), has developed standards for data transmission, i.e. STI (STI = Service Transport Interface, defined in EN 300 797) and ETI (ETI = Ensemble Transport Interface, defined in ETS 300 799). Both these standards describe how data are divided into frames or packets having a duration of 24 ms. Data streams according to STI and ETI are generated in a so called multiplexer (S-mux or E-mux) (S-mux = Service Component Multiplexer, defined in EN 300 797, and E-mux = Ensemble Multiplexer. Defined in ETS 300 799 and ETS 300401).

The multiplexer is a critical component in the system. Sometimes two multiplexers are used to generate the same signal in order to avoid silence in the radio transmission. If one of the multiplexers fails or is disturbed in some way the other multiplexer could be switched in while the failure on the first one is repaired.

PROBLEM WITH PRIOR ART SYSTEMS

However, the switching will cause strong disturbances of the radio signal unless the data streams are exactly synchronised both to their content and their timing. The content comprises besides data also counting and sometimes time stamps. These must be exactly alike. Also, the joints of the frames must correspond.

The two redundant multiplex systems are often placed near each other. However, they could also be positioned a long distance apart. A common reference could then be provided by the Global Position System (GPS). GPS can synchronise the data timing, but frame joints, counting, and time stamps demand more information to be coincident. GPS can also deliver a pulse having the repetition rate 1 sec, called PPS (one Pulse Per Second). This pulse is simultaneous for all GPS receivers. However, it is impossible to tell the difference between one pulse from another, i.e. the pulses have no identity. Time stamps are data fields, which are apparent in each frame. A time stamp can vary between 0 and 999.999939 ms and indicates when the frame was transmitted from the multiplexer in relation to the latest PPS pulse. The difference between the time stamp within two subsequent frames must always be exactly 24 ms, in accordance with present standards. The time stamp is used in order to synchronise the radio transmitters in a so-called SFN (Single Frequency Network).

A GPS receiver gives also a high precision clock signal having a rate of 10 MHz, which clock signal is synchronised with the PPS signal. Some kinds of receiver models are also able to generate a time notation, below called "absolute time".

DESCRIPTION OF RELATED ART

A system is described in EP 0 987 845 Al with the Applicant ITIS for aligning time in a network for digital transmission network on land, which will typically be used for the digital DAB radio or DNB-T television transmission. The timing information of the digital signal is modified at the transmission site before it reaches a modulator. The timing information is exchanged to a timing information of the signal, which is calculated after a delivery of a timing signal provided by a GPS receiver, and comprises the transmission delay at the site in question.

Use is made by the PPS signal and of the 10 MHz signal and of a datum signal called UTC (Universal Time Co-ordinated, a French statement corresponding to the definition "absolute time" stated above). This paper does only concern the problem of having the data output from different sites synchronous.

ITIS has an equipment on the market called SES (Seamless ETI Switch), which analyses the quality of two input ETI data streams and chooses the best of them. The input signals are presumed to be nearly in phase with each other, even to the content, otherwise each switching brings about that the following transmitters lose their synchronisation. Thus, there will be silence in the radio transmission during some seconds. This equipment is not intended for two redundant sources, such as multiplexers, but for redundant transmitting paths. The equipment is a so called switch product, which inputs two signals, analyse them and chooses one of them without changing the content. Thus, here the signals can in fact be synchronous, since they are assumed to have the same origin but are transmitted on two paths having nearly the same delay. The companies Avitel and DB Broadcast also have on the market an equipment of practically the same kind as ITIS, which analyses the quality of two input ETI data streams and chooses the best one of them. The equipment does not observe the phase, content, frame counting, time stamps etc of the inputs. When the signals are not exactly locked to each other each switching results in a lose of synchronisation, leading to a silence in the radio transmission for ca 9 sec.

THE INVENTION

An object of the invention is to provide a synchronisation method and system for data streams generated at different sites to be exactly synchronised both regarding their timing and content.

Another object of the invention is to provide a synchronisation method and system for redundant transmitted data streams to make frame joints, counting, and time stamps coincident.

Still another object of the invention is to provide a method and a system to synchronise two redundant multiplexers and to identify PPS pulses in a system using data packets having a predetermined duration, for example 24 ms.

The majority of these objects are fulfilled with a method having the characteristic features set forth in Claim 1. Further features and developments of the invention will be apparent from the remaining claims.

Thus, the invention relates to a method to synchronise a primary system and at least one redundant system, and to identify CLOCK-pulse cycles along a time synchronisation line using data frames having a predetermined repetition time, for example 24 ms. The method is characterized by, in order to synchronise the systems, letting the systems exchange information in a rate in dependence of a cycle comprising the time when a combined pattern of CLOCK-pulse cycles and frames repeat itself, called a timing reference cycle.

In order to derive the combined pattern of CLOCK-pulse cycles and frames, a calculation could be made regarding how many CLOCK-pulse cycles are needed until the pattern of frames and CLOCK-pulse cycle repeat itself, and setting the timing reference cycle comprising the number n of calculated CLOCK-pulse cycles. In order to synchronise the primary system and the at least one redundant system the following could be made:

• Letting the redundant system ask the primary system to give information of when one of its frames enters a determined CLOCK-pulse cycle in its timing reference cycle; and

• after being provided with the asked information, the redundant system calculates the number of frames, which should pass before the next timing reference cycle appears, and calculates for the redundant system a frame counting that a "synch frame" should have in order to be the same as in the primary system, • in relation with the redundant system asking the primary system, the redundant system also asks for a frame counter of the entered frame of the primary system; and

• the primary system gives the frame counter asked for to the redundant system.

Time stamps, i.e. the times when the frames could be transmitted, could be synchronised for the frames in the timing reference cycle, and the time stamps could be counted in relation to the CLOCK-pulses marking the timing reference cycle. In order to synchronise the time stamps to the timing reference cycle, for each n:th CLOCK-pulse, the first frame could be sent at the same time as the n:th CLOCK- pulse giving it the time stamp 0. The next frame could be given the time stamp equal to the predetermined frame repetition time, and giving the following frames the predetermined frame repetition time times the order in a sequence until the last frame in the reference time is reached. If the n frames are not exactly within the reference time of the CLOCK-pulse cycle, the first frame in the next CLOCK-pulse cycle could be given the time stamp equal to the excessive time for the last time frame in the earlier CLOCK-pulse cycle. The sequence of frames could be proceeding within this CLOCK-pulse cycle, and with every following CLOCK-pulse cycle until the end of the last time frame in a CLOCK-pulse cycle corresponds with the end of that cycle. The first frame could be sent in the next CLOCK-pulse cycle at the same time as the n:th pulse giving the time stamp 0, calling it "the synch frame".

A data stream could be adjusted by the redundant system, such that the same data content is provided in the synch frame as in the first frame in the timing reference cycle for the primary system. The time from the latest PPS-pulse could be counted by using a reference signal, which when GPS is used is 10 MHz.

The invention also relates to an arrangement to synchronise a primary system and at least one redundant system, and provided with control and management means to identify CLOCK-pulse cycles along a time synchronisation line. The arrangement is characterized in that the primary and redundant systems are adapted to exchange information in a rate dependent on a cycle, called a timing reference cycle, having a cycle time related to the time when a combined pattern of CLOCK-pulse cycles and frames repeat itself, in order to synchronise the systems. The arrangement according to the invention is adapted to make the features described for the method according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and for further objects and advantages thereof, reference is now made to the following description of examples of embodiments thereof- as shown in the accompanying drawings, in which:

FIG. 1 A shows the a method version of a first embodiment of the system according to the invention; FIG. IB shows a block schedule of the first embodiment of the system according to the invention; FIG. 2 A shows the a method version of a second embodiment of the system according to the invention; FIG. 2B shows a block schedule of the second embodiment of the system according to the invention; FIG. 2C shows a block schedule of a variation of the second embodiment of the system according to the invention;

FIG. 3 shows a timing schedule to illustrate synchronisation according to the invention; and FIG. 4 shows a block schedule of a system, which exemplifies a use of the method and device according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the prior art

For the digital broadcast systems (for example DAB or DNB) of today there is often provided a Single Frequency Network (SFN), into which all the transmitters belonging to a particular broadcasting system is transmitting on the same frequency. The transmitters then have to be exactly synchronised in relation to each other in order make this possible.

An SFN-network is here synchronous in itself, but this is achieved by use of timestamp field in the frames. Thus, in the prior art, an SFN-network becomes synchronous by sending the same data stream to all transmitters. Every transmitter uses the timestamp in order to decide when the information is to be broadcast, i.e. simultaneous with all the other transmitters. The output from the E-mux is a mix (or an "ensemble") of a number of audio and data signals, called ETI signal. There can be several mixes ("ensembles") in the air on different SFN-networks.

Thus, for the digital broadcast systems (for example DAB or DVB) of today, a standardised technique is used to multiplex or group different kinds of content into a few common bit streams.

In DAB (Digital Audio Broadcasting), audio services (radio stations) and data services to be broadcast are multiplexed together in common bit streams. A bit- stream consists of frames each having a duration of 24 ms, according to standard. Each frame contains a primary followed by data packages comprising the audio or data channels to be broadcast. Each radio or data channel to be broadcast is thus multiplexed with the other radio or data channels onto the same medium in a packet multiplexing technique.

The frame header of each frame contains a frame counter. Each frame also ends with a footer, which contains a field, which is called "time stamp". The time stamp is the time when the frame is transmitted, calculated from the most recent PPS-pulse.

The invention

In accordance with the invention, the time stamp feature is one of the qualities or fields in the data stream, which is to be synchronised. However, here it is the question of time stamps in at least two redundant data streams, which are separate.

Referring to FIG IB, which represents a system, several audio channels are provided. Each audio channel is digitised and fed into an audio coding device (ENC) 1 for a first channel, 2 for a second channel, and n for an n:th channel. There could be several audio channels, for example eight. There could alternatively be no audio channels and only data channels or a mixture of both radio and data channels. FIG 1 A illustrates the method, in accordance to which the invention works. In FIG 1 A the digitised and possibly coded channels are only represented as inputs to the multiplexing systems. In FIG. IB each reference having the similar meaning as in FIG. 1 A has been provided with an *.

All the channels are together with optional data channels fed into a primary multiplexing system 4 (4*), which adds all the incoming channel packets in a packet interval comprising one single bit stream. The software of the multiplexing system 4 (4*) adds a header, some control bits, and possibly also some padding, and creates a frame, which for example comprises 2048 kbit per second and a frame length of 24 ms, according to the standard of today (it is, however, to be noticed that the data rate could vary). However, the inventive idea is not limited to the use of these standard data.

The system is redundant. Therefore, another redundant multiplexing system 5 (5*) is provided with the same input data as the primary multiplexing system 4 (4*) and has the same circuitry. The references of the means inside the multiplexing systems 4 (4*) and 5 (5*) are therefore the same.

The multiplexing systems 4, 4* and 5, 5* need not be placed near each other. There could in fact be more than one redundant multiplexing system, which could be controlled and synchronised with the primary multiplexing system 4 (4*) in the same manner as and simultaneously with the redundant multiplexing system 5 (5*) (more than one redundant system is not shown of illustrative reasons). Even though the multiplexing system 4 (4*) is defined as the primary system in the description it is to be noted that the functions of the systems 4 (4*) and 5 (5*) could change place with each other. This is possible since the systems 4 (4*) and 5 (5*) work using the same method and have exactly the same circuitry.

The same input signals are fed to both multiplexing systems 4 (4*) and 5 (5*), and the two (ore more) systems can communicate with each other via a convenient protocol 6*, e.g. TCP/IP.

In the embodiment shown in FIG 1 A and IB the multiplexing systems are each provided with an internal managing means 21 (21 *) managing the synchronisation sequence. The multiplexer adds a header, some control bits, and possibly also some padding, and creates a frame for each packet in the bit stream representing the multiplexed inputs. The invention relates to synchronisation of the two multiplexing systems 4 (4*) and 5 (5*), such that a switching will be provided without any break in the output bit stream either regarding its content and/or its timing.

A common timing reference could be provided by the Global Position System GPS. GPS delivers a pulse having the repetition rate 1 sec, called PPS (one Pulse Per Second). This pulse is simultaneous for all GPS receivers. However, it is impossible to tell the difference between one pulse from another, i.e. the pulses have no identity.

However, it is to be noted that the invention is not limited to use the GPS, but could be co-operating also with other kind of systems providing a common timing reference. The timing reference could therefore be called CLOCK-pulses, and the duration between these pulses a CLOCK-pulse cycle.

According to the invention a calculation is made of how many CLOCK-pulse cycles are needed until the pattern of frames and CLOCK-pulse cycle repeats itself. A timing reference cycle is set comprismg the number n of calculated CLOCK-pulse cycles. Therefore, a frame or packet cycle amounts for example to 24 ms, since this has been made a standard. This frame or packet cycle has to be synchronised to the CLOCK-pulse cycle from a clocking device, which could be GPS, having a predetermined clocking rate, such as the PPS-cycle, i.e. the lsec cycle. Therefore, each multiplexing system 4 (4*) or 5 (5*) is provided with a synchronising means 10 shown in FIG 1 A. FIG 1 A illustrates means making the synchronisation. A CLOCK receiver 12 (12*), e.g. a GPS receiver, is connected to the means 10 in the primary multiplexing system 4 (4*), and another CLOCK receiver 13 (13*) of the same kind is connected to the means 10 in the redundant multiplexing system 5 (5*).

According to the method illustrated in FIG 1A, each means 10 receives the predetermined clocking rate, below called PPS (1 -second pulse), in a PPS cycle counter 14. By simplicity reasons, the description below is made having GPS as the time reference source, even though it is obvious that other clock pulse sources can be used instead. A high precision clock signal P MHz, e.g. 10 MHz if the clock source is GPS, is fed to a time stamp generator circuitry 16. The circuitry translates the P MHz external reference frequency to the actual time stamp counting frequency by using PLL (Phase Locked Loop). The method shown in FIG 1 A could be provided by software in a computer. However at least a part of it could be provided in a circuitry comprising both a computer and some hardware controlled by the computer. As apparent from FIG IB the method made inside the means 10 could be provided in the embodiment of the circuitry shown inside the multiplexing system 4* and 5*. The same references are used in each of the multiplexing systems 4* and 5*. The 10 MHz signal from the CLOCK 12* is fed to an internal oscillator 18, in order to hold the oscillator updated. An output from the oscillator 18 is connected to a time stamp generator 19, to which the PPS-pulses from the CLOCK 12* are fed as well. Another output from the internal oscillator 18 is connected to an input of a data frame composer 20. The composer 20 is controlled by a control & managing device 21*. It is the data frame composer 20 in the multiplexing system, which adds a header, some control bits, and possibly also some padding, and creates a frame. The data frame composer 20 has contact with a frame count generator 22, which also is controlled by the control & managing device 21.

The frame streams from the multiplexing systems 4 (4*) and 5 (5*) are fed to an individual input each into a switch 7.

The switch 7 could switch from the output of the primary multiplexing system 4 (4*) to the output of the redundant, multiplexing system 5 (5*), when there is a need for such a switching, such as when faultiness of some kind has occurred in the primary one. The system 5 (5*) could then be the primary one during the repair of the faulty system 4 (4*) function as such when the repaired system 4 (4*) should be installed again. After a probable another disturbance the systems 4 (4*) and 5 (5*) could change their functions again.

If, for instance, the multiplexing systems are placed in the same room the CLOCK- pulses can be delivered by a pulse source (not shown in FIGs 1 A and IB but is illustrated as the means 26 in FIG 2A, 2B) provided in the same room. It is also possible to use both the GPS signals and an extra pulse source, where the multiplexing systems normally use the GPS signals. However, in case of failure or change in GPS (for example scrambling of its signals), or if one or both the CLOCK receivers 12, 13 will be inaccurate, then the extra pulse source will be activated. Then, the extra pulse source could be set by the CLOCK signals at adequate times when it is not activated in order to have it synchronised with the CLOCK signals.

Another way to give a protection against the case, when GPS or other kinds of external CLOCK and/or control signals could disappear, is to provide each multiplexing system with an internal clock. The internal clock will then give the CLOCK-pulses, and this is particularly due for the multiplexing system, which is functioning for the moment being. The internal clock could be separate, but alternatively the internal oscillator 18 could be designed to act as such an internal clock, when needed.

When the external clock source has been functional again, restarting the system could cause some problems with a pause in the transmission. However, the restart could be provided at a time when there are few user of the system, for example when there are few listener to transmitted radio programs, such as in the middle of the night. Thus, the multiplexing units 4 (4*) and 5 (5*) could be provided with a software ordering restart to wait until a predetermined time, for example 2 o'clock in the night, or the like.

FIG 2A, 2B illustrates a method and a device of a second embodiment, in which a single and common control & managing device 27 (27*) is placed outside the primary multiplexing system 24 (24*) and the redundant multiplexing system 25 (25*). The inputs are fed to separate inputs of both the primary and the redundant multiplexing systems. The inputs are multiplexed there. The external, common control & managing device 27 (27*) controls and manages the internal work of the multiplexing systems.

FIG 2C illustrates a third embodiment of a device according to the invention, in which a control & managing device 28 and 29 each are placed outside the primary and redundant multiplexing systems 24* and 25*, respectively. The same input signals are fed to both the primary and redundant systems. The control & managing devices 28 and 29 can communicate with each other via a convenient protocol, e.g. TCP/IP or similar. Such a solution could be used when the multiplexing systems are placed at a distance from each other.

All components in FIG 2 A, 2B, and 2C corresponding to the components in FIG 1 A and IB, respectively, are provided with the same references.

The description below will illustrate the invention solving the problem to have the 24 ms cycle to divide equally with a lsec cycle. The 24 ms cycle divides equally with 3 sec, i.e. three PPS pulses, below called a three-PPS -pulse cycle, which is divided into the 0-PPS cycle, the 1-PPS cycle, and the 2-PPS cycle.

Assume that the primary multiplexing system 4 (4*) or 24 (24*) is working, and that the redundant multiplexing system 5 (5*) or 25 (25*) is to be synchronised with the primary system 4 (4*) or 24 (24*). The synchronisation will be provided in the following way:

1. Frame boundaries, the time stamp, and the three-PPS-pulse cycle shall be synchronised

The time stamp in a 24-ms frame is the notional delivery time when the frame leaves the generating source. The time is counted from the latest PPS-pulse. The counting is made using the reference signal as clock reference. When GPS is used the reference signal is 10 MHz. However, the reference signal could have another frequency.

The first task is to synchronise the frame boundaries and the time stamp to the three- PPS pulse cycle. Thus, for each third PPS pulse the first frame is sent at the same time as that third pulse and is given the time stamp 0. The time stamp is regularly count-up. Next frame gets the time stamp 24ms, and the next 48ms etc. After the next PPS-pulse (1 second later) the time stamp in the first frame is 8ms, the time stamp for the second frame is 32ms etc. After the next PPS-pulse (yet 1 sec, later) the time stamp in the first frame is 16 ms, for the second frame 40 ms etc. At last, after the third PPS-pulse, the three-PPS pulse cycle is completed. The next frame is sent exactly at the PPS-pulse and is given the time stamp 0. This frame is referred to as "the synch frame" below.

However, the present invention could be interesting and functional without time stamps. For example STI is completely without time stamps. The invention involves, however, that also time stamps are synchronised, if they are used. The frame counter is always due.

The redundant multiplexing system 5 (5*) or 25 (25*) asks the primary multiplexing system 4 (4*) or 24 (24*) about in which X-PPS-cycle it is operating (cycle X; 0, 1, or 2). Thereafter, the managing software in the system 5 (5*) or 25 (25*) adjusts the circuitry to let the next PPS-pulse trigger the cycle in the system 5 (5*) or 25 (25*) to be cycle (X+l). This is most simple to do for X=2 and for "(X+1)=0", i.e. when the next expected PPS pulse will trigger the sync frame containing the time stamp value "0". However, it could be done also in other sync frames.

The trigging of the sync frame containing the time stamp value "0" is illustrated in FIG 3, as an example. It is to be noted, however, that there are other ways to perform the task exemplified below. The essential is that there is an exchange of information between the primary and the redundant multiplexing systems. In the example, the system 5 (5*) or 25 (25*) happens to ask for this information when the system 4 (4*) or 24 (24*) is in its 0-PPS-cycle. After the PPS-pulse #2 the system 4 (4*) sends the answer "entering cycle 2. Frame counter: NN" to the system 5 (5*) or 25 (25*).

Thereafter, at GO, the systems 4 (4*) or 24 (24*) and 5 (5*) or 25 (25*) are synchronised both regarding their boundary timing and their time stamp content from the next PPS-pulse #0. By using the 3 seconds phase technique described above, a relation between the PPS pulse, the PPS phase, and the time stamp value is established. Thus, the time stamp value is automatically synchronised as soon as the PPS phases and frame boundaries are synchronised.

It is also convenient to let a system (e.g. 5 or 25), which has asked another system (e.g. 4 or 24) to send an answer to a demand, check that an answer from the asked system is received within a time corresponding to the timing reference cycle. If an answer has not been provided within the predetermined time, then either the demand could be repeated or an alarm could be sent to an operator or both these features could be provided.

2. Next step is to synchronise the frame counting

The redundant system 5 (5*) or 25 (25*) asks the primary system 4 (4*) or 24 (24*) about which frame counting (frame number) the system 4 (4*) or 24 (24*) is using in the first frame in the PPS-cycle #0, i.e. the cycle having the time stamp 0 ms. This frame will below be called the "synch frame".

The system 4 (4*) or 24 (24*) answers, as apparent from FIG 3.

The management software in the redundant system 5 (5*) or 25 (25*) is now able to calculate the number of frames, which should pass before the next "synch frame" appears. It is thus also able to calculate which frame counting the "synch frame" should have in order to be the same as in the primary system 4 (4*) or 24 (24*).

3. Data (content) synchronisation

The redundant system 5 (5*) or 25 (25*) can, if desired, also adjust its data stream such that the same data content is provided in the synch frame as in the primary system 4 (4*) or 24 (24*).

It is, however, more important for a DAB-system that frame joints, the frame counting, and the time stamps are synchronous than that the data content is synchronous. 4. The last step in the synchronising cycle

The redundant system 5 (5*) or 25 (25*) waits for the third PPS-pulse (the second PPS-pulse 0 in FIG 3), which indicates that the next cycle #0 begins. The third PPS- pulse also triggers the physical interface to start transmitting data.

The output from the switch 7 is transferred, via a network, to a COFDM-modulators and transmitters. For the digital radio transmission for example a so-called SFN (Single Frequency Network) (not shown) is provided. Practically all transmitters, which send the same content, send on the same frequency. To achieve this the transmitters must be exactly synchronised with each another. All the radio and data channels have therefore been assembled in an "ensemble". It is the "ensemble", which is transmitted. As mentioned above the same ensemble is generated from at least two multiplexing systems 4 (4*) or 24 (24*), 5 (5*) or 25 (25*) in order to have a redundancy making the transmitting system practically insensitive for failure or distortion. A pause in a radio transmission must be avoided.

EXAMPLE OF SYSTEM FOR WHICH THE INVENTION IS ADAPTED

Nowadays, when transmitting of the same radio programs within several nearby areas, the transmitting stations for the different areas are sending on different bearing frequencies even though they get their information from the same source. Thereby the different transmitting stations are not disturbing each other. However, it is possible to make transmittance from transmitters being placed distant from each other simultaneously on the same bearing frequencies without interference between them. The invention has been developed in order avoid irritating disturbances when changing mux-system. It is important that the time stamps are the same and coincident in the multiplexing systems.

The output from the common switch 7 in FIG 1 A could be sent to different sending stations supplying different areas. This can be done in different ways known per se. It could for example be sent by cable or by air transmission.

The embodiment shown in FIG 4 illustrates that the transmitting from the common switch to the SFN could be provided by means of satellites. The circuitry 1 to 5 and 12 and 13 could be practically the same as in FIG 1A and IB. The outputs from the redundant multiplexing systems 4 (4*) and 5 (5*) are sent to the intelligent switch 20 essentially corresponding to the switch 7 in FIG 1 A and IB. In this embodiment the switch 20 is provided with two outputs in order to provide a redundant system also for the output from the inventive system. However, a system for transferring the output signals need not be redundant.

The switch 20 sends the same output to a new redundant system comprising at least two satellite antennas 21 and 22. The output is the same whether it emanates from the multiplexing system 4 (4*) or from the multiplexing system 5 (5*).

The antenna 21 sends its information to a satellite 23. The antenna 22 sends its information to a satellite 24. The satellite 23 sends its received information on to a first ground based satellite antenna station 25 and to a second ground based satellite antenna station 26.

It is to be noted that for an application, for which the satellite transmission is not redundant, a satellite link could only comprise the satellite antenna 21, the satellite 23 and a single receiving satellite antenna station 26. Thus, the two illustrated receiving antenna arrangements for transmitting the received signals to a ground based transmitter station represent two different embodiments.

The satellite 24 sends its received information on to a third ground based satellite antenna station 27.

The satellite antenna stations 25 and 27, representing a redundant system, are connected to an input each of the same selecting or switching device 30. A selecting or switching device is able to look at two data streams simultaneously, investigate their quality, and choose the best of them. This can function if the two incoming signals to a selecting or switching device 30 are fairly synchronised regarding the frame joints.

The selecting or switching device 30 is connected to a COFDM (COFDM = Coded Orthogonal Frequency Division Multiplexing) 32, which is connected to a GPS- receiver 34, providing the PPS-pulses and a 10 MHz-pulse. The COFDM is connected to an individual ground based transmitter station 36.

The single satellite antenna station 26 is connected directly to a COFDM 33, which is connected to a GPS-receiver 35, providing the PPS-pulses and a 10 MHz-pulse. The COFDM 33 is connected to an individual ground based transmitter station 37.

It is also possible to have more stations either of redundant or not redundant kinds, to which the satellites 23 and 24 or a single of them are sending their information. Two ground stations are only shown for illustration purposes. As mentioned above, there is a frame joint at 0 sec, 24 sec, 48 sec, etc. The frame joint times could differ somewhat after the transmittances in the satellite system 21 to 28. However, it is important to make the signals exactly synchronised when transmitting them from the transmitter stations 36 and 37,

Therefore use is made by the PPS- and 10 MHz- signals from the GPS-receivers 34 and 35, respectively. To allow for different kinds of transportation routes and delays, the PPS signal allows the transportation time from the frame generation source (i.e. the multiplexer system) to the transmitter to be up to 1 se , while the 10 MHz reference signal provides a common timing reference for all the systems involved.

Claims

We claim

1. A method to synchronise a primary system (4 or 24) and at least one redundant system (5 or 25), and to identify CLOCK-pulse cycles along a time synclironisation line, and using data frames having a predetermined repetition time, for example 24 ms, characterized by in order to synchronise the systems, letting the systems exchange information in a rate in dependence of a cycle comprising the time when a combined pattern of CLOCK-pulse cycles and frames repeat itself, called a timing reference cycle.

2. The method according to claim 1, characterized by in order to derive the combined pattern of CLOCK-pulse cycles and frames, calculating how many CLOCK-pulse cycles are needed until the pattern of frames and CLOCK-pulse cycle repeat itself, and setting the timing reference cycle comprising the number n of calculated CLOCK-pulse cycles.

3. A method according to claim 1 or 2, characterized by

In order to synchronise the primary system (4 or 24) and the at least one redundant system (5 or 25) systems:

• Letting the redundant system (5 or 25) ask the primary system (4 or 24) to give information of when one of its frames enters a determined CLOCK-pulse cycle in its timing reference cycle; and

• after being provided with the asked information, the redundant system (5 or 25) calculates the number of frames, which should pass before the next timing reference cycle appears, and calculates for the redundant system (5 or 25) a frame counting that a "synch frame" should have in order to be the same as in the primary system.

4. The method according to claim 3, characterized by

• in relation with the redundant system (5 or 25) asking the primary system (4 or 24), the redundant system (5 or 25) also asks for a frame counter of the entered frame of the primary system (4 or 24); and

• the primary system (4 or 24) gives the frame counter asked for to the redundant system (5 or 25).

5. The method according to anyoneof the preceding claims, characterized in that a system (e.g. 5 or 25), which has asked another system (e.g. 4 or 24) to send an answer to a demand, checks that an answer from the asked system is received within a time corresponding to the timing reference cycle.

6. The method according to anyone of the preceding claims, characterized by

• Synchronising time stamps, i.e. the times when the frames could be transmitted, for the frames in the timing reference cycle;

• Counting the time stamps in relation to the CLOCK-pulses marking the timing reference cycle.

7. The method according to claim 6, characterized by

• In order to synchronise the time stamps to the timing reference cycle, for each n:th CLOCK-pulse, send the first frame at the same time as the n:th CLOCK- pulse giving it the time stamp 0;

• Giving the next frame the time stamp equal to the predetermined frame repetition time, and giving the following frames the predetermined frame repetition time times the order in a sequence until the last frame in the reference time is reached.

8. The method according to claim 6 or 7, characterized by

• If the n frames are not exactly within the reference time of the CLOCK-pulse cycle, giving the first frame in the next CLOCK-pulse cycle the time stamp equal to the excessive time for the last time frame in the earlier CLOCK-pulse cycle, and proceeding the sequence of frames within this CLOCK-pulse cycle, and with every following CLOCK-pulse cycle until the end of the last time frame in a

CLOCK-pulse cycle corresponds with the end of that cycle;

• sending the first frame in the next CLOCK-pulse cycle at the same time as the n:th pulse giving the time stamp 0, calling it "the synch frame".

9. The method according to one of the preceding claims, characterized by providing the CLOCK-pulses by the Global Position System GPS, which delivers a pulse train having the repetition rate 1 sec, called PPS (one Pulse Per Second).

10. The method according to claim 9, characterized by providing a frame time equal to 24 ms, setting the timing reference cycle to comprise three CLOCK-pulse cycles.

11. The method according to anyone of the preceding claims, characterized by adjusting a data stream by the redundant system (5 or 25), such that the same data content is provided in the synch frame as in the first frame in the timing reference cycle for the primary system (4 or 24).

12. The method according to anyone of the preceding claims, characterized by counting the time from the latest PPS-pulse, by using a reference signal, which when GPS is used is 10 MHz.

13. The method according to anyone of the preceding claims, characterized by making a restart at a time when there are few user of the system after a failure of the CLOCK-pulse and when the CLOCK-pulse source has been functional again.

14. An arrangement to synchronise a primary system (4* or 24*) and at least one redundant system (5* or 25*), and provided with control and management means

(21*, 27*, 28, 29) to identify CLOCK-pulse cycles along a time synchronisation line, characterized by the primary and redundant systems are adapted to exchange information in a rate dependent on a cycle, called a timing reference cycle, having a cycle time related to the time when a combined pattern of CLOCK-pulse cycles and frames repeat itself, in order to synchronise the systems.

15. The arrangement according to claim 14, characterized in that the control and management means (21 *, 27*,28,29) controls a data frame composer (20) to add a header, some control bits, and possibly also some padding, and create a frame.

16. The arrangement according to claim 14 or 15, characterized in that the control and management means (21*, 27*,28,29) having software managing incoming channel packets in packet intervals to constitute the frames and arranging the frames in a single bit stream.

17. The arrangement according to anyone of the claims 15 or 16, characterized in that the control and management means (21*, 27*,28,29) co-operates with a frame count generator (22), which also is connected to the data frame composer (20).

18. The arrangement according to anyone of the claims 14 to 17, characterized in that the primary system (4* or 24*) and the at least one redundant system (5* or 25*) each comprises an internal oscillator (18), which is kept updated by the CLOCK-pulses when they are present.

19. The arrangement according to anyone of the claims 15 to 18, characterized in that each of the primary system (4* or 24*) and the at least one redundant system (5* or 25*) comprise time stamp generator means (19), which is connected to the data frame composer (20).

20. The arrangement according to claim 19, characterized in that the control and management means (21*, 27*,28,29) synchronises the time stamps, i.e. the times when the frames could be transmitted, for the frames in the timing reference cycle and counts the time stamps in relation to the CLOCK-pulses marking the timing reference cycle.

21. The arrangement according to claim 19 or 20, characterized in that the control and management means (21*, 27*,28,29), in order to synchronise the time stamps to the timing reference cycle, for each n:th CLOCK-pulse controls the primary system (4* or 24*) to send the first frame at the same time as the n:th

CLOCK-pulse giving it the time stamp 0, and to give the next frame the time stamp equal to the predetermined frame repetition time, and to give the following frames the predetermined frame repetition time times the order in a sequence until the last frame in the reference time is reached.

22. The arrangement according to anyone of the claims 14 to 21, characterized in that the control and management means (21*, 27*,28,29), in order to derive the combined pattern of CLOCK-pulse cycles and frames, calculates how many CLOCK-pulse cycles are needed until the pattern of frames and CLOCK-pulse cycle repeat itself, and setting the timing reference cycle comprising the number n of calculated CLOCK-pulse cycles.

23. The arrangement according to anyone of the claims 14 to 22, characterized in that the control and management means, in order to synchronise the primary system (4* or 24*) and the at least one redundant system (5* or 25*): • controls the redundant system (5* or 25*) to ask the primary system (4* or 24*) to give information of when one of its frames enters a determined CLOCK-pulse cycle in its timing reference cycle; and

• controls the redundant system (5* or 25*), after being provided with the asked information, to calculate the number of frames, which should pass before the next timing reference cycle appears, and to calculate for the redundant system (5* or 25*) a frame counting that a "synch frame" should have in order to be the same as in the primary system.

24. The arrangement according to claim 23, characterized in that the control and management means (21*, 27*,28,29), in relation to the redundant system (5* or 25*) asking the primary system (4* or 24*), also controls the redundant system (5* or 25*) to ask for a frame counter of the entered frame of the primary system (4* or 24*) and controls the primary system (4) to give the frame counter asked for to the redundant system (5* or 25*).

25. The arrangement according to anyone of the claims 19 to 24, characterized in that if the n frames are not exactly within the reference time of the CLOCK-pulse cycle, the control and management means (21 *, 27*,28,29) gives the first frame in the next CLOCK-pulse cycle the time stamp equal to the excessive time for the last time frame in the earlier CLOCK-pulse cycle, and proceeds the sequence of frames within this CLOCK-pulse cycle , and with every following CLOCK-pulse cycle until the end of the last time frame in a CLOCK-pulse cycle corresponds with the end of that cycle, and controls the redundant system (5* or 25*) to send the first frame in the next CLOCK-pulse cycle at the same time as the n:th pulse giving the time stamp 0, calling it "the synch frame".

26. The arrangement according to one of the claims 14 to 25, characterized by a Global Position System GPS connection (12, 13) each for the primary system (4* or 24*) and the redundant system (5* or 25*), which delivers a pulse having the repetition rate 1 sec, called PPS (one Pulse Per Second) providing the CLOCK- pulses.

27. The arrangement according to claim 26, characterized in that the control and management means (21*, 27*,28,29) provides a frame time equal to 24 ms, setting the timing reference cycle to comprise three CLOCK-pulse cycles.

28. The arrangement according to anyone of the claims 14 to 27, characterized in that the control and management means (21*, 27*,28,29) adjusts a data stream by the redundant system (5* or 25*) such that the same data content is provided in the synch frame as in the first frame in the primary system (4* or 24*).

29. The arrangement according to anyone of the claims 14 to 28, characterized in that the control and management means (21*, 27*,28,29) counts the time from the latest PPS-pulse, by using a reference signal, which when GPS is connected is 10 MHz.

30. The arrangement according to anyone of the claims 14 to 29, characterized by having a common control and managing device (18) for both the primary multiplexing system (4* or 24*) and the redundant multiplexing system (5* or 25*).

31. The arrangement according to anyone of the claims 14 to 30, characterized in that the control and management means (21 *, 27*,28,29) makes a restart at a time when there are few users of the arrangement after a failure of the CLOCK-pulse and when the CLOCK-pulse source has been functional again.