Methods for transporting digital media
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.
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;
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;
wherein 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 and wherein at least one network device is configured to receive and transmit media signals to a media device; and
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.
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 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.
9. A data network according to claim 8 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 8 wherein the media clock signal is used for conversion between analogue and digital media signals.
11. A data network according to claim 8 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 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.
13. 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.
14. 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.
15. A data network device for connection to other network devices to transport media packets between them, the network device comprising:
a media port for receiving media signals from a media device and transmitting media signals to a media device;
a clock port coupled to receive a system time signal; and
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.
16. 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;
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 receiving media signals from a media device, and transmitting media signals to a media device;
connecting each network device to receive the system time signal; and,
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.