Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J3/00—Time-division multiplex systems
  • H04J3/02—Details
  • H04J3/06—Synchronising arrangements
  • H04J3/0635—Clock or time synchronisation in a network
  • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
  • H04J3/0658—Clock or time synchronisation among packet nodes
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W56/00—Synchronisation arrangements
  • H04W56/001—Synchronization between nodes
  • H04W56/0015—Synchronization between nodes one node acting as a reference for the others

Definitions

• the invention relates to signal distribution systems comprising at least two devices coupled together in a continuous manner by a wireless link.
• Bluetooth wireless links registered trademark of Bluetooth SIG, Inc.
• Bluetooth specifications offer the possibility of remotely controlling a remote device via a bidirectional wireless link. In practice, this is most often a mobile phone or a computer device, but Bluetooth specifications are not limited to this type
• Profile A2DP Advanced Audio Distribution Profile
• AVRCP profile Audio Video Remote Control Profile
• Bluetooth technology is of particular interest in view of its universal and evolving nature, the existence of numerous functionalities, as well as numerous, inexpensive components especially designed for its implementation. This choice is however in no way limiting, and the wireless link can be implemented at
• WiFi wireless fidelity
• the invention particularly relates, but is not limited to, systems for distributing audio signals between a plurality of active speakers communicating with each other by wireless transmission means, for reproducing the signals of an audio source on the various respective speakers.
• each enclosure comprises means, advantageously of the Bluetooth type, wireless interfacing with any other speaker of the system.
• the speakers automatically configure themselves into a network, establishing a table of mutual visibilities and defining from this table a hierarchical network topology, one of the speakers will be designated "headend".
• the function of the network head is, among other things, to establish a connection (wired or, preferably, radio) with a remote device producing a coded audio signal or a remote control signal.
• the other loudspeakers receive signals addressed to them, encapsulated in messages including routing data and broadcast in the network from the head speaker, directly or after these signals have been relayed by another speaker of the speaker. system.
• each enclosure has its own clock, ensuring the control of its various circuits, in particular the Bluetooth transmit / receive and interface circuit and the digital signal processing circuits, generally grouped together in a specific DSP processor.
• the audio data is streamed, that is to say in a continuous stream, between the different speakers from the headend enclosure that acts as the server of a client / server architecture, the other speakers being client speakers.
• a streaming broadcast requires that the client (receiver) consumes the data transmitted to it at the same rate as these are broadcast by the server (the source, transmitter). If the client consumes the data too slowly, it will sooner or later overload unused data, with overflow of the interfacing buffer; conversely, if the data consumption is too fast, the client will sooner or later be lacking data, with underflow of the buffer.
• Bluetooth specifications require that all paired devices have a very accurate clock at ⁇ 20 ppm. This requirement applies to all types of devices, even those that play a privileged role such as master devices in a piconet or scatternet master / slave network.
• the remaining variability between the clocks of the different devices is managed using a mechanism of recognizing a particular pattern (binary configuration) in low level packets, and then adding or subtracting an offset (offset) to correct the sequencing for get the desired synchronization.
• the US 2005/181729 A1 thus provides a particular servocontrol of the clock of a receiver device so that it can operate at low level in certain circumstances, and thus optimize energy consumption.
• the processing performed is performed exclusively at the RF circuit, internally, so as to correct transmission anomalies occurring at the RF link, but without affecting other application circuits placed downstream of this RF circuit.
• WO 02/073851 A1 describes a device in which the actual frequency of the clock is not modified, the device merely updating an offset value making it possible to manage the addition or the withdrawal of compensation pulses. .
• the desynchronization management technique of the Bluetooth specifications is implemented in the chipset (chip set) providing Bluetooth radio / interfacing functions.
• Bluetooth technology is used somewhat different from its original purpose - however without modification of the protocols, so as to remain perfectly compatible with the specifications of the Bluetooth specifications. Indeed, in this particular system, the Bluetooth technology is used to establish a permanent link in the system, a link that lasts until the speakers are unpowered, that is to say physically disconnected from the power outlet. Specifically, when the system is idle, ie apparently inactive for a user, the wireless links between the various speakers remain active, albeit with reduced information traffic, so as to allow the maintenance the network configuration and the detection at any time of the appearance of an external signal by the network.
• the link can thus be maintained continuously, persistently, for several hours or several days without interruption.
• each device is equipped with its own clock (chipset external clock)
• chipset external clock if the RF link is continuously maintained without interruption - an unusual situation for a Bluetooth connection - a progressive desynchronization will appear between the timing of the signal flow received (which has already been corrected internally by the resynchronization process of the RF chipset) and that of the DSP processing these signals.
• This drift even small, can cause signal processing incidents by the DSPs, especially since the clock is imprecise, due to the faster accumulation of various desynchronizations.
• the solution proposed by the invention consists essentially of:
• the invention relates to a signal distribution system of the type disclosed for example in the aforementioned WO 07/074245 A1, that is to say comprising at least two devices coupled continuously by a wireless link, with a master device and at least one slave device.
• the master device comprises first transmission and interfacing means, a first digital signal processing processor, and a first clock, for the common timing of the first transmission and interfacing means and the first digital processor, transmitting signals wirelessly at a rate defined by said first clock.
• the slave device comprises second transmission and interfacing means, a second digital signal processing processor, and a second clock for the common timing of the second transmission and interfacing means and the second digital processor.
• the second transmission and interfacing means include internal resynchronization means, with internal correlator and compensator means, for determining a frequency offset between, on the one hand, the rate of the signals received by the second transmission means and on the other hand. interfacing and, secondly, the frequency of the second clock, and for correcting the bit rate of the signals delivered to the second digital processor by the second transmission and interfacing means as a function of the frequency offset thus determined.
• the second clock is an external clock with respect to the aforementioned second transmission and interfacing means, the frequency of which is controllable around its nominal value
• the slave device furthermore comprises interrogating means, for extracting correlating and compensating means the value of said frequency offset, and servo means, for applying to the second clock the offset thus extracted, so as to drive the frequency of the second clock in response to said frequency offset value, in the direction of minimizing said offset.
• the interrogator means are very advantageously means, incorporated in the second digital processor, for reading the contents of an internal register of the correlating and compensating means.
• the second digital processor comprises means for delivering a control signal of the frequency of the second clock, in particular a digital signal such as a pulse width modulation signal delivered to the second clock via a pass filter. low.
• the accuracy of the first and second clocks can be as low as 10 ppm at most, preferably 20 ppm at most.
• Figure 1 schematically illustrates the different devices of a system in which signal distribution is provided by wireless means.
• Figure 2 is a schematic view of the various elements of an audio signal distribution system in a wireless speaker array.
• Figure 3 is a block block diagram of the various elements providing implementation of the invention in the system of Figure 2.
• FIG. 1 schematically illustrates a network consisting of various devices exchanging signals with each other wirelessly, for example Bluetooth type links.
• Bluetooth specifications enable not only point-to-point links between two elements, but also establish and manage more or less complex networks consisting of a number of these elements.
• a first type of network is the piconet, or micro-network, which is created automatically when several Bluetooth compatible elements are in the same radius.
• the piconet follows a star topology, with a master 10 and several slaves 20: the communications are direct between the master 10 and the slaves 20, and the slaves 20 can not communicate with each other.
• each master device 10 or slave 20 comprises a transmitter / receiver and Bluetooth interface circuit, respectively designated 12, 22.
• the master circuit M (the circuit 12) communicates with each of the slave circuits Si, S2, S3 ,.
• the circuits 12 and 22 thus form a wireless network 30 comprising a master M (with the circuit 12) and a plurality of slaves Si, S 2 , S 3 (together with the circuits 12). circuits 22).
• Each device 10, 20 also comprises, in addition to the transmitter / receiver and interfacing circuit 12, 22, a specific application circuit 14, 24 for processing the data exchanged within the wireless network 30 by the various devices.
• these application circuits 14, 24 are outside the network 30, which is as such intended for the exchange of signals, without application role.
• Figure 2 there is illustrated a concrete example of such a network, applied to the distribution of audio signals.
• the devices are active speakers mutually configured in a wireless network
• one of the speakers 10 is the master speaker and the other (the other) speaker (s) 20 is (are) a ) slave enclosure (s).
• the illustrated system is a stereo system with only two speakers, but a system with a larger number of speakers can be made in the same way, especially for "home theater” installations. with "5.1”, “7.1”, etc. sound configurations, including various surround satellite speakers, subwoofers, etc.
• the master speaker 10 may be coupled to various peripherals, for example a digital music player 40 provided with a Bluetooth module (internal or external) transmitting to the system an audio stream according to an A2DP profile, or a mobile phone 42 sending the same way audio data to the system.
• the peripherals can also be control devices sending the system control signals according to an AVRCP profile.
• the device can also be a device capable of sending both audio signals and commands, for example the mobile phone 42 whose keyboard keys can be used to control the system (selection of sources, volume, balance, .. .).
• the system can also be connected to conventional elements such as FM tuner, CD / DVD player / recorder, TV, etc., by a wired connection by means of plugs connected to corresponding input jacks provided on the radio. 10.
• the transmitter / receiver and Bluetooth interfacing circuit 12 of the speaker 10 delivers signals to an application circuit 14 digital signal processing (DSP).
• DSP digital signal processing
• the two digital circuits 12, 14 of the enclosure 10 are sequenced by a common clock 16, for example a clock of 26 MHz nominal frequency with a stability of the order of ⁇ 20 ppm.
• the slave speaker 20 also comprises a Bluetooth transmitter / receiver and interfacing circuit 22 which delivers signals to an application circuit 24 for digital signal processing (DSP).
• the two digital circuits 22, 24 of the enclosure 20 are sequenced by a common clock 26, for example a clock of 26 MHz nominal frequency with a stability of the order of ⁇ 20 ppm.
• the clocks 16 and 26 of the two speakers 10 and 20 of the system are of the same nominal frequency but, in practice, their actual frequency always has a slight deviation from the nominal value.
• the Bluetooth specifications provide means for compensating for this difference, by adding or subtracting, on the slave side, to each data packet an offset or offset corresponding to the observed frequency deviation.
• the deviation from the nominal frequency is detected by a correlator capable of recognizing in the data stream a particular pattern or sequence of bits sent to the beginning of the packets, analyzing the temporal position, and consequently determining the offset. necessary.
• the correction has the effect that the data stream at the output of the circuit 22 has exactly the same timing as that at the output of the circuit 12 of the remote master device.
• the offset determined by the circuit 22 is used to synchronize also the digital application circuit 24 of the slave device.
• the circuit 24 is controlled to read a register 50, internal to the circuit 22, containing the offset value determined by the Bluetooth correlator.
• This register designated ClockJDffset in the Bluetooth specification, contains the value of the time offset between the master-side clock signal and the slave-side clock signal, as well as a validity flag.
• the link 52 symbolizes the reading of the contents of this register 50 by the circuit 24.
• the read value thus enables the circuit 24 to know if the clock 26 on the slave side is moving ahead or behind the clock 16 on the master side. From this indication, the base frequency of the slave-side clock 26 will be adjusted to make it as synchronous as possible with that of the master-side clock (note that this adjustment is only slave-side because Bluetooth specifications prohibit any action on the master clock).
• This operation consists, in other words, in externalizing Voffset contained in the register to apply it to the main clock.
• the regulation of the clock 26 is for example controlled by a digital output 54 of the circuit 24 delivering a pulse width modulated signal PWM.
• This digital signal is used to generate a control voltage of the clock, for example by means of a low-pass filter 56 of the second order with a cut-off frequency of the order of 10 Hz.
• the DC voltage obtained at the output of the filter 56 for example between 0.5 and 2.5 V, thus reflects the value of the offset between the clocks of the two devices 10 and 20.
• This voltage is applied to a control input 58 of the clock 26, which is a voltage controllable quartz oscillator (VCXO).
• the application circuit 24, responsible for processing the received data can thus be perfectly synchronized to the clock frequency of the wireless network ensuring the transfer of data between the various devices.
• the application circuit 24 responsible for processing the received data, can thus be perfectly synchronized to the clock frequency of the wireless network ensuring the transfer of data between the various devices.
• all the devices will have the same internal structure, and it will only be at the time of the automatic configuration of the network that a device will be defined as being master or slave. Insofar as the corrections are applied only to the slave devices and not to the master device, it suffices for the latter to apply a zero voltage to the clock oscillator control input 16.
• the VXCO side master will behave like a simple XO, oscillating on its nominal frequency with some inaccuracy, but which will be compensated on the slave side.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Synchronisation In Digital Transmission Systems (AREA)
• Mobile Radio Communication Systems (AREA)
• Transceivers (AREA)

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• 2007
  • 2007-09-06 FR FR0706236A patent/FR2920930B1/fr not_active Expired - Fee Related
• 2008
  • 2008-08-26 EP EP08848370A patent/EP2186367A1/de not_active Withdrawn
  • 2008-08-26 WO PCT/FR2008/001207 patent/WO2009060136A1/fr active Application Filing
  • 2008-08-26 US US12/675,931 patent/US20110129048A1/en not_active Abandoned

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