MXPA97008810A - System to reduce the variation of gain and income in return transmissions of c - Google Patents

Abstract

The present invention relates to a CATV system having a network of communication paths for upstream and downstream communications between a header and a plurality of subscriber terminals, so that a communication path between the header and each terminal is provided. of subscriber, the system is characterized in that it comprises: for each plurality of selected subscriber terminals, associated loss means disposed within the communication path for each selected subscriber terminal to attenuate the upstream communication signals between the selected subscriber terminal and the header, and each of the loss media selected to match the gain between the upstream communication signals originating from the selected subscriber terminals

Description

SYSTEM TO REDUCE THE VARIATION OF GAIN AND INCOME IN CATV RETURN TRANSMISSIONS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates generally to cable television communication systems. More particularly, the invention relates to a system that produces gain and noise ingress variations in the return transmission path of a bidirectional cable television communication system.

DESCRIPTION OF THE RELATED TECHNIQUE A number of different types of cable television communication (CATV) systems exist to provide a variety of CATV services. The conventional one-way CATV systems primarily provide broadcast video services, which are sent over the CATV transmission network in a downstream direction, from a CATV system header to a plurality of subscriber terminals. Bidirectional CATV systems have a common place in the industry as the diversity of services has grown. The pay-per-view channels are an example of bidirectional CATV service, where the subscriber transmits the information back to the header with respect to the selection of the program. New services that are in development will be completely based on the use and development of return trajectory communication. Bidirectional CATV systems support path communication both downstream and back. Accordingly, individual subscribers can communicate with the head, other subscribers or service providers within the system. These systems also allow subscribers to select specific video or consumer programming services and pay only for those services they are using. Typical CATV systems in the United States are capable of transporting a frequency range from about 5 MHz to 750 MHz. Signal frequencies above 50 MHz are commonly reserved for distributing signals in the downstream direction from a header to terminals of subscriber (front frequencies). Signal frequencies below 40 MHz are commonly reserved for the transport of signals in the direction of return path from individual subscriber terminals to the header (inverse frequencies). Signal frequencies between 40 and 50 MHz are used as a safety band to separate the upstream from the downstream service. A CATV transmission distribution installation includes coaxial cables, signal couplers / separators, amplifiers and subscriber connections. When placed in the signal path, passive transmission devices attenuate the signal. Normally, the system is designed from the header to each subscriber observing signal losses and gains through the main circuit, branch lines and subscriber drops. The final distribution of the CATV signal to subscribers is usually via a coaxial cable that is connected at specific locations to provide subscriber drops. The signal gain from the header to a subscriber terminal for front frequencies and from the terminal to the header for inverse frequencies will vary depending on the intervention devices and the length of the transmission due to the inherent cable and loss of device insertion. The signal attenuation varies with respect to the frequency, since the attenuation is greater at higher frequencies than at lower frequencies. One of the main objects in the design of a cable distribution system is to provide the appropriate signal strength levels at a subscriber connection. Normally, amplifiers are installed in the cable installation to periodically restore the lost signal strength due to cable attenuation and charging of other CATV devices. The output levels of each amplifier are set to the same signal strength. This concept is known as unit gain. Since coaxial cable causes greater attenuation at higher frequencies, the loss at the highest frequency, typically determines the gain of an amplifier. The frequency response of an in-line amplifier is oscillated slightly towards the higher frequencies to anticipate the attenuation of higher frequencies. At the point in the cable installation where the high frequency loss is greater than the design limits, an amplifier is inserted to reset all frequencies to the unit gain. One current technique used to provide uniform forward signal levels from a number of subscribers is to select a specific attenuation value for each subscriber connection. The connection values for each subsequent subscriber are gradually reduced in attenuation, since the coaxial cable also contributes to the attenuation of the signal due to the length of the transmission. Referring to Figure 1, a subscriber connection 15 of the prior art includes an input signal port 1 7, an output signal port 19 and four subscriber connection ports "F" 21. Figure 2 shows the electrical components of the subscriber connection 15. The input signal port 1 7 is coupled to the directional coupler 23, which passes the CATV signal to an output signal port 19 and to a signal separator 25. The signal is separated again depending on the number of ports "F" connection 21. To provide the various connection values, an attenuation value 27 can be inserted between the directional coupler 23 and the first signal separator 25. The downstream signal is attenuated by the connection value when it is entered by each subscriber through a port "F" connection 21. The connection output port 19 is connected to the next connection 15 by another length of coaxial cable. Each subsequent connection value is reduced in descending order resulting in the same signal level present in the last "F" port of subscriber connection as it was in the first connection. The calculation of the connection values is mainly based on the highest transported front frequency. The downstream signal transmission is designed in this manner to ensure that the appropriate signal strength levels are provided to each subscriber terminal. However, for lower frequencies in the transmission spectrum, the connection output level increases with each successive connection. The loss at the highest frequency will be controlled by the output values of the connection port imposed by the system design. However, since the coaxial cable has fewer losses at lower frequencies, the output level of connection to these frequencies varies enormously at each connection. This problem is evident to a greater degree in the return path, since only low frequencies are used. Therefore, there is a large variation in the loss for each connection in the return direction. With reference to Figure 3A, there is shown a connected feeder line 29 of the prior art consisting of a bidirectional amplifier 31 and four subscriber connections 33, 35, 37 and 39. Referring to Figure 3B, a table is shown. comparison of forward frequency loss of 750 MHz and corresponding inverse frequency of 40 MHz for the connected feeder line 29 of the prior art. At 40 MHz, the variation of signal loss back between connections 4 (39) and 1 (33) is 7.7dB. The amount of gain (or loss) on the return path of a particular home to the headset varies from house to house. This gain diversity can be referred to as a gain variation. In addition to the loss of variation of the return path due to system design, other variables such as differences in loss of separation at the subscriber's house, individual drop lengths, system frequency response, link loss difference, temperature coefficients and header receiver tolerances, all are added to the gain variation of the return path. This gain variation places an extra load on the design of the terminal return transmitters. To compensate for the variation of gain in the return path, a terminal return transmitter must have a wide range of transmission levels (output power). A current technique used to compensate for the frequency of variation against the attenuation property and to provide the appropriate signal level is cable equalization. Cable equalization is typically performed within in-line amplifiers. Since the forward and reverse frequencies in the cable installation can be separated using diplex filters, the front frequencies can be separated from the inverse frequencies. Using diplexers, fixed attenuators, separate equalizers and amplifiers for both forward and reverse frequencies, both directions can be equalized. The front and reverse equalizers are used to add loss to reduce the frequency response through each respective bandwidth. The equalizer characteristics are the inverse part of the cable frequency loss. However, the return path line equalization of the prior art does not address the problem of returning the frequency gain variation in each subscriber connection. Another problem is that the return frequencies are subjected to the combined interference of all return signals. As shown in Figure 4, the front frequencies of the header 41 to a particular subscriber terminal 43 are susceptible to interference 45 along a discrete path defined via the main circuit 47, branches 49, and drop lines 51 of the system CATV 53. However, the return frequencies are subject to greater interference due to the combination of all return signals that are sent via system drop lines 51, branches 49 and main circuit line 47. In addition, the Return signals are also more susceptible to interference due to the frequency bandwidth used in the return path transmission that overlaps with the frequency bandwidth of the out-of-air broadcast signals, which propagate around the world . Further mixing this problem is the solution of the prior art to maintain uniform front frequency signal levels. By design, subscriber drops at the end of a feeder line will have the lowest connection attenuation values. Since this attempts to solve the problem of front frequency gain variations (referring to the table shown in Figure 3B), as shown in Figure 5, and is tabulated in Figure 3B, the connections with the attenuation more low 39 allow greater interference input at the inverse frequencies than connections that have higher attenuation. The lack of an attenuation barrier allows more sources of income such as CB radios, noise from electrical items or discrete sub-band transmitters in the network. The income varies with time and is a serious damage to the operation of the receivers 55 connected to the return path. The return transmission resistance of a subscriber terminal must be sufficiently greater than the input that is correctly received in the header. Since the connections in the subscriber's homes at the end of a connected feeder have less attenuation, any income originating from these locations dominates the entire income of the system. As existing CATV systems will provide advanced bidirectional services such as interactive television, high-speed data transfer, advanced telephony services, it is desirable to limit the amount of interference on the return path of a CATV system.

BRIEF DESCRIPTION OF THE INVENTION The CATV system of the present invention reduces the total revenue of the entire return path of the CATV system and allows a reduction in the transmitter power back from the terminal. The system of variation of gain and reduction of income places a calculated loss in each subscriber connection location. By placing these losses at each subscriber location, the header receives a more consistent return path gain thereby reducing the transmitter power range requirement back from the terminal. In addition, losses reduce return path income. Accordingly, it is an object of the present invention to provide a system for reducing the gain variations and the interference income manifested in CATV return signal transmissions. It is a further object of the invention to provide an interference reduction method by which a loss is placed at each subscriber connection location. It is a further object of the invention to provide a method of interference reduction whereby the loss can be a flat loss or a passive equalizer.

Other objects and advantages of the system and method will be apparent to those skilled in the art after reading the detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a CATV subscriber connection of the prior art. Figure 2 is a block diagram of a CATV subscriber connection of the prior art. Figure 3A is a block diagram illustrating the prior art CATV feeder line downstream of a bidirectional amplifier. Figure 3B is a front frequency loss comparison table of 750 MHz and a 40 MHz inverse frequency of the prior art. Figure 4 is a block diagram illustrating typical typical CATV forward frequency / return frequency distribution facility. Figure 5 is a block diagram illustrating the interference input in the return path transmission of a CATV distribution facility. Figure 6A is a block diagram illustrating a typical CATV feeder line from a bidirectional line amplifier with the preferred embodiment of the present invention. Figure 6B is a comparison table of forward frequency loss of 750 MHz and inverse frequency of 40 MHz. Figure 7 is a plot of magnitude versus frequency of a flat loss. Figure 8 is a plot of magnitude versus frequency of a flat loss with a notch. Figure 9 is a plot of magnitude versus frequency of an equalizer loss. Figure 10 is a block diagram of an alternative location of the present invention. Figure 11 is a block diagram of an alternative location of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY The preferred embodiment will be described with reference to the drawings in which similar numbers represent similar elements therethrough. With reference to Figure 4, the topology of a CATV 53 distribution facility for distributing downstream cable television signals to a subscriber and for receiving return path messages from a subscriber CATV terminal 43 is shown. CATV distribution module 53 connects a header to a plurality of subscribers through subscriber terminals 43. The distribution facility 53 begins with a coaxial or fiber optic main circuit line 47 coupled to header 41. Some portions of the CATV installation may use fiber optic cables instead of coaxial transmission cables. At points where the transmission cable is divided, signal separators / combiners are installed 25. Fall lines 51, the branch lines 49 and the main circuit line 47 provide the bi-directional transport medium between the header 41 and the subscriber terminals 43 within the CATV system. All components are susceptible to interference input. All CATV subscribers typically receive the same broadband CATV signal sent from the 41st header. Signal amplifiers 31 are distributed through the cable installation 53 to promote and equalize a transmitted signal and ensure that appropriate levels of signal strength are maintained. Signal amplifiers 31 are capable of bidirectional amplification. An agile RF data return transmitter of frequency 57 is included in each terminal 43 and allows a subscriber to communicate with the headerer 41 by transmitting messages at the return address of the CATV 53 installation. Header 41 includes a data receiver Agile frequency RF 55 for receiving messages transmitted by multiple return transmitters 57. Due to the topology of a CATV installation, the noise input of all locations in cable installation 53 is accumulated in header 41. This noise affects the return signal transmissions for more forward transmissions. For example, the interference 45 in lines 59 and 61 will be combined in the interference in the signal separator 25. Since the signals travel to the header 41, the noise will be further combined with noise in each of the other lines in the installation of CATV 53. Due to this accumulation of noise, it can be difficult to discriminate a data signal in the headend 41 from the accumulated noise. The gain variation and income reduction system 63 of the present invention is shown in Figure 6A. The distribution of the CATV signal to individual subscribers lies on a coaxial cable 65 that is connected at specific locations 67, 69, 71 and 73 to provide service to each subscriber. As the length of the cable increases, fewer lower frequencies than higher frequencies attenuate. Since the design of the cable installation ensures that the connection levels are more constant for front frequencies, the loss of return frequencies from a subscriber connection to an amplifier is reduced as more cable is found. This is shown in the loss comparison table in Figure 3B. As a result, there is a large range of losses from any particular connection to the amplifier. This requires that the CATV equipment at home have a large range of transmission levels. The preferred mode equals the forward and reverse frequencies at each subscriber connection in addition to relying on the forward and reverse equalization performed on a signal amplifier. Each individual subscriber equalizer could be individually calculated in relation to the cable loss downstream of the previous line amplifier. This technique maintains a uniform loss of front and inverse frequency. In addition, each passive equalizer reduces the revenue that originates at each subscriber location from entering the cable installation. As shown in Figure 6B, connection losses are designed so that a more constant signal level is maintained at each subscriber connection port, without considering how much cable exists between a specific connection and the previous amplifier. As cable insertion and connection losses increase, lower connection values should be used. The level at 750 MHz in each connection remains relatively more constant. Eventually, the low frequency levels should be attenuated. This is achieved through the equalizers 75, 77 and 79 (EQ). In this modality, the equalizers are chosen in steps of 6dB. At 40 MHz, the variation of signal loss back between connections 4 (73) and 1 (67) is only 1.8 dB. Preferably, a loss is placed in each subscriber connection port or on the side of or inside each house. The loss must be of two general types: 1) flat loss in the return band, or 2) a broadband passive equalizer, which has a higher loss in the return band. Referring to Figure 7, the characteristic filter of the flat loss has a more constant loss in the return frequencies and a minimum loss in the front frequencies. A transverse inclination occurs within the safety band. The amount of loss in the return band will be one of several normal values with one being the closest chosen for each particular connection location. As shown in Figure 8, the characteristic flat loss filter with notch is similar to the flat loss filter, however, a particular portion of the return frequency spectrum is blocked. The notch will completely block any entry in the notch frequencies of a particular house. In this way, subscribers do not need to have access to a particular portion of the return path and do not contribute to the total system revenue in the blocked frequency range. Referring to Figure 9, the characteristic equalizer is the inverse part of the cable frequency response. The equalizer compensates for the cable, which is between the respective connection and the next amplifier upstream. The amount of equalization will be one of the various normal dB values with the closest one being chosen for each particular connection location. A variation of the passive equalizer is an equalizer with return frequencies with a specific notch. In this mode, subscriber contributions can be reduced for income. In addition, entry into a particular portion of the return frequency spectrum is blocked. As one skilled in the art can appreciate, each of the above loss types can be fixed or variable. Depending on the specific requirements, any type that allows greater flexibility can be used. Any of the above modalities can be located in one of the four subscriber areas: 1) outside the subscriber's house, near the subscriber signal separator or in the ground block; 2) inside the subscriber's house attached to its terminal; 3) inside the feeder connection between the directional coupler and the signal separator; or 4) coupled to each connection port "F" of the feeder connection. In the first location, the loss device is placed on the side of a subscriber's house. This location will block the entrance of the house to the cable installation, but not the fall cable that leads to the subscriber connection. Each subscriber can have a device designed for the services offered. In the second location, the loss device is coupled to a subscriber terminal within the house. This location will not allow for a significant reduction in income, but will reduce the gain variation. Similarly, each subscriber may have the device designed to the services offered in their home. Referring to Figure 10, at the third location, the loss device 81 is located within the connection and coupled between the directional coupler and the separator. In this location, the filter can not be designed to each individual subscriber served by the common connection, however, only one loss device is required per connection instead of one "F" connection port. Referring to Figure 11, in the fourth location, the loss device 81 is placed in the outlet of each connection port "F". This location will block the cable entry of both home and drop. Each subscriber can have an installed device that is designed to the services offered to the subscriber. Greater flexibility can be obtained by combining one of the previous loss locations with each other. Depending on the services desired by a particular subscriber and the response needed to match the return frequency gain, a subscriber connection, having a plurality of "F" ports, serving multiple subscribers, may have losses placed in a variety of locations . A subscriber may have a particular type of loss located in the subscriber connection, another subscriber may have a type of loss located within the subscriber's house and yet another subscriber may have a loss located in the separator or ground block of the signal of subscriber. Since specific embodiments of the present invention have been shown and described, many modifications and variations may be made by those skilled in the art without departing from the spirit and scope of the invention. The above description serves to illustrate and not to limit the particular form in any way.

Claims (21)

1 . A CATV system that has a network of communication paths for current communications 5 upstream and downstream between a header and a plurality of subscriber terminals, so that a communication path between the header and each subscriber terminal is provided, the system is characterized in that it comprises: IO for each plurality of selected subscriber terminals associated lmeans disposed within the communication path for each selected subscriber terminal to attenuate the upstream communication signals between the subscriber terminal 15 selected and the header; and each of the lmeans selected to equalize the gain between the upstream communication signals originating from the selected subscriber terminals.

2. The CATV system, according to claim 1, characterized in that the communication path of a selected first subscriber terminal includes a first subscriber connection that 25 has a plurality of connection ports; the first selected subscriber terminal is coupled to one of the subscriber connection ports first; first lmeans associated with the first selected subscriber terminal disposed within or downstream of the first subscriber connection; the communication path of a second subscriber terminal includes a first subscriber connection and, downstream thereof, a second subscriber connection having a plurality of ports; the second subscriber terminal is coupled to one of the ports of the second subscriber connection; and second lmeans associated with the first selected subscriber terminal disposed within or downstream of the second subscriber connection.

3. The CATV system, according to claim 2, characterized in that the first lmeans are located between the first connection and the first subscriber terminal.

4. The CATV system, according to claim 3, characterized in that the second lmeans are located between the second connection and the second subscriber terminal.

5. The CATV system, according to claim 2, characterized in that the first lmeans are located near the first subscriber terminal.

6. The CATV system, according to claim 5, characterized in that the second lmeans are located near the second subscriber terminal.

7. The CATV system, according to claim 2, characterized in that the first lmeans are located near the first subscriber connection ports.

8. The CATV system, according to claim 7, characterized in that the second lmeans are located near the second subscriber connection ports.

9. The CATV system, according to claim 2, characterized in that the first lmeans are located within the first subscriber connection and the second lmeans are located within the second subscriber connection.

10. The CATV system, according to claim 2, characterized in that the upstream communication signals are transported within a predetermined range of upstream frequencies and the downstream communication signals are transported within a predetermined range of current communication frequencies. below, which are greater than the upstream communication frequencies, and wherein the first subscriber connection has a different attenuation value than the second subscriber connection to equalize the downstream communication signal gain.

11. The CATV system, according to claim 10, characterized in that at least one of the lmeans is a filter having a flat lat the upstream frequencies and a minimum lat the downstream frequencies.

12. The CATV systems, according to claim 10, characterized in that one of the means of loss is a filter having a flat loss with a notch in the upstream frequencies and a minimum loss in the downstream frequencies and the notch blocks a particular portion of the upstream frequency range.

13. The CATV system, according to claim 2, characterized in that the communication path of the first subscriber terminal has a characteristic response at the location of the first loss means, and the first loss means have a characteristic response to the Inverse form of the first characteristic communication path response.

14. The CATV system, according to claim 2, characterized in that the means of loss are an equalizer with a return frequency with specific notch; and the specific notch blocks a particular portion of the upstream communication signals.

15. A CATV subscriber connection for use in a CATV system, characterized in that it comprises: an input signal port; a plurality of subscriber ports; a directional coupler; a signal separator; an attenuation value; and loss means arranged to equalize the gain between the upstream communication signals.

16. A CATV subscriber connection, according to claim 15, further characterized in that it comprises an output signal port.

17. A CATV subscriber connection, according to claim 15, characterized in that the loss means is an equalizer.

18. A CATV subscriber connection, according to claim 15, characterized in that the loss means is an equalizer with a notch.

19. The CATV system, according to claim 1, characterized in that the means of loss are fixed.

20. The CATV system, according to claim 1, characterized in that the means of loss are variable.

21. In a CATV system having a network of communication paths for upstream and downstream communications between a headend and a plurality of subscriber terminals, so that a communication path between the headend and each subscriber terminal is provided, the method is characterized in that it comprises: for each plurality of selected subscriber terminals, providing associated loss means for attenuating upstream communication signals between the selected subscriber terminal and the header; and each of the loss means, selected to equalize the gain between the upstream communication signals originating from the selected subscriber terminals; and arranging each of the loss media within the communication path for each selected subscriber terminal.