Abstract:
In an audio network system connecting a plurality of nodes in a ring form, a master node transmits a packet of frame data regularly every sampling cycle, such that the packet circulates through the nodes during the sampling cycle. The packet is provided with a plurality of regions for containing audio sample data in correspondence to a plurality of channels. A first node reads audio sample data from a particular region of the packet, which corresponds to a particular channel allocated to the first node, and stores the read audio sample data in a buffer. The first node acquires positional information indicating whether a second node which has written the audio sample data into the particular region is located upstream or downstream of the first node. The first node reads and outputs a previous one of the audio sample data from the buffer if the second node is located upstream of the first node, and outputs a current one of the audio sample data if the second node is located downstream of the first node.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field of the Invention 
         [0002]    The present invention relates to a lag correction technology of audio samples in which sample mismatching between nodes in a ring audio network system is corrected based on the positional relationship of the nodes. 
         [0003]    The present invention also relates to a technology for a ring audio network system in which transmission delay times in the system are taken into consideration and the output times of audio signals from nodes are corrected, thereby correcting the phase difference of all the nodes in the system. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventional technologies for audio signal communication in an audio network system used for PA such as plays and concerts, music production, and private broadcasting include CobraNet (registered trademark) described in Non-Patent Reference 1, SuperMAC (registered trademark) described in Non-Patent Reference 2, and EtherSound (registered trademark) described in Non-Patent Reference 3. Any of them is a technology for transmitting audio signals over the Ethernet. 
         [0006]    [Non-Patent Reference 1]
   http://www.balcom.co.jp/cobranet.htm   
 
         [0008]    [Non-Patent Reference 2]
   http://www.sonyoxford.co.uk/pub/supermac/   
 
         [0010]    [Non-Patent Reference 3]
   http://www.ethersound.com/news/getnews.php?enews_key=101   
 
         [0012]    Conventional technologies used in a telephone line network include a ring communication network such as FDDI/TokenRing. In this scheme, a token (i.e., the right to transmit data) circulates through nodes connected in a ring so that only a node which obtains the token is permitted to transmit data. 
         [0013]    One can consider an audio signal (audio sample data) transmission scheme using the conventional ring communication network.  FIG. 13  illustrates an example of a ring audio network system. A signal circulates through nodes reciprocally along the nodes in the order of node  1301           node  1302           node  1303           node  1302           node  1301 .  FIG. 14  illustrates another example of the same ring audio network system. In this example, nodes  1401  to  1406  are physically connected in a ring in the named order. A packet transmitted from the master node  1401  circulates once in one sampling cycle (or time) in the order of  1401             1402             1403             1404             1405             1406 . A storage region of audio sample data of a plurality of channels (Ch) is allocated in the packet and each node transmits data by loading it into a storage region of a suitable channel in the packet. 
         [0014]    However, in the system in which data is transmitted in a ring as described above, sample data of the same sampling cycle in which corresponding sample data is stored in the circulated packet by the node  1404  is extracted from the node  1406 , while sample data of a different sampling cycle is extracted from the node  1403 . This is because, when the sample data stored in the node  1404  reaches the master node  1401 , the sample data is incorporated into a packet that circulates in the next sampling cycle, and the node  1403  then receives the packet. By such a manner, there is caused sample time mismatching between nodes in a conventional ring audio network system. Usually, such sample mismatching causes almost no problem. However, a variety of professional audio devices may not permit such sample mismatching. 
         [0015]    Further in the conventional system in which data is transmitted in a ring as described above, it is difficult to accurately match output timings of audio signals of all nodes of the system. Particularly, as long as the system uses transmission lines, the system suffers from delay in signal transmission through the transmission lines. Processing in each node also causes a delay. Usually, such a small delay causes almost no problem. However, a variety of professional audio devices may not permit even such a small time delay, and there may be a desire to eliminate even the phase difference of audio signals output from all nodes. For example, in order to strictly design and set up a line array speaker system, it may be necessary to take into consideration even such a small delay. 
       SUMMARY OF THE INVENTION 
       [0016]    It is a first object of the present invention to provide a sample mismatching correction technology for a ring audio network system in which, when audio sample data is transmitted while circulating a packet through nodes, sample mismatching is corrected so that signals input at an absolute time are output from all the nodes in the same sampling cycle (or time), regardless of the positional relationship of the nodes. 
         [0017]    It is a second object of the present invention to provide a delay correction technology which ensures that a ring audio network system can output audio signals without any phase difference of nodes by taking into consideration a very small delay such as a delay on a transmission line when performing transmission of audio sample data while circulating a packet through the nodes. 
         [0018]    In order to achieve the first object, the invention provides an audio network system that connects a plurality of nodes in a ring so as to allow loop transmission of data and that performs data transmission in one direction through the ring of the nodes to perform communication between any ones of the plurality of the nodes, wherein one of the plurality of the nodes is a master node and the other nodes are slave nodes, wherein the master node transmits a packet of frame data regularly every sampling cycle, such that the packet circulates through the plurality of the nodes connected in the ring during the sampling cycle, wherein the packet is provided with a plurality of regions for containing audio sample data in correspondence to a plurality of channels, and wherein each of the nodes includes: a reading part that reads audio sample data from a particular region of the packet, which corresponds to a particular channel allocated to the node, the audio sample data being written into the particular region by another node; a storage part that stores the read audio sample data of the channel allocated to the node, wherein the storage part stores a current one of the audio sample data read in a current sampling cycle and a previous one of the audio sample data read in one sampling cycle ago; an acquiring part that acquires positional information which indicates whether said another node which writes the audio sample data of the allocated channel into the packet is located upstream or downstream of the node along a stream of the packet which is transmitted from the master node, then flows through the nodes and returns to the master node; and an output part that outputs the audio sample data of the allocated channel stored in the storage part, wherein the output part outputs the previous one of the audio sample data of the allocated channel from the storage part if said another node which writes the audio sample data of the allocated channel into the packet is located upstream of the node, and the output part outputs the current one of the audio sample data of the allocated channel from the storage part if said another node is located downstream of the node. 
         [0019]    In an extended form, the storage part stores n+1 number of the audio sample data ranging from a current one of the audio sample data read in a current sampling cycle to previous ones of the audio sample data read in 1 through n sampling cycles ago, where “n” is an integer equal to or greater than 1. The output part outputs a previous one of the audio sample data of the allocated channel which has been stored n sampling cycles ago if said another node which writes the audio sample data of the allocated channel into the packet is located upstream of the node, and the output part outputs another previous one of the audio sample data of the allocated channel which has been stored n−1 sampling cycles ago if said another node is located downstream of the node. 
         [0020]    In order to achieve the second object, the invention provides an audio network system that connects a plurality of nodes in a ring so as to allow loop transmission of data and that performs data transmission in one direction through the plurality of the nodes to perform communication between any ones of the plurality of the nodes, wherein one of the plurality of the nodes is a master node and the other nodes are slave nodes, wherein the master node transmits a packet of frame data regularly every sampling cycle, such that the packet circulates through the plurality of the nodes connected in the ring during the sampling cycle, wherein the packet is provided with a plurality of regions for containing audio sample data in correspondence to a plurality of channels allocated to the respective nodes for performing transmission of the audio sample data between the respective nodes, wherein each of the nodes calculates a correction time that elapses until the master node receives the packet after the node receives the packet, and wherein, when each of the nodes outputs the audio sample data of the allocated channel to an external device, the node outputs the audio sample data at an output time which is adjusted by the correction time. 
         [0021]    In a specific form, the inventive audio network system connects a plurality of nodes in a ring so as to allow loop transmission of data and performs data transmission in one direction through the plurality of the nodes to perform communication between any ones of the plurality of the nodes, wherein one of the plurality of the nodes is a master node and the other nodes are slave nodes, wherein the ring of the nodes is constructed such that a first part of the plurality of the slave nodes are connected in chains in a forward direction so that data is transmitted from the master node in the forward direction and the first part of the plurality of the slave nodes are connected in chains in a backward direction so that the data is transmitted from a terminal slave node of the forward direction in the backward direction until the data reaches the master node after the data turns around upon reaching the terminal slave node in the forward direction, and the second part of the plurality of the slave nodes are connected in chains in a backward direction so that data is transmitted from the master node in the backward direction and the second part of the plurality of the slave nodes are connected in chains in a forward direction so that the data is transmitted from a terminal slave node of the backward direction in a forward direction until the data reaches the master node after the data turns around upon reaching the terminal slave node in the backward-direction, wherein the master node transmits a packet of frame data regularly every sampling cycle, such that the packet circulates through the plurality of the nodes connected in the ring during the sampling cycle, wherein the packet is provided with a plurality of regions for containing audio sample data in correspondence to a plurality of channels which are allocated to the respective nodes, wherein the master node includes: an acquiring part that acquires delay information including a total network delay time during which the packet circulates through the ring of the nodes and then returns to the master node, a forward-side delay time that elapses until the packet returns to the master node in the backward direction after being transmitted in the forward direction, and a backward-side delay time that elapses until the packet returns to the master node in the forward direction after being transmitted in the backward direction; and a notifying part that notifies all the slave nodes of the acquired delay information including the total network delay time, forward-side delay time, and backward-side delay time, wherein each of the slave nodes includes: a first calculating part that calculates a reception time difference between a reception time of the packet received in the forward direction and another reception time of the packet received in the backward direction; a second calculating part that calculates a correction time that elapses from a time when the node receives the packet to another time when the master node receives the packet using the notified delay information from the master node and the calculated reception time difference; a reading part that reads the audio sample data from a particular region of the packet which corresponds to the allocated channel; a storage part that stores the read audio sample data of the allocated channel; and an output part that outputs the audio sample data stored in the storage part at a proper output time which is adjusted by the correction time. 
         [0022]    According to the first aspect of the invention, it is possible to extract data of the same sampling cycle of all channels from every node in a synchronous ring audio network, without depending on the positional relationship of nodes to which data is input and nodes from which data is output. 
         [0023]    According to the second aspect of the invention, even in an audio network having a transmission delay, it is possible to output audio signals from respective nodes at the same time and to output in-phase audio signals from all the nodes. Thus, the invention is suitable for application to an audio system (for example, a line array speaker system) in which the phase difference between output audio signals of nodes is seriously considered. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a module configuration diagram of an audio network system according to the present invention. 
           [0025]      FIG. 2  is a hardware configuration diagram of each node. 
           [0026]      FIG. 3  is a flow chart of an audio data input routine. 
           [0027]      FIG. 4  is a flow chart of an audio data output routine. 
           [0028]      FIG. 5  is a flow chart of a delay time calculation routine for a master node. 
           [0029]      FIG. 6  is a flow chart of a delay time calculation routine for a slave node. 
           [0030]      FIG. 7  is a flow chart of an audio data output routine for a slave node. 
           [0031]      FIG. 8  illustrates a sample mismatching correction method. 
           [0032]      FIG. 9  illustrates the definitions of delay and transmission/reception times. 
           [0033]      FIGS. 10   a,    10   b  and  10   c  illustrate delay times calculated by a master node. 
           [0034]      FIG. 11  illustrates delay times calculated by a slave node. 
           [0035]      FIG. 12  illustrates an example of correction time calculation for each node. 
           [0036]      FIG. 13  illustrates an example of a ring audio network system. 
           [0037]      FIG. 14  illustrates another example of the ring audio network system. 
           [0038]      FIGS. 15   a  and  15   b  illustrate an example occurrence of mismatching of samples. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    Embodiments of the present invention will now be described with reference to the drawings. 
         [0040]      FIG. 1  illustrates a module configuration of an audio network system according to the invention. Reference numerals “101” to “104” denote four nodes that are connected so as to circulate a packet  150  in a ring as shown by a dotted line  151 . A microphone  112  is connected to the node  101  through an analog to digital converter (ADC)  111 . The node  101  can transmit audio data input through the ADC  111  to another node using a channel of the circulating packet  150 . The node  102  is connected to a mixer  121 . The node  102  can read audio data from a channel of the circulating packet  150  and transmit the audio data to the mixer  121 . The mixer  11  performs a variety of signal processing, such as mixing, audio volume level adjustment, and effecting, on the input audio data. The mixer  121  inputs the resulting audio data of the signal processing to the node  102 . The node  102  then can transmit the input audio data to another node using a channel of the circulating packet  150 . The node  103  is connected to a power amplifier  132  and a speaker  133  through a digital to analog converter (DAC)  131 . Similarly, the node  104  is connected to a power amplifier  142  and a speaker  143  through a DAC  141 . The node  103  can read audio data from a channel of the circulating packet  150  and transmit the audio data to the power amplifier  132  through the DAC and then output its sound through the speaker  133 . The same is true for the node  104 . 
         [0041]    Each of the nodes  101  to  104  includes two-stage buffers (buffer A and buffer B). Each of the buffers stores audio data that can be read by a corresponding node. The purpose of providing the two-stage buffers is to store both sample data of one sampling cycle ago and latest sample data for each channel. Most recently read data is stored in one buffer (for example,  110 A) with the symbol “A” attached thereto and sample data of one sampling cycle ago is stored in another buffer (for example,  110 B) with the symbol “B” attached thereto. 
         [0042]      FIG. 2  illustrates a hardware configuration of each node. One node includes a central processing unit (CPU)  201 , a random access memory (RAM)  202 , a read only memory (ROM)  203 , a communication interface (I/F)  204 , an audio I/O (input/output interface)  206 , and a communication bus  208 . 
         [0043]    The CPU  201  is a processing unit that controls overall operations of the node. The RAM  202  is a volatile memory that is used as a loading region of programs executed by the CPU  201  or as a work region for setting a variety of buffers. The ROM  203  is a nonvolatile memory that stores a variety of programs to be executed by the CPU  201 , constant number data, and the like. The communication interface  204  is connected to another node  205  that is an external device. Since, for example, the node  102  is connected to both the nodes  103  and  101  although the node  205  is illustrated as one block in  FIG. 2 , the node  102  includes a plurality of communication interfaces  204  for connection to the nodes  103  and  101 . The audio I/O  206  is an interface for connection with an external device such as an ADC, a DAC, a mixer, or the like. 
         [0044]    A sample mismatching correction method or sampling cycle lag correction method for an audio network system of this embodiment will now be described.  FIG. 8  illustrates the sample mismatching or sampling cycle lag correction method. Nodes A to D are connected in a ring allowing signals to reciprocate through the nodes. The node B is a master node. The master node B transmits a packet at the start time of one sampling cycle. Here, it is assumed that the node A stores data in a channel Ch 1  of the circulating packet, the node B stores data in a channel Ch 2  of the circulating packet, the node C stores data in a channel Ch 3  of the circulating packet, and the node D stores data in a channel Ch 4  of the circulating packet. It is also assumed that each of the nodes A to D reads data of all the channels Ch 1  to Ch 4 . Reference numeral “801” is data which has returned to the master node B in a sampling cycle “t-1” and which contains data set in each channel in the sampling cycle “t-1”. When a new sampling cycle “t” starts, the node B stores data of a cycle “t” in a Ch 2  region in the packet and transmits the packet as a packet  802  to the next node C. The node C receives the packet as a packet  803  and stores data of the cycle “t” in a Ch 3  region in the packet and then transmits it as a packet  804  to the next node D. The node D receives the packet as a packet  805  and stores data of the cycle “t” in a Ch 4  region in the packet and then transmits it as a packet  806  to the path of a backward line. The packet only passes through the nodes C and B on the backward line without change as shown by “807” and “808”. The next node A receives the packet as a packet  809  and stores data of the cycle “t” in a Ch 1  region in the packet and then transmits it as a packet  810  to the next node B. The master node B receives the packet as a packet  811  and waits until the next sampling cycle starts and then performs the same processes. 
         [0045]    Each of the nodes includes buffers A and B as illustrated in  FIG. 1 . For example, in the master node B, one row denoted by “822A” is a buffer A which stores latest sample data of each channel and another row denoted by “822B” is a buffer B which stores sample data of each channel of one sampling cycle ago. The same is true for other nodes. When transmitting a packet to the next node, each of the nodes A to D stores data of the channels Ch 1  to Ch 4  of the packet in the buffer A. Data that has been stored in the buffer until then is copied to the buffer B. The data storage in the buffer A is performed after the data copy is performed. Accordingly, each of the nodes A to D always contains a latest sample and a sample of one sampling cycle ago of each of the channels Ch 1  to Ch 4  in its buffers. 
         [0046]    Each of the nodes A to D outputs sample data stored in the buffers A and B without causing mismatching of samples and, for example, delivers it to a mixer or outputs its sound through an amplifier. To prevent the sample mismatching, each node outputs data of one sampling cycle ago, which is stored in the buffer B, for channels which are subjected to writing at its upstream nodes (including a range of nodes from the master node to an immediately previous node) along the flow of data, which flows through the nodes in a ring, and outputs data of the current sampling cycle, which is stored in the buffer A, for channels which are subjected to writing at its downstream nodes (including a range of nodes from an immediately subsequent node to a node immediately prior to the master node). In the example of  FIG. 8 , each node outputs data of a corresponding channel in the buffer B since the node stores data for writing of the current sampling cycle in the corresponding channel and thereafter writes it together with data of the other channels in the buffer A. Of course, each node may update data of a channel for writing at the node after storing its received packet in the buffer. When the procedure is performed in this order, the node outputs the data from the buffer A. In  FIG. 8 , each item surrounded by an ellipse indicates a sample that each node outputs according to the rule described above. In this manner, mismatching of samples can be prevented and the samples of the cycle “t-1” can be output simultaneously. 
         [0047]    Here, it is assumed that information of connection positions of the nodes and information about which channel is used for writing at each node are already shared among all the nodes. For example, when the system has started up, when a change has been made to the membership of nodes, or when a change has been made to channels set for writing or reading to or from a packet at each node, the nodes share this information, for example by exchanging control data with each other at the initiative of the master node. 
         [0048]    Namely, the master node notifies all the slave nodes of control information representing at least a connecting position of each node in the audio network system and a channel allocated to each node, at the time when the audio network system starts up. 
         [0049]      FIG. 3  is a flow chart of an audio data input routine that each of the nodes performs upon receiving a packet circulating through the nodes. At step  301 , audio data of a channel for inputting at the node is input to the node. In this process, for example, an audio signal from the microphone  112  is input to the node  101  of  FIG. 1  through the ADC  111  or a digital audio signal output from the mixer  121  is input to the node  102 . At step  302 , data of the channel in the received packet is updated by overwriting the data of the channel with the latest sample input at step  301 . At step  303 , the node discards data of the channel in the buffer B in which samples of one sampling cycle ago have been stored. At step  304 , the data of the channel of the buffer B is overwritten with data of the channel of the buffer A in which samples of the current sampling cycle are stored. At step  305 , data of the channel of the buffer A is overwritten with data of the channel of the packet. When data of a plurality of channels is input, the node repeats the above procedure. The data discarding at step  303  is automatically performed by the overwriting at step  304 . 
         [0050]    As described above, the node  101  is connected to a microphone  112  through an analog-to-digital converter  111  such that the analog-to-digital converter  111  converts an audio signal fed from the microphone  112  into audio sample data, and the node  101  writes the audio sample data fed from the analog-to-digital converter  111  into a region of a packet  150  so as to transmit the audio sample data to another node  102 . 
         [0051]      FIG. 4  is a flow chart of an audio data output routine in which each node outputs sample data read into a buffer to an external device. In this procedure, for example, the node  102  of  FIG. 1  outputs sample data to the mixer  121  or the node  103  or  104  outputs its sound through the DAC and the amplifier. At step  401 , the node determines whether or not output data of all channels, which will be output from the node, have been updated. This process is to determine whether or not all sample data of channels to be output has been gathered at a specific work area (for output data). If all the data has not yet been updated, the node (i.e., the current node) obtains, at step  402 , position information of a corresponding node at which a sample of the corresponding channel was stored. At step  403 , the current node determines whether the corresponding node is its upstream node (i.e., one in the range from the master node to its immediately previous node) or its downstream node (i.e., one in the range from its immediately subsequent node to a node immediately prior to the master node). If the corresponding node is an upstream node, the current node obtains sample data of the corresponding channel from the buffer B for one cycle ago at step  404 . If the corresponding node is a downstream node, the current node obtains sample data of the corresponding channel from the buffer A for the current cycle at step  405 . Then, the node updates the output data with the obtained data at step  406  and returns to step  401 . If the sample data of all channels to be output is gathered together as output data, the node proceeds from step  401  to step  407  and performs an audio data output time correction process which will be described later. Then, at step  408 , the node outputs audio data of channels, which is set to be output from the node, to a specified external device. 
         [0052]    As described above, one node  102  is connected to a mixer  121 . The node  102  outputs the audio sample data of the allocated channel to the mixer  121  so that the mixer  121  applies a predetermined signal process to the audio sample data transmitted from the node  102  and feeds audio sample data applied with the predetermined signal process. The node  102  includes an updating part that writes the audio sample data fed from the mixer  121  a region of a packet  150  so as to transmit the audio sample data to another node  103 . 
         [0053]    The node  103  is connected to an amplifier  132  of a speaker  133  through a digital-to-analog converter  131 . The node  103  outputs the audio sample data of the allocated channel to the digital-to-analog converter  131  so that the digital-to-analog converter  131  converts the outputted audio sample data to an analog audio signal, and the amplifier  132  amplifies the analog audio signal for driving the speaker  133 . 
         [0054]    In the above manner, each node performs the sampling cycle lag correction described with reference to  FIG. 8  and simultaneously outputs samples of the same sampling cycle. 
         [0055]    Although 2-stage buffers are provided in  FIG. 8 , (n+1)-stage buffers may be provided, where “n” is an integer equal to or greater than 1. In this case, n+1 audio sample data including a range of samples from samples read in the current sampling cycle to samples read in n sampling cycles ago are stored. When a sample in a packet is to be output from the current node which has received the packet, the current node may read and output a corresponding sample of n sampling cycles ago if a corresponding node which incorporated the sample into the packet is upstream of the current node and may read and output a corresponding sample of n−1 sampling cycles ago if the corresponding node is downstream of the current node. 
         [0056]    The audio data output time correction process of step  407  will now be described in detail. The above sample deviation correction can match output timings on a sample basis. However, each transmission path between nodes on the ring network causes a delay since sample data of each channel is transmitted by incorporating the data into a packet that circulates through the ring network. Although this delay may be very small and thus be negligible, even such a small delay may not be permitted in professional audio devices. The audio data output time correction process is performed to correct the delay due to the transmission path, thereby very accurately matching the output timings of audio signals from the node. 
         [0057]    The following is a detailed description of the audio data output time correction process. The following description will be given with reference to a network having a connection structure allowing signals to reciprocate through the network as shown in  FIG. 13 . It is also assumed that each node performs writing and reading of data to and from a packet on a forward path and the packet only passes through each node on a backward path. In the following description, “t(a)” denotes a delay time for a line length “a”, “T FT ” and “T FR ” denote forward-side transmission/reception times of each of the nodes  901  and  902 , and “T BT ” and “T BR ” denote backward-side transmission/reception times as shown in  FIG. 9 . In addition, “Total Delay” denotes a total delay time of the network, “Forward Delay” and “Backward Delay” denotes delay times of the forward and backward sides of the master node, and “Node (Name) Delay” denotes the difference between packet reception times of each node. That is, “Total Delay” is a delay time corresponding to a period of time during which the packet circulates through the network once. “Forward Delay” is a delay time corresponding to the time that elapses until the master node receives data at its forward side after the data is transmitted from the master node at its forward side. “Backward Delay” is a delay time corresponding to the time that elapses until the master node receives data at its backward side after the data is transmitted from the master node at its backward side. 
         [0058]    The term “forward side” refers to one side of the master node at which the master node starts transmitting a packet at the start of a sampling cycle and the term “backward side” refers to the opposite side. In  FIG. 9 , the right side is the forward side and the left side is the backward side. The same is true for  FIGS. 10 and 11 . 
         [0059]      FIGS. 10   a  to  10   c  illustrate delay times calculated by a master node.  FIG. 10   a  illustrates an example where a master node  1001  is disposed at a backward end of a network and a backward-side output of the master node  1001  is connected directly to a backward-side input thereof inside the master node  1001 .  FIG. 10   b  illustrates an example where a master node  1012  is connected between slave nodes  1011  and  1013  and the slave nodes  1011  and  1013  are connected respectively to a backward-side input and output and a forward-side input and output of the master node  1012 .  FIG. 10   c  illustrates an example where a master node  1023  is disposed at a forward end of a network and a forward-side output of the master node  1023  is connected directly to a forward-side input thereof inside the master node  1023 . The respective delay times “Total Delay”, “Forward Delay”, and “Backward Delay” of the examples are calculated as illustrated in  FIGS. 10   a  to  10   c.  Although the delay time calculation is exemplified by three nodes in these examples, the delay times are calculated in the same manner also when a larger number of nodes are connected. 
         [0060]    As described above, the master node determines the total network delay time, the forward-side delay time, and the backward-side delay time according to a length of lines “a” and “b” for connecting the nodes. The master node may determine either of the forward-side delay time and the backward-side delay time to be zero in case that either of the first part or the second part of the plurality of the slave nodes is not connected to the master node as depicted in  FIGS. 10   a  and  10   c.    
         [0061]      FIGS. 11   a  to  11   f  illustrate delay times calculated by slave nodes. Each slave node calculates, as a delay time, the difference between packet data reception times of the forward and backward sides of the slave node. This delay time is calculated using different calculation methods depending on the positional relationship with the master node. For example, slave nodes  1102  and  1103  (entitled “S3” and “S4”) which are located on the forward side of the master node  1101  calculate the delay time as “T FR -T BR ” as shown in  FIGS. 11   d  and  11   e.  On the other hand, slave nodes  1104  and  1105  (entitled “S1” and “S2”) which are located on the backward side of the master node  1101  calculate the delay time as “T BR -T FR ” as shown in  FIGS. 11   b  and  11   c.  In the connection example of  FIG. 11   a,  the master node  1101  calculates delay times as shown in  FIG. 11   f.    
         [0062]    As described above, the terminal slave nodes S 1  and S 4  calculate their reception time difference to be zero as depicted in  FIGS. 11   b  and  11   e.    
         [0063]      FIGS. 12   a  to  12   c  illustrate examples of calculation of a correction time for each node, which is the time by which the node delays the output time of sample data when outputting the sample data to an external device such as a mixer or an amplifier. The horizontal axis of  FIG. 12   a  represents elapsed time and symbols “S1”, “S2”, “M”, “S3”, and “S4” arranged along the vertical axis represent the master and slave nodes of  FIG. 11   a.  Arrows in “M         S 3           S 4           S 3           M         S 2           S 1           S 2           M”  FIG. 12   a  indicate that packet data circulates through the nodes along the arrows in the network structure of  FIG. 11   a,  where the right-down arrows indicate a forward path and the left-down arrows indicate a backward path. “1024” denotes delay times between the nodes when packet data circulates along the arrows. “1201” denotes the total delay, “1202” denotes the forward delay, and “1203” denotes the backward delay. 
         [0064]    Reference numeral “1213” in  FIG. 12   a  denotes the timing when the slave node S 3  receives a packet transmitted from the master node M. Upon receiving the packet, the node S 3  performs the above procedure of  FIG. 4 . Similarly, the node S 4  performs the procedure of  FIG. 4  at timing  1214 , the node S 1  performs the procedure at timing  1211 , and the node S 2  performs the procedure at timing  1212 . If the nodes do not perform the output time correction of step  407  in the procedure of  FIG. 4 , the nodes output audio data at different timings since the node S 3  outputs audio data at the timing  1213 , the node S 4  outputs audio data at the timing  1214 , and so on. Accordingly, in this embodiment, the timings  1213 ,  1214 ,  1211 , and  1212  of the nodes are delayed until the timing  1210 . The timing  1210  is the timing when the packet returns to the master node. If they wait until the timing  1210 , all the nodes gather together samples of the same sampling cycle. By simultaneously outputting the samples, it is possible to very accurately match the output timings of audio signals from the nodes. The delay time of each node until the timing  1210  is reached after it receives the packet is referred to as a correction time. In  FIG. 12   a, “ 1221” to “1224” indicate respective correction time calculation methods of the nodes. 
         [0065]      FIG. 12   b  illustrates a correction time calculation method for the forward-side slave nodes. The forward-side slave nodes are for example the nodes S 3  and S 4 . The correction time of each of the nodes S 3  and S 4  can be basically calculated as the time that elapses until a packet returns to the master node after the packet is input to the node through its backward-side input (T BR ) so that it can be calculated as “Total Delay−{(Total Delay)−(Backward Delay)−(Node (Name) Delay)}/2”. This expression can be rearranged to obtain an expression of  FIG. 12   b.  Equations  1223  and  1224  are obtained by substituting the equations of  FIG. 11   f  into the expression of  FIG. 12   b.    
         [0066]      FIG. 12   c  illustrates a correction time calculation method for the backward-side slave nodes. The backward-side slave nodes are for example the nodes S 1  and S 2 . The correction time of each of the nodes S 1  and S 2  can also be basically calculated as the time that elapses until a packet returns to the master node after the packet is input to the node through its backward-side input (T BR ) so that it can be calculated as “{(Total Delay)−(Forward Delay)−(Node (Name) Delay)}/2”. This expression can be rearranged to obtain an expression of  FIG. 12   c.  Equations  1221  and  1222  are obtained by substituting the equations of  FIG. 11   f  into the expression of  FIG. 12   c.    
         [0067]      FIG. 5  is a flow chart of a delay time calculation routine for a master node. At step  501 , the master node calculates a total delay time (Total Delay) of the network. At step  502 , the master node calculates a forward-side delay time (Forward Delay). At step  503 , the master node calculates a backward-side delay time (Backward Delay). At step  504 , the master node notifies all nodes of the calculation information. Through this procedure, the master node calculates each delay time described above with reference to  FIG. 10 . 
         [0068]      FIG. 6  is a flow chart of a delay time calculation routine for each slave node. At step  601 , each slave node obtains total network, forward-side, and backward-side delay time information (Total Delay, Forward Delay, and Backward Delay). At step  602 , the node determines whether it is located on the forward side or the backward side. If it is located on the forward side, the node obtains its packet time reception difference at step  603  and calculates a correction time using the mathematical expression of FIG.  12   b  at step  604 . If it is located on the backward side, the node obtains its packet time reception difference at step  605  and calculates a correction time using the mathematical expression of  FIG. 12   c  at step  606 . 
         [0069]    Through the procedures of  FIGS. 5 and 6 , the nodes can calculate their delay times described above with reference to  FIGS. 10 and 11 . Before audio data transmission is initiated, the slave nodes perform the procedures of  FIGS. 5 and 6  and calculate their correction times. 
         [0070]      FIG. 7  is a flow chart of an audio data output routine for a slave node. The node performs this procedure at step  407  of  FIG. 4 . The node resets a timer at step  701  and waits, at steps  702  and  703 , until the timer value exceeds the correction time which the node has calculated at step  604  or  606 . If the timer value has exceeded the correction time, the node returns to the procedure of  FIG. 4 . Thereafter, the node outputs audio data at step  408  of  FIG. 4 . In this manner, each node outputs audio data by delaying it by a correction time for the node as described above with reference to  FIG. 12 , thereby matching the output timings of all the nodes. 
         [0071]    While the above embodiments have been exemplified by the audio network system having the connection relationship as shown in  FIG. 13 , the invention can be applied to any audio network system which connects a plurality of nodes in a ring to circulate a packet through the nodes. In this case, each node obtains the time that elapses from when the node receives a packet to when the master node receives the packet and determines it to be a correction time. For example, when the system starts up, it may circulate a correction time measurement packet so that each node obtains, as a correction time, the time that elapses from a time when the node receives a packet to another time when the master node receives the packet. 
         [0072]    Lastly, the following is a detailed mechanism of time lags of samples created among the nodes. This description is simply intended to facilitate better evaluation of the invention.  FIG. 15   b  illustrates a ring audio network system in which nodes A to D are connected in such a manner that a signal reciprocates along the nodes as shown in  FIG. 13 . The node B is a master node. The master node B transmits a packet at the start time of one sampling cycle. 
         [0073]    Here, it is assumed that, as shown in  FIG. 15   a,  the node A stores data in a channel Ch 1  of the circulating packet, the node B stores data in a channel Ch 2  of the circulating packet, the node C stores data in a channel Ch 3  of the circulating packet, and the node D stores data in a channel Ch 4  of the circulating packet. It is also assumed that each of the nodes A to D reads data of all the channels Ch 1  to Ch 4 . Reference numeral “1501” is data which has returned to the master node B in a sampling cycle “t-1” and which contains data set in each channel in the sampling cycle “t-1”. When a hew sampling cycle “t” starts, the node B stores data of a cycle “t” in a Ch 2  region in the packet and transmits the packet as a packet  1502  to the next node C. The node C receives the packet as a packet  1503  and stores data of the cycle “t” in a Ch 3  region in the packet and then transmits it as a packet  1504  to the next node D. The node D receives the packet as a packet  1505  and stores data of the cycle “t” in a Ch 4  region in the packet and then transmits it as a packet  1506  to the path of a backward line. The packet only passes through the nodes C and B on the backward line without change as shown by “1507” and “1508”. The next node A receives the packet as a packet  1509  and stores data of the cycle “t” in a Ch 1  region in the packet and then transmits it as a packet  1510  to the next node B. The master node B receives the packet as a packet  1511  and waits until the next sampling cycle starts and then performs the same processes. 
         [0074]    While the packet circulates in the above manner, the nodes A to D read data of the channels Ch 1  to Ch 4  of the packets  1510 ,  1511 ,  1504 , and  1506  surrounded by ellipses, respectively. In this case, while the master node A and its immediately previous node B obtain samples of the cycle “t”, sample mismatching occurs in the nodes C and D. That is, each of the nodes C and D reads a mix of samples of the cycle “t-1” and samples of the cycle “t”. The sample mismatching occurs for the following reason. In the case where a packet circulates through nodes on a network, starting from a master node, once in a sampling cycle, data stored in a node in a sampling cycle “t” may be extracted from one different node in the sampling cycle “t” while it may be extracted from another different node in the next sampling cycle, depending on the arrangement of the nodes on the network (i.e., depending on the positional relationship of nodes in which data is stored and nodes from which data is extracted).