Method for transporting digital media

• US 7,747,725 B2
• Filed: 04/21/2006
• Issued: 06/29/2010
• Est. Priority Date: 04/22/2005
• Status: Active Grant

• Current Assignee: Audinate Holdings Pty Limited
• Original Assignee: Audinate Pty Ltd (Audinate Group Ltd.)
• Inventors: Witana, Varuni, Williams, Aidan
• Primary Examiner(s): Jacobs; Lashonda T
• Application Number:
  US11/409,190
• Publication Number:
  US 20060280182A1
• Time in Patent Office: 1,530 Days
• Field of Search: 709/248, 709/203, 709/209, 709/231, 370/350, 370/503
• US Class Current: 709/223
• CPC Class Codes: H04J 3/0632 Synchronisation of packets and cells, e.g. transmission of voice via a packet network, circuit emulation service [CES] queuing arrangements in packet switching elements H04L49/90; synchronising systems for the synchronous transmission of a pulse code modulated video signal with one or more other pulse code modulated signals H04N7/56
• CPC Class Codes: H04J 3/0661 using timestamps
• CPC Class Codes: H04J 3/0667 Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays arrangements for monitoring round trip delays in packet switching networks H04L43/0864
• CPC Class Codes: H04L 43/106 using time related information in packets, e.g. by adding timestamps
• CPC Class Codes: H04L 65/1101 Session protocols
• CPC Class Codes: H04L 65/60 Network streaming of media packets
• CPC Class Codes: H04L 65/762 at the source reformatting of additional data in video distribution servers H04N21/2355
• CPC Class Codes: H04L 65/80 Responding to QoS
• CPC Class Codes: H04L 67/1095 Replication or mirroring of data, e.g. scheduling or transport for data synchronisation between network nodes
• CPC Class Codes: H04L 7/0025 interpolation of clock signal
• CPC Class Codes: H04L 9/12 Transmitting and receiving encryption devices synchronised or initially set up in a particular manner
• CPC Class Codes: H04N 21/2381 Adapting the multiplex stream to a specific network, e.g. an Internet Protocol [IP] network transmission of MPEG streams over ATM H04L12/5601
• CPC Class Codes: H04N 21/242 Synchronization processes, e.g. processing of PCR [Program Clock References] arrangements for synchronising broadcast or distribution via plural systems in broadcast distribution systems H04H20/18
• CPC Class Codes: H04N 21/4305 Synchronising client clock from received content stream, e.g. locking decoder clock with encoder clock, extraction of the PCR packets arrangements for synchronising receiver with transmitter by comparing receiver clock with transmitter clock H04L7/0012; arrangements for synchronising receiver with transmitter wherein the receiver takes measures against momentary loss of synchronisation H04L7/0083

Abstract

A networked system is provided for transporting digital media packets, such as audio and video. The network includes network devices interconnected to send and receive packets. Each network device can receive and transmit media signals from media devices. A master clock generates a system time signal that the network devices use, together with a network time protocol to generate a local clock signal synchronized to the system time signal for both rate and offset. The local clock signal governs both the rate and offset of the received or transmitted media signals. The system, which can be implemented using conventional network equipment enables media signals to be transported to meet quality and timing requirements for high quality audio and video reproduction.

Claims

1. A data network suitable for transporting media packets, the network comprising:

a master clock device to generate a system time signal for the network, wherein the system time signal and media packets are both communicated in a same band on the network;

a plural number of network devices interconnected such that each network device can send media packets to other network devices, and to receive media packets from other network devices;

each network device also being configured to receive media signals from a media device, to transmit media signals to a media device, or to do both;

wherein, each network device is coupled to the master clock device and uses the system time signal and a network time protocol to generate a local clock signal 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; and

wherein a media clock signal is synthesized from at least the local clock signal, such that the media clock signal directly governs the rate and offset at which media signals are received or transmitted.

2. A data network according to claim 1 wherein, in use, each network device that receives media signals from a media device uses its local clock signal to packetise the media signals, and to timestamp the media packets with system time before transmitting them to another network device.

3. A data network according to claim 2 wherein the timestamp of a media packet includes the earliest system time at which the media signal contained within the media packet was generated.

4. A data network according to claim 1 wherein each network device that receives media packets uses the packet timestamps to reorder media packets as required, and in addition to perform at least one of (1) to align and combine media signals received from different network devices, and (2) to determine the appropriate playout time for media signals contained in the media packets.

5. A data network according to claim 4 wherein the appropriate playout time for media signals contained in the media packets is different for two network devices that receive the media packets.

6. A data network according to claim 1 wherein the media packets contain at least one of audio and video media signals.

7. A data network according to claim 1 wherein the network comprises an Ethernet network.

8. A data network according to claim 1 wherein the rate of the local clock signal of a network device is different from the rate of a local clock signal of another network device.

9. A data network according to claim 1 wherein the rate of the media clock signal of a network device is different from the rate of a media clock signal of another network device.

10. A data network according to claim 1 wherein the media clock signal is used for conversion between analogue and digital media signals.

11. A data network according to claim 1 wherein the media clock signal is synthesized by one of pulse width modulated clock synthesis involving a pulse width modulated counter, direct digital synthesis, a digitally controlled oscillator, a voltage controlled oscillator controlled by a digital to analogue converter, and a software timer caused to track a synchronized local clock signal.

12. A data network according to claim 1 wherein at least one media device is connected to a network device for carrying out at least one of (1) to generate and deliver media signals, (2) to receive, to process and to deliver media signals, and (3) to receive and playout media signals.

13. A data network according to claim 1 wherein a quality of service scheme of the data network operates to prioritise the transmission of packets that contain network time information.

14. A data network device for connection to other network devices to transport media packets between them, the network device comprising:

a media port for performing at least one of (1) receiving media signals from a media device and (2) transmitting media signals to a media device;

a clock port coupled to receive a system time signal, wherein the system time signal and media signals are both communicated in a same band on the network;

a local clock generator coupled to use the system time signal and a network time protocol to generate a local clock signal 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, and

wherein a media clock signal is synthesized from the local clock signal, such that the media clock signal directly governs the rate and offset at which media signals are received or transmitted.

15. A data network device according to claim 14 wherein when the network device receives media signals from a media device it uses the local clock signal to packetise the media signals and to timestamp the media packets with system time before sending them to another network device.

16. A data network device according to claim 15 wherein the timestamp of a media packet includes the earliest system time at which the media signals contained in the media packet were generated.

17. A data network device according to claim 14 wherein when the network device receives media packets it uses the packet timestamps to reorder packets as required, and in addition to do at least one of (1) to align and combine media signals received from different network devices, and (2) to determine the appropriate playout time for media signals contained in the media packets.

18. A data network device according to claim 17 wherein the appropriate playout time for media signals contained in the media packets is different than the playout time determined by another network device that received the media packets.

19. A data network device according to claim 14 wherein the media packets contain at least one of audio and video media signals.

20. A data network device according to claim 15 wherein the rate of the local clock signal of the network device is different than the rate of a local clock signal of another network device.

21. A data network device according to claim 14 wherein the rate of the media clock signal of the network device is different than the rate of a media clock signal of another network device.

22. A data network device according to claim 14 wherein the media clock signal drives the conversion between analogue and digital media signals.

23. A data network device according to claim 14 wherein the media clock signal is synthesized by at least one of pulse width modulated clock synthesis involving a pulse width modulated counter, direct digital synthesis, a digitally controlled oscillator, a voltage controlled oscillator controlled by a digital to analogue converter, and a software timer caused to track the synchronized local clock signal.

24. A data network device according to claim 14 wherein at least one media device is connected to the data network device to perform at least one of (1) generate and deliver media signals, (2) to receive, process and deliver media signals, and (3) to receive and playout media signals.

25. A data network device according to claim 14 wherein the playout time of a media signal of a media packet is delayed to allow for late arrival of media packets.

26. A method of operating a data network device to transport media packets, comprising the steps of:

generating or receiving a system time signal for the network, wherein the system time signal and media signals are both communicated in a same band on the network;

configuring a plural number of the network devices such that each is able to perform at least one of (1) sending packets to other network devices, and (2) receiving packets from other network devices;

configuring the network devices to perform at least one of (1) receiving media signals from a media device, and (2) transmitting media signals to a media device;

connecting each network device to receive the system time signal;

operating each network device to use the system time signal and a network time protocol to generate a local clock signal 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; and

synthesizing a media clock signal from the local clock signal, such that the media clock signal directly governs the rate and offset at which media signals are received or transmitted.

27. A method according to claim 26 further comprising:

receiving at a network device media signals from a media device,

packetising the media signals; and

timestamping the media packets with system time using the local clock signal of the network device, and

then transmitting the media packets over the network to another network device.

28. A method according to claim 27 wherein the timestamp of the media packet is the earliest system time at which the media signals contained in the media packet was generated.

29. A method according to claim 26 further comprising:

receiving media packets at a network device and using the packet timestamps to reorder media packets; and

performing at least one of;

aligning and combining media signals from different network devices, and determining the appropriate playout time for the media signals contained in the media packets.

30. A method according to claim 29 wherein the appropriate playout time for the media signals contained in the media packet is different for each network device that receives the media packets.

32. A method according to claim 26 wherein the media packets contain at least one of audio and video media signals.

33. A method according to claim 26 further comprising synthesizing a media clock by using at least one of pulse width modulated clock synthesis, direct digital synthesis, a digitally controlled oscillator, a voltage controlled oscillator controlled by a digital to analogue converter, or a software timer caused to track the synchronized local clock signal.

34. A method according to claim 26 further comprising using the network time protocol supporting redundant master clocks.

35. A method according to claim 26 further comprising configuring the network devices to compute a playout time at a receiver by adding a latency to the source timestamp.

36. A method according to claim 26 the method further comprising prioritising the transmission of packets on the network that contain network time information.

37. A method according to claim 26 further comprising the step of configuring the network devices selecting a random start time and to transmit media packets from their respective random start time.

31. A method according to 26 further comprising using a network time protocol involving a bidirectional exchange of messages to enable the calculation of both clock offset and network delay between a master clock and local slave clocks.

38. An article of manufacture comprising a computer readable storage medium having computer program instructions configured to cause a processing system to execute the steps of:

generating or receiving a system time signal for the network, wherein the system time signal and media signals are both communicated in a same band on the network;

configuring a plural number of the network devices such that each is able to perform at least one of (1) sending packets to other network devices, and (2) receiving packets from other network devices;

configuring the network devices to perform at least one of (1) receiving media signals from a media device, and (2) transmitting media signals to a media device;

connecting each network device to receive the system time signal;

operating each network device to use the system time signal and a network time protocol to generate a local clock signal 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; and

synthesizing a media clock signal from the local clock signal, such that the media clock signal directly governs the rate and offset at which media signals are received or transmitted.