TV and data cable system ingress noise blocker

• US 6,094,211 A
• Filed: 08/15/1996
• Issued: 07/25/2000
• Est. Priority Date: 08/15/1996
• Status: Expired due to Fees

• Current Assignee: Arris International, Inc. (CommScope Holding Company, Inc.)
• Original Assignee: Com21, Inc.
• Inventors: Baran, Paul, Bunya, George K., Hollimon, Marshall H., Heinzmann, F. Jud
• Primary Examiner(s): Faile, Andrew I.
• Assistant Examiner(s): Srivastava, Vivek
• Application Number:
  US08/699,888
• Time in Patent Office: 1,440 Days
• Field of Search: 455/3.1, 455/5.1, 455/501, 455/63, 455/67.3, 455/135, 455/226.3, 455/296, 348/12, 348/10, 348/6, 348/470, 348/533, 348/534, 348/535, 348/607, 348/608, 348/609, 348/610, 348/611, 348/614, 348/622, 345/12, 345/10, 345/6
• US Class Current: 725/125
• CPC Class Codes: H04N 21/6118 involving cable transmission, e.g. using a cable modem
• CPC Class Codes: H04N 21/6168 involving cable transmission, e.g. using a cable modem
• CPC Class Codes: H04N 7/102 Circuits therefor, e.g. noise reducers, equalisers, amplifiers H04N7/108 takes precedence
• CPC Class Codes: H04N 7/173 with two-way working, e.g. subscriber sending a programme selection signal
• CPC Class Codes: H04N 7/17309 Transmission or handling of upstream communications

Abstract

This invention permits the upstream transmission of short packets of information in a cable TV system, and blocking upstream noise at all other times, and does so without interfering with image quality of normal TV viewing. A remotely operable ingress noise blocking filter is placed at the terminating junction between a subscriber'"'"'s coaxial drop cable and a corresponding feeder tap in a cable TV system. The ingress noise blocking filter contains a high pass filter to pass the normal TV band. This high pass filter is bypassed by a section containing low pass filters and a switch operated when receiving a control signal from a cable modem during those short durations the cable modem is authorized to transmit an upstream signal. Low pass filters isolate the switching elements so that switching transients cannot occur in the downstream TV band. The level of the amplitude of the downstream signal is unaffected whether the switching elements are open or closed. Further, the switching arrangement provides a relatively constant impedance, both when the switching element is on and off to provide a high return loss and avoid signal reflections and maintain signal quality. In a diagnostic mode, the location of each noise source is determined by correlating the energy measured at each a set of frequencies across the band at the headend during those periods when the cable modem is allowed to transmit versus the energy received during those periods when the same cable modem is not authorized to transmit.

Claims

1. A subscriber subsystem for use with a cable TV system, said cable TV system having a headend from which downstream signal emanates and to which an upstream signal is directed and a shared bidirectional subsystem connected to said headend and extending downstream therefrom to conduct both downstream and upstream signals, said downstream signal being in a first band and said upstream signal being in a lower second band wherein the lowest frequency of said first band is higher than the highest frequency of said second band, said subscriber subsystem connected to said shared bidirectional subsystem downstream from said headend to receive said downstream signal and to add a user initiated upstream signal to said shared bidirectional subsystem, said subscriber subsystem comprising:

a drop cable connected to said shared bidirectional subsystem and extending downstream therefrom;

an ingress noise blocker serially connected with said drop cable at a selected point there along, said ingress noise blocker having;

a first high pass filter having a cut-off frequency that is lower than said lowest frequency of said first band and higher than said highest frequency of said second band to pass said downstream signal and block said upstream signal; and

a bypass section connected in parallel across said first high pass filter, said bypass section including;

a pair of low pass filters each having a cut-off frequency that is lower than said lowest frequency of said first band, higher than said highest frequency of said second band, and lower than said cut-off frequency of said first high pass filter to pass said upstream signal and to block the transmission of visible switching noise artifacts and said downstream signal; and

a remotely operable attenuator serially connected intermediate said pair of low pass filters to pass upstream signals following receipt of a remote control signal; and

a subscriber facility connected to a downstream end of said drop cable to receive said downstream signal therefrom, to generate and present said upstream signal to said drop cable, and to generate and apply said remote control signal to said drop cable to activate said remotely operable attenuator prior to presenting said upstream signal to said drop cable;

wherein said remote control signal has a frequency that is lower than the lowest frequency of said second band, begins before said upstream signal is presented to said drop cable and lasts at least until said upstream signal terminates.

2. A subscriber subsystem as in claim 1 wherein:

said subscriber facility includes;

an upstream signal generator responsive to user supplied data to be transmitted upstream;

a remote control signal generator to generate said remote control signal;

a signal adder coupled to said upstream signal generator and said remote control signal generator to provide a combination of said upstream signal and said remote control signal to said drop cable;

a trigger circuit coupled to said upstream signal generator and said remote control signal generator to trigger said remote control signal generator and said adder to begin applying said remote control signal to said drop cable a preselected period of time prior to applying said upstream signal to said drop cable when said upstream signal generator generates an upstream signal; and

said remotely operable attenuator includes a detector to monitor said remote control signal and to activate said remotely operable attenuator when said remote control signal is detected to permit transients in said remotely operable attenuator to settle out prior to receipt of said upstream signal and to maintain said remotely operable attenuator active for the duration of said upstream signal.

3. A subscriber subsystem as in claim 2 wherein said remote control signal generator is a tone generator and said remote control signal is a tone having a frequency that is lower than said lowest frequency of said second band.

4. A subscriber subsystem as in claim 3 wherein:

said detector of said remotely operable attenuator is a tone detector, said tone detector includes;

a tuned series resonate circuit connected between one of said pair of low pass filters closest to said subscriber facility and a signal return including a series connected capacitor and inductor with an intermediate junction therebetween; and

a rectifier circuit coupled to said intermediate junction of said tuned series resonate circuit to generate a DC power signal from said tone received from said subscriber facility; and

said remotely operable attenuator further includes;

a second high pass filter having a first end and a second end with said second end connected the same one of said pair of low pass filters to which said tuned series resonate circuit is connected, and having a cut-off frequency that is lower than said lowest frequency of said second band and higher than said frequency of said tone; and

an externally powered constant impedance gate circuit coupled to said first end of said second high pass filter to receive said upstream signal, and connected to said rectifier circuit to receive operating power therefrom.

5. A subscriber subsystem as in claim 4 wherein said constant impedance gate includes:

a first resistive element of a first selected impedance having a first end and second end, with said first end coupled to said first end of said second high pass filter;

a second resistive element of a second selected impedance having a first end and a second end with said first end coupled to said second end of said first resistive element;

a third resistive element of said first selected impedance having a first end and a second end with said first end coupled to said second end of said second resistive element and said second end coupled to the one of said low pass filters that is coupled to said drop cable in the upstream direction;

a fourth resistive element of a third selected impedance having a first end and second end with said first end coupled to said second end of said first resistive element and said second end coupled to said signal return;

a fifth resistive element of said third impedance having a first end and second end with said first end coupled to said first end of said third resistive element and said second end coupled to said signal return;

a first switching element connected across said first resistive element being normally open and closing when power is received from said rectifier circuit;

a second powered switching element connected across said third resistive element being normally open and closing when power is received from said rectifier circuit;

a third powered switching element connected across said fourth resistive element being normally closed and opening when power is received from said rectifier circuit; and

a fourth powered switching element connected across said fifth resistive element being normally closed and opening when power is received from said rectifier circuit;

wherein said second selected impedance is lower than said first selected impedance, and said first selected impedance is lower than said third selected impedance.

6. A subscriber subsystem as in claim 5 wherein:

said first selected impedance is 75 ohms;

said second selected impedance is 10 ohms; and

said third impedance is 680 ohms.

7. A subscriber subsystem as in claim 2 wherein said lowest frequency of said second band is 5 MHZ and said frequency of said tone is 1 MHZ.

8. A subscriber subsystem as in claim 1 wherein said ingress noise blocker is located at the junction of, and between, said shared bidirectional subsystem and said drop cable.

9. A subscriber subsystem as in claim 1 wherein said remotely operable attenuator includes:

a signal detector coupled to one of said pair of low pass filters closest to said subscriber facility to detect said remote control signal and to generate a power signal from said remote control signal; and

a constant impedance gate coupled to the same one of said pair of low pass filters coupled to said signal detector to receive said upstream signal from said subscriber facility and coupled to said signal detector to receive operational power therefrom, said constant impedance gate having;

a first selected impedance having a first end and second end, with said first end coupled to said first end of said first low pass filter;

a second resistive element of a second selected impedance having a first end and a second end with said first end coupled to said second end of said first resistive element;

a third resistive element of said first selected impedance having a first end and a second end with said first end coupled to said second end of said second resistive element and said second end coupled to said drop cable in the upstream direction;

a fourth resistive element of a third selected impedance having a first end and second end with said first end coupled to said second end of said first resistive element and said second end coupled to said signal return;

a fifth resistive element of said third impedance having a first end and second end with said first end coupled to said first end of said third resistive element and said second end coupled to said signal return;

a first switching element connected across said first resistive element being normally open and closing when a power signal is received from said signal detector;

a second powered switching element connected across said third resistive element being normally open and closing when said power signal is received from said signal detector;

a third powered switching element connected across said fourth resistive element being normally closed and opening when said power signal is received from said signal detector; and

a fourth powered switching element connected across said fifth resistive element being normally closed and opening when said power signal is received from said signal detector;

wherein said second selected impedance is lower than said first selected impedance, and said first selected impedance is lower than said third selected impedance.

10. A subscriber subsystem as in claim 9 wherein:

said first selected impedance is 75 ohms;

said second selected impedance is 10 ohms; and

said third impedance is 680 ohms.

11. A subscriber subsystem as in claim 2 wherein said remote control signal generator is a DC generator and said remote control signal is a DC level.

12. A subscriber subsystem as in claim 11 wherein:

said detector of said remotely operable attenuator is a DC voltage level detector connected between one of said pair of low pass filters closest to said subscriber facility and a signal return to monitor said remote control signal for a DC voltage level greater than a present threshold value; and

said remotely operable attenuator further includes;

a second high pass filter having a first end and a second end with said second end connected to said first end of said first low pass filter, and having a cut-off frequency that is lower than said lowest frequency of said second band; and

an externally powered constant impedance gate circuit coupled to said first end of said second high pass filter to receive said upstream signal, and connected to said DC voltage level detector to receive operating power therefrom when said DC voltage level exceeds said preset threshold.

13. A subscriber subsystem as in claim 12 wherein said constant impedance gate includes:

a first resistive element of a first selected impedance having a first end and second end, with said first end coupled to said first end of said second high pass filter;

a second resistive element of a second selected impedance having a first end and a second end with said first end coupled to said second end of said first resistive element;

a third resistive element of said first selected impedance having a first end and a second end with said first end coupled to said second end of said second resistive element and said second end coupled to said drop cable in the upstream direction;

a fourth resistive element of a third selected impedance having a first end and second end with said first end coupled to said second end of said first resistive element and said second end coupled to said signal return;

a fifth resistive element of said third impedance having a first end and second end with said first end coupled to said first end of said third resistive element and said second end coupled to said signal return;

a first switching element connected across said first resistive element being normally open and closing when power is received from said DC voltage level detector;

a second powered switching element connected across said third resistive element being normally open and closing when power is received from said DC voltage level detector;

a third powered switching element connected across said fourth resistive element being normally closed and opening when power is received from said DC voltage level detector; and

a fourth powered switching element connected across said fifth resistive element being normally closed and opening when power is received from said DC voltage level detector;

wherein said second selected impedance is lower than said first selected impedance, and said first selected impedance is lower than said third selected impedance.

14. A subscriber subsystem as in claim 13 wherein:

said first selected impedance is 75 ohms;

said second selected impedance is 10 ohms; and

said third impedance is 680 ohms.

15. A subscriber subsystem as in claim 1 wherein said remotely operable attenuator has a response time that is less than the sweep time of a single TV horizontal line.

16. A subscriber subsystem as in claim 1 wherein said ingress noise blocker has a worst case return loss in excess of 14 dB in both an upstream frequency band and a downstream frequency band, as measured with said remotely operable attenuator conductive and non-conductive.

17. A subscriber subsystem as in claim 1 wherein said bypass section of said ingress noise blocker further includes a selectable upstream attenuator serial connected with said first low pass filter, said selectable upstream attenuator disposed to match an optimum signal level to be applied to said shared bidirectional subsystem and to further reduce ingress noise.

18. A subscriber subsystem as in claim 5 wherein said second resistive element is a selectable attenuator that is disposed to match an optimum signal level to be applied to said shared bidirectional subsystem and to further reduce ingress noise.

19. A subscriber subsystem as in claim 9 wherein said second resistive element is a selectable attenuator that is disposed to match an optimum signal level to be applied to said shared bidirectional subsystem and to further reduce ingress noise.

20. An ingress noise blocker to be used in a bidirectional TV cable system connected serially between a shared bidirectional subsystem and a subscriber subsystem, said subscriber subsystem generates and presents an upstream signal to said ingress noise blocker, and Generates and provides a remote control signal to said ingress noise blocker wherein said remote control signal has a frequency that is lower than any upstream or downstream signal and is provided to said ingress noise blocker before said upstream signal is presented to the ingress noise blocker, said ingress noise blocker comprising:

a high pass filter designed to pass downstream signals; and

a bypass section connected in parallel across said high pass filter, said bypass section including;

a low pass filter designed to pass upstream signals and block the transmission of visible switching artifacts; and

a remotely operable attenuator serially connected to said first low pass filter to pass upstream signals beginning a preselected period of time following receipt of said remote control signal from said subscriber subsystem to permit switching transitions to settle out before said upstream signal is received by said remotely operable attenuator, wherein said remotely operable attenuator further includes a gate section serially connected to said low pass filter and a DC voltage level detector with said gate section non-conductive when said DC voltage is below said preset threshold and conductive when said DC voltage exceeds said preset thresholds;

wherein;

said remote control signal is a DC voltage level; and

said DC voltage level detector monitors said remote control signal for a DC voltage level greater than a preset threshold.

21. An ingress noise blocker to be used in a bidirectional TV cable system connected serially between a shared bidirectional subsystem and a subscriber subsystem, said subscriber subsystem generates and presents an upstream signal to said ingress noise blocker, and generates and provides a remote control signal to said ingress noise blocker wherein said remote control signal has a frequency that is lower than any upstream or downstream signal and is provided to said ingress noise blocker before said upstream signal is presented to ingress noise blocker, said ingress noise blocker comprising:

a high pass filter designed to pass downstream signals; and

a bypass section connected in parallel across said high pass filter, said bypass section including;

a low pass filter designed to pass upstream signals and block the transmission of visible switching artifacts; and

a remotely operable attenuator serially connected to said first low pass filter to pass upstream signals beginning a preselected period of time following receipt of said remote control signal from said subscriber subsystem to permit switching transitions to settle out before said upstream signal is received by said remotely operable attenuator, wherein said remotely operable attenuator includes;

a signal detector coupled to said low pass filter to detect said remote control signal and to generate a power signal from said remote control signal; and

a constant impedance gate serially coupled to said low pass filter to receive said upstream signal from said subscriber facility and coupled to said signal detector to receive operational power therefrom, said constant impedance gate having;

a first selected impedance having a first end and second end, with said first end coupled to said low pass filter;

a second resistive element of a second selected impedance having a first end and a second end with said first end coupled to said second end of said first resistive element;

a third resistive element of said first selected impedance having a first end and a second end with said first end coupled to said second end of said second resistive element and said second end coupled to said drop cable in the upstream direction;

a fourth resistive element of a third selected impedance having a first end and second end with said first end coupled to said second end of said first resistive element and said second end coupled to said signal return;

a fifth resistive element of said third impedance having a first end and second end with said first end coupled to said first end of said third resistive element and said second end coupled to said signal return;

a first switching element connected across said first resistive element being normally open and closing when a power signal is received from said signal detector;

a second powered switching element connected across said third resistive element being normally open and closing when said power signal is received from said signal detector;

a third powered switching element connected across said fourth resistive element being normally closed and opening when said power signal is received from said signal detector; and

a fourth powered switching element connected across said fifth resistive element being normally closed and opening when said power signal is received from said signal detector;

wherein said second selected impedance is lower than said first selected impedance, and said first selected impedance is lower than said third selected impedance.

22. An ingress noise blocker as in claim 21 wherein:

said first selected impedance is 75 ohms;

said second selected impedance is 10 ohms; and

said third impedance is 680 ohms.

23. An ingress noise blocker as in claim 21 wherein said remote control signal generator is a tone generator and said remote control signal is a constant frequency tone.

24. An ingress noise blocker as in claim 21 wherein:

said signal detector is a tone detector, said tone detector including;

a tuned series resonate circuit connected between said low pass filter and a signal return including a series connected capacitor and inductor with an intermediate junction therebetween; and

a rectifier circuit coupled to said intermediate junction of said tuned series resonate circuit to generate a DC power signal from said constant frequency tone received from said subscriber facility; and

said remotely operable attenuator further including a second high pass filter having a first end and a second end with said second end connected to said low pass filter, and having a cut-off frequency that is lower than a lowest frequency of said upstream signal and higher than said constant frequency of said tone.

25. An ingress noise blocker as in claim 21 wherein:

said remote control signal generator is a DC generator and said remote control signal is a DC level; and

said signal detector is a DC voltage threshold detector that generators said power signal as a DC voltage signal when said remote control signal exceeds a present DC threshold.