Synchronization of decoders in a bi-directional CATV network

DETAILED DESCRIPTION The present invention relates to a method and apparatus for synchronizing a number of decoders with a network sync signal in a bi-directional communication network such as a cable television network.
In one embodiment, a method is presented for synchronizing a decoder in a communication network to allow upstream transmission of a message from the decoder to a headend of the communication network in a time slot, wherein the decoder is adapted to receive a television signal via the communication network.
The method includes the steps of: providing a network sync signal having a network sync reference point at the headend, and providing a delay value at the headend.
The delay value may be pre-programmed (e.
g.
, stored in memory), and is selected such that the start time of the time slot follows the network sync reference point by the delay value.
A first horizontal scan line of the television signal at the headend that corresponds to the network sync reference point is determined.
A particular horizontal scan line of the television signal that corresponds to the start time is also determined at the headend using the delay value and the first horizontal scan line.
Identifying data which identifies the particular horizontal scan line is encoded and inserted into a blanking interval of the television signal.
The method may include the further step of inserting the identifying data into a blanking interval of the television signal which is between the first horizontal scan line and the particular horizontal scan line.
The blanking interval of the television signal may be a vertical blanking interval (VBI) or a horizontal blanking interval (HBI).
The delay value is greater than a field or frame period of the television signal when the blanking interval of the television signal is a vertical blanking interval.
This ensures that a VBI will be available for inserting the identifying data prior to the designated scan line.
The method may include the further steps of transmitting the television signal to the decoder via the communication network; recovering the television signal from the communication network at the decoder; extracting the identifying data of the recovered television signal; and providing a start signal using the extracted identifying data to indicate the start time for the decoder to start transmitting the message upstream to the headend in the time slot