CA1212456A - Catv upstream signal transmission at nonharmonic video frequencies - Google Patents

Abstract

Abstract of the Disclosure In a cable television (CATV) system improved upstream data communication is provided by the use of preferred frequencies. Harmonics of downstream video and audio carrier signals arising from cable discontinuities produce common mode nonlinear distortion. Cable discontinuities caused by a loose, defective or corroded connector operate as a downstream mixing diode. The cross-and intermodulation products thus produced are avoided in the present invention by the use of upstream frequencies displaced from these harmonics. In a preferred embodiment, two harmonically-related upstream carriers generated by a single signal source and centered between adjacent harmonic noise peaks provide reliable upstream data channels in a two-way CATV system. In another arrangement, each subscriber terminal transmits upstream in an alternating manner, first at one frequency and then at a second frequency, until an acknowledge signal is received from the CATV headend.

Description

Zoo S SUFFOCATION

background ox the Invention This invention relates generally to Carve systems and is more specifically directed to the upstream transmission of data in a two-way CAT system.
Cable television systems typically include a central master source of television programming information, or headed, which imparts programs at different assigned frequencies to a network of cable connections. The network, or cable plant, includes a main distribution cable to which is coupled a plurality of "drop" cables, each of which is coupled to an individual subscriber terminal. Particularly in large metropolitan areas, CAT systems may include one or more master sources which are patched into a common distribution system. CAT signals are broadcast from the headed to individual subscribers in a "downstream' direction over different assigned carrier frequencies typically from 50 to 450 MHz. These downstream signals are in conventional television video signal format and include synchronization audio, video etc., components These downstream signals are transmitted in a branched-out arrangement where all headed initiated signals are provided to all remote subscribers, if authorized.
In a Tao TV system each subscriber is able to transmit signals back to the CAT headed. These subscriber-initiatea signals may include program purchasing requests opinion poll responses and TV converter swept information. These upstream signals typically wake use of out of band frequencies such as in the 5-32.5 MHz band.
The upstream signal distribution network is in the form of a nerving tree topology" in which the signals generated by many sources, or subscribers, converge and are transmitted on a single transmission line back to the CAT
headed.
In an effort to increase the number of CAT
channels available, several types of coherent headwinds have been devised One coherent headed is termed Harmonically Related Coherent (ARC), while another is termed Interval Related Coherent (IRCt. The ARC headed requires all cable channel picture carriers to be shifted to harmonics of a 6 MHz reference signal. IRK headed channels operate in normally assigned frequencies, except channels 5 and 6, which must be shifted 2 MHz to fall in line with the other channels. In yet another arrangement all cable channel picture carriers are spaced 6 MHz apart from an adjacent channel with the exception of channels 5 and 6. Channels 4 and 5 are separated by less than 6 MHz, while channels 6 and 7 are spaced greater than 6 MHz apart. ID all of the aforementioned TV signal distribution arrangements, ~2~Z~S~

essentially all TV downstream signals are harmonically related In general, the main distribution cable is well maintained in a TV system. However, the connectors coupling either the drop cable to the distribution cable or coupling a subscriber terminal to a drop cable are frequently subject to wear and corrosion. defective connector produces nonlinearities in the signal handling characteristics of the cable. Under such conditions, a faulty connector operates as a mixing diode in generating sum and difference products of all CAT downstream signals.
Only those subscribers having a faulty connector will experience the aforementioned video signal distortion due to the thus generated cross- and inter modulation products However, these unwanted harmonics are coupled upstream tug the CAT headed and in a two-way CAT system seriously degrade upstream communication. Because of the various components of the video signal and side bands associated therewith, the number of modulation products thus produced is very large and substantially contributes to the noise figure across the entire video band. In addition, because television signals employ only downward modulation from a clamped synchronization signal the effective carrier level of each channel varies according to the video content of the signal. Thus, the various modulation products are a function of video signal content as well as carrier and sub carrier signal frequencies and, as such, present a continually varying source of noise which is extremely difficult to eliminate, or even minimize.
An example of a two-way CAT system utilizing selected frequencies for upstream and downstream communication is disclosed in US. Patent No. 3j943,447 to Shim The system described therein takes advantage of a transparent window in both the Upstream and downstream directions, to communications signals over a frequency band from 10-300 KHz wherein the communications signals follow the path of a 60 Ho power signal throughout the system.
It is claimed that these communications signals do not interfere with the high frequency television signals being transmitted, nor are they subject to interference by such television signals. This system permits bidirectional communication between any two or more points linked by such a CAT system The low frequencies of the signals utilized in this system limit its data handling capacity and thus the amount of information which can be transmitted.
The present invention is intended to overcome the limitations of the prior art and solve the aforementioned problems by providing a system which makes use of non harmonic regencies for upstream signal transmission in a two-way CAT system The present invention is used in a -two-way TV
system wherein a plurality of modulated signals each cores-pounding to a CAT channel and having associated -therewith a plurality of carrier frequencies are transmitted downstream in a band between 50-~50 MHz on a cable from a headed to a plurality of subscriber terminals at spaced frequency intervals of 6 Mllz and wherein modulation products of the downstream modulated signals arising from discontinuities in the cable are coupled upstream -to the headed. The invention relates to the improvement wherein each of -the subscriber terminals comprises: means for generating a data signal and a carrier select signal, the carrier select signal being generated independently of the headed; means for generating a 5.5 MHz carrier signal and an 11 MHz carrier signal; switch means coupled to the carrier signal generating means, the switch means having an output -terminal and being responsive to the carrier select signal for alternatively developing the 5,5 MHz and 11 MHz carrier signals a-t the output terminal, means coupled -to the output -terminal for transmitting the data signal upstream to the headed as modulation of the alternately developed 5.5 MHz and 11 MHz carrier signals; and means a-t the headed for receiving the transmitted upstream data signals.
Brief Description of the Drawings The appended claims set forth these novel features believed characteristic of the invention. However, the invention itself, as well as further objects and ad-vantages thereof, will best be understood by reference -to the so following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a simplified block diagram of a two-way CAT system embodying the principles of -the present invention; and FIG. 2 illustrates various modulation products resulting from video and audio carrier and sub carrier downstream frequencies in a CAT system as well as the frequencies of signals used for upstream transmission in the CAT system in accordance with the present invention.
Description of the Preferred Embodiment Referring to FIG. 1, there is shown in simplified block diagram form a two-way cable television (CAT) system 10 for transmitting upstream signals at uniquely determined frequencies in accordance with the present invention.
The CAT system 10 includes a headed portion 12 from which control and programming signals are provided to a plurality of CAT subscriber terminals 14, 16 and 18.
Although FIG. 1 shows only three subscriber terminals included in the TV system 10, it is to be understood that the present invention contemplates the distribution of the cable television signals to a large number of subscriber terminals, each having associated therewith a respective cable -television service controller and television receiver ~Z1~5~

which is not shown in FIG. 1 as it does not form 'd part of the present invention.
The CAT system 10 includes a distribution cable 20 and a plurality of drop cables 22, 24 and 26~ Typically, each subscriber terminal it couple to the distribution cable 20 via a respective drop cable The CAT headed 12 includes a 5.5 MHz receiver 38 and a 11.0 MHz receiver 40 coupled to the distribution cable 20 via a duplex filter 36~ Therefore, a 5.5 MHz upstream signal is provided to receiver 38 via duplex filter 36, while an 11.0 MHz upstream signal is provided to receiver 40 via duplex filter 36.
The 5.5 and 11.0 MHz receivers 38, 40 are each coupled to a microcomputer 34 in headed 12. Microcomputer 34 is responsive to the upsetter signals provided to respective receivers 38, 40 and performs various functions in response thereto. For example, microcomputer 34 may implement a subscriber authorization operation following a subscriber-initiated request for CAT programming. On the other hand, the primary function of the CAT headed 12 with respect to the upstream data may be merely to store this information fox later use such as in the case of subscriber polling responses or CAT converter status information, ego, power, program viewed, etc. Microcomputer 34 is further coupled to a transmitter I which, in turn, is coupled by jeans of duplex filter 36 to toe distribution cable 20.

2~6 Therefore, microcomputer-generated control data fur controlling Mach of the subscriber terminals 14, 16 and 18 may be provided to the distribution cable 20 via duplex filter 36 and transmitter 421 The downstream data provided by transmitter 42 to each of the subscriber terminals may further include subscriber address and program authorization data as well as CAT video program signals. CAT headed 12 may include addition components such as an encoderJdecoder, disc drives and a tape backup. These elements of the CAT headed 12 are not shown in Eli 1 as they do not form a part of the present invention.
For simplicity, only the details of TV
subscriber terminal 14 are discussed herein since the retaining subscriber terminals 16 and 18 are similarly configured for performing similar functions. Only the signal generation portion of CAT subscriber terminal 14 is discussed in detail herein, since the manner in which a subscriber terminal receives and responds to TV headed-initiated downstream data is not a part of the present invention. Subscriber terminal 14 includes a subscriber input device 48~ such as a keyboard, by means of which a viewer may enter data. This data may typically be in the form of a program request or headend-reques~ed subscriber polling information. Subscriber input device 48 is coupled to a microcomputer 46 which may either generate data in ~L2~Z~S~

response to inputs from the subscriber input device 48, or may generate upstream data in response to downstream headed initiated requests Two outputs are generated by microcomputer 46: one is a data output provided to modulator 44, the other is a control output provided to switch 50. The data output represents information to be communicated to CAT headed I while the control output controls the operation of switch 50. Switch 50 may take on any of a larger number of switching device configurations and may generally be termed an electronic switch. Two additional inputs are provided to switch 50. One input is from an 11 MHz oscillator 52, while the other input is from a divide by two circuit 54 which, in turn, is also coupled to the 11 MHz oscillator. The output of the divide-by-two circuit I to switch 50 it a 5.5 MHz signal.
Switch 50, in response to the control input from microcomputer 46, provides either an 11 MHz or a 505 MHz signal to modulator 44. In one embodiment of the present invention, the 5.5 and 11 MHz signals are alternately provided by switch 50 to modulator I Modulator I it also coupled to microcomputer 46 and is controlled by the data output therefrom. Therefore, the 5.5 and 11 MHz output signals from switch 50 are alternately modulated by modulator 44 in accordance with data from microcomputer 46 for transmission via drop cable 22 to CAT headed 12 I
By alternately transmitting upstream a-t two different frequencies, upstream data transmission reliability is increased while -tire effects of interference on upstream signal -transmissions are reduced. Each CAT subscriber terminal will continue to -transmit upstream until its upstream data signal is successfully communicated to CAT
headed 12 and an acknowledge signal is received from the transmitter 42 of CAT headed 12. One example of -the manner in which such a CAT subscriber terminal control signal is generated and provided -thereto is described in applicants cop ending patent application Serial No. 469,597, "Tao CAT System With AWL Commands", filed December 7, 19~4.

I

Downstream signal 5 from the cable headed 12 to individual subscribers typically include subscriber address and program authorization information which are transmitted on frequencies between 50 and 450 Mllz in a preferred embodiment Upstream date from each subscriber terminal to the CAY head end 12 is preferably transmitted within a frequency band of 5-32.5 MHz. Data transmitted upstream to the CAT headed I
may include such information as subscriber responses to headed initiated polling signals, subscriber terminal power status information, and subscriber-initiated program requests. the 17.S MY between the upstream and downstream signal frequencies represents a guard band for providing frequency isolation between these two groups of CAT signals.
Ivory, complete isolation between the upstream and downstream channels is not available in a typical TV system For example, the drop cable connector 28, 30 and I may become damaged, corroded, or subject to a physical strain so as to impair the electrical connection resulting in degraded connector performance. Typically, under any of the aforementioned situations, the drop connector will function electrically as a mixing diode in adding and subtracting all of the downstream CITY video carriers and sidebands associated therewith. With in excess of 50 channels available it some CAT systems and each channel having associated therewith several carrier frequencies, a large number of cross- and inter modulation products will be formed by a malfunctioning or defective drop connect These unwanted products appear as noise in the downstream video signals and degrade the video signal provided to the subscriber terminal coupled to the drop cable via the malfunctioning or ~21~
damaged drop connector. Thus, a defective drop connector affects only the quality of the downstream video signal provided by its associated subscriber -terminal. Ilowever, the effect of a defective drop connector is substantially different with respect to upstream -transmitted signals.
The inter- and cross modulation products generated by the defective drop connector are no-t only transmitted downstream, but are also coupled upstream to -the CAT
headed 12. Thus, a single defective drop connector in the two-way CAT system 10 will degrade the upstream data channel for all subscriber terminals by providing various CAT signal modulation products to the CAT headed 12.
Referring to FIG. 2, there is shown the frequencies of various interfering modulation products generated by a defective drop connector which are reflected upstream to the CAT headed 12. Because the downstream video carrier signals are 6 MHz apart over the band of 50-450 MHz, the interfering signal spectrum shown in FIG. 2 is repeated each 6 MHz increment over the upstream frequency band. Because this interference spectrum repeats itself every 6 MHz, only the presence of interfering signals in the band from approximately 6-12 MHz will be considered herein, with the understanding that a similar interference spectrum and the following discussion applies over the entire upstream frequency band of 5-32.5 MHz.
As shown in FIG. 2, strong modulation products resulting from downstream video carrier signals are present at 6 and 12 mHz. Each downstream CAT channel has an audio carrier associated therewith displaced 4.5 MHz above its video carrier. The modulation product at 10.5 Mllz, as well as -the modulation product at 4.5 Mllz, thus results from inter modulation of such downstream audio and video carriers. Finally, a modulation product is shown at 7.5 Ml-lz which represents cross-modulation of audio and adjacent channel video downstream carrier signals.
The frequency distribution of the various signals shown in FIG. 2 falls off exponentially due to the various sideband frequencies displaced + 15 KHz, 30 Claus, etc.
from the respective downstream carrier frequencies. The modulation products discussed thus far represent the primary upstream interfering signals in a two-way CAT
system. In addition, various secondary interfering signals are seen by the CAT headed. These secondary interfering signals arise primarily from cross and inter modulation of the chrome, or color, components of the downstream video carrier signals and are shown in FIG. 2 at frequencies of 9.58 My or 3.58 Mllz above the 6 Mllz modulation product, and 8.42 M~lz, or 3.58 Mllz below the 12.0 Miss modulation product. Additional secondary sources of interference occur at 6.58 Mllz resulting from modulation products of downstream audio and chrome carriers.
Similarly, a secondary source of interference occurs at 11.4 Mllz also resulting from modulation products of downstream audio and chrome carriers. Thus, it can be seen from FIG. 2 that various sources of interference occur between the video carrier induced modulation products in a two-way CAT system having a downstream signal transmission discontinuity.

I I
The present invention improves upstream data communication in a -two-way CAT system by minimizing the effects of the aforementioned primary and secondary sources of interference. This is accomplished by determining the optimum frequencies for upstream signal transmission in view of the various aforementioned sources of interference encountered in a two-way CAT system. Application of the present invention can be seen from FIG. 2 wherein are shown in dotted line form -the upstream signal frequencies utilized in a preferred embodiment. From FIG. 2, it can be seen that at 5.5 and 11.0 MHz the absence of either primary or secondary interfering signals permits the upstream signals to be transmitted free of unwanted inter-furriness. In addition, by utilizing two harmonically related frequencies, a single frequency source such as aconvenbional oscillator circuit 52 in combination with a frequency divider 54 which may be selectively engaged for generating both upstream signal frequencies. finally, switch 50 under -the control of microcomputer 46 permits alternate upstream signal transmission at 5.5 and 11.0 MHz in order to provide a second backup channel for more reliable upstream data communication. Thus, the use of the present invention in a two-way CAT system not only substantially improves upstream communication performance but also simplifies and reduces the cost of each individual subscriber terminal.
There has thus been shown an improved arrangement for upstream signal transmission in a -two-way CAT system.
Upstream carrier frequencies are utilized for the trays-mission of upstream signals for minimizing the effects of cross- and inter modulation products of -the downstream video and audio carrier signals which are coupled upstream by discontinuities in the cable of -the two-way CAT
system. While particular embodiments of -the present invention have been shown and described, i-t will be obvious to those skilled in -the art -that changes and modifications may be made without departing from the invention in its broader aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and no-t as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a two-way CATV system wherein a plurality of modulated signals each corresponding to a CATV channel and having associated therewith a plurality of carrier frequencies are transmitted downstream in a band between 50-450 MHz on a cable from a headend to a plurality of subscriber terminals at spaced frequency intervals of 6 MHz and wherein modulation products of said downstream modulated signals arising from discontinuities in said cable are coupled upstream to said headend, the improvement wherein each of said subscriber terminals comprises:
means for generating a data signal and a carrier select signal, the carrier select signal being generated independently of the headend;
means for generating a 5.5 MHz carrier signal and an 11 MHz carrier signal;
switch means coupled to said carrier signal generating means, said switch means having an output terminal and being responsive to said carrier select signal for alternately developing said 5.5 MHz and 11 MHz carrier signals at said output terminal;
means coupled to said output terminal for transmitting the data signal upstream to the headend as modulation of said alternately developed 5.5 MHz and 11 MHz carrier signals; and means at the headend for receiving the transmitted upstream data signals.