METHOD AND APPARATUS FOR USING RF DETECTOR AND DELAY CIRCUIT TO REDUCE NOISE BUILDUP IN RF RETURN SYSTEMS

Abstract: This method and apparatus minimizes noise buildup in a wireless or wireline RF return system of a cable television system through which set-top terminals communicate with the cable system headend. An RF detector detects the signal from the set-top terminal to the headend. The RF detector, through a delay circuit, closes an RF switch or turns on a transverter allowing the signal from the set-top terminal to be communicated to the RF return system. The delay circuit prevents the link between the set-top terminal and the return system from being broken for a predetermined period of time after the RF detector stops detecting a signal from the set-top terminal. This prevents delays in transmitting signals, particularly time division multiplexed signals.

Description: TITLE OF THE INVENTION METHOD AND APPARATUS FOR USING RF DETECTOR AND DELAY CIRCUIT TO REDUCE NOISE BUILDUP IN RF RETURN SYSTEMS FIELD OF THE INVENTION The present invention relates to the field of cable television systems. More particularly, the present invention relates to the field of minimizing noise buildup in RF return systems by which the set -top terminals of individual cable subscribers communicate with the cable signal headend. The RF return systems involved with the present invention may be either wired or wireless . BACKGROUND OF THE INVENTION In a cable television system, the cable television service provider establishes a signal headend from which television signals are distributed over the cable system to the population of subscribers. Typically, each subscriber receives a set-top terminal that is connected between an outlet to the cable system and the subscriber's television set. The set-top terminal allows the subscriber's television to make use of the cable television signal. In some instances, the set-top terminal also sends signals to communicate with the headend. These signals are typically radio frequency (RF) signals. Thus, the system by which the set-top terminal communicates with the headend is known as an RF return system. The return signals are generated by the set -top terminal and are sent to the headend via an RF transmitter in a wireless return system, or over an RF wire-line link in a wired return system. A problem arises in RF return systems due to the buildup of unwanted signal noise. The active electronics of the set-top terminals and, in wireless return systems, the RF transmitters, generate a small amount of random noise or other spurious signals that propagate upstream over the RF return system. This unwanted signal noise is generated even if the RF transmitter is inactive, i.e., not transmitting a signal. Spurious noise can also be created in wired return systems by signal leakage or interference caused by over-the-air short wave or two-way radio or other signal broadcasts. The amount of noise generated by or through any one set-top terminal may be almost negligible, particularly with careful design, shielding and construction. However, the buildup of noise from the totality of set- top terminals of all the subscribers to the cable system creates a substantial problem in the RF return system. A conventional means of addressing this problem is illustrated in Fig. 3. As shown in Fig. 3, a set-top terminal (101) is provided for each subscriber and connected between the cable system and the subscriber's television set (not shown) . The set-top terminal (101) is connected by an RF signal cable (105) to the RF return system (106) . In order to prevent the random noise generated by the set-top terminal (101) from being introduced to the RF return system (106) , an RF switch (102) is interposed along the cable (105) between the set-top terminal (101) and the RF return system (106) . The switch (102) is opened when the set-top terminal (101) is not signaling the headend thereby preventing any noise generated from propagating into the RF return system (106) . The set-top terminal (101) is also provided with a separate wire connection (301) for controlling the switch (102) . When the set-top terminal (101) needs to signal the headend, the terminal (101) generates a switch control signal on this connection (301) that signals the switch (102) to close. The set-top terminal is then linked through the switch (102) to the RF return system (106) . When the set-top terminal (101) is finished signaling the headend, the switch (102) opens to again prevent noise from the terminal (101) from reaching the RF return system (106) . Fig. 4 illustrates a solution to noise buildup using the same principles in a wireless cable system. As shown in Fig. 4, the set-top terminal (101) is connected via cable (105) with a transverter (201) . The transverter (201) is an RF transmitter for wirelessly signaling the headend as an RF return system (116) . To prevent unwanted noise from being transmitted as part of the RF return system (116) , the set-top terminal (101) has a separate wired connection (401) over which an ON/OFF signal is sent to the transverter (201) to shut the transverter (201) down when the set- top terminal (101) is not using the RF return system (116) to signal the head end. While these arrangements limit the noise buildup in the RF return system, they also raise several problems. It must be noted that the transverter (201) (Fig. 4) , or the RF switch (102) (Fig. 3) , is typically mounted outside the subscriber's house remote from the set-top terminal (101) . Thus, the systems of Figs. 3 and 4 require additional wiring between the set -top terminal (101) and the components exterior to the subscriber's home in order to provide the control connections (301) and (401) . Additionally, there is a delay required to signal the switch (102) or the transverter (201) over the connections (301) or (401) , respectively, before the set-top terminal can begin using the RF return system (106) or (116) . Consequently, there is need in the art for a method and apparatus of preventing unwanted noise buildup in the RF return system of a cable television system that does not require additional wiring between the set -top terminal and the components exterior to a subscriber's house. There is a further need in the art for a method and apparatus of regulating the connection between a set -top terminal and an RF return system that avoids unnecessary delay in providing access to the RF return system when needed. SUMMARY OF THE INVENTION It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide a method and apparatus of preventing unwanted noise buildup in the RF return system of a cable television system that does not require additional wiring between the set -top terminal and the components exterior to a subscriber's house. It is a further object of the present invention to provide a method and apparatus of regulating the connection between a set-top terminal and an RF return system that avoids unnecessary delay in providing access to the RF return system when needed. Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims. To achieve these stated and other objects, the present invention may be embodied and described as an apparatus for minimizing noise buildup in a signal return system of a cable television system through which a set-top terminal signals a cable system headend. In a preferred embodiment, the apparatus of the present invention includes: a signal detector connected to the output of a set-top terminal; and a selective link between the set-top terminal and the signal return system. The selective link is activated by the signal detector so as to operatively connect the set-top terminal with the signal return system when the signal detector detects a signal output by the set-top terminal. A delay circuit is preferably used for delaying deactivation of the selective link for a predetermined period of time after the signal detector no longer detects the signal output by the set-top terminal . Where the signal return system is a wireline system, the selective link includes a switch for selectively coupling the set-top terminal with the signal return system. The signal detector closes the switch so as to connect the set-top terminal to the signal return system when the signal detector detects a signal output by the set-top terminal. The signal detector also signals the switch to open when the output signal from the set-top terminal is no longer detected. If the delay circuit is used, the delay circuit maintains the switch in a closed state for a predetermined period of time after the signal detector stops detecting the signal output by the set-top terminal. This is particularly important if the output signal from the set-top terminal is time division multiplexed and comprises a series of spaced signal bursts. If the signal return system is a wireless system, the selective link preferably includes a transverter for transmitting signals from the set-top terminal to the headend. The signal detector turns the transverter on when the signal detector detects a signal output by the set-top terminal. The signal detector also signals the transverter to deactivate when the output signal from the set-top terminal is no longer detected. If the delay circuit is used, the delay circuit maintains the transverter in an on state for a predetermined period of time after the signal detector stops detecting the signal output by the set-top terminal. Again, this is particularly useful if the output signal from the set -top terminal is time division multiplexed. The present invention also encompasses a method for minimizing noise buildup in a signal return system of a cable television system through which a set-top terminal signals a cable system headend. The method includes activating a selective link between the set- top terminal and the signal return system in response to detection of a signal output by the set -top terminal. Preferably, the method also includes delaying deactivation of the selective link for a predetermined period of time after the signal output by the set-top terminal ceases. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate the present invention and are a part of the specification. Together with the following description, the drawings demonstrate and explain the principles of the present invention. Fig. 1 is a block diagram of a device for preventing unwanted noise in a wired RF return system of a cable television network according to the present invention. Fig. 2 is a block diagram of a device for preventing unwanted noise in a wireless RF return system of a cable television network according to the present invention. Fig. 3 is a block diagram of a conventional device for preventing unwanted noise in a wired RF return system of a cable television network. Fig. 4 is a block diagram of a conventional device for preventing unwanted noise in a wireless RF return system of a cable television network. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Using the drawings, the preferred embodiments of the present invention will now be explained. In the drawings, identical numbers indicate identical system elements . Fig. 1 illustrates the principles of the present invention as applied to a wired cable television network with a wired RF return system (106) . As shown in Fig. 1, a set-top terminal (101) is connected via a cable (105) to an RF return system (106) . An RF switch (102) is employed between the set -top terminal (101) and the RF return system (106) to prevent spurious noise from the set-top terminal (101) from entering the RF return system (106) when the set-top terminal (101) is not using the RF return system (106) . In contrast to the prior systems described above, the set-top terminal (101) has no direct control over the open/closed status of the switch (102) . Rather, an RF signal detector (103) is connected to the cable (105) . When the detector (103) detects an RF signal from the set-top terminal (101), it signals the switch (102) to close, thereby connecting the set-top terminal (101) to the RF return system (106) . This arrangement eliminates the need for additional wiring between the set-top terminal (101) and the RF switch (102) which is usually located on the exterior of a subscriber's house remote from the set-top terminal (101) . As shown in Fig. 1, the link between the RF detector (103) and the RF switch (102) also includes a delay circuit (104) . A typical RF return system (106) uses time-based multiplexing to accommodate the large number of subscriber set-top terminals which may be needed to use the RF return system (106) to signal the headend. Therefore, a signal from the set-top terminal (101) to the RF return system (106) may actually be transmitted as a number of brief transmissions using time division multiple access (TDMA) . This can create a problem for the RF detector (103) which would try to rapidly switch the RF switch (102) on and off to accommodate each of the brief spring of transmissions in the string of transmission bursts from the set-top terminal (101) . Each transmission burst may experience a delay during which the detector (103) activates the switch (102) . This problem is avoided, however, by adding the turn-off delay circuit (104) between the detector (103) and the switch (102) . The turn-off delay circuit (104) keeps the switch (102) closed, i.e. keeps the set-top terminal (101) connected to the RF return system (106), for a predetermined period of time after the RF detector (103) has ceased detecting an RF signal on the cable (105) . The predetermined period during which the turn-off delay circuit (104) maintains the link between the set- top terminal (101) and the RF return system (106) should be minimized. For example, if the transmission bursts in the TDMA scheme being used are separated by 10 seconds, the delay circuit (104) will hold the switch (102) closed for 10 seconds or more after the RF detector (103) ceases to detect an RF signal on the cable (105) . In this manner, only the first transmission burst may experience a delay in the activation of the switch (102) . All the transmission bursts in the signal thereafter will find the switch (102) held closed by the turn-off delay circuit (104) . Fig. 2 illustrates the principles of the present invention as applied to a cable system with a wireless RF return system (116) . As shown in Fig. 2, a set-top terminal (101) is connected via a cable (105) to a wireless RF return system (116) . A transverter (201) is used to wirelessly transmit signals from the terminal (101) over the RF return system (116) . To prevent spurious noise from the transverter (201) from propagating over the RF return system (116) , it is desired to turn off the transverter (201) when the set- top terminal (101) is not using the RF return system (116) . In contrast to the prior systems described above, the set-top terminal (101) has no direct control over the ON/OFF status of the transverter (201) . Rather, an RF signal detector (103) is connected to the cable (105) . When the detector (103) detects an RF signal from the set-top terminal (101) , it turns on the transverter (201) , thereby enabling transmissions from the set-top terminal (101) to the RF return system (116) . This arrangement eliminates the need for additional wiring between the set-top terminal (101) and the transverter (201) which is typically mounted on the exterior of a subscriber's house remote from the set-top terminal (101) . As shown in Fig. 2, the link between the RF detector (103) and the transverter (201) also includes a delay circuit (104) . As before, the turn-off delay circuit (104) keeps the transverter (201) turned on for a predetermined period of time after the RF detector (103) has ceased detecting an RF signal on the cable (105) . The predetermined period during which the turn-off delay circuit (104) maintains the transverter (201) in the "on" state should be minimized. As in the preceding example, if the transmission bursts in the TDMA scheme being used are separated by 10 seconds, the delay circuit (104) will keep the transverter (201) on for 10 seconds or more after the RF detector (103) ceases to detect an RF signal on the cable (105) . In this manner, only the first transmission burst may experience a delay in the activation of the transverter (201) . All the transmission bursts in the signal thereafter will find the transverter (201) turned on. Consequently, the present invention allows optimal control of the RF switch (102) or transverter (201) so as to minimize spurious noise in the return system (106 or 116) without a control connection between the set- top terminal (101) and the return system (106 or 116) . Moreover, the delay circuit (104) avoids the problem of delay in activating the selective link between the set-top terminal (101) and the return system (106 or 116) . Thus, communication from the set-top terminal (101) to the headend is made swift, efficient and free of noise. The preceding description has been presented only to illustrate and describe the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. The preferred embodiment was chosen and described in order to best explain the principles of the invention and its practical application. The preceding description is intended to enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.

Claims: WHAT IS CLAIMED IS : 1. An apparatus for minimizing noise buildup in a signal return system of a cable television system through which a set-top terminal signals a cable system headend, the apparatus comprising: a signal detector connected to an output of a set- top terminal; and a selective link between said set-top terminal and said signal return system; wherein said selective link is activated by said signal detector so as to operatively connect said set- top terminal with said signal return system when said signal detector detects a signal output by said set-top terminal . 2. The apparatus of claim 1, further comprising a delay circuit for delaying deactivation of said selective link for a predetermined period of time after said signal detector no longer detects said signal output by said set-top terminal. 3. The apparatus of claim 1, wherein said selective link comprises a switch for selectively coupling said set-top terminal with said signal return system, said signal detector closing said switch so as to connect said set-top terminal to said signal return system when said signal detector detects a signal output by said set-top terminal. 4. The apparatus of claim 3, wherein said signal detector signals said switch to open when said output signal from said set-top terminal is no longer detected. 5. The apparatus of claim 3, wherein said selective link further comprises a delay circuit connected between said signal detector and said switch, said delay circuit maintaining said switch in a closed state for a predetermined period of time after said signal detector stops detecting said signal output by said set-top terminal. 6. The apparatus of claim 5, wherein said output signal from said set-top terminal is time division multiplexed. 7. The apparatus of claim 1, wherein said signal return system is a wireless signal return system and said selective link comprises a transverter for transmitting signals from said set-top terminal to said headend, said signal detector turning said transverter on when said signal detector detects a signal output by said set-top terminal. 8. The apparatus of claim 7, wherein said signal detector signals said transverter to deactivate when said output signal from said set-top terminal is no longer detected. 9. The apparatus of claim 7, wherein said selective link further comprises a delay circuit connected between said signal detector and said transverter, said delay circuit maintaining said transverter in an on state for a predetermined period of time after said signal detector stops detecting said signal output by said set-top terminal. 10. The apparatus of claim 9, wherein said output signal from said set-top terminal is time division multiplexed. 11. A method for minimizing noise buildup in a signal return system of a cable television system through which a set -top terminal signals a cable system headend, the method comprising activating a selective link between said set-top terminal and said signal return system in response to detection of a signal output by said set-top terminal. 12. The method of claim 11, further comprising delaying deactivation of said selective link for a predetermined period of time after said signal output by said set-top terminal ceases. 13. The method of claim 11, wherein activating said selective link comprises closing a switch for selectively coupling said set-top terminal with said signal return system. 14. The method of claim 13, further comprising deactivating selective link by opening said switch when said output signal from said set-top terminal is no longer detected. 15. The method of claim 13, further comprising maintaining said switch in a closed state for a predetermined period of time after said signal output by said set-top terminal ceases. 16. The method of claim 15, further comprising time division multiplexing said output signal from said set-top terminal. 17. The method of claim 11, wherein said signal return system is a wireless signal return system and activating said selective link furhter comprises wirelessly transmitting signals from said set-top terminal to said headend. 18. The method of claim 17, further comprising deactivating a transverter used for said wireless transmitting when said output signal from said set-top terminal ceases. 19. The method of claim 17, further comprising maintaining a transverter used for said wireless transmitting in an on state for a predetermined period of time after said signal output by said set-top terminal ceases . 20. The method of claim 19, further comprising time division multiplexing said output signal from said set-top terminal. 21. An apparatus for minimizing noise buildup in a signal return system of a cable television system through which a set-top terminal signals a cable system headend, the apparatus comprising: a signal detection means connected to an output of a set-top terminal; and a selective linking means between said set-top terminal and said signal return system; wherein said selective linking means is activated by said signal detection means so as to operatively connect said set-top terminal with said signal return system when said signal detection means detects a signal output by said set-top terminal. 22. The apparatus of claim 21, further comprising a delay means for delaying deactivation of said selective linking means for a predetermined period of time after said signal detection means no longer detects said signal output by said set-top terminal. 23. The apparatus of claim 21, wherein said selective linking means comprises a switching means for selectively coupling said set-top terminal with said signal return system. 24. The apparatus of claim 23, wherein said selective linking means further comprises a delay means for maintaining said switching means in a closed state for a predetermined period of time after said signal detection means stops detecting said signal output by said set-top terminal. 25. The apparatus of claim 21, wherein said selective linking means comprises a transmitting means for wirelessly transmitting said signal output by said set -top terminal to said headend. 26. The apparatus of claim 25, wherein said selective linking means further comprises a delay means for maintaining said transmitting means in an activated state for a predetermined period of time after said signal detection means stops detecting said signal output by said set-top terminal.

Inventors: KOLBER MARK ALLAN, KOLBER, MARK, ALLAN
Applicants: GEN INSTRUMENT CORP, GENERAL INSTRUMENT CORPORATION
Application year: 2000
Main classification: H04N7
IPC codes: H04N7