Patent Publication Number: US-2022224431-A1

Title: Methods for Transporting Digital Media

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Australian Provisional Application No. 2005902065, filed Apr. 22, 2005, and Australian Provisional Application No. 2005906272, filed Nov. 11, 2005. 
     BACKGROUND OF THE INVENTION 
     Technical Field 
     This invention concerns the transporting of digital media packets, such as audio and video. In a first aspect the invention concerns a data network able to carry high fidelity audio. In another aspect the invention concerns a network device for sending and receiving digital media packets; such as a Digital Signal Processing (DSP) chip for public address systems. In a further aspect the invention concerns a method of operating the data network or network devices and software for performing the method, for instance, on a personal computer. 
     Background Art 
     Audio and video signals have long been transmitted using application specific cables. For instance two-core speaker cable is used to carry left and right audio signals from amplifiers to speakers. 
     The time at which a received media signals are played out by a media device is called the playout time. Typically, a media device that receives media signals will playout the media signals by rendering them in some way. For example, if the media device is a loudspeaker it will render the audio media signal into sound. If the media device is a video screen it will render frames of the video media signal onto a screen. Alternatively, if the media device is a lighting control system it will render the lighting media signal by turning a spotlight on and off. 
     Real time transporting of digital audio and video and other digital media over data networks creates a new set of problems compared to non-media data. For instance, data networks may use packet switching in which data is divided into packets for separate transmission. As the packets are transmitted, sequential packets may take different routes and have different transit times. The packets are numbered to ensure they can be reordered correctly after arrival. This technique, however, does not suffice when left and right audio signals are to be received at different destinations, for instance at different speakers. 
     Unlike non-media data, digital media must be played out in synchronisation. For example, video and audio must be aligned in time so that when they are played out the images match the sound. 
     The concept of a network clock has been used to address timing problems in data networks. A network clock signal is typically generated at a specific point in the network and this becomes the system time signal received by devices on the network. The system time signal is then used as a time reference for every device that receives the system time signal. Because of the topology of the network, devices at different locations on the network will receive the clock signal with a phase offset from the network clock, depending on the propagation delay from the clock to the device. A further consequence is that the different remote devices will have received clock signals that have phase offsets with respect to each other, as well as with respect to the network clock. 
     Digital media transmission has historically embedded clocking information in the transmitted data. Embedding and recovering clocking information from data signal transitions or packet timing (e.g. AES3, SP/DIF, Gibson MaGIC) works well for point to point links between a small numbers of devices, but as the number of devices increases, clock jitter cascades and builds through each device that recovers and re-transmits the clocking information. Large systems employ a separate clock network to avoid such problems with clock jitter. 
     Digital media transmissions may alternatively employ a Time Division Multiplexing (TDM) approach. In TDM systems (e.g. MADI, CobraNet), a master clock device initiates periodic transmission cycles and each device is allocated one or more time slots within that cycle for transmission. This limits the total available number of channels. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a method of transporting multiple media channels over a network in a manner that meets quality and timing requirements for high quality audio and video reproduction without the need for specialised equipment, such as a separate clock network. The invention has the advantage of being able to use low cost data network cabling and equipment, such as switched Ethernet, to transport digital media. 
     The invention provides network devices having a local clock signal that is synchronised to a system time signal of the network for both rate and offset. Rate synchronisation helps to ensure the rate that the local clock ticks over at is the same as the rate of the system time signal. Offset synchronisation helps to ensure both the system time signal and local time signal share a common time reference point. The local clock signal governs the rate and offset of the received or transmitted media signals. Because each network device has a local clock signal that is synchronised to the system time signal, all received and transmitted media signals of the network are synchronised for both rate and offset. 
     Accordingly, it is a feature of this invention to provide a network and suitable devices for transporting digital media. The network includes a master clock device to generate a system time signal for the network and a plural number of network devices interconnected such that each device is able to send packets to other network devices, and to receive packets from other network devices. Preferably, each network device is also configured to receive media signals from a media device, to transmit media signals to a media device, or to do both. Typically, each network device is connected to the master clock device and uses the system time signal and a network time protocol to generate a local clock signal that is synchronised to the system time signal&#39;for both rate and offset, the local clock signal governing both the rate and offset of the received or transmitted media signals. The media packets may be audio or video, or a mixture of both. 
     Typically, each network device that receives media signals from a media device uses its local clock which is synchronised to the system time signal to packetise the media signals, and to timestamp the media packets with system time before sending them to another network device. In this way, media packets are generated and sent by all network devices in the network at a local rate that is synchronised to the rate of the system time signal. This helps to eliminate buffer overrun and underrun in the network. Further, the packets are timestamped with a synchronised time value. The timestamp may be the earliest system time at which the media signal contained in the packet was converted into digital form (i.e. generation time). 
     Each network device that receives media packets uses the packet timestamp to coordinate the media signals in time for playout. The coordination of media signals is achieved using the packet timestamps to re-order the media packets if required; and in addition to align and combine media signals received from different network devices, or to determine the appropriate playout time for media signals contained in the media packets, or both. Aligning media signals may comprise delaying the playout of one media signal with respect to another. 
     The appropriate playout time for media signals contained in a media packet may be different for each network device that receives the media packet. In this way, where there are multiple playout media devices, the playout time can be controlled based on the spatial location of each playout media device, so that media signals are played out by each media device is received at one particular spatial location at the same time. For, example in a rock concert the sound from the speakers on stage and in the auditorium should arrive at the audience at the same time. To achieve this, sound from the closer auditorium speakers can be delayed to allow the sound from the stage speakers to travel the extra distance through the air. 
     A media clock signal may be provided that is synthesized from the local clock signal. The media clock signal is able to drive conversion between analogue and digital media signals and to directly govern the rate and offset at which media signals are received or transmitted (e.g. are produced or consumed) by the network device. Because the media clock is synthesised from the local clock signal, which is in turn synchronised to the system time signal, the rate that media signals are produced and consumed is governed in a manner that eliminates buffer overrun and underrun in the network devices. 
     Techniques that can be applied to synthesise a media clock from the local clock signal include Direct Digital Synthesis (DDS), Digitally Controlled Oscillators (DCO) or Voltage Controlled Oscillators (VCO) controlled by a Digital to Analogue Converter (DAC). The media clock may be synthesized by using a software timer that is caused to track the synchronised local clock, to directly govern the rate and offset of the received or transmitted media signals. 
     The rate of the media clock signal may be different to the rate of the local clock. For example, the rate of the media clock may be a multiple of the rate at which the media signals are generated by the media device, for example a multiple of the rate at which digital audio samples are produced. Use of a network time protocol to synchronise the local time to the system time signal on the network de-couples clocking and clock synchronization from data transmission. This simplifies support for simultaneously transmitting at multiple data rates, such as 44.1 kHz/48 kHz/96 kHz audio by different media clocks, and eliminates the need to run a separate clock network to each network device. Because the media clock can derive any sample rate or frame rate it likes from the system time signal, the network as a whole is not constrained to a single sample rate. One or more media devices may be connected to a network device to generate and deliver media signals, to receive, to process and to deliver media signals, or to receive and playout media signals. 
     The jitter experienced by network time protocol packets is minimised by using a Quality of Service (QoS) technique. The QoS technique may operate in the network to expedite the forwarding of packets having a higher priority than other packets. For example, packets may be prioritised based on three categories. Packets that contain network time information may be prioritised over audio media packets. In turn, audio media packets may be prioritised over best effort traffic. By placing the highest priority on packets that contain network time information, the QoS scheme is able to optimise the maintenance of the timing synchronising of the network devices. 
     According to one embodiment of this invention, if two network devices seek to send media packets to another network device, they each may select a random start time and send media packets from their respective random start time at a regular rate. In this way, two network devices sending media signals to be played in synchronization do not cause a bottleneck in the network that would otherwise increase the jitter in the network. 
     The network time protocol (e.g. according to IEEE 1588) distributes absolute time from a master clock. The network time protocol may use a bi-directional exchange of messages to enable the calculation of the transmission delay between master and local slave clocks which can be used to calculate a more accurate estimate of the local clock offset. This enables the local clocks to compensate for variable network delay and achieve tighter synchronization regardless of their location in the network topology. 
     The network time protocol may operate to elect a master clock device where multiple possible master clock devices exist on the network. For example, in the IEEE 1588 network protocol, the best master clock and backup master clock are selected from a set of potential candidates. The backup master clock uses the IEEE 1588 network time protocol to synchronise the backup clock with the elected master clock. If the selected master clock fails, the backup master clock operates to take over by providing the system time signal for the network. Network devices that depend on a master clock can continue to operate smoothly. 
     Further, the network time protocol may support multiple master clock devices such that different network devices are connected to different master clocks. For example, the data network may be comprised of two distinct audio networks in different locations each having a IEEE 1588 master clock. The distinct networks may coordinate their master clocks via another mechanism, for example GPS time synchronisation. This allows the two networks to share a common notion of time and send timestamped media packets to each other. 
     The network may include an Ethernet network. Ethernet networks are able to support multicast transmission in which network packets are duplicated in network switches, thus efficiently supporting large numbers of receiving network devices. The network devices may be connected directly, or indirectly to the master clock. The architecture of this invention scales with increasing Ethernet bandwidth (e.g. 100 Mbit/second to 1 Gbit/second). Increasing bandwidth implies increased total throughput as well as decreased latency and jitter. A heterogeneous network combining network devices connected via 100 Mbit/sec links interconnected by 1 Gbit/sec trunks is also enabled. 
     The playout time for media signals must take account of network transmission delays, network time protocol synchronization errors, media clock synthesis errors, sender timer jitter and network jitter. These are all factors which may delay or produce the appearance of the delay in the receiving of media packets. The playout time of received media signals must be delayed enough to allow for late arrival of media packets due to any of these causes since if a playout time is selected that is too early, any delayed media packets containing media signals that must be played out in synchronization with the received media signals will not be available for playout when required. The media packets are buffered until their playout time in order to compensate for delays in transmission and clock synchronization errors. Each network device may include a jitter compensation buffer provisioned according to the maximum expected end-to-end delay such that there is never a buffer underrun. Media packets arriving are stored in the jitter compensation data buffer until the correct playout time arrives. 
     In another aspect the invention provides a data network device for connection to other network devices to transport media packets between them. The network device includes a media port to receive media signals from a media device, to transmit media signals to a media device, or both; a master clock to generate a system time signal, or a clock port to receive a system time signal; and a local clock generator to use the system time signal and a network time protocol to generate a local clock signal that is synchronised to the system time signal for both rate and offset, the local clock signal governing both the rate and offset of the received or transmitted media signals. 
     In use, when the network device receives media signals from a media device it may use its local clock to packetise the media signals, and to timestamp the media packets with system time before transmitting them to another network device. 
     In a further aspect, the invention provides a method of operating a data network device to transport media packets. The method includes steps of generating or receiving a system time signal for the network, and configuring a plural number of the network devices such that each is able to send packets to and/or receive packets from other network devices. It further includes steps of configuring the network devices to receive media signals from a media device, transmitting media signals to a media device, or both, and connecting each network device to receive the system time signal. Finally a step of operating each network device to use the system time signal and a NTP to generate a local clock signal that is synchronised to the system time signal for both rate and offset, the local clock signal governing the rate and offset of the received or transmitted media signals is performed. 
     The method may further comprise receiving media signals at a network device, packetising the media signals and timestamping the media packets with system time, using the device&#39;s local clock, and then transmitting the media packets over the network to another network device. The method may further comprise receiving a media packet at a network device and using the packet timestamp to reorder media packets; and, to align and combine media signals from different network devices, or to determine the appropriate playout time for media signals contained in the media packets, or both. If desired a software program can be used to implement the method as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a simple network using the invention; 
         FIG. 2  is a block diagram of a network device that is able to receive, send and process media packets; 
         FIG. 3  is a block diagram of a programmable logic device configured for direct clock synthesis; 
         FIG. 4  is a clock diagram showing PMW modulation clock synthesis technique; 
         FIG. 5  is a waveform diagram, showing PWM modulation; 
         FIG. 6  is a waveform diagram showing the effect of the local high frequency clock being interrupted; and 
         FIG. 7  is a block diagram of a typical implementation of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Overview of the Components of the Network 
     Referring first to  FIGS. 1 and 2 , a data network  100  comprises a master clock device  102  to generate a system time signal  104  for the network  100 . Two network devices  108  and  110  are connected to each other by a network  106  so that they are able to send and receive media packets. The network devices  108  and  110  are also connected to media devices  112  and  114  respectively which are able to generate and/or playout media signals. The network device  108  is able to receive and packetize media signals to be sent as media packets over the network  106 . The network device  110  is able to receive pacts and transmit media signals contained in the media packets to the media device  114 . Network devices  108  and  110  each contain a local clock  122  and  124  and a media clock  123  and  125  respectively. 
     The network includes a network time protocol (NTP)  120 . A NTP  120  is a set of network messages used to synchronise a clock of one device with a clock of another device. In this case, the local clocks  122  and  124  are synchronised with the master clock  1 Q 2  using the NTP  120  and the system time signal  104 . The network messages sent by the NTP  120  includes the sending of packets on the network  106  that relate to the system time. There are various known standard NTPs, for example, the IEEE 1588 Precision Time Protocol, and the IETF NTP. 
     Media clock signals  130  and  132  are derived (i.e. synthesized) from the local clock signals (i.e. Local TOD signal)  126  and  128  respectively. The NTP uses a bi-directional exchange of messages to enable the calculation of both clock offset and rate. 
     Clock rate synchronisation ensures that the rate at which the network devices  108  and  110  send and/or receive data packets is the same to within a desired accuracy. Clock offset synchronisation ensures that the time difference from the master clock  102  to the local clocks  122  and  124  is the same to within a desired accuracy. In this way any two clocks in the network have a bounded rate and offset error. Rate synchronisation also ensures that the media signal is produced and consumed by the network devices at a rate that is derived from the rate of the master clock. The derived rate of the network device may be different from the rate derived by another network device on the network. The derived rate (local clock frequency) may be related to the sample rate of the media signals that the network device is converting into packets (i.e. 256 for 48 Hz or 256 for 44.1 Hz). Rate and offset synchronisation enables the local clocks  122  and  124  to compensate for variable delays (i.e. days in the reception of media packets) and achieve tighter synchronisation regardless of the location of the network device  108  and  110  in the network  100  topology. 
     The synchronized local clock  122  is used to timestamp media packets with the earliest system time (that is also the local time  126 ) at which the media signal contained in the packet was converted into digital form by the data converter  140 . Using the timestamp, the network device  110  that receives the media packet can coordinate the correct playout time for the media signals. 
     Overview of the Operation of the Network 
     In reference to  FIG. 1 , operation of the sending network device  108  will now be described. Incoming analogue media signals produced by the media player  112  reach the analogue to digital converter  140  of the network device  108 . The rate that the analogue to digital converter  140  converts the media signal is governed by the media clock  130 . The digital signal produced by the analogue to digital converter  140  is passed to a data packetise and timestamp buffer  142  for collection into media packets. The media packets are timestamped with the earliest system time at which the media signal contained in the packet was converted into digital form. 
     The local clock  122  supplies rate control, and offset control to the packetising and time stamping of the media packets via link  130 . Since the local clock  122  is synchronised with the master clock  102 , the rate that the media packets are produced is at the same rate as the system time signal  104  of the master clock  102 . This will also be the same rate of the local clock  124  of the receiving network device  110  so the rate that the entire network  100  produces and consumes media packets is synchronised. The local clock  122  is also synchronised to the master clock  102  for offset. The adjustment of the local time to synchronise with the time of the master clock  102  is achieved using an offset amount. The local time offset from some epoch (e.g. seconds since 00:00:00 Jan. 1, 1970) tracks the master clock time. The packets are then passed to the network port  144  for transmission on the network  106  for delivery to the network device  110 . 
     Operation of the receiving network device  110  will now be described. Incoming packets are received from the network  106  in a jitter compensation data buffer  148  (described in more detail below) where they are delayed to account for the maximum expected latency variation (or jitter) between the sender  108  and the receiver  110 . The receiver  110  uses the time stamps of the received packets to reorder the packets if necessary. The receiver  110  may align and combine media signals received from different sources. Further, the device  110  also determines the playout time for the media signals. The media signals are then passed to the digital to analogue converter  150  for conversion to analogue at a rate controlled by the media clock signal  132  of the media clock  125 . The media signal is then sent to a media device  114 , for instance, for playout. 
     The device  111  shown in  FIG. 2  is able to perform the functions of both network devices  108  and  110 . Further, this network device  111  can be used for processing media signals in a digital form. In this case media packets are received from one or more senders at the network port  144  and processed within the network device  111  at processor  146 . The timestamps of the received media packets are used to align the digital media signals of the packets in time, if necessary. Processing takes place to produce a new set of digital media signals (e.g. mixing a set of audio channels to a left/right stereo channel pair). This processing takes place at the rate and offset controlled by the local clock  122 / 124  via link  149 . The new digital media signals are placed into packets and transmitted at a rate determined by the local clock signal  149  and with timestamps that are determined by the local clock offset. The processed packets are then retransmitted from the network port  144 . Rather than generating a new timestamp for outgoing packets from the local clock  122  and  124 , it is also possible to copy a timestamp (i.e. the offset) from an incoming packet to the outgoing packet, thus preserving the time that the media signal was originally generated. In this way, media signals can be processed purely in a computer domain by the components enclosed by  147  which may be part of a personal computer. 
     Any clock left to itself will tick along at a certain rate which will be slightly different for each piece of hardware. The process of disciplining a clock adjusts the rate and offset of the clock to track another reference clock (in this case the master clock  102 ). The process of clock synchronisation and synthesis will now be described in more detail. 
     Local Clock Rate Synchronization 
     The local clock signal  126  is a local representation of time at the network device  108 . The local clock  122  and  124  is synchronised to the data network master clock  102  using the NTP  120 . The local clock signals  126  and  128  are generated by a local oscillator. Even if each of the local oscillators have the same nominal frequency (e.g. 12.288 Mhz), their actual frequency (or rate) may be slightly different. In addition this rate may drift with time due to effects such as ambient temperature variations. A local clock  122  and  124  is considered synchronized to the master clock  102  if its rate is actually the same as the master clock  102  to within a desired accuracy. 
     The local clock signal  126  and  128  may be an electrical signal (e.g. produced by a Voltage Control Oscillator (VCO)) or it may be represented as software counters maintained by an operating system date/time facility. In all cases, the local clock signal  126  and  128  is governed (i.e. disciplined) so that it advances (ticks) at the same rate as the master clock  102 . 
     Local Clock Offset Synchronization 
     The local clock signal  126  drives a time of day (TOD) clock which can be expressed as a number of seconds since an epoch. For example, Network Time Protocol version 3 (NTPv3), expresses time as two 32 bit numbers corresponding to the number of seconds and fraction of a second elapsed since 00:00:00 Jan. 1, 1900. The TOD clock is used to timestamp media packets. The epoch for this TOD clock is a global constant for the network. The availability of a global timestamp enables media signals originating from different sources to be time-aligned and combined accurately by the receiving network device  110 , for example, for playout. A local clock  122  and  124  is considered to be offset synchronized to the master clock  102  if its time difference from the master clock  102  is the same to within a desired accuracy. If the time difference between master clock  102  and any local clock  122  and  124  is less than a ¼ of the sample period (5 μs for a 48 Khz signal) then any two local clocks  122  and  124  are synchronised to each other to within ½ of the sample period. This enables samples generated simultaneously at two different sources  112  to be accurately aligned for playout. 
     When the local clock signal  126  is an electrical signal (e.g. from a VCO), clock pulses increment a counter value which represents absolute time. The counter can be read to produce a timestamp which can be compared with timestamps from the master clock  102  (via the network time protocol  120 ) in a phase locked loop to achieve absolute time (offset) synchronisation in addition to rate synchronisation. 
     Accurate clock offset synchronization requires a network time synchronization protocol with a two way exchange of messages. This enables the local clock  122  and  124  to calculate the network delay between it and the master clock  102  and compensate for it. 
     Two standard network time protocols  120  which can be used with this invention are the Network Time Protocol (NTP), and the IEEE 1588 Precision Time Protocol, NTP Version 3 is widely implemented and has been documented by the Internet Engineering Task Force (IETF) in RFC1305 as an internet draft standard. Additional NTP information can be found at http://www.ntp.org/. IEEE 1588 is a published standard of the IEEE (Std 1588-2002) and is available from http://standards.ieee.org/. Additional IEEE 1588 information can be found at http://ieee1588.nist.gov/. 
     IETF NTP  120  is used on general purpose computer systems and can readily achieve millisecond time synchronization accuracy in a local area network. IETF NTP  120  is slave driven in the sense that a request from the slave (i.e. here the network device  108  or  110 ) results in a two way message exchange, which enables the slave to calculate both its time offset from the master clock  102  and the network delay. 
     Herein, IEEE 1588 is the preferred time synchronisation protocol  120 , although many NTPs may also be used. In general, timestamping packets as close to the network transmission or reception time as possible reduces the error due to end-system jitter. Timestamping can be performed (in preference order) in hardware, in a device driver or in an application. 
     IEEE 1588 was designed for use with industrial control and measurement systems and is suited to accurate hardware implementation. Hardware implementations have been shown to achieve sub-microsecond time synchronization accuracy. Implementation of IEEE 1588 Border Clocks in switches eliminates the jittering of clock packets as they pass through them. IEEE 1588 uses frequent multicast messages from the master to calculate the offset. It uses a less frequent delay request message from the slave, resulting in a delay response message from the master to calculate the delay. 
     It may also support redundant master clocks by including a mechanism where another master clock takes over if the original master clock  102  fails. The invention can use the combination of two separate IEEE 1588 networks each having their own master clock, for example, a recording studio in Sydney, Australia and a recording studio in Melbourne, Australia. These two networks may coordinate their master clocks via another mechanism, for example GPS time synchronisation. This allows the two networks to share a common notion of time and send timestamped packets between each other. 
     The system may also be implemented using a set of network devices which synchronize with one master, while another set of devices synchronize with a different master on the same network, either at different times or simultaneously. As an example, a set-top box synchronized to an external video source may act as a master clock when viewing a video, whereas the amplifier in an audio system may act as the master clock for music. 
     The network time protocol  120  exchanges messages at a regular rate (e.g. every 1 second). By making this interval a non-multiple of media sample rates (i.e. 48 khz or 44.1 Khz) the possibility of the clock packets being encountered and being jittered by a media packet in transmission on the network  106  is minimised. 
     The NTP IEEE 1588 is discussed further next. In the IEEE 1588 protocol, the best master clock  102  and a backup master clock (not shown) are elected from a set of potential candidates. The elected master clock  102  may be a local clock  122  of a network device  108 . The backup master clock uses the IEEE 1588 network time protocol  129  to synchronise its clock with the elected master  102 . In the case that the master fails  102 , the backup master takes over and other local clocks that were previously synchronised with the elected master clock  102  will now synchronise with the backup master clock and the network  100  continues to operate smoothly. 
     Media Clock Synthesis 
     Clock synthesis is a widely studied problem and various techniques can be applied to synthesise a media clock  123  and  125  from the local clock signal, including Direct Digital Synthesis (DDS), Digitally Controlled Oscillators (DCO) or Voltage Controlled Oscillators (VCO) controlled by a Digital to Analogue Converter (DAC). 
     A digital media signal has an implied clock. For an audio media signal, this can be the audio sample rate (e.g. 48 kHz). For a video media signal, this can be the number of frames per second. Hardware that produces or consumes digital media signals often uses a multiple of the sample rate or frame rate. For example, analogue-to-digital converters  140  commonly need a clock that is 128× or 256×the rate at which audio samples are actually produced. We term this clock multiple the media clock  123  and  125 . Media clock synthesis is the process of deriving a media clock signals  130  and  132 , that is a digital to analog converter (DAC) word clock, from the network time protocol  120 . 
     At least three techniques can be used to derive a media clock: (i) directly disciplining a hardware clock from the network time protocol, (ii) controlling a pulse width modulated media clock  123  and  125  with a master clock  102  that has been disciplined by NTP/1588, and (iii) controlling a software timer from a master clock  102 . These techniques are discussed next. 
     (i) Direct Clock Synthesis 
     Direct media clock synthesis can be implemented by using a programmable logic device such as an field programmable gate array (FPGA). Referring to  FIG. 3 , the frequency synthesizer block  200  generates the local clock f LOCAL  from a clock source f SRC    199  under the control of the network time protocol  120 . The ratio of the nominal frequencies of f SRC /f LOCAL  is preferably greater than two. Preferably, either of two possible implementations of the frequency synthesis block  200  to generate the local clock frequency f LOCAL  can be used. Both implementations use an M bit accumulator. 
     In a first implementation, the most significant bit (MSB) of the M bit accumulator is a square wave of frequency f LOCAL . This waveform is jittered (i.e. delayed), however, by 1/f SRC  whenever the accumulator builds up enough phase error to output an extra pulse. If f SRC  is selected to be an integer multiple of f LOCAL  then these phase shifts are required only to compensate for the frequency offset from nominal and drift of the source clock f SRC    199 . These occur at a very low frequency and while they are large, have been shown to be inaudible jitter. 
     In the second implementation very low jitter f LOCAL  can be synthesized by using “Direct Digital Synthesis” (DDS). DDS involves using P significant bits of the M bit phase accumulator to address a sine wave lookup table. This is then input to a DAC  208  whose output is an analog sine wave of frequency f LOCAL . This sine wave is filtered to remove harmonics and a comparator is used to generate a local clock with very low jitter. The frequency of f LOCAL  can be tuned to the resolution of 2 M . 
     For both implementations, the output of the frequency synthesis block  200  is the local clock f LOCAL . This signal is used to clock an N bit counter  202 . The N bit counter  202  provides the local TOD clock and is used by the timestamper  204  to timestamp sent and received network time protocol and media packets. The local clock frequency is further divided by the frequency divider  206  to generate the media clock f MEDIA , which drives the ADC/DAC  208 . Alternatively, the local clock signal f LOCAL  could be provided directly to the ADC/DAC  208  from the frequency synthesis block. In this case there would be no separate media clock signal. Instead, the local clock signal could be used to control the rate that media signals are produced and consumed by the network device. 
     The method is as follows:
         1. The network time protocol runs at T second intervals and calculates the offset O between the local and master clocks. This offset may be a result of both a time offset between the local and master clock, and an offset error due to a frequency difference between the two clocks. Calculate a frequency f LOCAL  at which the local clock needs to run at over the next T seconds to compensate for this error as:       

     
       
      
       f 
       LOCAL 
       =f 
       LOCAL_NOMINAL 
       +O/T  
      
     
     2. Calculate a phase accumulator tuning word 
         TW= 2 M   *f   LOCAL   /f   SRC            where TW is the input to the M bit accumulator  200  and is added to its output every 1/f SRC  seconds. The NTP protocol time processes the network time protocol messages and uses the locally generated transmit and receive timestamps to compute an updated tuning word (TW). The tuning word adjustments may be smoothed to remove the impact of jitter in the time offsets due to network jitter by using a PLL, instead of making instantaneous adjustments.       
     (ii) Pulse Width Modulation (PWM) Clock Synthesis 
     The PWM clock synthesis technique for media clock synthesis can use commercially available digital processor (DSP) chips  220  and  222  (see  FIG. 4 ), a time of day clock disciplined by NTP or IEEE 1588 as found in many embedded operating systems, a local high frequency (f SRC  Hz) clock source  199 , a PWM counter output pin and a programmable counter that can generate interrupts every P  180  cycles of the local high frequency clock. The PWM counter can be programmed to repeatedly output logic HI for a count of M cycles of f SRC , then logic LOW for N cycles of f SRC . 
     As shown in  FIGS. 5 and 6 , the method involves: 
     1. Estimate, as described above, the frequency of the local high frequency clock f SRC    199  using the programmable counter  220 . For example, if the programmable counter generates an interrupt every P clock cycles and the system time of day clock is recorded each time an interrupt is received, f SRC  may be estimated: 
         f   SRC   =P /( t   INTERRUPT[N+1]   −t   INTERUPT[N] ) 
     2. Compute nominal PWM counter values for N  181  &amp; M  182  that will produce a clock frequency as close as possible to the desired f MEDIA : 
       ( N+M )= f   SRC   /f   MEDIA    
     3. Set the programmable counter to throw an interrupt every P=k*(N+M) cycles of f SRC  as shown in  FIG. 6 . Each time this interrupt fires, the values of M  182 , N  181  and P  180  are updated. For an audio system, preferably P  180  is chosen so that jitter introduced by clock adjustments is inaudible (e.g. a 1 Hz interrupt rate). Note that in each clock update period, the clock can be corrected in multiples of k/f SRC  seconds. 
     4. Initialise the residual synthesized clock error (to zero, say): 
       E RESIDUAL =0 
     5. Each time the interrupt fires: 
     a. Estimate the source clock rate f SRC  as in step 1. 
     b. Compute new (M+N)=f SRC /f MEDIA . 
     c. Compute the error between the synthesized clock and the time of day clock. 
     
       
      
       t 
       EXPECTED 
       =t 
       INTERRUPT[N] 
       +k/f 
       MEDIA  
      
     
     
       
      
       E=t 
       EXPECTED 
       −t 
       INTERRUPT[N+1] 
      
     
     d. Add computed error to residual synth clock error 
     
       
      
       E 
       RESIDUAL 
       =E 
       RESIDUAL 
       +E  
      
     
     e. Amortise residual clock error over the next k periods of the media clock 
       ( M+N )=( M+N )+quotient( E   RESIDUAL /( k/f   SRC )) 
         E   RESIDUAL =remainder( E   RESIDUAL /( k/f   SRC )) 
     f. Program the PWM counter with the new values for N and M. 
     g. Update the value of P=k*(M+N). 
     h. Program the interrupt period with the new value of P. 
     (iii) Software-Only Systems 
     The software-only system approach for media clock synthesis is applicable in systems that do not discipline a hardware media clock  123  and  125 . A software master clock  102  is a timer implemented in software. The timer is a common feature of operating systems. An application may request to be signaled after T microseconds have elapsed, or may request to be signaled at a particular (future) TOD instant. An example is a general purpose computer sending packetised audio data coming from a file over a network. The “media clock” in these systems is the timer used to schedule the transmission of the next media packet. It will be apparent that a software timer may be caused to track the synchronized local TOD clock. 
     One approach for implementing such a system is:
         1. T=1/f S —the period of the sample clock
           S PP =number of samples per packet (e.g. 10)   Record a start time t START =Read-TOD-current-value( )   Initialize a packet counter n PKTS =0   
           2. Set timer to fire in S PP *T seconds from now   3. [TIMER FIRES]
           Send a packet with S PP  samples of audio data   
               

         n   PKTS   =n   PKTS +1 
     
       
      
       t 
       EXPECTED 
       =t 
       START 
       +n 
       PKTS 
       *T*S 
       PP  
      
     
         t   NOW =Read-TOD-current-value( )         4. Set timer to fire in S PP *T−(t NOW −t EXPECTED ) seconds from now   5. Back to step 3 when the timer fires.       
     In many systems, software timers are not always guaranteed to arrive promptly. If such a software timer arrives late, packets may have not been sent because the timer signal did not arrive. In this case the system “catches up” by sending the packets that were not sent. It will be apparent that a system in which timers may fire later than one packet-time from when they are scheduled to arrive will need to send additional packets at step 3 from time to time to catch up after late timer arrivals. 
     Media Clock Offset Synchronisation 
     Media clocks offset synchronisation is achieved by: 
     1. counting ticks of the media clock since a nominated start time, 
     2. periodically computing a media clock absolute time 
       ( T =start-time+ N *media_clock_period), 
     3. comparing the media clock absolute time with the master clock time to produce a time difference, and 
     4. increasing or decreasing the media clock frequency to minimise the time difference. 
     A wide variety of known phase locked loop techniques may be employed to increase or decrease the media clock frequency and so implement this control loop. The process of counting media clock ticks since a designated start time assigns an absolute time to each edge of the media clock and the control loop acts to ensure that the media clock edges are aligned with the master clock. 
     To illustrate, the counter  202  in  FIG. 3  counts pulses of f LOCAL  (a multiple of f MEDIA ). This count in combination with a start time is used to produce a local TOD clock. Timestamps taken using the local TOD clock are then compared to timestamps from the master clock in the network time protocol messages  121  and the difference used to update the DDS tuning word (TW)  200  thus varying media clock frequency and offset. 
     Jitter Constraining QoS Scheme 
     Further to clock synchronisation, a QoS scheme can be used to control overall jitter and delay in a network. Typical components of a media networking QoS scheme technique used for this invention include: 
     1. Classification of Packets According to Priority 
     This can be implemented by inspecting packet fields (e.g. 802.1Q priority bits, IP Diffsery Code Point, UDP or TCP port numbers) set by the source. Packets not labeled with a priority value are classified as “best effort” traffic. 
     2. Expedited Forwarding of High Priority Packets 
     In this approach, high priority packets are transmitted preferentially, minimizing the time they spend queued in switches of the network  106 . Various mechanisms are commonly available including “Strict Priority” and “Weighted Fair Queuing” scheduling. A source may also implement a priority scheduler whereby it ensures that high priority packets are transmitted preferentially. 
     3. Preventing Over-Subscription of the Network Using Admission Control 
     To ensure a given upper bound for jitter and to ensure that the network is operated within total available capacity, bounds on maximum network utilization of each traffic class are enforced. Clients may use a signaling protocol such as RSVP to discover whether the network will support additional network flows before transmitting packets into the network. In simple network topologies where a network of switches is connected via an over-provisioned backbone, the link between the client and the next hop switch is the bottleneck. In this case the client may perform a local admission check without needing to use hop by hop RSVP signaling 
     Three approaches for QoS techniques in a network transmitting digital audio data are: 
     1. Three levels ofpriority—no NTP/1588 support in the switch. 
     Highest Priority: network time protocol traffic (NTP/1588) 
     Middle Priority: packets containing digital audio data 
     Lowest Priority: best effort traffic 
     Expedited forwarding is enabled using Weighted Fair Queuing or Strict Priority. Highest priority clock packets may still experience some jitter due to queuing behind low priority packets already in transit in a switch. While filtering of jittered packets can be used to improve performance, this reduces the clock synchronization accuracy achievable. The network diameter (maximum number of hops) is typically restricted to stay within desired clock synchronization accuracy, end-to-end latency and accuracy of sample alignment in the network. This may only be required for certain critical paths of the network with laxer timing requirements being acceptable on other paths. The bandwidth requirement for an audio stream can be calculated knowing its bit depth and sample rate. The admission control mechanism is used to ensure that the bandwidth required by the sum of all streams traversing any particular network link does not exceed the link capacity. 
     2. Two levels of priority—1588 border clocks in switches 
     If the network switch supports the IEEE 1588 network protocol, it will consume and regenerate the network time protocol packets rather than forwarding them. In this case, only two priority levels are needed: 
     Highest Priority: packets containing digital audio data 
     Lowest Priority: best effort traffic 
     Expedited forwarding is enabled using Strict Priority or Weighted Fair Queuing. The bandwidth requirement for an audio stream can be calculated knowing its bit depth and sample rate. The admission control mechanism is used to ensure that the bandwidth required by the sum of all streams traversing any particular network link does not exceed the link capacity. 
     3. Over-provisioned, audio-only network. 
     For networks with few devices, few audio channels and constructed using high speed links, such as a set of devices connected to a single gigabit Ethernet switch, the network has more capacity than is required. Provided the network is only carrying audio and network time protocol traffic, priority schemes, expedited forwarding and admission control are not necessary. This is because the network is so lightly loaded that it does not introduce significant jitter. 
     The above list of QoS schemes for transmitting digital audio data is not exhaustive, and may be extended to support video traffic or multiple classes of audio traffic with different latency requirements by adding additional priority levels. 
     Playout Time 
     Receivers  110  compute a playout time by adding a latency time interval, compensating for network jitter and timing errors, to the source timestamp. The additional latency required can be easily measured at each receiver  110  by examining the incoming timestamps from each distinct source to the local clock  124 . Pre-recorded data can be sent early, relying on the receiver to use the timestamp to compute the correct playout time. Sequences of event based data, such as MIDI or lighting control commands, can be supported. 
     An appropriate playout time can be represented as NOW+D. The value for D can by estimated by recording the worst case difference between the time at which a packet was received, as measured by taking a timestamp using the local clock at the instant the packet arrives, and the timestamp placed in the packet by the sender  108 . 
     Pre-recorded audio data, or event sequences can be sent with a timestamp in the future, enabling them to be buffered by the receiver  11 Q until the playout time arrives. For example, consider lighting control. The message TURN_ON can be sent to three different lights, one after the other, with the same timestamp, 1 second in the future from now. Each light will buffer the message until the playout time arrives, and although the messages were sent at different times, the lights will all turn on at the same time. In another example, a sequence of TURN_ON messages can be sent to a light with a timestamp of NOW+1s, and immediately another message is sent with a TURN_OFF message to the same light, but with a timestamp of NOW+10s. The light processes each message when the playout time arrives, turning the light on for 9 seconds at one second after the messages were sent. In this example, the lighting control messages are sent “early,” with timestamps in the future. 
     Jitter Compensation Data Buffers  142  and  148   
     The playout time for a particular packet must take account of network transmission delays, network time protocol synchronization errors, media clock synthesis errors, sender timer jitter and network jitter. The playout time of received packets must be delayed enough to allow for late arrival of packets due to any of these causes. The length of the jitter compensation buffer  148  must be provisioned according to the maximum expected end-end delay between that sender  108  and receiver  110  such that there is no buffer underrun. Packets arriving are stored in the jitter compensation data buffer  148  until the playout time arrives. 
     For example, if packets may be delayed by at most D microseconds traveling between the source and a receiver, the system can safely choose a playout time of NOW+D microseconds. The packets that are received between NOW and NOW+D are stored in the buffer to ensure that by the time NOW+D arrives, all delayed packets have been received. Playout of each packet occurs at D microseconds after the timestamp it contains. 
     Each receiver  110  will independently determine an appropriate playout time for audio from a particular sender  108 . Under system control, two receivers  110  may choose to synchronize their playout times. For example the right and left channel in a home stereo would choose the same playout time for the right and left channel sound to be synchronized. Receivers  110  in different parts of the network also may choose differing playout times. For example in a live concert, the delay from the performer to the speaker providing the musicians mix is critical, thus the network would be designed (i.e. by ensuring that high speed links are used and that the number of switch hops is minimised) such that the network delay in this path is minimal and the speaker uses an earlier playout time. On the other hand, the delay to the reinforcement speakers in the hall is less critical and may involve more network hops. These receivers could use a later playout time than the musicians speaker. In addition, arrays of reinforcement speakers in a hall could also select different playout delays selected such that the phase relationship between them makes for optimal listening. 
     Source timestamps can also be used to align and combine data from different sources even when the delays through their respective network paths to the receiver are different, for instance in audio mixing. Audio playout may also be synchronized with non-streaming data such as MIDI by aligning the timestamps in the MIDI data with the timestamps in the audio data. This same approach applies for any timestamped media, video, lighting control. A given receiver  110  may pick different playout times for each audio channel as described above. If the system is minimising the latency through the network  100  for each audio channel independently, then the delay (D) can be estimated for each channel and the playout time NOW+D can be calculated for each channel separately. The above functionality is not possible if the audio packets are timestamped with a playout time, rather than the generated time. 
     An implementation on a general purpose computer system requires a jitter compensation latency of the order of 5-10 ms, primarily due to timer jitter in the sending system and NTP synchronization errors. The minimum jitter compensation latency for a tightly synchronized system (e.g. a hardware implementation using IEEE 1588) is one packet interarrival time—just as the current packet is consumed, the next packet arrives. 
     Since media packets are generated at regular intervals, if all sources connected to a switch generate a packet at the same time, they would be passed through the switch as a bunch. If this bunch of packets then encounters a similar bunch of packets at the next switch hop, then the latency through a switch would increase at each hop. To avoid this effect sender  108  randomises start times and thereafter generate packets at a regular rate with respect to this random start time at the governed rate. This helps to reduce bottlenecks in the network  106 . 
     More than one sender  108  is able to send packets over the network  106  at any given time—in keeping with the usual mode of operation of packet switched networks like Ethernet. The maximum network latency encountered by audio packets between any sender  108  and a receiver  110  may by bounded by using well understood Quality of Service (QoS) techniques. These include using admission control to limit the number of audio streams on each link as well as deploying scheduling techniques in switches to prioritise audio packets over other non-real time data as discussed above. Therefore a system with tight clock synchronization with network QoS should be able to achieve a smaller end-to-end latency than a system with looser clock synchronization and no QoS. An implementation on a general purpose computer system requires a jitter compensation latency of the order of 5-10 ms which is primarily due to timer jitter in the sending system and NTP synchronization errors. 
     The architecture of the invention also supports non-streamed data that requires time synchronization. Examples are MIDI and lighting control, as discussed next. 
     EXAMPLE 
       FIG. 7  is a block diagram of a typical application of the invention. The master clock device  102  produces a system time signal  104  which is relayed to the network devices  108 ,  110  and  111 . These network devices may be connected to a variety of media devices  112  and  114  as shown here, or alternatively they may be incorporated into the media device. Some of the media devices may be digital sources  112  or analog sources microphones  170  and a guitar  172 . Others may be audio playout devices  114  such as speakers  174  and the redundant speaker  176 . Still others may be media processing devices such as a MIDI sound console  178 , a mixer  160  or a lighting control module  179 . A computer  162  may be used to record and send audio, and also to send MIDI and lighting command signals. Another computer  164  may be used as a configuration console for the network  100 . 
     The invention supports a wide range of application domains from low cost devices with lax timing requirements, such as audio distribution in an airport, to professional audio production systems requiring multi-channel sample accurate timing. Further suitable applications of the invention include public address system, live music systems, recording studios including professional and home recording studios, and home theatre systems. 
     Advantages of the Invention 
     Reduced cabling costs—Using this invention, each cable of the network is able to carry multiple channels, which leads to reduced cabling complexity and cost. Further, the cables may contain a mix of different speed links, such as low speed device links and a highspeed backbone. A simple interface on a computer may be provided for equipment needing many channels, providing reduced setup time, including plug and play connection. 
     The “audio snake” is a common component in audio systems. It is a bundle of audio cables wrapped in a sheath and is terminated with a sprout of connectors or a patch bay. There is a one-to-one correspondence between sockets on the patch bay and connectors in the sprout. An audio snake is an expensive cable due to the high cost of the connectors and the labour involved terminating the many wires. Using Ethernet vastly reduces the cabling cost and reduces the number of connectors. 
     Speaker cabling carries high-power audio signals. Speakers are fairly low impedance devices (typically 4 or 8 Ohm) and high cable impedances cause power losses and changes the frequency response of the speaker. Every metre of cable in a long run adds to the cable impedance. Low impedance speaker cable is thick and expensive. The closer the power amplifier is to the speaker the lower the losses. Placing the power amplifier inside the speaker is increasingly common, but requires power and the audio signal to be routed to every speaker. Digital audio networking as used by the invention is a scalable way of routing audio sources to powered speakers. 
     Easier maintenance—Using the invention, moving a piece of equipment from one network port to another port does not require audio routing configuration to be changed. This is in contrast to audio networking solutions where audio routing is configured hop-by-hop at each “switch.” Changing the location of a device in such systems involves either re-cabling to make it appear that the device is still in the old location, or reconfiguring the “switches” to send the audio to the new location, rather than the old location. The “switches” in such systems do not conceptually route packets (e.g. as an Ethernet switch would do), rather they route audio channels. 
     Producing an audio track is not a set and forget affair. A sound engineer at a mixing console adjusts the levels of each instrument, fades between different tracks and adjusts effects during each track to get the best combined sound from the recorded sources. These adjustments can be considered part of the final performance. Recording and replaying the actions of the audio engineer avoids tedious repetition and reduces errors. An audio network facilitates the transport of automated or recorded mixing commands to equipment attached to the network (e.g. equalisers, effects processors). 
     Scalability—At least one embodiment of the invention provides a multicast network architecture that can be scaled to thousands of receivers, supporting large venues like sporting stadia. 
     Flexibility—The invention may be implemented in a variety of different situations as described above. This includes, high quality hardware implementations that require low latency and sample accurate timing. Sample accurate timing is the capability of two (or more) devices attached to a network to play out an audio sample at the same time—to within a single sampling period of the digital audio. For example, left and right speakers must play out audio in synchrony otherwise poor sound reproduction will occur. Sample accurate timing ensures that digital audio data from the left and right channels having the same timestamp will be played out the left and right speakers at the same time, to within the time period for a single sample. Samples with the same timestamp will be “aligned in time” at the moment they are played out on all devices connected to the network. At the highest sample rates supported by audio systems today (192 kHz), this translates to a time synchronisation accuracy of about 1 microsecond. On the other hand cost-conscious software implementations with relaxed timing can still send and receive audio data into the network. 
     Allowing different audio mixes—The invention has applications in situations where there are different audio mixes coming from different speakers, such as “Audience Mix” and “Musicians Mix.” For example, musicians performing live (e.g. a rock band) commonly use amplification to boost the level of sound for the audience. The “Audience Mix” is an audio signal containing all of the instruments and vocals which is played out the speakers to the audience. The “Audience Mix” is sometimes called the “Front of House” mix. The musicians, however, also need to hear themselves clearly to play well. The “Musicians Mix” is the audio signal amplified and played out the speakers pointing at the musicians on stage. Often, each musician has an individual speaker and mix in which their instrument is louder than the others. Using the invention a tenfold increase in bandwidth leads to a tenfold decrease in latency, and a tenfold increase in the number of channels. 
     By using networks such as Ethernet networks, the invention allows direct interfacing with computer equipment. Standard computers can participate in the network timing protocol and remain in synchronization with other devices in the network. Use of UDP/IP for audio data encapsulation allows interfacing with computer systems. Using the computer interface moving a piece of equipment is also simplified. 
     In traditional systems, each digital audio link type has a different data format, different cabling and different plugs. The result is that audio equipment usually has a variety of different connectors and plugs to transport the same basic PCM encoded audio data. Each component of an audio system must have an intersecting set of connectors with another component in the audio system. Each component in the system usually needs to be configured with basic information like sample rate and bit depth. Digital audio networking as used by the invention provides a way of transporting audio streams of different types over a common network. Further, the same network can be used for transporting network time information. 
     Support for MIDI Data—The Musical Instrument Digital Interface is referred to as MIDI. Pressing and releasing keys on an electronic piano generates MIDI messages describing the pitch, duration and velocity of each note played. MIDI is a popular interconnection technology now used in areas other than instrument control. Today it is used to control audio effects units (reverberation, equalisation, etc), lighting equipment, and for configuration parameters for musical equipment. This invention supports the transmission of event data, like MIDI, over a single audio network. 
     Accurate Timing for MIDI—The MIDI standard specifies a 31.25 kbaud/second serial cable supports 16 channels. Low bus bandwidth has two effects: first it limits the number of control events that can be sent, and second, the timing of simultaneous events is impaired as the bus load increases. These two issues are routinely avoided by using multiple MIDI busses. 
     Precise timing of MIDI events output from each port is achieved by clock synchronisation between the PC and the MIDI box, and by embedding timing information in the MIDI events traveling over, e.g. a USB bus. A digital audio network can replace the USB bus, and can further directly transport MIDI events to the devices that consume them with accurate timing. 
     Timing Errors—Timing errors can result in phase errors in audio output. The human ear is highly sensitive to phase errors and can detect changes in sounds produced by timing errors of less than a millisecond. The invention supports precise time timing for MIDI and audio data allowing tight synchronisation to be achieved. 
     Peer-to-peer capability—Current audio systems are centralised. All source audio data is usually routed to a mixing desk, processed and then provided to speakers. A peer-to-peer audio network architecture removes the requirement for a centralised mixing desk—audio sinks can locate and consume audio data directly. 
     A peer-to-peer digital audio network architecture also makes recording of digital audio data simpler. A recorder can locate and sink all sources of audio data in the network. This is enabled for a standalone recording device or PC-based audio production software. 
     Data networking technologies focus on error free reliable communication. As an example, Ethernet boasts high bandwidth, low bit error rates and low cost cabling. In addition, robustness against component failure is important in critical communication systems. A wide variety of protocol techniques for failing over between redundant links and services are available. 
     Although the invention has been described with reference to particular examples and applications it should be appreciated that it may be put into effect in many other forms and for many other applications. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.