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 network device configured to be connected to a data network suitable for transporting media packets, the network device comprising:
a transmitter configured to send media packets to other network devices;
a receiver configured to receive media packets from other network devices; and
a local clock synchronized to a system time signal for both rate and offset;
wherein the network device is configured to be coupled to a master clock device to generate a system time signal for the network;
wherein the each network device is configured to receive media signals from a media device and transmit media signals to a media device; and
wherein, the network device uses the system time signal and a network time protocol to generate the local clock signal synchronized 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. The network device according to claim 1 wherein, in use, the network device that receives media signals from a media device uses its local clock signal to packetize the media signals, and to timestamp the media packets with system time before transmitting them to another network device.
3. The network device 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. The network device according to claim 1 wherein the 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. The network device 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. The network device according to claim 1 wherein the media packets contain at least one of audio and video media signals.
7. The network device according to claim 1 wherein the network comprises an Ethernet network.
8. The network device 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. The network device 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. The network device according to claim 8 wherein the media clock signal is used for conversion between analogue and digital media signals.
11. The network device 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. The network device 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. The network device according to claim 1 wherein the 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. The network device according to claim 1 wherein a quality of service scheme of the data network operates to prioritize the transmission of packets that contain network time information.
15. A network device for connection to other network devices to transport media packets between them, the network device comprising:
a media port configured to receive media signals from a media device and transmit media signals to a media device;
a clock port coupled to receive a system time signal; and
a local clock generator coupled to use at least the system time signal and 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 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:
receiving a system time signal for the network;
configuring the network device to send packets to other network devices, and receive packets from other network devices;
configuring the network device to receive media signals from a media device, and transmit media signals to a media device;
connecting the network device to receive the system time signal; and,
operating the network device to use the system time signal and 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 governing the rate and offset of the received or transmitted media signals.