CATV video and data transmission system with automatic dispersion compensation
Improved systems and methods for delivering CATV content over a fiber optic network from a transmitter by compensating for optical dispersion over the fiber optic network.
1. A head end for initiating dispersion compensation, said head end comprising:
(a) the head end operatively connected to a fiber optic transmission having a length from the head end to at least one of a hub or a node, the head end adjusting a data rate of the fiber optic transmission line between the head end and the at least one of the hub or the node to a data rate that is lower than that of an operating rate of the fiber optic transmission line;
(b) a transmitter that selectively sends a first message along said fiber optic transmission line in accordance with the data rate of the fiber optic transmission line that is adjusted to be lower than that of the operating rate of the fiber optic transmission line;
(c) a receiver capable of receiving a second message from the at least one of the hub or the node, the second message having content based on said first message and returned to the transmitter at a data rate that is lower than that of an operating rate of the fiber optic transmission line, the data rate of the second message being the same or different from the data rate at which the first message is sent along the transmission line;
(d) the head end configured for determining an amount of dispersion along said length of said fiber optic transmission line based on the messages transmitted at the adjusted lower data rate; and
(e) a processor that automatically compensates for the amount of dispersion along said length of said transmission line based on said returned second message.
2. The head end of claim 1 where said transmitter has a first mode and a second mode, said first mode configured for a low data rate transmission and said second mode configured for a normal data rate transmission faster than said low data rate transmission.
3. The head end of claim 2 where said first message is sent, and said second message is received, while said transmitter is in said first mode.
4. The head end of claim 3 where said processor instructs said transmitter to enter said first mode and said transmitter automatically sends said message when it switches to said first mode.
5. The head end of claim 4 where said processor is located in said transmitter.
6. The head end of claim 4 where said processor is not located in said transmitter.
7. The head end of claim 3 where said head end receives a third message from a remote location instructing said transmitter to enter said first mode.
8. The head end of claim 1 where said second message is used to calculate said length.
9. The head end of claim 1 where said processor automatically compensates for said dispersion by applying a filter that shifts a minimum-dispersion window such that it is centered on a receive window based upon a calculated delay between a time said first message is sent and a time that said second message is received.
10. The head end of claim 9 where said filter is applied using a network of switched compensation elements.
11. The head end of claim 9 where said filter is applied using a tunable dispersion control element.
12. The head end of claim 9 where said filter is applied using a combination of a network of switched compensation elements and a tunable dispersion control element.
13. The head end of claim 8 where said filter uses pre-distortion of said signal prior to electrical-optical conversion.
14. A method of compensating for dispersion over a fiber optic link between a transmitter and a receiver, said method comprising:
(a) entering into a first mode of operation of said transmitter and said receiver;
(b) adjusting a data rate of the fiber optic link between the transmitter and the receiver to a data rate that is lower than that of an operating rate of the fiber optic link;
(c) calculating an amount of dispersion over the length of the fiber optic link between the transmitter and the receiver based on a second message returned from the receiver to the transmitter in response to a first message sent from the transmitter to the receiver, the first and second messages transmitted over the fiber optic link at a data rate adjusted to be lower than that of the operating rate of the fiber optic link, wherein the data rate of the first message may be the same or different from the data rate of the first message;
(d) entering into a second mode configured for transmitting CATV data; and
(e) compensating for said amount of dispersion while in said second mode.
15. The method of claim 14 where said first mode of operation is configured for a low data rate transmission between said transmitter and said receiver, slower than a normal data rate.
16. The method of claim 15 including the step of sending a first message over said fiber optic link, and receiving a second message based on said first message, while in said first mode.
17. The method of claim 14 where said step of compensating for said dispersion uses a network of switched compensation elements.
18. The method of claim 14 where said step of compensating for said dispersion uses a tunable dispersion control element.
19. The method of claim 14 where said step of compensating for said dispersion uses a combination of a network of switched compensation elements and a tunable dispersion control element.
20. The method of claim 14 where said step of compensating for said dispersion uses pre-distortion of said signal prior to electrical-optical conversion.