JP2007104347A - Clock synchronization system and method of audio transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a clock synchronization system capable of minimizing the audio output time difference between receiving devices by eliminating variation of a delay time of audio of the audio transmission system. <P>SOLUTION: A master device 11 corresponds to a main device and a slave device 12 corresponds to a remote device. The master device 11 transmits a synchronous packet in which time stamp information is embedded to a plurality of slave devices 12 through a network 13. Each of the slave devices 12 receives the synchronous packet, calculates the difference between a cumulative time stamp value To of the master device 11 and a cumulative time stamp value Ti of the slave device 12 itself, and adjusts the frequency of a variable frequency clock source 20 according to the difference (To-Ti). Further, the slave device 12 controls the frequency by using a variation coefficient (c) proportional to the degree of a gradient of the difference (To-Ti) to one direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a clock synchronization system and method provided in an audio transmission system including a main device and a remote device connected via a network, and in particular, from one preset main device (clock master device). The present invention relates to a clock synchronization system and method in which each remote device (slave device) performs clock synchronization with reference to a synchronization packet transmitted separately from a voice packet.

  As the network environment is improved, the expansion of communication services utilizing the Internet Protocol (IP) network is progressing. As an example using these, attention has been paid to a voice transmission system for transmitting voice over an IP network. These systems are mainly used for broadcasting to remote locations and multipoint broadcasting such as multi-store commercial facilities.

  In an audio transmission system including a plurality of devices connected via a network, audio data to be transmitted is packetized and transmitted. Therefore, it is necessary for the receiving apparatus to perform buffering until audio data that can be reliably output is prepared. At this time, if the clock frequencies used in the transmission device and the reception device do not match, there is a problem that the audio buffer of the reception device overflows or underflows and the output audio is interrupted. .

  In order to solve this point, clock synchronization must be performed between the transmission device and the reception device. This method will be described with reference to FIG.

  FIG. 12 is a diagram illustrating an example of a clock synchronization method in a conventional audio transmission system. As shown in FIG. 12, a conventional audio transmission system 50 includes a master device 51 and a slave device 52, which are connected via a network 53. As the master device 51, one device is set in advance in the system. The master device 51 has a function of transmitting a packet containing its own clock signal information to another slave device 52 via the network 53. On the other hand, one or more slave devices 52 exist on the system and have a function of receiving the packet via the network 53.

  The master device 51 includes a clock source 54, a time stamp extraction unit 55, and a packet transmission unit 56. If the clock synchronization is to be started, the time stamp extraction unit 55 transmits time stamp information obtained by counting the clock frequency from the clock source 54 to the packet transmission unit 56. The packet transmission unit 56 packetizes the received time stamp information and transmits it to the network 52. There are various types of communication protocols to be transmitted on the network. Here, an example using the IP protocol is considered. In the case of an audio transmission system including a large number of slave devices 52, a plurality of slave devices can easily receive packets by transmitting packets from the master device 51 by IP multicast.

  The slave device 52 includes a packet receiver 57, a packet transmission interval calculator 58, a frequency variable clock source 59, a time stamp extractor 60, a packet reception interval calculator 61, and a calculation corrector 62. When the packet reception unit 57 receives a packet from the master device 51, the packet transmission interval calculation unit 58 takes the difference between the time stamp information included in the packet and the time stamp information at the time of arrival of the previous packet, and the packet transmission interval Ts is calculated. At the same time, the packet reception interval calculation unit 61 extracts the time stamp information obtained by counting the clock frequency from its own frequency variable clock source 59 by the time stamp extraction unit 60, and takes the difference from the previous time stamp extraction to obtain the packet reception interval. Tr is calculated. The arithmetic correction unit 62 uses the packet transmission interval Tr and the packet reception interval Ts to calculate a frequency variable clock based on a value calculated by an arithmetic expression of a * (Tr−Ts) / Tr (a is a predetermined constant). The source 59 is controlled so that the clock frequency of the slave device 52 matches the clock frequency of the master device 51.

  As described above, the conventional clock synchronization system performs clock synchronization for the purpose of matching the phases of the transmission device and the reception device and preventing the audio buffer of the reception device from failing.

Patent Document 1 discloses a clock synchronization method for matching the clock signal frequency of one terminal device with the clock signal frequency of another terminal device. In the method described in this document, the clock signal frequency is easily matched by controlling the frequency variable clock source based on the value calculated from the difference between the packet transmission interval of the master device and the packet reception interval of the slave device. Can do.
JP 2004-153546 A

  However, when performing the clock synchronization method in the above-described audio transmission system, there are the following problems.

  The above prior art is a technique for matching the phases of the transmission side and the reception side. In the prior art, the clock frequency is controlled based on the difference between the packet interval on the transmission side and the packet interval on the reception side, and the clock frequencies on the transmission side and the reception side are matched. Thus, in the prior art, the phases of the transmission side and the reception side can be matched. However, the conventional clock synchronization system does not realize clock synchronization that accurately matches the audio output times between a plurality of receiving apparatuses. For this reason, there is a problem in that a difference in audio output time may occur between a plurality of receiving apparatuses. This problem will be described in more detail below.

  As a precondition for controlling the frequency variable clock source, there is a restriction that the control value for the frequency variable clock source cannot be set to a value smaller than the resolution allowed by the oscillator. For this reason, every time the frequency variable clock source is controlled, an error of a unit size equal to or smaller than the resolution occurs between a value originally desired to be set and a value actually set through truncation by the resolution.

  This error becomes a problem in the prior art. In the prior art, the frequency variable clock source is controlled based on a value calculated from the difference between the packet transmission interval of the master device and the packet reception interval of the slave device. Therefore, an error less than the above resolution occurs every time a packet is received, and this error is accumulated. When the error is accumulated, the total time stamp value from the start of synchronization on the slave side is separated from the total time stamp value on the master side. Such a difference between the total time stamps cannot be determined by a conventional control method using a packet interval. Since errors are accumulated in each of the plurality of slave devices, the total time stamp value is shifted between the plurality of slave devices. As a result, there is a problem that expansion and contraction exist in the audio delay time in a plurality of slave devices. Further, there is a problem that the difference in delay time causes a difference in audio output time between a plurality of slave devices.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to reduce the expansion and contraction of the audio delay time and to minimize the audio output time difference between the receiving apparatuses. It is to provide a clock synchronization system and method.

  It is another object of the present invention to provide a clock synchronization system and method capable of minimizing a difference in audio output time between receiving apparatuses even in a network environment having a disturbance element.

 The clock synchronization system of the present invention is a system that uses a synchronization packet to synchronize clocks between a main device that performs voice transmission over a network and a remote device, and the main device receives a time stamp from a clock source. A packet receiving unit for receiving the synchronization packet from the network, comprising: a time stamp generating unit for generating information; and a packet transmitting unit for transmitting the synchronization packet in which the time stamp information is embedded to the network. A main device cumulative time stamp calculating unit that calculates a main device cumulative type stamp value To that is a cumulative time stamp value from the start of synchronization from the time stamp information included in the synchronization packet, and frequency variable control is possible Time stamp information from various clock sources A remote device cumulative time stamp calculation unit for calculating a remote device cumulative time stamp value Ti, which is a time stamp value, and a frequency of the clock source capable of variable frequency control, the main device cumulative time stamp value To and the remote device cumulative time. A correction calculation unit that performs correction based on the difference between the stamp values Ti.

  Thus, by referring to the time stamp (To, Ti) from the start of synchronization, the frequency of the clock source is controlled while monitoring how far the total time stamp is apart. Therefore, errors that accumulate each time the frequency variable clock source is controlled, that is, errors that are caused by the difference between a value that is originally desired to be set and a value that is actually set after being truncated by the resolution, are accumulated. There is no. Only the error for one packet that arrives at that time is included in the control. This one-packet error has little effect on the total time stamp difference. In this way, the difference between the time stamp values of the master device and the slave device can be minimized, and the audio output time difference between the slave devices can be minimized.

  In the clock synchronization system of the present invention, the correction calculation unit sets the control value P for controlling the frequency of the clock source capable of variable frequency control to the main device accumulated time stamp value To and the remote device accumulated time stamp value Ti. The difference (To-Ti) and the difference (To-Ti) are determined based on the degree of continuous tilting in one direction.

  With this configuration, by controlling the clock frequency based on the degree of time stamp difference tilted in one direction, when a difference occurs, the difference is pulled back more rapidly in the direction opposite to the generated difference. Reduced control can be activated. For this reason, it is possible to prevent the time stamp difference from spreading greatly, and the audio output time difference between the slave devices can be stably kept to a minimum.

  In the clock synchronization system of the present invention, the correction calculation unit determines the control value P according to a calculation formula of P = a * (To−Ti) + c, the constant a is a fixed constant, and the variation coefficient c Is a value that fluctuates in proportion to the degree that the difference (To-Ti) is continuously tilted in one direction.

  In this way, by using the coefficient of variation c and changing the coefficient of variation c in proportion to the degree that the time stamp difference is tilted in one direction, when the difference occurs, the difference is generated in the opposite direction. The control that pulls back the difference more quickly can be activated. For this reason, it is possible to prevent the time stamp difference from spreading greatly, and the audio output time difference between the slave devices can be stably kept to a minimum. The value of the constant a used at this time is a fixed constant. When the value of the constant a is set to be large, a control is performed in which the time stamp difference closely vibrates up and down with respect to 0 as time passes. Also, when the constant a is set to a small value, a control is performed in which the time stamp value difference gently oscillates with 0 as a boundary. The constant a is set in consideration of such a phenomenon, and suitable control corresponding to the time stamp difference can be performed using the variation coefficient c as described above.

  In the clock synchronization system of the present invention, the correction calculation unit determines the variation coefficient c according to the number of times the difference (To-Ti) is continuously tilted. More specifically, the correction calculation unit determines that the variation coefficient c is c = (previous c) + k and (To−Ti) <0 when (To−Ti)> 0 continues N times. Is defined as c = (previous c) −k when N continues for N times. Here, N may be a predetermined number of times, and k may be a predetermined constant.

  By using such a variation coefficient c, for example, when N = 3, when the difference in time stamps becomes To> Ti for three consecutive times, the frequency of the clock source is controlled by increasing the value of c. The value P to be set is set to be large, the oscillation frequency on the slave device side is set to be large, and Ti is controlled so as to approach To sooner. On the other hand, when the difference between the time stamps becomes To <Ti for three consecutive times, the value P for controlling the frequency of the clock source is set small by decreasing the value of c, and the oscillation frequency on the slave device side is set small. And Ti is controlled to approach To earlier. By these methods, the spread of the time stamp difference can be detected earlier, and the difference can be pulled back and reduced in the direction in which the difference becomes smaller. As for the value of k used here, the larger the value k, the faster the difference in the time stamp is pulled back in the reverse direction. However, the difference when tilted in the reverse direction also increases. On the other hand, as the value k is smaller, control is performed such that the time stamp difference is slowly pulled back in the reverse direction as time elapses, but the difference when tilted in the reverse direction can be reduced. The value k is set in consideration of such a phenomenon, and suitable control corresponding to the time stamp difference can be performed using the variation coefficient c as described above.

  In the clock synchronization system of the present invention, the correction calculation unit is configured to determine the variation coefficient c as c = (P set last time) when (To−Ti) = 0.

  Thus, when (To−Ti) = 0, the time stamp values of the master device and the slave device are substantially the same. In such a state, the correction calculation unit does not change the value P that controls the frequency of the previously set clock source as it is. Thereby, the synchronization established state can be maintained for a while. However, even if this state continues, it is unlikely that the frequency of the master device and the frequency of the slave device exactly match over a long time. Therefore, the difference should occur in either direction. In that case, the clock synchronization control shown in the present invention is again performed in the same manner, and the frequencies of the master device and the slave device can be matched. In this way, suitable synchronization control between the master device and the slave device can be performed.

  In the clock synchronization system of the present invention, the main device is configured to transmit the synchronization packet by IP multicast or IP broadcast.

  By adopting this configuration, even if there are a large number of slave devices in the system, all slaves can be easily maintained while keeping the load on the network constant by transmitting synchronization packets by IP multicast or IP broadcast. It is possible for the device to receive a synchronization packet. Even when a slave device is newly added to the system, the clock synchronization can be started while keeping the network traffic constant without particularly increasing a new load.

  In the clock synchronization system of the present invention, the main device transmits the synchronization packet at regular intervals, and the remote device monitors a scheduled arrival time of the synchronization packet received from the main device, and receives a packet arrival. The synchronization packet that has arrived beyond a predetermined range from the scheduled time is excluded from the processing target for controlling the frequency of the clock source capable of the frequency variable control.

  With this configuration, the scheduled arrival time of the next packet is monitored, and packets that arrive outside the allowable range W are invalidated. Therefore, a packet including a large arrival delay can be excluded. Although the synchronization control is not performed for the invalid packet times, it can be easily corrected by the normal packet that arrives the next time, so that clock synchronization is not greatly affected. By referring to only the accurate time stamp in this way, it is possible to prevent a decrease in synchronization accuracy due to a disturbance element such as a network.

  In the clock synchronization system of the present invention, when the difference (To−Ti) exceeds a predetermined threshold value R, the remote device accumulates the main device accumulated time stamp value To and the remote device accumulated so far. The cumulative total time stamp value Ti is reset and accumulation of the new time stamp value is resumed.

  This configuration provides the following advantages. If the reference time stamp value acquired immediately after the start of synchronization includes a large delay, the slave device performs synchronization control using the incorrect value, and there is a problem that normal synchronization control cannot be performed. In order to solve this point, it is preferable to adopt the above configuration. With the above configuration, even if the reference time stamp value to be acquired for the first time includes a delay exceeding the threshold value, if the normal time stamp value is received even once, the difference value exceeds the threshold value, and A new accurate time stamp value can be obtained again. In this way, it is possible to further enhance resistance to delay due to disturbance elements such as a network.

  Another aspect of the present invention is a clock synchronization method in which clock synchronization between a main apparatus and a remote apparatus that performs voice transmission via a network is performed using a synchronization packet. In this method, the main device generates a time stamp information for generating time stamp information from a clock source, transmits the synchronization packet in which the time stamp information is embedded to the network, and the remote device transmits the time stamp information from the network. A synchronization packet is received, a main device cumulative type stamp value To which is a cumulative time stamp value from the start of synchronization is calculated from the time stamp information included in the synchronization packet, and a time from a clock source capable of variable frequency control is calculated. The stamp information is extracted, the remote device cumulative time stamp value Ti which is the cumulative time stamp value from the start of synchronization is calculated, and the frequency of the clock source capable of the frequency variable control is set as the main device cumulative time stamp value To and the Correction is performed according to the difference of the remote device cumulative time stamp value Ti. This method also provides the advantages of the present invention described above.

  Another aspect of the present invention is a remote device that is provided in a system that performs voice transmission via a network and that performs clock synchronization using a synchronization packet received from a main device. The device includes a packet receiving unit that receives the synchronization packet in which the time stamp information generated from the clock source of the main device is embedded from the network, and synchronization start from the time stamp information included in the synchronization packet. The main device cumulative time stamp calculating unit for calculating the main device cumulative type stamp value To, which is a cumulative time stamp value of, and the time stamp information is extracted from the clock source capable of variable frequency control on the remote device side, and the synchronization is started. A remote device cumulative time stamp calculation unit for calculating a remote device cumulative time stamp value Ti, which is a cumulative time stamp value of the clock, and a frequency of the clock source capable of frequency variable control, the main device cumulative time stamp value To and the frequency Correction calculation unit for correcting based on the difference of the remote device cumulative time stamp value Ti , And a. This configuration also provides the advantages of the present invention described above.

  As described above, according to the present invention, it is possible to reduce the audio output time difference between slave devices.

  Further, even if the number of slave devices existing in the system increases, clock synchronization can be performed without changing the network traffic load.

  Furthermore, even when a wide area network such as WAN is used, packet information including large fluctuations can be eliminated, and only an accurate time stamp value can be always referred to, and a clock synchronization system with high clock synchronization accuracy can be provided. it can.

  Hereinafter, a clock synchronization system according to an embodiment of the present invention will be described with reference to the drawings. The clock synchronization system of the present embodiment is provided in an audio transmission system as shown in FIG. 2 and realizes clock synchronization between audio transmission / reception devices 33.

  The voice transmission system in FIG. 2 includes a sound source 31, a speaker 32, a voice transmission / reception device 33, a local area LAN 34, and a WAN 35. The voice transmission / reception device 33 is a device capable of sending and receiving voice data via the local LAN 34 and the WAN 35, and has a function of packetizing the voice data in order to stream the voice data and transmit it over the network. Further, the sound source 31 is connected to the audio transmission / reception device 33 as a signal source, and the speaker 32 is connected to the audio transmission / reception device 33 as sound emitting means.

  As such a system, multipoint broadcasting is mainly assumed, and this broadcasting realizes voice communication in buildings such as office buildings, multi-store commercial facilities, stations, and airports. As the sound source 31, for example, a CD player, an MD player, an IC player, a remote control microphone, or the like for broadcasting BGM or an emergency broadcast sound source is connected. As the speaker 32, for example, various types of speakers such as a ceiling-embedded speaker and a suspended speaker are connected. The configuration via the WAN 35 enables voice transmission between separate LANs via a wide area network such as the Internet.

  In addition to the broadcast application as described above, by realizing sound field control using the Haas effect on the speaker 32 side, it is possible to provide a more sophisticated service.

  FIG. 3 is a diagram for explaining the phenomenon of the Haas effect. The Haas effect is a phenomenon in which sound is given a sense of direction by utilizing the characteristics of human hearing. When sound is output from a plurality of speakers, a human recognizes that the sound is sounding in the direction of the speaker that has emitted the sound that first arrived. When the same sound is output from two speakers that are equidistant from the receiving point, if one speaker sound is delayed, humans will recognize that the sound is heard from the other speaker. . In the example of FIG. 3A, when the speakers A and B emit sound simultaneously, a human recognizes that the sound is being played by the nearby speaker B. However, as shown in FIG. 3B, when a delay is applied so that the sound of the near speaker B arrives later than the far speaker A, the human recognizes that the far speaker A is sounding.

  By using this phenomenon to intentionally control the direction of sound, a service using sound field control can be provided. Specifically, various services such as evacuation guidance broadcasting in office buildings, acoustic design in theaters and concert halls, and Dolby Digital 5.1 channel acoustic broadcasting can be realized via a network.

  FIG. 4 is a diagram showing an embodiment using evacuation guidance broadcasting in an office building. An audio transmission unit 41 is arranged on the broadcast room side, and a plurality of audio reception units 42 are arranged on the reception side. The voice transmission unit 41 and the plurality of voice reception units 42 are arranged via the local area LAN 34. The audio transmission unit 41 includes a sound source and an audio transmission device. The voice receiving unit 42 includes a voice receiving device and a speaker provided on the aisle ceiling of the building.

  Now, when evacuation guidance broadcasting is started, a delay time is individually set for each of the audio receiving units 42 (speakers). Thereby, it is possible to make a human recognize that sound is heard from the direction in which he / she wants to evacuate. In this case, the audio streams output from the respective speakers are data having the same content. However, the output delay time differs among a plurality of speakers. In order to realize evacuation guidance, it is necessary to set the delay time in the direction of evacuation to be small and to deliver the sound of the speaker in the same direction to the human ear earliest.

  FIG. 5 is a diagram showing an embodiment in which Dolby Digital 5.1 channel sound broadcasting is broadcast from a remote place to a plurality of points. An audio transmission unit 41 is arranged on the broadcast room side, and the audio transmission unit 41 includes a sound source and an audio transmission / reception device. A plurality of Dolby Digital 5.1 channel spaces 44 are arranged via the local area LAN 34. In each 5.1 channel space, a plurality of speakers are present corresponding to 5.1 channels. Each speaker is equipped with an audio receiving device. In the case of Dolby Digital 5.1 channel, unlike evacuation guidance broadcasting, each stream is used to provide sound effects used in movies, etc., rather than broadcasting the same stream in the same 5.1 channel space. There are many occasions where independent audio is broadcast on the channel.

  In order to realize these services via a network, it is necessary to minimize the original audio output time difference existing between speakers even when audio data is transmitted over a network having disturbance elements. The original audio output time difference is a time difference in a state in which the delay of the Haas effect is not applied (performance value). If the original audio output time difference is not small, the Haas effect will not occur well when a delay is applied. Specifically, when the speaker installation interval is several meters to several tens of meters, it is said that the limit value of the audio output time difference that can effectively use the Haas effect is 2 milliseconds. In order to minimize the original audio output time difference, it is necessary that clock synchronization be accurately realized between the transmitting and receiving apparatuses in the audio transmission system.

  Hereinafter, a clock synchronization system according to an embodiment of the present invention will be described with reference to the drawings. This system is intended to enable the provision of the above services. Specifically, this system can be suitably applied to services using the Haas effect. Further, this system can be suitably applied to sound field control such as 5.1 channel broadcasting.

  FIG. 1 is a block diagram showing a configuration of a clock synchronization system 10 according to the present embodiment. As shown in FIG. 1, the clock synchronization system 10 includes a master device 11 and a slave device 12, and the master device 11 and the slave device 12 are connected via a network 13.

  The clock synchronization system 10 in FIG. 1 is realized by the configuration of the audio transmission system in FIG. The master device 11 is composed of one of the plurality of audio transmission / reception devices 33 shown in FIG. The slave device 12 is also composed of one of the plurality of audio transmission / reception devices 33 in FIG. The network 13 corresponds to the LAN 34 and the WAN 35 in FIG.

  In FIG. 1, one slave device 12 is shown. In practice, a plurality of slave devices 12 may be provided. That is, each of the plurality of audio transmission / reception devices 33 in FIG. 2 may function as the slave device 12. The plurality of slave devices 12 have the same function and realize clock synchronization by the same operation. Therefore, in the following description, the clock synchronization system 10 according to the present embodiment will be described focusing on one slave device 12.

  Further, the master device 11 and the slave device 12 in FIG. 1 correspond to the main device and the remote device of the present invention, respectively.

  In FIG. 1, as the master device 11, one device is set in advance in the system. The master device 11 has a function of transmitting a packet containing its own time stamp information to the slave device 12 via the network 13.

  The clock synchronization system 10 according to the present embodiment is not a type that adds time stamp information for clock synchronization to an audio stream. In the clock synchronization system 10, one master device 11 existing in the system transmits a unique clock synchronization packet. This is because a case where a plurality of audio channels exist in one apparatus is considered. Assume that slave devices that own a plurality of channels perform clock synchronization from time stamp information embedded in different audio streams. In this case, the configuration of the number of audio channels is required for all systems including the processor, and thus the configuration is not effective. A configuration in which all slave devices 12 and all audio channels existing in the device are synchronized in accordance with the clock of one set master device 11 is effective. In addition, mounting a plurality of audio channels in one apparatus can reduce the number of apparatuses existing in the system, and can provide an effective configuration from the viewpoint of cost and system construction. From this point of view, the synchronization packet is advantageously used.

  In addition, since the clock synchronization of the present embodiment aims to make the entire system subordinate to one clock, the master device 11 is not necessarily limited to the voice transmission device. In the audio transmission system 10, all devices other than the master device 11 become slave devices 12. Accordingly, one or more slave devices 12 exist on the system and have a function of receiving a synchronization packet from the master device 11 via the network 13. As the network 13, for example, a LAN or WAN capable of IP protocol communication can be used.

  The master device 11 includes a clock source 14, a time stamp extraction unit 15, and a packet transmission unit 16. The clock source 14 is configured by an element capable of generating a system reference clock such as a crystal oscillator. The time stamp extraction unit 15 includes an interface for extracting a clock from the clock source 14 and an interface for discharging time stamp information counted from the frequency of the obtained clock to the subsequent packet transmission unit 16. The packet sending unit 16 includes an interface for obtaining time stamp information from the time stamp extracting unit 15 and an interface for packetizing the information and discharging it to the network 13. As a packetization method, for example, communication based on the IP protocol can be performed. Further, it is preferable to implement a protocol that can use IP multicast. In addition, IP broadcast may be used. As a result, it is possible to receive packets by a plurality of specific slave devices while keeping the load on the network constant.

  On the other hand, the slave device 12 includes a packet receiving unit 17, a master device reference time stamp storage unit 18, a master device cumulative time stamp calculation unit 19, a frequency variable clock source 20, a slave device reference time stamp storage unit 21, and a slave device cumulative time stamp. A calculation unit 22 and a calculation correction unit 23 are provided.

  The packet receiving unit 17 includes an interface that receives a clock synchronization packet from the network 13. The packet receiving unit 17 uses, for example, an interface having a protocol capable of receiving an IP multicast packet. Thereby, the plurality of slave devices 12 can receive the clock synchronization packet from the master device 11 with a constant load. A packet may be received by IP broadcast.

  The master device reference time stamp storage unit 18 is configured by a memory that stores time stamp information of the master device received at the first time when clock synchronization starts as a reference value. This time stamp value becomes reference information for calculating the accumulated time stamp value of the master device when performing clock synchronization thereafter.

  The master device cumulative time stamp calculation unit 19 refers to the time stamp information of the master device received after the second time and the reference time stamp information at the start of clock synchronization obtained from the master device reference time stamp storage unit 18. It has an interface for calculating the difference of the stamp information and thereby discharging the total time stamp of the master device from the start of the clock synchronization to the calculation correction unit 23.

  The frequency variable clock source 20 is a clock source in the slave device 12 and is configured by an element capable of changing the clock frequency. The clock source 20 can be composed of, for example, a crystal oscillator capable of changing the frequency by voltage fluctuation.

  The slave device reference time stamp storage unit 21 is configured by a memory that stores, as a reference value, time stamp information of the slave device when clock synchronization starts and a synchronization packet is received for the first time. This time stamp value becomes reference information for calculating the time stamp accumulated value of the slave device when performing clock synchronization thereafter.

  The slave device cumulative time stamp calculation unit 22 refers to the time stamp information of the slave device received after the second time and the reference time stamp information at the start of clock synchronization obtained from the slave device reference time stamp storage unit 21. An interface for calculating the difference of the time stamp information and discharging the total time stamp of the slave device from the start of clock synchronization to the calculation correction unit 23 is thereby obtained.

  The arithmetic correction unit 23 has an interface for extracting the time stamp value To from the master device cumulative time stamp calculation unit 21 and an interface for extracting the time stamp value Ti from the slave device cumulative time stamp calculation unit 22. . The arithmetic correction unit 23 adjusts the frequency of the frequency variable clock source 20 based on the integer value. Then, the arithmetic correction unit 20 controls the frequency variable clock source 20 based on the value calculated from the difference value (To−Ti) between the two time stamp values, thereby mastering the accumulated time stamp value of the slave device 12. The accumulated time stamp value of the device 11 is approaching.

  In this way, the clock synchronization system according to the present embodiment can minimize the difference between the cumulative time stamp values of the master device and the slave device, and further minimize the audio output time difference between the slave devices. To the limit. The accumulated time stamp value of the master device and the accumulated time stamp value of the slave device correspond to the main device accumulated time stamp value To and the remote device accumulated time stamp value Ti of the present invention, respectively.

  The operation of the embodiment will be described below. Here, an operation in which the slave device 12 depends on the clock of the master device 11 via the network 13 will be described with reference to each drawing.

  In order to maintain the state where the audio output time difference is always kept to a minimum, as shown in FIG. 6, the slave device 12 is configured so that the accumulated time stamp values of the master device 11 and the slave device 12 are always close to each other. It is effective to control the clock source. Such control is realized by the clock synchronization system 10 and its clock synchronization method of the present embodiment.

  When the slave device 12 receives a synchronization packet for the first time, the time stamp information included in the packet is stored in the master device reference time stamp storage unit 18 as a reference value for clock synchronization control to be performed later. At the same time, the slave device 10 stores the reference time stamp value acquired from the frequency variable clock source 20 of its own device in the slave device reference time stamp storage unit 21. Thereafter, since the master device 11 transmits the synchronization packet at regular intervals, the slave device 12 also periodically receives the synchronization packet.

  However, packet arrival may be delayed more than usual due to the occurrence of disturbance factors such as a network. Considering this, as shown in FIG. 10, when the packet arrives, the slave device 12 monitors the scheduled arrival time of the next packet from the arrival time. Then, the slave device 12 determines whether or not the corresponding next packet arrives outside the allowable range W. In the present embodiment, as shown in the figure, the allowable range is determined based on the estimated arrival time of the next packet, and specifically, is set to the estimated arrival time ± W. Then, the slave device 12 invalidates the packet that has arrived outside the allowable range W. On the other hand, a packet that arrives within the allowable range W is determined not to affect the audio output reception difference. Then, this normal and valid packet is processed as an object of clock synchronization control.

  As a result of such packet invalidation processing, synchronization control is not performed for invalidated packets. However, since correction can be made easily by normal packets that arrive next time, there is almost no influence of packet invalidation.

  With the above processing, a packet that has arrived normally is subject to clock synchronization control. Here, the accumulated time stamp values To (master device) and Ti (slave device) from the time of receiving the first packet are expressed as shown in FIG. 7 on the time axis.

  The arithmetic correction unit 23 of the slave device 12 controls the oscillation frequency by performing voltage control on the frequency variable clock source 20. In the present embodiment, the calculation correction unit 23 performs control according to the difference between the accumulated time stamp value To of the master device 11 and the accumulated time stamp value Ti of the slave device 12. The correction calculation unit 23 is configured to adjust the frequency based on the integer value. By this control, the cumulative time stamp value Ti of the slave device 12 is brought close to the cumulative time stamp value To of the master device 11.

  More specifically, the calculation correction unit 23 performs voltage control from the control value P calculated by the following calculation formula. a is a fixed constant (coefficient), and c is a coefficient of variation described later.

  P = a * (To-Ti) + c

  Here, FIG. 8 shows an error when controlling the frequency variable clock source 20. As shown in FIG. 8, the frequency variable clock source 20 has a set resolution. Therefore, it should be noted that an error occurs due to the resolution in the control of the frequency clock source 20. This error is an error due to a difference between a value that is originally set and a value that is actually set in the frequency variable clock source. This error occurs every time each packet is processed. In the prior art, this error is accumulated, and the accumulated type stamp value is largely shifted between the master device and the slave device. Further, the accumulated error differs between slave devices, and the accumulated time stamp value also shifts between slaves. Therefore, the accuracy of clock synchronization is reduced, and the audio output time difference between slave devices is increased.

  However, in the present embodiment, control is performed with reference to the accumulated time stamp value from the start of synchronization. Therefore, as shown in FIG. 9, even if an error occurs, only the error for one packet that arrives at that time is included. This error has little effect on the difference between the accumulated time stamp values. Therefore, the cumulative type stamp values of the master device 11 and each slave device 12 can be made closer. As a result, the time stamp values can be made closer even between the plurality of slave devices 12.

  The constant a is a fixed constant. When the value of the constant a is set to a large value, control is performed in which the time stamp difference closely oscillates vertically with time 0 as the time elapses. Also, when the constant a is set to a small value, a control is performed in which the time stamp value difference gently oscillates with 0 as a boundary. The constant a is set in consideration of such a phenomenon, and thus suitable control according to the time stamp difference can be performed.

  Further, as shown in the above arithmetic expression, in the present embodiment, the variation coefficient c is used for frequency control. The variation coefficient c is added to a * (To−Ti) (a value corresponding to the difference between the time stamp values), whereby the control value P is calculated. The change coefficient c changes in proportion to the degree to which the time stamp difference is tilted in one direction. By changing the value of the coefficient of variation c in this way, as shown in FIG. 9, it is possible to obtain the effect of rapidly pulling back the generated difference in the reverse direction and preventing the time stamp difference from spreading greatly. be able to. For this reason, it is possible to prevent the time stamp difference from spreading greatly, and the audio output time difference between the slave devices can be stably kept to a minimum.

  As described above, the variation coefficient c changes in proportion to the degree to which the time stamp difference is tilted in one direction. This degree is typically expressed by the number of tilts (the number of continuous tilts) as described below. In this case, the correction calculation unit 23 determines the variation coefficient c according to the number of times that the difference (To−Ti) continuously tilts. More specifically, the correction calculation unit 23 determines the variation coefficient c as c = (previous c) + k when (To−Ti)> 0 continues N times. Further, the correction calculation unit 23 determines the variation coefficient c as c = (previous c) −k when (To−Ti) <0 continues N times. Here, N is a predetermined number of times, and k is a predetermined constant.

  For example, assume that N = 3. When the difference in time stamps becomes To> Ti for three consecutive times, the value P for controlling the frequency of the clock source is increased by increasing the value of c, and the oscillation frequency on the slave device side is set to be large. Control is performed so that Ti approaches To earlier. On the other hand, when the difference between the time stamps becomes To <Ti for three consecutive times, the value P for controlling the frequency of the clock source is set small by decreasing the value of c, and the oscillation frequency on the slave device side is set small. And Ti is controlled to approach To earlier. By these methods, the spread of the time stamp difference can be detected earlier, and the difference can be pulled back in the direction in which the difference becomes smaller.

  As for the value of k used here, the larger the value k, the faster the difference in the time stamp is pulled back in the reverse direction. However, the difference when tilted in the reverse direction also increases. On the other hand, as the value k is smaller, control is performed such that the time stamp difference is slowly pulled back in the reverse direction as time elapses, but the difference when tilted in the reverse direction can be reduced. Considering such a phenomenon, the value k is appropriately set. As described above, it is possible to perform suitable control according to the time stamp difference using the variation coefficient c.

  Moreover, the correction | amendment calculating part 23 determines the variation coefficient c as c = (P set last time), when (To-Ti) = 0. By this processing, synchronous control is performed as follows.

  When (To−Ti) = 0, the time stamp values of the master device and the slave device are substantially the same. In such a state, the correction calculation unit 23 does not change the value P that controls the frequency of the clock source set previously. Thereby, the synchronization established state can be maintained for a while. However, even if this state continues, it is unlikely that the frequency of the master device and the frequency of the slave device exactly match over a long time. Therefore, the difference should occur in either direction. In that case, the clock synchronization control shown in the present invention is again performed in the same manner, and the frequencies of the master device and the slave device can be matched. In this way, suitable synchronization control between the master device and the slave device can be performed.

  As described above, the master device 11 transmits the synchronization packet, the slave device 12 receives the synchronization packet, and performs synchronization control on the clock source. The master device 11 and the slave device 12 adjust the accumulated time stamp values by repeating this operation.

  In the above embodiment, the master device 11 and the slave device 12 have a one-to-one relationship. However, the actual configuration may be one-to-many. The other slave devices 12 have the same configuration and perform the same operation. In this way, it is possible to minimize the difference in audio output time among the plurality of slave devices 12.

  Further, FIG. 11 is a diagram illustrating a coping method when a large delay is included in the packet received by the slave device 12 for the first time. Assume that the reference time stamp value acquired for the first time includes a large delay. In this case, the system continues the synchronization control while using the erroneous reference time stamp value. In order to solve this point, a predetermined threshold R is set in the present embodiment. When the time stamp difference (To-Ti) exceeds the threshold value R, the slave device 12 initializes the accumulated time stamp value accumulated so far, and resumes accumulating a new accumulated time stamp value. More specifically, the accumulated time stamp value To of the master device 11 and the accumulated time stamp value Ti of the slave device 12 are reset, and accumulation of the new time stamp value is resumed.

  This configuration provides the following advantages. It is assumed that the time stamp value that is acquired for the first time and becomes a reference includes a delay exceeding the threshold value R. In this case, when a normal time stamp value arrives thereafter, the difference value (To-Ti) exceeds the threshold value, and a new accurate time stamp value can be obtained again. In this way, it is possible to further enhance resistance to delay due to disturbance elements such as a network.

  As described above, by using the clock synchronization system or method according to the present embodiment, it is possible to construct an audio transmission system with no audio output time difference between slave devices.

  Further, even if the number of slave devices existing in the system increases, clock synchronization can be performed without changing the network traffic load.

  Further, even when a wide area network such as a WAN is used, it is possible to provide a voice transmission system with high clock synchronization accuracy by eliminating packet information including large fluctuations and always referring to only an accurate time stamp value. .

  The clock synchronization system and the audio transmission system including the clock synchronization system according to the present embodiment are used in office buildings in addition to uses such as broadcasting to remote locations and multipoint broadcasting systems such as multi-store commercial facilities. It can be applied to various services using sound field control via a network, such as evacuation guidance broadcasting, acoustic design in theaters and concert halls, and Dolby Digital 5.1 channel acoustic broadcasting.

   The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and it goes without saying that those skilled in the art can modify the above-described embodiments within the scope of the present invention.

  As described above, the clock synchronization system according to the present invention can increase the clock synchronization accuracy, and is useful as a clock synchronization system such as an audio transmission system that performs sound field control and the like.

The block diagram which shows the clock synchronization system in embodiment of this invention Block diagram of audio transmission system with clock synchronization system (A) (b) Diagram for explaining the outline of the Hearth effect The figure which shows the example which applied the voice transmission system to evacuation guidance broadcasting The figure which shows the Example which broadcasts the acoustic broadcast of Dolby Digital 5.1 channel to multiple points from a remote place with an audio transmission system. Diagram to show the transition of the time stamp value in the clock synchronization system The figure for showing the outline of the synchronous packet transmission and reception in the clock synchronous system Diagram to show setting error of variable frequency clock source in clock synchronization system (A) Diagram for showing error when clock synchronization is performed (b) Diagram for showing time stamp value transition when clock synchronization is performed Diagram for showing an overview of processing when a packet containing a delay arrives The figure for showing the outline of the processing when the time stamp value difference exceeds the threshold Diagram for showing a conventional clock synchronization system

Explanation of symbols

10 Clock synchronization system (synchronization means)
DESCRIPTION OF SYMBOLS 11 Master apparatus 12 Slave apparatus 13, 34, 35 Network 31 Sound source 32 Speaker 33 Audio | voice transmission / reception apparatus 41 Audio | voice transmission unit 42 Audio | voice reception unit 44 Dolby digital 5.1 channel space

Claims (11)

A clock synchronization system that uses a synchronization packet to perform clock synchronization between a main apparatus and a remote apparatus that perform voice transmission via a network,
The main unit is
A time stamp generator for generating time stamp information from a clock source;
A packet transmission unit that transmits the synchronization packet in which the time stamp information is embedded to the network;
The remote device is:
A packet receiver for receiving the synchronization packet from the network;
A main device cumulative time stamp calculating unit that calculates a main device cumulative type stamp value To that is a cumulative time stamp value from the start of synchronization from the time stamp information included in the synchronization packet;
A remote device cumulative time stamp calculating unit that extracts time stamp information from a clock source capable of variable frequency control on the remote device side and calculates a remote device cumulative time stamp value Ti that is a cumulative time stamp value from the start of synchronization;
A correction calculation unit that corrects the frequency of the clock source capable of variable frequency control based on the difference between the main device cumulative time stamp value To and the remote device cumulative time stamp value Ti;
A clock synchronization system comprising:   The correction calculation unit sets a control value P for controlling the frequency of the clock source capable of frequency variable control as a difference (To−Ti) between the main device cumulative time stamp value To and the remote device cumulative time stamp value Ti. The clock synchronization system according to claim 1, wherein the difference (To−Ti) is determined based on a degree of continuous tilting in one direction.   The correction calculation unit determines the control value P according to a calculation formula of P = a * (To−Ti) + c, the constant a is a fixed constant, and the variation coefficient c is the difference (To−Ti). The clock synchronization system according to claim 2, wherein the value fluctuates in proportion to the degree of continuous tilting in one direction.   4. The clock synchronization system according to claim 3, wherein the correction calculation unit determines the variation coefficient c according to the number of times the difference (To−Ti) is continuously tilted. 5.   The correction calculation unit sets the variation coefficient c as c = (previous c) + k when (To−Ti)> 0 continues N times, and (To−Ti) <0 continues N times. 5. The clock synchronization system according to claim 4, wherein c = (previous c) −k.   6. The clock according to claim 3, wherein the correction calculation unit determines the variation coefficient c as c = (P set last time) when (To−Ti) = 0. Synchronous system.   The clock synchronization system according to claim 1, wherein the main device transmits the synchronization packet by IP multicast or IP broadcast. The main device transmits the synchronization packet at regular intervals,
The remote device monitors a scheduled arrival time of the synchronization packet received from the main device, and a clock source capable of the frequency variable control for the synchronization packet that has arrived beyond a predetermined range from the scheduled packet arrival time. The clock synchronization system according to claim 7, wherein the clock synchronization system is excluded from processing targets for controlling the frequency of the clock.   When the difference (To−Ti) exceeds a predetermined threshold R, the remote device resets the main device accumulated time stamp value To and the remote site accumulated time stamp value Ti accumulated so far, 8. The clock synchronization system according to claim 7, wherein accumulation of the time stamp value is newly resumed. A clock synchronization method for performing clock synchronization between a main apparatus and a remote apparatus that performs voice transmission via a network using a synchronization packet,
The main unit is
Generate time stamp information to generate time stamp information from the clock source,
Sending the synchronization packet in which the time stamp information is embedded to the network;
The remote device is:
Receiving the synchronization packet from the network;
A main device cumulative type stamp value To which is a cumulative time stamp value from the start of synchronization is calculated from the time stamp information included in the synchronous packet,
Extract time stamp information from a clock source capable of variable frequency control and calculate a remote device cumulative time stamp value Ti, which is a cumulative time stamp value from the start of synchronization,
A clock synchronization method, wherein a frequency of a clock source capable of variable frequency control is corrected according to a difference between the main device accumulated time stamp value To and the remote device accumulated time stamp value Ti. A remote device that is provided in a system that performs voice transmission via a network and that performs clock synchronization using a synchronization packet received from a main device,
A packet receiver for receiving from the network the synchronization packet in which the time stamp information generated from the clock source of the main device is embedded;
A main device cumulative time stamp calculating unit that calculates a main device cumulative type stamp value To that is a cumulative time stamp value from the start of synchronization from the time stamp information included in the synchronization packet;
A remote device cumulative time stamp calculating unit that extracts time stamp information from a clock source capable of variable frequency control on the remote device side and calculates a remote device cumulative time stamp value Ti that is a cumulative time stamp value from the start of synchronization;
A correction calculation unit that corrects the frequency of the clock source capable of variable frequency control based on the difference between the main device cumulative time stamp value To and the remote device cumulative time stamp value Ti;
A remote device characterized by comprising:

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