Interactive video distribution systems – Video distribution system with upstream communication – Transmission network
Reexamination Certificate
1998-02-09
2001-07-10
Grant, Chris (Department: 2611)
Interactive video distribution systems
Video distribution system with upstream communication
Transmission network
C725S116000
Reexamination Certificate
active
06260193
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for synchronizing a number of decoders in a bi-directional communication network such as a cable television network. In particular, synchronizing data that is inserted into a blanking interval of each television signal designates a horizontal line which is synchronized with a network synchronization (sync) clock. By synchronizing each television signal with the network sync clock, each subscriber terminal (e.g., decoder) in the network can also be synchronized with the network sync clock, and an upstream channel can be time-shared by one or more subscriber terminals to communicate messages synchronously with the network clock to a headend or other network control center.
Interactive applications for television communication networks allow users to transmit messages upstream from a decoder/transmitter terminal (e.g., set-top box) to a headend, or central control point, of the television network. For example, in cable television networks, one or more upstream or return paths are provided for such upstream messages. The upstream path may be provided in the frequency range of 5-30 MHz, for example. Moreover, many set-top boxes which are currently deployed transmit in the range of 8-12 MHz or 8-15 MHz. Generally, the upstream signals do not interfere with the television signals, which are transmitted at carrier frequencies from about 50-500 MHz.
The provision of an upstream channel allows users to transmit messages relating to pay-per-view (PPV) requests, near-video-on-demand (NVOD) requests, requests for account status, Internet access commands, electronic mail, opinion poll data, game or contest data, home shopping data, or other interactive services, for example. Additionally, the above data may be accumulated by the decoder and transmitted on a near-real-time basis or non-real-time basis. Moreover, maintenance functions of the decoder which do not require a user input may also require transmission of upstream messages. This non-user data may relate to monitoring of viewing selections for marketing purposes. Alternatively, non-user data may relate to a decoder status, for example, whether the decoder has been tampered with, or whether the decoder has been updated with particular software.
However, to optimize the throughput of the upstream channel, it is necessary to have a scheme for coordinating the upstream messages of each decoder. With one conventional scheme, the headend polls a particular decoder using an out-of-band signal (e.g., out of the television signal band) to inform that decoder that it can begin transmitting its upstream message. Since the headend controls the transmission of all upstream messages, no further time synchronization between the headend and the decoders is required.
However, the polling scheme is problematic since it is not known when a particular set-top box will have a message to transmit. Allocation of an upstream transmission period which is not used is bandwidth-inefficient. Additionally, the transmission of the polling signal consumes bandwidth.
With another conventional scheme, a decoder can tune to a particular channel and synchronize to the television signal on that channel. However, the user is forced to tune to the particular channel. This can be inconvenient if the upstream message which is to be transmitted is not related to the content of the particular television signal. For example, the user may be forced to tune to a channel for ordering pay-per-view programming, but the user may wish to transmit Internet commands or other messages which are not related to the pay-per-view programming.
Additionally, with the conventional schemes, the headend must also synchronize to the particular television signal, and therefore cannot synchronize to an independent network sync signal. Moreover, the conventional schemes do not account for variations in timing of the television signal due to standard tolerances which are acceptable in the industry, and do not allow the use of an independent network sync signal which can be designed to higher accuracy tolerances.
With other conventional schemes, such as the unslotted or unsynchronized ALOHA approach, no synchronization signal is used, and each decoder begins transmitting data packets at arbitrary times. The data packets are broadcast back to the decoders when received by the headend. Thus, a decoder can verify that its packet has been received at the headend. If the packet has not been received, the decoder simply tries to re-transmit it to the headend. However, due to the probability of collisions between different packets, the system throughput is relatively low.
Accordingly, it would be desirable to provide a system for synchronizing one or more decoders in a television network with a network sync signal at a headend of the network. The system should provide synchronization of each decoder without requiring the decoders to tune to a particular television channel. The system should be effective with a plurality of asynchronous television signals. The system should avoid the need for out-of-band polling signals to be sent to the decoders. Furthermore, the system should provide real-time compensation for variations in the timing accuracy of each television signal.
The present invention provides a system having the above and other advantages.
SUMMARY OF THE INVENTION
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.
The method may include the further steps of monitoring horizontal scan lines of the television signal; and providing the start signal when the particular horizontal scan line of the television signal is detected.
The recovering of the television signal at the decoder may be responsive to a user comma
Chang Kevin T.
Chodelka David E.
Reichgott Sam
Schell, III Charles E.
General Instrument Corporation
Grant Chris
Lipsitz Barry R.
McAllister Douglas M.
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