CA1336340C - Catv network with filters - Google Patents
Catv network with filtersInfo
- Publication number
- CA1336340C CA1336340C CA000616759A CA616759A CA1336340C CA 1336340 C CA1336340 C CA 1336340C CA 000616759 A CA000616759 A CA 000616759A CA 616759 A CA616759 A CA 616759A CA 1336340 C CA1336340 C CA 1336340C
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- Canada
- Prior art keywords
- upstream
- signal
- bandpass filter
- filter
- electronic switch
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N7/00—Television systems
- H04N7/16—Analogue secrecy systems; Analogue subscription systems
- H04N7/173—Analogue secrecy systems; Analogue subscription systems with two-way working, e.g. subscriber sending a programme selection signal
- H04N7/17309—Transmission or handling of upstream communications
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- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Signal Processing (AREA)
- Two-Way Televisions, Distribution Of Moving Picture Or The Like (AREA)
Abstract
For use in a CATV network, an upstream filter circuit comprised of at least one bandpass filter for receiving an upstream signal, an electronic switch connected to the output of the at least one bandpass filter for applying a filtered upstream signal from the at least one bandpass filter to an upstream line, apparatus for detecting the energy level of the filtered upstream signal at a point between the at least one bandpass filter and the electronic switch, and apparatus for enabling the electronic switch to close in the event the energy level is above a predetermined threshold.
Description
This application is a divisional application of Canadian application S.N. 564,762 filed April 21, 1988.
This invention relates to cable television (CATV) networks and in particular to a reliable bidirectional CATV network.
CATV networks have in the past been structured with a ~lead end which provides the various signals, connected to a trunk, distribution lines connected to the trunk, and sometimes subdistribution lines connected to the distribution lines (herein being grouped with distribution lines). Subscriber drops are connected to the distribution lines. CATV
signals pass from the head end along the trunk, through the distribution lines and the subscriber drops to television sets or other subscriber terminals at various subscriber locations.
In order to facilitate enhanced subscriber services, some CATV systems have been made bidirectional, that is, they contain bidirectional amplifiers to enable signals to be transmitted both from the suhscriber locations to the head end and from the head end to the subscriber locations. Such enhanced services were envisioned to facilitate e.g.
pay per view of TV programs, ordering products displayed on television channels, playing of interactive video games, responding to polls, answering questions provided by a lecture course, etc.
It has been found that bidirectional systems have been unsuccessful because of a major noise gathering problem which was encountered. Noise caused by electronic or radio interference, poor terminal connections, ground currents, power lines and noise carried thereon, automobile ignitions, etc.
arises on the subscriber drops and distribution lines and all feed into the trunk and head end in the upstream direction. The noise is random and interferes to a prohibitive extent with legitimate signals transmitted upstream via the system from the var ious subscr ibers . ~
- 1 - ~
. r~. ~ ~ ~
t __ ~36340 01 In order to make an existing bidirectionaL
02 system work, CATV operators have had to rebuild their 03 distribution systems entirely to eliminate all 04 possible sources of interference, or use code operated 05 switches in bridgers addressed from the head end to 06 isolate distribution lines from the trunk and open 07 onLy one distribution line at a time for transmission;
08 or on a retained dLstribution system, inspect every 09 aspect, ground the various units properly and permanently and increase shielding at various 11 locations, in order to reduce as much as possible the 12 environment input of the noise signals.
13 It has also been found that excess noise 14 in the upstream direction can overload the upstream portion of the bidirectional amplifiers, which can 16 cause oscillation in the bidirectional amplifiers in 17 the trunk and/or the distribution lines. Since the 18 level of noise cannot be predicted because both its 19 timing and amplitude is random, this has posed a major problem.
21 The present invention is a CATV network 22 which allows for the first time an existing cabling 23 structure to be able to be used in a bidirectional 24 mode, by substantially reducing or eliminating the effect of noise gathering.
26 In a typical system, the downstream 27 signals are contained within a high requency band, 28 e.g. typically between 50 and 500 mHz, while the 29 upstream signals are contained within a low frequency band, e.g. between 5 and 30 mHz. In the present 31 invention the downstream directional signals are 32 untouched. The trunk retains its bidirectional 33 amplifiers, that is, amplifying downstream signals in 34 the high band and amplifying upstream signals in the low band.
36 According to a first embodiment of the 37 present invention, the upstream signals are contained t ._ 1 ~36340 01 in one or more narrow bands within the low band, 02 preferably centered at two frequencies, e.g. 11 mHz 03 and 26 mHz, with a bandwidth of e.g. 1 mHz. It should 04 be noted however that the present invention is not 05 limited to narrow band or to the use of two upstream 06 signalling frequencies, since one, or more than two 07 frequencies could be used.
08 Narrowband upstream filters are located in 09 the distribution lines, preerably but not necessarily just next to the points that the distribution lines 11 are connected to the trunk. They may also be 12 connected in the trunk, and the filters may be 13 connected in series within the network. The 14 downstream signals are unafected.
Preferably the upstream signals are 16 carriers modulated-by digital signals, e.g. at a 17 500 kb/sec rate using minimum shift keying or an 18 equivalent frequency modulation technique.
19 The resuLt of the above is that all upstream signals outside the narrow bandwidth of the 21 upstream signalling bands are blocked. Thus all 22 noise, which will include the vast majority of the 23 noise, which is outside the narrow signalling band of 24 the narrow bands is blocked. The utilization of the rest of the bandwidth outside the n~arrow bands for 26 video transmission is thus made possible.
27 Because the resulting amount of total 28 energy (signal plus noise) within the narrow 29 signalling bands is made low, the likelihood of overloading the upstream amplifiers by noise signals 31 causing oscillation of the downstream amplifiers in 32 the trunk will be very low, by the use of the present 33 invention.
34 Thus by the use of the present invention the problem o~ overloading in a bidirectional CATV
36 network is substantially reduced.
37 According to a second embodiment of the 01 invention, each of the upstream filters of the kind 02 described above contains a gate, which blocks all 03 upstream signals. Thus all noise entirely across the 04 low frequency upstream band will be blocked. Each of 05 the filters contains a circuit for detecting the 06 energy level of upstream signals carried in the narrow 07 frequency band, which is detected at the input to the 08 filter. When the energy exceeds a predetermined 09 threshold, it is assumed that the energy consists of a signal to be transmitted to the head end. Below the 11 threshold the energy is considered to be noise. When 12 the energy level is above the predetermined thres,hold, 13 the gate opens, allowing signals within the narrow 14 frequency band of the filter to pass to the trunk and thus to the head end.
16 Thus in accordance with this second 17 embodiment, there is no contribution to the trunk of 18 any noi~e whatsoever, even in the narrow signalling 19 bands, unless an actual upstream signal is being transmitted. The gate then opens automatically 21 without intervention from the head end, allowing the 22 signal and only the small energy content of noise 23 within the narrow signalling band of the low band to 24 pass through. Since all other filter gates are closed at that time, the noise passed upstream to the head 26 end is reduced substantially even in comparison to the 27 passive filter embodiment. The interval for sensing 28 of energy level and the opening of the gate should be 29 very short, within a,5 microseconds interval or less to permit implementation of a multi-access protocol 31 (the use of the same channel by several subscribers 32 with the head end recognizing and dealing with 33 collisions - see Canadian Patent Application 550,764 34 filed November 2, 1987 invented by Samir Sammoun) and realize the result of little effect on performance.
36 It should be noted, however, that for optimum noise 37 blocking, this interval should be maximized.
38 It is important to recognize that in the ~9 - 4 -`r336340 01 prior art, distribution switches are polled from the 02 head end in order to determine which subscriber is 03 transmitting, in order to obtain the upstream 04 transmitted data. That is both dif~icult and is a 05 very slow technique for receiving system signals due 06 to the time required to complete the polling, in 07 contrast to the present invention which does not 08 require polling.
09 A prototype system built according to the second embodiment was tested on an actual CATV network 11 with about 4,000 subscribers per switched filter and 12 was found to be reliable at least 99% of the time, 13 which is believed to be a substantial advance in the 14 state of the art.
According to a further embodiment passive 16 filters as described with respect to the first 17 embodiment above can be used in the distribution 18 lines, and an active filter containing the gate and 19 energy sensor described with respect to the second embodiment can be used in the upstream direction in 21 the trunk.
22 In a fourth embodiment, the active ~ilters 23 in the distribution lines can be addressable. The 24 gates in the filters can be purposefully opened or closed by selectively addressing them from the head 26 end. Thus in this embodiment each of the filters 27 should have an unique address. A signal passed from 28 the head end downstream and received by the filter 29 would cause the gate to open or close as desired from the head end. This allows the head end to selectively 31 test individual distribution lines, or perform a 32 remote program source switching operation.
33 While the upstream signalling bandwidth 34 has been described above as pre~erably being comprised of two narrowband signalling frequencies within the 36 upstream low frequency band, the upstream signal can 37 instead be comprised of video signals or other program 1 336~40 01 signals. Distribution lines can be split off from the 02 trunk adjacent a single (or multiple) subscriber 03 location, which can connect to a remote studio 04 containing several cameras. Each of the cameras can 05 be connected to a separate distribution line which 06 contains a selectably addressable filter. By enabling 07 the individual filters from the head end, their 08 individual gates can be opened, and individual signals 09 from selectable cameras transmitted to the head end.
At the same time noise gathering from other 11 distribution lines is blocked. This type of system is 12 useful for the remote controlling from the head end of 13 remote broadcasts, e.g. the transmission of university 14 courses using several cameras without requiring local switching personnel, the provision of video 16 conferences from various locations, remote control of 17 surveillance cameras, the reception of different types 18 of signals for simultaneous or later transmission to 19 other subscribers, the remote gathering of news, the provision o~ audio or video forums, etc. of course 21 the filters in this case should have bandwidth 22 sufficient to accommodate the upstream signal.
23 All of the above is made feasible using the 24 present invention avoiding the problem of noise gathering.
26 In accordance with the present invention, 27 an embodiment thereof is a CATV network comprising a 28 bidirectional trunk for connection to a head end and 29 bidirectional distribution lines connected to the trunk to which subscriber drops can be connected, the 31 distribution lines including means such as an 32 amplifier for transmitting upstream signals in at 33 least one narrow frequency band, a bandpass filter 34 connected in the upstream direction in each of the distribution lines having passbands corresponding to 36 the narrow frequency band or bands for blocking 37 upstream noise and signals which may appear on the ~ 1 336340 distribution lines except those contained in the narrow frequency band or bands. Preferably the filters are connected in the distribution lines immediately adjacent their connections to the trunk. However in some cases it may be desirable to connect the filters at the subscriber locations, e.g. within the drops or immediately adjacent the subscriber equipment connection facility.
In accordance with a preferred embodiment, such filter includes a gate for blocking all upstream signals. A circuit is included for detecting the energy level of upstream signals carried in the narrow frequency band or bands on a corresponding distribution line, and opens the gate to allow upstream signals to pass along the corresponding distribution line within the passband in the event the energy level is above a predetermined threshold.
In accordance with a further embodiment a circuit is included in each filter for receiving an address signal passing downstream on the distribution lines, and a circuit is included for opening the gate upon receipt of the addressed signal. Preferably addresses unique to each of the corresponding filters are used.
In accordance with a further embodiment the gate opens to allow video or other wideband signals to pass upstream.
In accordance with another embodiment, the passive (gateless) filters are connected in the distribution lines, but an active (gate included) filter is connected in the trunk between the head end and the location of the closest distribution line connection to the trunk.
.~
In accordance with an embodiment of the invention, for use in CATV network, an upstream filter circuit is comprised of at least one bandpass filter for receiving an upstream signal, an electronic switch 5 connected to the output of at least one bandpass filter for applying a filtered upstream signal from at least one bandpass filter to an upstream line, apparatus for detecting the energy level of the filtered upstream signal at a point between at least one bandpass filter and the electronic switch, and apparatus for enabling the electronic switch to close in the event the energy level is above a predetermined threshold.
A better understanding of the invention will be obtained by reference to the detailed description 15 below in conjunction with the following drawings, in which:
- 7a -~ 1 336340 01 Figure 1 is a block schematic of a CATV
02 network according to the prior art, 03 Figures 2A and 2B respectively illustrate 04 the low and high frequency bands, and the preferred 05 signal frequencies of the upstream signals, 06 Figure 3 is a block schematic illustrating 07 at least one embodiment the present invention, and 08 appears on the same page as Figure 1, 09 Figure 4 is a block schematic illustrating the preferred filter structure of the present 11 invention, 12 Figure 5 illustrates how Figures 6 and 7 13 should be placed together, and appears on the same 14 page as Figure 7, Figures 6 and 7 placed together form a 16 schematic diagram of an active filter according to the 17 preferred embodiment of the present invention, 18 Figure 8 is a block diagram of an 19 addressable active filter according to an embodiment of the present invention, and appears on the same page 21 as Figures 2A and 2B, 22 Figure 9A is an example of a spectrum 23 diagram of the upstream signal received at the head 24 end as in a prior art network, ~igure 9B is a spectrum diagram of the 26 upstream signal received at the head end using the 27 preferred embodiment of the present invention, in the 28 absence of an upstream signalling signal, and 29 Figure 9C is a spectrum diagram of the upstream signal received at the head end, using the 31 preferred embodiment of the present invention, in the 32 pr~n~ of an up~tr~m fli~n~llin~ si~n~l ~entered ~t 33 26 mHz.
34 Figure 1 illustrates a bidirectional CATV
system in accordance with the prior art. A head end 1 36 is connected to a trunk 2, to which distribution lines 37 3 are connected. Subscriber drops 4 connect to the -1 33634~
01 distribution lines, and subscriber station equipment 5 02 such as TV sets are connected to the subscriber drops.
03 The station equipment 5 in the present 04 embodiment includes upstream signal generation means 05 as described in Canadian Patent 1,177,558 issued 06 November 6th, 1984, invented by Michel Dufresne et 07 al. At least one bidirectional amplifier 6 is usually 08 connected in series with the trunk, for ampLifying the 09 downstream signals illustrated by arrow 7 and by the upstream signals illustrated by arrows 8.
11 In the system according to the prior art, 12 in addition to upstream signalling signals, 13 significant noise is passed upstream frbm the 14 distribution lines 3, drops 4 and station equipment 5. This noise as described earlier in this 16 specification is typically caused by automotive 17 ignitions, ground currents, poor ground connectors, 60 18 Hz powerline signals which themselves carry additional 19 noise, radio frequency pagers, radio telephones, other radio frequency signals, etc. The distribution lines 21 act as large distributed antennae, all feeding their 22 noise signals into the upstream portion of amplifier 23 6, where the noise signals are ampliied and are fed 24 to the head end 1. Clearly this can overwhelm any legitimate signal generated at the subscriber station 26 equipment to be transmitted to the head end, can 27 overload the trunk and head end amplifiers, and has 28 generally resulted in an unsatisfactory system.
29 Figure 2A illustrates a signalling scheme for use on a CATV network. A high frequency band 31 having 1 dB down points between 50 and 400 mHz 32 contains downstream television and/or other signals 33 from the head end through the trunk and distribution 34 lines to the subscriber station receiving equipment.
A low frequency band having 1 dB down points between 5 36 and 30 mHz carries upstream signals between the 37 subscriber station transmitting equipment and the head 38 _ 9 _ "--~36340 ' 01 end via the subscriber drops, distribution lines and 02 trunk.
03 In the present invention at least one but 04 preferably 2 narrowband upstream signalling 05 frequencies carrying digital signals are used, e.g. in 06 one successful prototype having carrier center 07 frequencies at 11 and 26 mHz, and each being about 1 08 mHz wide. The positions of these two narrow 09 signalling bands with respect to the low frequency band are shown in Figure 2B, with reference to Figure 11 2A.
12 Turning now to Figure 3, a network is 13 shown which is generally similar to that shown in 14 Figure 1, but has the addition of narrowband upstream filters 9, connected in series with each of the 16 distribution lines 3 adjacent the point at which they 17 are connected to the trunk 2. The upstream filters 9 18 should be bandpass filters having narrow passbands 19 centered on the upstream signalling carrier frequencies, e.g. in the preferred embodiment at Il 21 and 26 mHz, and having the bandwidth of the upstream 22 signalling signals, i.e. 1 mHz wide.
23 In one embodiment the filters 9 are 24 passive, allowing the full high band downstream signal to pass, but only allowing the narrowband upstream 26 signalling signals to pass upstream. Thus nearly all 27 of the upstream noise from each of the distribution 28 lines will be blocked; the only noise which will pass 29 upstream is the relatively small amount of noise energy contained within the narrow passband of the 31 filters. Clearly this substantially reduces the 32 amount of noise gathered and applied to the trunk and 33 head çnd amplifiers, substantially improving the 34 signal to noise ratio in the trunk outside ~he narrow band, and substantially reducing the possibility of 36 overloading the trunk bidirectional amplifiers which 37 would reduce the signal to noise ratio within the ~ 336340 01 narrow band(s) at the head end.
02 In a second embodiment the upstream 03 filters 9 contain active circuits, to be described 04 below, which detect the energy content of the signal 05 within the passbands of the upstream filters. In the 06 case of noise, almost all of the time the energy 07 content of those passbands will be low. However if a 08 signal is being transmitted from the subscriber 09 station equipment, it will be concentrated within the filter passband. The energy level in that case will 11 thus be much higher.
12 Each of the upstream filters contains a 13 gate which stops all signals from passing upstream 14 into the trunk. Thus the noise passed into the trunk from the distribution lines will be reduced to a 16 negligible value. When the energy which results from 17 the transmission of a signal from the subscriber 18 station equipment, which is concentrated within the 19 passband of the filters, is received, its energy level will be high, and above a predetermined threshold.
21 When this threshold has been exceeded, the gate opens, 22 allowing those signals within the passband of the 23 filter to pass upstream. Since typically only one 24 subscriber will be transmitting at a time (although this is not universally true), only one upstream 26 filter will be open at a time and thus the signal and 27 the very small amount of noise energy w~ithin the 28 upstream signalling narrowband will be carried from 29 the single distribution line through trunk 2 to the head end 1. The amount of total noise energy thus 31 received by the head end will be reduced to the 32 portion contributed by that distribution line, and 33 only the noise within the narrow signalling band.
34 Clearly also any excessive noise in one subnetwork is kept isolated from affecting the other 36 subnetworks by the filters.
37 In accordance with a third embodiment, the ~336340 01 upstream filters 9 are passive, as described with 02 respect to the first embodiment, but active filters 10 03 and lOA are connected in upstream series with the 04 trunk 2 to isolate and divide the network.
05 Subnetworks are subdivided by series active filters 06 lOA. Active filters 10 and lOA are similar to filter 07 9 as described with reference to the second 08 embodiment, that is, each contains a gate which stops 09 the transmission of all upstream signal~ in the associated subnetwork until an upstream signal 11 exceeding a predetermined threshold within the 12 narrowband upstream signalling bands is received, then i3 it will open, allowing the signalling signals to pass 14 upstream.
It should be noted that active filters 9 16 and 10 and lOA in whatever form are used, are 17 transparent in the downstream direction, allowing the 18 high frequency band signals to pass downstream, 19 unimpeded.
If filters 10 and lOA are present, they 21 are active, and filters 9 may be active or passive (as 22 described above). If filters 10 and lOA are not 23 present, filters 9 can be either active (as in the 24 second embodiment) or passive (as in the first embodiment).
26 In accordance with a fourth embodiment, 27 active filters 10 and lOA may or may not be present, 28 but at least upstream filters 9 are individually 29 addressable from head end 1. When addressed to either be purposely opened or closed, they allow the head end 31 to test or control the transmission of signals passing 32 along one or more distribution lines.
33 In accordance with another embodimen~ ~
34 separate conditioned line for carrying upstream video is connected to the upstream filter, and the upstream 36 filter bandwidth is wider, sufficient to accommodate a 37 video signal. The filter senses the video signal 01 presence as exceeding a threshold, and opens. While 02 the video camera can be locally or remotely 03 controlled, the filters are controlled by sensing of 04 the video (or indeed any other upstream signal such as OS a conferencing signal) from the cameras. Indeed the 06 video filter can be in the same narrow band upstream 07 signalling filter described above. This allows 08 subscriber station equipment connected to the trunk 09 through reconditioned or individual drops on the distribution line to generate video signals, which can 11 be switched into the trunk and head end by the 12 automatic sensing of the signal pressure or by the 13 head end addressing the upstream filters 9. It may be 14 desirable in this or in other embodiments to locate the upstream filters adjacent or at the subscriber 16 station equipment or to make the station equipment 17 remotely controlled from the head end.
18 By arranging several subscriber stations 19 together in one room, with video cameras connected as the subscriber station equipment, and by the use of 21 the upstream filters described above, remote studios 22 can be formed, remotely controlled from the head end 1 23 by using the upstream filters as remote switchers or 24 by automatic sensing of the video signals, switching remote selectable video signals generated at the 26 subscriber station video equipment into the trunk and 27 into the head end as desired at the head end. The 28 CATV network is facilitated for this upstream 29 transmission also because of the reduction of noise gathering due to use of the filters in the remaining 31 subnetworks or distribution lines of the system.
32 Figure 4 is a block diagram of a preferred 33 form of an active filter for use in the present 34 invention. The lead 3A represents the input of the upstream part of a cable distribution line and the 36 lead 3B represents the input of the downstream part of 37 the cable distribution line. A high band bandpass 01 filter 13, e.g. for passing signals between 50 and 400 02 mHz is connected in the trunk or distribution line 03 between leads 3A and 3B, to allow high band signals to 04 pass downstream, but to block low band upstream 05 signals. The arrows represent the signal direction.
06 60 Hz power is typicalLy pa~sed from 07 either direction along the cable to power remote line 08 amplifiers. A 60 Hz low pass filter 14 is connected 09 in parallel with filter 13. This allows 60 Hz power signals to pass around filter 13 along the 11 distribution line or the trunk from lead 3A to lead 3B
12 or vice versa.
13 Part of the 60 Hz signal is tapped, and is 14 applied to AC/DC converter 15, which generates e.g. 12 volts DC for operation of the active filter to be 16 described below.
17 Also connected in parallel with filter 13 18 is an upstream low band bandpass filter 16. This 19 filter is powered from the 12 volts DC generated in converter 15. Filter 16 passes the upstream 21 signalling signals from lead 3B to lead 3A along the 22 distribution line. It also blocks all upstream 23 signals outside its pass bands. In the preferred 24 embodiment the filter 16 has one mHz passband centered at 11 and 26 mHz, as shown in Figure 2B.
26 With the use of the embodiment of Figure 27 4, it may be seen that upstream noise within the low 28 frequency upstream band 5-30 mHz is blocked from 29 passing upstream through filter 13 because it has a passband only between 50 and 500 mHz. Similarly 31 filter 14, being a 60 Hz filter, will not pass 32 upstream signals between 5 and 30 mHz. The upstream 33 bandpass filter 16 blocks all signals except for those 34 within the upstream signalling bands, which can be one or more narrowbands, but which preferably is at the 11 36 and 26 mHz center frequencies as described above.
37 Thus virtually all noise is blocked from being passed 01 upstream.
02 The downstream high band bandpass filter 03 is a conventional LC filter and need not be described 04 in detail as it is known to those persons skilled in 05 the art.
06 Also in the preferred embodiment, upstream 07 bandpass filter 16 contains a gate which blocks noise 08 even within the upstream signalling bands, unless the 09 energy level is above a predetermined threshold. A
detailed description of this active filter will be 11 given below.
12 Referring to Figures 6 and 7 placed 13 together as shown in Figure 5, the upstream signal is 14 received on lead 3B and is amplitude limited by inversely poled parallel connected diodes 20A and 20B
16 connected between lead 3B and ground. The signal is 17 then passed into a preamplifier 21, where it is 18 amplified, through a multiple segment L-C filter 22, 19 having a 26 mEIz or 11 mE~z center frequency and 1 m~Iz passband, and passes through an output amplifier 23 21 from which it is capacitively coupled by capacitor 24 22 to a signal splitter 25. The circuit so far described 23 provides an output signal at 11 or 26 mHz, one mHz 24 broad.
The output signal is amplified further in 26 amplifier 26, from which it is applied to the input of 27 an electronic switch 27. The electronic switch 27 28 must switch very quickly, e.g. within a microsecond.
29 A switch which has been found very satisfactory for this is type 4053B.
31 The output of electronic switch 27 is 32 capacitively coupled by capacitor 28 to a 75 ohm pad 33 29 to the distribution line 3A, which typically will 34 be operating at an impedance of 75 ohms.
Thus, when electronic switch 27 is 36 enabled, the input signal from lead 3B, restricted to 37 the passband of the filter e.g. at 11 or 26 mHz center , 01 frequency and 1 mEIz wide, will be switched with great 02 speed to distribution line lead 3A, from which it 03 passes upstream to the trunk.
04 In order to control the operation of the 05 electronic switch 27, the signal from splitter 25 is 06 applied via capacitor 30 bypassed by resistors 31 and 07 32 at both ends thereof, to a half wave rectiEier 08 diode 23, the output signal of wl-ich is integrated by 09 capacitor 34 bypassed by discharge resistor 35. This accumulates the energy in the input signal and 11 provides a variable DC output voltage which is 12 proportional thereto. The resulting variable DC
13 output voltage is amplified in operational amplifier 14 36, the output signal of which is applied to the noninverting input of operational amplifier 38.
16 A circuit identical to that described 17 above connec~ed between elements 30 and 36 is 18 connected to the inverting input of operational 19 amplifier 38, the elements of which having been similarly ~abelled followed by the de~ignation A, that 21 is, elements 30A - 36A. The purpose of elements 30A -22 36A is to provide reference level compensation for the 23 rectifying circuit in the previously described branch 24 connected to the noninverting input of operational amplifier 38. This provides both temperature 26 compensation and level compensation for the diode 27 threshold and for the element variances caused by 28 temperature variation. Resistors 37 and 37A
29 respectively connect the outputs of amplifiers 36 and 36A to the noninverting and inverting inputs of 31 amplifier 38. Resistor 37C connects the noninverting 32 input to -lOV and resistor 37B connects the inverting 33 input of amplifier 38 to its output.
34 The integrating circuit described above determines the speed of detection and should have a 36 time constant of 5 microseconds or less. It should be 37 short enough to open, so as not to cut off a signal t 33634~
01 but long enough to avoid triggering by noise. In case 02 the input signal is video (requiring the filter 03 comprised of elements 21, 22 and 23 to be 04 substantially broader in bandwidth than described 05 earlier to accommodate the video), the switching speed 06 of the integrator can be substantially relaxed. It 07 should be noted that the longer the time constant, the 08 greater the reliability will be.
09 The output signal of amplifier 38 is connected to a level detector. This is preferably 11 comprised of an operational amplifier 39, to the 12 inverting input of which the output signal of 13 amplifier 38 is applied. The detection threshold is 14 set by the noninverting input to amplifier 39 being connected through resistor 40 to the tap terminal of a 16 potentiometer 41, which itself is connected in series 17 with resistors 42 and 43 at its opposite terminals 18 respectively between a source of -20 V and ground.
19 The tap terminal of potentiometer 41 is by-passed to ground by capacitor 44.
21 The threshold setting circuit will cause 22 amplifier 39 to output either 0 volts or -20 volts 23 when the rectified and integrated input signal from 24 splitter 25 is above or below the level set on potentiometer 41 between -20 V and ground (0 V). This 26 output signal is applied to the cathode of diode 45 27 through resistor 46. The anode is connected to -17.2 28 V. Thus when the output of amplifier 39 is 0 V diode 29 45 will be reverse biased (off), and when the output of amplifier 39 is -20 V, diode 45 will be biased into 31 its conducting state and the voltage on its cathode 32 will be approximately -17 V. With the cathode of 33 diode 45 connected to the control input 48 of 34 electronic switch 27 the electronic switch will be turned on when diode 45 is conducting, i.e. when the 36 output of amplifier 39 is -20 V, and off when diode 45 37 is reverse biased.
~ 1 336340 01 With the time constant of the integrator 02 less then 5 microseconds, and the switching time of 03 electronic switch 27 less than 1 microsecond, clearly 04 when the input signal results in the threshold set by 05 potentiometer 41 is exceeded, electronic switch 27 06 will cause the input signal passing through the filter 07 and appearing at splitter 25 to be switched very 08 rapidly to the upstream distribution line lead 3A.
09 The switch state monitor lead could also be connected to a control input of filter 22 (if it 11 were made on active filter), to control and thereby 12 open or close its bandwidth.
13 The above-described circuit should be 14 duplicated for each of the narrowband signalling signal frequencies appearing on the same distribution 16 line or trunk. In the case of two signals transmitted 17 at different sequential times at the 11 and 26 mHz 18 signalling bands e.g. using MSK, only one or the other 19 of the filters will be transmitting at a time. This can further reduce the amount of noi~e carried 21 upstream of the filter into the trunk over the case in 22 which both signals are transmitted simultaneously 23 (mf)-24 Additional enhancements may be made to the above-described circuit. For example the control 26 input to electronic switch 27 can also be connected to 27 the output of an addressable receiver-controller 47, 28 which responds to an unique code received in the 29 downstream direction from the head end in the high frequency band. This will be described in more detail 31 with respect to Figure 8. However with respect to the 32 present figure, controller 47 receives a signal, 33 demodulates it, decodes it and applies a constant open 34 or constant close level enable 5 ignal (0 or -17 V) to the control input of electronic switch 27. This can 36 forcibly control the transmission of signals upstream 37 through electronic switch 27 for testing or for 3~ - 18 -~ 33634Q
01 signalling source control purposes, as described 02 earlier.
03 A second contact 27A in electronic switch 04 27 can be connected through capacitor 48A to a switch 05 state monitor lead. This switch state monitor lead 06 can provide signals input to it e.g. to the upstream 07 lead 3A, or to another switch to provide 08 acknowledgement of the receipt of the remotely 09 controlled electronic switch control command which is received by controller 47, or to provide a second 11 switch state indicative to the head end and or to 12 local control equipment.
13 The switch state monitor lead can be 14 connected to the input of an encoder 49, the output of which is connected to a modulator 50, which has its 16 output connected through a 75 ohm line matching pad 17 29A, to upstream lead 3A. In this case the receipt of 18 an addressable command causes switch 27 to operate.
19 As a result a pulse appears at the input of encoder 49, which generates a code, modulated in modulator 50, 21 which is applied through output pad 29A to the 22 upstream lead 3A. This can be used to provide 23 acknowledgement of the receipt of a command to close 24 switch 27, since in the case of testing of switch 27, there may otherwise not be any input signal passing 26 therethrough to be received and noted by the head end.
27 Alternatively the output of modulator 50 28 can be connected into the input of switch 27 for 29 transmission of an acknowledgement code generated by encoder 49 through switch 27 as if it were a signal 31 received through the filter comprised of elements 21, 32 22 and 23.
33 A block diagram illustrating a variation 34 of the above is shown in Figure 8. In this figure high pass filter 13 is connected between leads 3A and 36 3B as described with reference to Figure 4. The 37 upstream signal is passed through a low band bandpass __ ~ .
01 filter 52 which passes signals between e.g. 5 - 30 02 mHz. The upstream signal then passes through an 03 amplifier 53 and a narrowband upstream signal filter 04 54 such as that described with respect to elements 21, 05 22 and 23. The output of filter 54 is connected to 06 the input of electronic switch 55, which is controlled 07 by a threshold detector/control circuit 56 such as 08 that described earlier with respect to elements 30 -09 45 in Figures 5, 6 and 7. The output of switch 55 is connected to the input of another filter 57 which is 11 similar to filter 54. The output of filter 57 is 12 connected to the upstream lead 3A, which is the 13 distribution line connected to the trunk.
14 A downstream control signal within the high band is received on lead 3A, passes through 16 control signal frequency filter 57A and is received in 17 demodulator 58. The demodulated signal is decoded in 18 decoder 59, and the decoded control signal is applied 19 to the control input of switch 55, in a manner similar to that described with reference to controller 47 21 controlling switch 27.
22 The switch state monitor output of switch 23 55 similar to that described with respect to switch 27 24 is connected to one of several input terminals of an encoder 60 which has its output connected to modulator 26 61, which has its output, in the low frequency band, 27 connected to the input of filter 57, from where the 28 output signal of modulator 61 is applied to upstream 29 3A for application to the trunk and to the head end.
It should be noted that several decoders 31 similar to decoder 59 can be connected to the output 32 of demodulator 58, each one of which can be connected 33 to a separate input of encoder 60 through controlling 34 of another switch similar to switch 55. This can clearly be used to apply various input signals to the 36 upstream distribution line and trunk. For example, 37 such signals can be from power meters, video signals, ~ 336340 .
01 acknowledgement of appliance turn on at subscriber 02 locations, etc. Further, decoded signals from the 03 head end can be used to switch appliances on and off, 04 activate power circuits, etc. at the subscriber 05 location. Indeed, control signals from subscriber 06 stations can be sent upstream to the head end to 07 command certain functions, such as the transmission of 08 control signals to control or monitor units at the 09 same or different subscriber stations.
The above description of embodiments of 11 the invention have clearly shown that the present 12 invention can be used for a variety of various 13 functions, but at its heart provides means for 14 providing signals to a head end with reliability and substantially reduced of noise. Figure 9A illustrates 16 the spectrum of the signal received at a head end with 17 a prior art network and is comprised virtually 18 entirely of noise; Figure 9B illustrates the spectrum 19 of the signal received at the head end utilizing the present filters and is clearly almost clear of 21 significant noise; and Figure 9C illustrates the 22 spectrum of signal received at the head end with a 26 23 mEIz upstream signalling signal used in accordance with 24 the present invention, and clearly illustrates a very high signal to noise ratio. The improvement over the 26 prior art is seen to be substantial.
27 A person understanding this invention may 28 now conceive of other alternatives or embodiments 29 using the principles described herein. A]l ~re considered to be within the sphere and scope o~ tlle 31 invention as defined in the claims appended hereto.
This invention relates to cable television (CATV) networks and in particular to a reliable bidirectional CATV network.
CATV networks have in the past been structured with a ~lead end which provides the various signals, connected to a trunk, distribution lines connected to the trunk, and sometimes subdistribution lines connected to the distribution lines (herein being grouped with distribution lines). Subscriber drops are connected to the distribution lines. CATV
signals pass from the head end along the trunk, through the distribution lines and the subscriber drops to television sets or other subscriber terminals at various subscriber locations.
In order to facilitate enhanced subscriber services, some CATV systems have been made bidirectional, that is, they contain bidirectional amplifiers to enable signals to be transmitted both from the suhscriber locations to the head end and from the head end to the subscriber locations. Such enhanced services were envisioned to facilitate e.g.
pay per view of TV programs, ordering products displayed on television channels, playing of interactive video games, responding to polls, answering questions provided by a lecture course, etc.
It has been found that bidirectional systems have been unsuccessful because of a major noise gathering problem which was encountered. Noise caused by electronic or radio interference, poor terminal connections, ground currents, power lines and noise carried thereon, automobile ignitions, etc.
arises on the subscriber drops and distribution lines and all feed into the trunk and head end in the upstream direction. The noise is random and interferes to a prohibitive extent with legitimate signals transmitted upstream via the system from the var ious subscr ibers . ~
- 1 - ~
. r~. ~ ~ ~
t __ ~36340 01 In order to make an existing bidirectionaL
02 system work, CATV operators have had to rebuild their 03 distribution systems entirely to eliminate all 04 possible sources of interference, or use code operated 05 switches in bridgers addressed from the head end to 06 isolate distribution lines from the trunk and open 07 onLy one distribution line at a time for transmission;
08 or on a retained dLstribution system, inspect every 09 aspect, ground the various units properly and permanently and increase shielding at various 11 locations, in order to reduce as much as possible the 12 environment input of the noise signals.
13 It has also been found that excess noise 14 in the upstream direction can overload the upstream portion of the bidirectional amplifiers, which can 16 cause oscillation in the bidirectional amplifiers in 17 the trunk and/or the distribution lines. Since the 18 level of noise cannot be predicted because both its 19 timing and amplitude is random, this has posed a major problem.
21 The present invention is a CATV network 22 which allows for the first time an existing cabling 23 structure to be able to be used in a bidirectional 24 mode, by substantially reducing or eliminating the effect of noise gathering.
26 In a typical system, the downstream 27 signals are contained within a high requency band, 28 e.g. typically between 50 and 500 mHz, while the 29 upstream signals are contained within a low frequency band, e.g. between 5 and 30 mHz. In the present 31 invention the downstream directional signals are 32 untouched. The trunk retains its bidirectional 33 amplifiers, that is, amplifying downstream signals in 34 the high band and amplifying upstream signals in the low band.
36 According to a first embodiment of the 37 present invention, the upstream signals are contained t ._ 1 ~36340 01 in one or more narrow bands within the low band, 02 preferably centered at two frequencies, e.g. 11 mHz 03 and 26 mHz, with a bandwidth of e.g. 1 mHz. It should 04 be noted however that the present invention is not 05 limited to narrow band or to the use of two upstream 06 signalling frequencies, since one, or more than two 07 frequencies could be used.
08 Narrowband upstream filters are located in 09 the distribution lines, preerably but not necessarily just next to the points that the distribution lines 11 are connected to the trunk. They may also be 12 connected in the trunk, and the filters may be 13 connected in series within the network. The 14 downstream signals are unafected.
Preferably the upstream signals are 16 carriers modulated-by digital signals, e.g. at a 17 500 kb/sec rate using minimum shift keying or an 18 equivalent frequency modulation technique.
19 The resuLt of the above is that all upstream signals outside the narrow bandwidth of the 21 upstream signalling bands are blocked. Thus all 22 noise, which will include the vast majority of the 23 noise, which is outside the narrow signalling band of 24 the narrow bands is blocked. The utilization of the rest of the bandwidth outside the n~arrow bands for 26 video transmission is thus made possible.
27 Because the resulting amount of total 28 energy (signal plus noise) within the narrow 29 signalling bands is made low, the likelihood of overloading the upstream amplifiers by noise signals 31 causing oscillation of the downstream amplifiers in 32 the trunk will be very low, by the use of the present 33 invention.
34 Thus by the use of the present invention the problem o~ overloading in a bidirectional CATV
36 network is substantially reduced.
37 According to a second embodiment of the 01 invention, each of the upstream filters of the kind 02 described above contains a gate, which blocks all 03 upstream signals. Thus all noise entirely across the 04 low frequency upstream band will be blocked. Each of 05 the filters contains a circuit for detecting the 06 energy level of upstream signals carried in the narrow 07 frequency band, which is detected at the input to the 08 filter. When the energy exceeds a predetermined 09 threshold, it is assumed that the energy consists of a signal to be transmitted to the head end. Below the 11 threshold the energy is considered to be noise. When 12 the energy level is above the predetermined thres,hold, 13 the gate opens, allowing signals within the narrow 14 frequency band of the filter to pass to the trunk and thus to the head end.
16 Thus in accordance with this second 17 embodiment, there is no contribution to the trunk of 18 any noi~e whatsoever, even in the narrow signalling 19 bands, unless an actual upstream signal is being transmitted. The gate then opens automatically 21 without intervention from the head end, allowing the 22 signal and only the small energy content of noise 23 within the narrow signalling band of the low band to 24 pass through. Since all other filter gates are closed at that time, the noise passed upstream to the head 26 end is reduced substantially even in comparison to the 27 passive filter embodiment. The interval for sensing 28 of energy level and the opening of the gate should be 29 very short, within a,5 microseconds interval or less to permit implementation of a multi-access protocol 31 (the use of the same channel by several subscribers 32 with the head end recognizing and dealing with 33 collisions - see Canadian Patent Application 550,764 34 filed November 2, 1987 invented by Samir Sammoun) and realize the result of little effect on performance.
36 It should be noted, however, that for optimum noise 37 blocking, this interval should be maximized.
38 It is important to recognize that in the ~9 - 4 -`r336340 01 prior art, distribution switches are polled from the 02 head end in order to determine which subscriber is 03 transmitting, in order to obtain the upstream 04 transmitted data. That is both dif~icult and is a 05 very slow technique for receiving system signals due 06 to the time required to complete the polling, in 07 contrast to the present invention which does not 08 require polling.
09 A prototype system built according to the second embodiment was tested on an actual CATV network 11 with about 4,000 subscribers per switched filter and 12 was found to be reliable at least 99% of the time, 13 which is believed to be a substantial advance in the 14 state of the art.
According to a further embodiment passive 16 filters as described with respect to the first 17 embodiment above can be used in the distribution 18 lines, and an active filter containing the gate and 19 energy sensor described with respect to the second embodiment can be used in the upstream direction in 21 the trunk.
22 In a fourth embodiment, the active ~ilters 23 in the distribution lines can be addressable. The 24 gates in the filters can be purposefully opened or closed by selectively addressing them from the head 26 end. Thus in this embodiment each of the filters 27 should have an unique address. A signal passed from 28 the head end downstream and received by the filter 29 would cause the gate to open or close as desired from the head end. This allows the head end to selectively 31 test individual distribution lines, or perform a 32 remote program source switching operation.
33 While the upstream signalling bandwidth 34 has been described above as pre~erably being comprised of two narrowband signalling frequencies within the 36 upstream low frequency band, the upstream signal can 37 instead be comprised of video signals or other program 1 336~40 01 signals. Distribution lines can be split off from the 02 trunk adjacent a single (or multiple) subscriber 03 location, which can connect to a remote studio 04 containing several cameras. Each of the cameras can 05 be connected to a separate distribution line which 06 contains a selectably addressable filter. By enabling 07 the individual filters from the head end, their 08 individual gates can be opened, and individual signals 09 from selectable cameras transmitted to the head end.
At the same time noise gathering from other 11 distribution lines is blocked. This type of system is 12 useful for the remote controlling from the head end of 13 remote broadcasts, e.g. the transmission of university 14 courses using several cameras without requiring local switching personnel, the provision of video 16 conferences from various locations, remote control of 17 surveillance cameras, the reception of different types 18 of signals for simultaneous or later transmission to 19 other subscribers, the remote gathering of news, the provision o~ audio or video forums, etc. of course 21 the filters in this case should have bandwidth 22 sufficient to accommodate the upstream signal.
23 All of the above is made feasible using the 24 present invention avoiding the problem of noise gathering.
26 In accordance with the present invention, 27 an embodiment thereof is a CATV network comprising a 28 bidirectional trunk for connection to a head end and 29 bidirectional distribution lines connected to the trunk to which subscriber drops can be connected, the 31 distribution lines including means such as an 32 amplifier for transmitting upstream signals in at 33 least one narrow frequency band, a bandpass filter 34 connected in the upstream direction in each of the distribution lines having passbands corresponding to 36 the narrow frequency band or bands for blocking 37 upstream noise and signals which may appear on the ~ 1 336340 distribution lines except those contained in the narrow frequency band or bands. Preferably the filters are connected in the distribution lines immediately adjacent their connections to the trunk. However in some cases it may be desirable to connect the filters at the subscriber locations, e.g. within the drops or immediately adjacent the subscriber equipment connection facility.
In accordance with a preferred embodiment, such filter includes a gate for blocking all upstream signals. A circuit is included for detecting the energy level of upstream signals carried in the narrow frequency band or bands on a corresponding distribution line, and opens the gate to allow upstream signals to pass along the corresponding distribution line within the passband in the event the energy level is above a predetermined threshold.
In accordance with a further embodiment a circuit is included in each filter for receiving an address signal passing downstream on the distribution lines, and a circuit is included for opening the gate upon receipt of the addressed signal. Preferably addresses unique to each of the corresponding filters are used.
In accordance with a further embodiment the gate opens to allow video or other wideband signals to pass upstream.
In accordance with another embodiment, the passive (gateless) filters are connected in the distribution lines, but an active (gate included) filter is connected in the trunk between the head end and the location of the closest distribution line connection to the trunk.
.~
In accordance with an embodiment of the invention, for use in CATV network, an upstream filter circuit is comprised of at least one bandpass filter for receiving an upstream signal, an electronic switch 5 connected to the output of at least one bandpass filter for applying a filtered upstream signal from at least one bandpass filter to an upstream line, apparatus for detecting the energy level of the filtered upstream signal at a point between at least one bandpass filter and the electronic switch, and apparatus for enabling the electronic switch to close in the event the energy level is above a predetermined threshold.
A better understanding of the invention will be obtained by reference to the detailed description 15 below in conjunction with the following drawings, in which:
- 7a -~ 1 336340 01 Figure 1 is a block schematic of a CATV
02 network according to the prior art, 03 Figures 2A and 2B respectively illustrate 04 the low and high frequency bands, and the preferred 05 signal frequencies of the upstream signals, 06 Figure 3 is a block schematic illustrating 07 at least one embodiment the present invention, and 08 appears on the same page as Figure 1, 09 Figure 4 is a block schematic illustrating the preferred filter structure of the present 11 invention, 12 Figure 5 illustrates how Figures 6 and 7 13 should be placed together, and appears on the same 14 page as Figure 7, Figures 6 and 7 placed together form a 16 schematic diagram of an active filter according to the 17 preferred embodiment of the present invention, 18 Figure 8 is a block diagram of an 19 addressable active filter according to an embodiment of the present invention, and appears on the same page 21 as Figures 2A and 2B, 22 Figure 9A is an example of a spectrum 23 diagram of the upstream signal received at the head 24 end as in a prior art network, ~igure 9B is a spectrum diagram of the 26 upstream signal received at the head end using the 27 preferred embodiment of the present invention, in the 28 absence of an upstream signalling signal, and 29 Figure 9C is a spectrum diagram of the upstream signal received at the head end, using the 31 preferred embodiment of the present invention, in the 32 pr~n~ of an up~tr~m fli~n~llin~ si~n~l ~entered ~t 33 26 mHz.
34 Figure 1 illustrates a bidirectional CATV
system in accordance with the prior art. A head end 1 36 is connected to a trunk 2, to which distribution lines 37 3 are connected. Subscriber drops 4 connect to the -1 33634~
01 distribution lines, and subscriber station equipment 5 02 such as TV sets are connected to the subscriber drops.
03 The station equipment 5 in the present 04 embodiment includes upstream signal generation means 05 as described in Canadian Patent 1,177,558 issued 06 November 6th, 1984, invented by Michel Dufresne et 07 al. At least one bidirectional amplifier 6 is usually 08 connected in series with the trunk, for ampLifying the 09 downstream signals illustrated by arrow 7 and by the upstream signals illustrated by arrows 8.
11 In the system according to the prior art, 12 in addition to upstream signalling signals, 13 significant noise is passed upstream frbm the 14 distribution lines 3, drops 4 and station equipment 5. This noise as described earlier in this 16 specification is typically caused by automotive 17 ignitions, ground currents, poor ground connectors, 60 18 Hz powerline signals which themselves carry additional 19 noise, radio frequency pagers, radio telephones, other radio frequency signals, etc. The distribution lines 21 act as large distributed antennae, all feeding their 22 noise signals into the upstream portion of amplifier 23 6, where the noise signals are ampliied and are fed 24 to the head end 1. Clearly this can overwhelm any legitimate signal generated at the subscriber station 26 equipment to be transmitted to the head end, can 27 overload the trunk and head end amplifiers, and has 28 generally resulted in an unsatisfactory system.
29 Figure 2A illustrates a signalling scheme for use on a CATV network. A high frequency band 31 having 1 dB down points between 50 and 400 mHz 32 contains downstream television and/or other signals 33 from the head end through the trunk and distribution 34 lines to the subscriber station receiving equipment.
A low frequency band having 1 dB down points between 5 36 and 30 mHz carries upstream signals between the 37 subscriber station transmitting equipment and the head 38 _ 9 _ "--~36340 ' 01 end via the subscriber drops, distribution lines and 02 trunk.
03 In the present invention at least one but 04 preferably 2 narrowband upstream signalling 05 frequencies carrying digital signals are used, e.g. in 06 one successful prototype having carrier center 07 frequencies at 11 and 26 mHz, and each being about 1 08 mHz wide. The positions of these two narrow 09 signalling bands with respect to the low frequency band are shown in Figure 2B, with reference to Figure 11 2A.
12 Turning now to Figure 3, a network is 13 shown which is generally similar to that shown in 14 Figure 1, but has the addition of narrowband upstream filters 9, connected in series with each of the 16 distribution lines 3 adjacent the point at which they 17 are connected to the trunk 2. The upstream filters 9 18 should be bandpass filters having narrow passbands 19 centered on the upstream signalling carrier frequencies, e.g. in the preferred embodiment at Il 21 and 26 mHz, and having the bandwidth of the upstream 22 signalling signals, i.e. 1 mHz wide.
23 In one embodiment the filters 9 are 24 passive, allowing the full high band downstream signal to pass, but only allowing the narrowband upstream 26 signalling signals to pass upstream. Thus nearly all 27 of the upstream noise from each of the distribution 28 lines will be blocked; the only noise which will pass 29 upstream is the relatively small amount of noise energy contained within the narrow passband of the 31 filters. Clearly this substantially reduces the 32 amount of noise gathered and applied to the trunk and 33 head çnd amplifiers, substantially improving the 34 signal to noise ratio in the trunk outside ~he narrow band, and substantially reducing the possibility of 36 overloading the trunk bidirectional amplifiers which 37 would reduce the signal to noise ratio within the ~ 336340 01 narrow band(s) at the head end.
02 In a second embodiment the upstream 03 filters 9 contain active circuits, to be described 04 below, which detect the energy content of the signal 05 within the passbands of the upstream filters. In the 06 case of noise, almost all of the time the energy 07 content of those passbands will be low. However if a 08 signal is being transmitted from the subscriber 09 station equipment, it will be concentrated within the filter passband. The energy level in that case will 11 thus be much higher.
12 Each of the upstream filters contains a 13 gate which stops all signals from passing upstream 14 into the trunk. Thus the noise passed into the trunk from the distribution lines will be reduced to a 16 negligible value. When the energy which results from 17 the transmission of a signal from the subscriber 18 station equipment, which is concentrated within the 19 passband of the filters, is received, its energy level will be high, and above a predetermined threshold.
21 When this threshold has been exceeded, the gate opens, 22 allowing those signals within the passband of the 23 filter to pass upstream. Since typically only one 24 subscriber will be transmitting at a time (although this is not universally true), only one upstream 26 filter will be open at a time and thus the signal and 27 the very small amount of noise energy w~ithin the 28 upstream signalling narrowband will be carried from 29 the single distribution line through trunk 2 to the head end 1. The amount of total noise energy thus 31 received by the head end will be reduced to the 32 portion contributed by that distribution line, and 33 only the noise within the narrow signalling band.
34 Clearly also any excessive noise in one subnetwork is kept isolated from affecting the other 36 subnetworks by the filters.
37 In accordance with a third embodiment, the ~336340 01 upstream filters 9 are passive, as described with 02 respect to the first embodiment, but active filters 10 03 and lOA are connected in upstream series with the 04 trunk 2 to isolate and divide the network.
05 Subnetworks are subdivided by series active filters 06 lOA. Active filters 10 and lOA are similar to filter 07 9 as described with reference to the second 08 embodiment, that is, each contains a gate which stops 09 the transmission of all upstream signal~ in the associated subnetwork until an upstream signal 11 exceeding a predetermined threshold within the 12 narrowband upstream signalling bands is received, then i3 it will open, allowing the signalling signals to pass 14 upstream.
It should be noted that active filters 9 16 and 10 and lOA in whatever form are used, are 17 transparent in the downstream direction, allowing the 18 high frequency band signals to pass downstream, 19 unimpeded.
If filters 10 and lOA are present, they 21 are active, and filters 9 may be active or passive (as 22 described above). If filters 10 and lOA are not 23 present, filters 9 can be either active (as in the 24 second embodiment) or passive (as in the first embodiment).
26 In accordance with a fourth embodiment, 27 active filters 10 and lOA may or may not be present, 28 but at least upstream filters 9 are individually 29 addressable from head end 1. When addressed to either be purposely opened or closed, they allow the head end 31 to test or control the transmission of signals passing 32 along one or more distribution lines.
33 In accordance with another embodimen~ ~
34 separate conditioned line for carrying upstream video is connected to the upstream filter, and the upstream 36 filter bandwidth is wider, sufficient to accommodate a 37 video signal. The filter senses the video signal 01 presence as exceeding a threshold, and opens. While 02 the video camera can be locally or remotely 03 controlled, the filters are controlled by sensing of 04 the video (or indeed any other upstream signal such as OS a conferencing signal) from the cameras. Indeed the 06 video filter can be in the same narrow band upstream 07 signalling filter described above. This allows 08 subscriber station equipment connected to the trunk 09 through reconditioned or individual drops on the distribution line to generate video signals, which can 11 be switched into the trunk and head end by the 12 automatic sensing of the signal pressure or by the 13 head end addressing the upstream filters 9. It may be 14 desirable in this or in other embodiments to locate the upstream filters adjacent or at the subscriber 16 station equipment or to make the station equipment 17 remotely controlled from the head end.
18 By arranging several subscriber stations 19 together in one room, with video cameras connected as the subscriber station equipment, and by the use of 21 the upstream filters described above, remote studios 22 can be formed, remotely controlled from the head end 1 23 by using the upstream filters as remote switchers or 24 by automatic sensing of the video signals, switching remote selectable video signals generated at the 26 subscriber station video equipment into the trunk and 27 into the head end as desired at the head end. The 28 CATV network is facilitated for this upstream 29 transmission also because of the reduction of noise gathering due to use of the filters in the remaining 31 subnetworks or distribution lines of the system.
32 Figure 4 is a block diagram of a preferred 33 form of an active filter for use in the present 34 invention. The lead 3A represents the input of the upstream part of a cable distribution line and the 36 lead 3B represents the input of the downstream part of 37 the cable distribution line. A high band bandpass 01 filter 13, e.g. for passing signals between 50 and 400 02 mHz is connected in the trunk or distribution line 03 between leads 3A and 3B, to allow high band signals to 04 pass downstream, but to block low band upstream 05 signals. The arrows represent the signal direction.
06 60 Hz power is typicalLy pa~sed from 07 either direction along the cable to power remote line 08 amplifiers. A 60 Hz low pass filter 14 is connected 09 in parallel with filter 13. This allows 60 Hz power signals to pass around filter 13 along the 11 distribution line or the trunk from lead 3A to lead 3B
12 or vice versa.
13 Part of the 60 Hz signal is tapped, and is 14 applied to AC/DC converter 15, which generates e.g. 12 volts DC for operation of the active filter to be 16 described below.
17 Also connected in parallel with filter 13 18 is an upstream low band bandpass filter 16. This 19 filter is powered from the 12 volts DC generated in converter 15. Filter 16 passes the upstream 21 signalling signals from lead 3B to lead 3A along the 22 distribution line. It also blocks all upstream 23 signals outside its pass bands. In the preferred 24 embodiment the filter 16 has one mHz passband centered at 11 and 26 mHz, as shown in Figure 2B.
26 With the use of the embodiment of Figure 27 4, it may be seen that upstream noise within the low 28 frequency upstream band 5-30 mHz is blocked from 29 passing upstream through filter 13 because it has a passband only between 50 and 500 mHz. Similarly 31 filter 14, being a 60 Hz filter, will not pass 32 upstream signals between 5 and 30 mHz. The upstream 33 bandpass filter 16 blocks all signals except for those 34 within the upstream signalling bands, which can be one or more narrowbands, but which preferably is at the 11 36 and 26 mHz center frequencies as described above.
37 Thus virtually all noise is blocked from being passed 01 upstream.
02 The downstream high band bandpass filter 03 is a conventional LC filter and need not be described 04 in detail as it is known to those persons skilled in 05 the art.
06 Also in the preferred embodiment, upstream 07 bandpass filter 16 contains a gate which blocks noise 08 even within the upstream signalling bands, unless the 09 energy level is above a predetermined threshold. A
detailed description of this active filter will be 11 given below.
12 Referring to Figures 6 and 7 placed 13 together as shown in Figure 5, the upstream signal is 14 received on lead 3B and is amplitude limited by inversely poled parallel connected diodes 20A and 20B
16 connected between lead 3B and ground. The signal is 17 then passed into a preamplifier 21, where it is 18 amplified, through a multiple segment L-C filter 22, 19 having a 26 mEIz or 11 mE~z center frequency and 1 m~Iz passband, and passes through an output amplifier 23 21 from which it is capacitively coupled by capacitor 24 22 to a signal splitter 25. The circuit so far described 23 provides an output signal at 11 or 26 mHz, one mHz 24 broad.
The output signal is amplified further in 26 amplifier 26, from which it is applied to the input of 27 an electronic switch 27. The electronic switch 27 28 must switch very quickly, e.g. within a microsecond.
29 A switch which has been found very satisfactory for this is type 4053B.
31 The output of electronic switch 27 is 32 capacitively coupled by capacitor 28 to a 75 ohm pad 33 29 to the distribution line 3A, which typically will 34 be operating at an impedance of 75 ohms.
Thus, when electronic switch 27 is 36 enabled, the input signal from lead 3B, restricted to 37 the passband of the filter e.g. at 11 or 26 mHz center , 01 frequency and 1 mEIz wide, will be switched with great 02 speed to distribution line lead 3A, from which it 03 passes upstream to the trunk.
04 In order to control the operation of the 05 electronic switch 27, the signal from splitter 25 is 06 applied via capacitor 30 bypassed by resistors 31 and 07 32 at both ends thereof, to a half wave rectiEier 08 diode 23, the output signal of wl-ich is integrated by 09 capacitor 34 bypassed by discharge resistor 35. This accumulates the energy in the input signal and 11 provides a variable DC output voltage which is 12 proportional thereto. The resulting variable DC
13 output voltage is amplified in operational amplifier 14 36, the output signal of which is applied to the noninverting input of operational amplifier 38.
16 A circuit identical to that described 17 above connec~ed between elements 30 and 36 is 18 connected to the inverting input of operational 19 amplifier 38, the elements of which having been similarly ~abelled followed by the de~ignation A, that 21 is, elements 30A - 36A. The purpose of elements 30A -22 36A is to provide reference level compensation for the 23 rectifying circuit in the previously described branch 24 connected to the noninverting input of operational amplifier 38. This provides both temperature 26 compensation and level compensation for the diode 27 threshold and for the element variances caused by 28 temperature variation. Resistors 37 and 37A
29 respectively connect the outputs of amplifiers 36 and 36A to the noninverting and inverting inputs of 31 amplifier 38. Resistor 37C connects the noninverting 32 input to -lOV and resistor 37B connects the inverting 33 input of amplifier 38 to its output.
34 The integrating circuit described above determines the speed of detection and should have a 36 time constant of 5 microseconds or less. It should be 37 short enough to open, so as not to cut off a signal t 33634~
01 but long enough to avoid triggering by noise. In case 02 the input signal is video (requiring the filter 03 comprised of elements 21, 22 and 23 to be 04 substantially broader in bandwidth than described 05 earlier to accommodate the video), the switching speed 06 of the integrator can be substantially relaxed. It 07 should be noted that the longer the time constant, the 08 greater the reliability will be.
09 The output signal of amplifier 38 is connected to a level detector. This is preferably 11 comprised of an operational amplifier 39, to the 12 inverting input of which the output signal of 13 amplifier 38 is applied. The detection threshold is 14 set by the noninverting input to amplifier 39 being connected through resistor 40 to the tap terminal of a 16 potentiometer 41, which itself is connected in series 17 with resistors 42 and 43 at its opposite terminals 18 respectively between a source of -20 V and ground.
19 The tap terminal of potentiometer 41 is by-passed to ground by capacitor 44.
21 The threshold setting circuit will cause 22 amplifier 39 to output either 0 volts or -20 volts 23 when the rectified and integrated input signal from 24 splitter 25 is above or below the level set on potentiometer 41 between -20 V and ground (0 V). This 26 output signal is applied to the cathode of diode 45 27 through resistor 46. The anode is connected to -17.2 28 V. Thus when the output of amplifier 39 is 0 V diode 29 45 will be reverse biased (off), and when the output of amplifier 39 is -20 V, diode 45 will be biased into 31 its conducting state and the voltage on its cathode 32 will be approximately -17 V. With the cathode of 33 diode 45 connected to the control input 48 of 34 electronic switch 27 the electronic switch will be turned on when diode 45 is conducting, i.e. when the 36 output of amplifier 39 is -20 V, and off when diode 45 37 is reverse biased.
~ 1 336340 01 With the time constant of the integrator 02 less then 5 microseconds, and the switching time of 03 electronic switch 27 less than 1 microsecond, clearly 04 when the input signal results in the threshold set by 05 potentiometer 41 is exceeded, electronic switch 27 06 will cause the input signal passing through the filter 07 and appearing at splitter 25 to be switched very 08 rapidly to the upstream distribution line lead 3A.
09 The switch state monitor lead could also be connected to a control input of filter 22 (if it 11 were made on active filter), to control and thereby 12 open or close its bandwidth.
13 The above-described circuit should be 14 duplicated for each of the narrowband signalling signal frequencies appearing on the same distribution 16 line or trunk. In the case of two signals transmitted 17 at different sequential times at the 11 and 26 mHz 18 signalling bands e.g. using MSK, only one or the other 19 of the filters will be transmitting at a time. This can further reduce the amount of noi~e carried 21 upstream of the filter into the trunk over the case in 22 which both signals are transmitted simultaneously 23 (mf)-24 Additional enhancements may be made to the above-described circuit. For example the control 26 input to electronic switch 27 can also be connected to 27 the output of an addressable receiver-controller 47, 28 which responds to an unique code received in the 29 downstream direction from the head end in the high frequency band. This will be described in more detail 31 with respect to Figure 8. However with respect to the 32 present figure, controller 47 receives a signal, 33 demodulates it, decodes it and applies a constant open 34 or constant close level enable 5 ignal (0 or -17 V) to the control input of electronic switch 27. This can 36 forcibly control the transmission of signals upstream 37 through electronic switch 27 for testing or for 3~ - 18 -~ 33634Q
01 signalling source control purposes, as described 02 earlier.
03 A second contact 27A in electronic switch 04 27 can be connected through capacitor 48A to a switch 05 state monitor lead. This switch state monitor lead 06 can provide signals input to it e.g. to the upstream 07 lead 3A, or to another switch to provide 08 acknowledgement of the receipt of the remotely 09 controlled electronic switch control command which is received by controller 47, or to provide a second 11 switch state indicative to the head end and or to 12 local control equipment.
13 The switch state monitor lead can be 14 connected to the input of an encoder 49, the output of which is connected to a modulator 50, which has its 16 output connected through a 75 ohm line matching pad 17 29A, to upstream lead 3A. In this case the receipt of 18 an addressable command causes switch 27 to operate.
19 As a result a pulse appears at the input of encoder 49, which generates a code, modulated in modulator 50, 21 which is applied through output pad 29A to the 22 upstream lead 3A. This can be used to provide 23 acknowledgement of the receipt of a command to close 24 switch 27, since in the case of testing of switch 27, there may otherwise not be any input signal passing 26 therethrough to be received and noted by the head end.
27 Alternatively the output of modulator 50 28 can be connected into the input of switch 27 for 29 transmission of an acknowledgement code generated by encoder 49 through switch 27 as if it were a signal 31 received through the filter comprised of elements 21, 32 22 and 23.
33 A block diagram illustrating a variation 34 of the above is shown in Figure 8. In this figure high pass filter 13 is connected between leads 3A and 36 3B as described with reference to Figure 4. The 37 upstream signal is passed through a low band bandpass __ ~ .
01 filter 52 which passes signals between e.g. 5 - 30 02 mHz. The upstream signal then passes through an 03 amplifier 53 and a narrowband upstream signal filter 04 54 such as that described with respect to elements 21, 05 22 and 23. The output of filter 54 is connected to 06 the input of electronic switch 55, which is controlled 07 by a threshold detector/control circuit 56 such as 08 that described earlier with respect to elements 30 -09 45 in Figures 5, 6 and 7. The output of switch 55 is connected to the input of another filter 57 which is 11 similar to filter 54. The output of filter 57 is 12 connected to the upstream lead 3A, which is the 13 distribution line connected to the trunk.
14 A downstream control signal within the high band is received on lead 3A, passes through 16 control signal frequency filter 57A and is received in 17 demodulator 58. The demodulated signal is decoded in 18 decoder 59, and the decoded control signal is applied 19 to the control input of switch 55, in a manner similar to that described with reference to controller 47 21 controlling switch 27.
22 The switch state monitor output of switch 23 55 similar to that described with respect to switch 27 24 is connected to one of several input terminals of an encoder 60 which has its output connected to modulator 26 61, which has its output, in the low frequency band, 27 connected to the input of filter 57, from where the 28 output signal of modulator 61 is applied to upstream 29 3A for application to the trunk and to the head end.
It should be noted that several decoders 31 similar to decoder 59 can be connected to the output 32 of demodulator 58, each one of which can be connected 33 to a separate input of encoder 60 through controlling 34 of another switch similar to switch 55. This can clearly be used to apply various input signals to the 36 upstream distribution line and trunk. For example, 37 such signals can be from power meters, video signals, ~ 336340 .
01 acknowledgement of appliance turn on at subscriber 02 locations, etc. Further, decoded signals from the 03 head end can be used to switch appliances on and off, 04 activate power circuits, etc. at the subscriber 05 location. Indeed, control signals from subscriber 06 stations can be sent upstream to the head end to 07 command certain functions, such as the transmission of 08 control signals to control or monitor units at the 09 same or different subscriber stations.
The above description of embodiments of 11 the invention have clearly shown that the present 12 invention can be used for a variety of various 13 functions, but at its heart provides means for 14 providing signals to a head end with reliability and substantially reduced of noise. Figure 9A illustrates 16 the spectrum of the signal received at a head end with 17 a prior art network and is comprised virtually 18 entirely of noise; Figure 9B illustrates the spectrum 19 of the signal received at the head end utilizing the present filters and is clearly almost clear of 21 significant noise; and Figure 9C illustrates the 22 spectrum of signal received at the head end with a 26 23 mEIz upstream signalling signal used in accordance with 24 the present invention, and clearly illustrates a very high signal to noise ratio. The improvement over the 26 prior art is seen to be substantial.
27 A person understanding this invention may 28 now conceive of other alternatives or embodiments 29 using the principles described herein. A]l ~re considered to be within the sphere and scope o~ tlle 31 invention as defined in the claims appended hereto.
Claims (18)
1. For use in a CATV network, an upstream filter circuit comprising at least one bandpass filter for receiving an upstream signal, an electronic switch connected to the output of said at least one bandpass filter for applying a filtered upstream signal from said at least one bandpass filter to an upstream line, means for detecting the energy level of said filtered upstream signal at a point between said at least one bandpass filter and said electronic switch, and means for enabling the electronic switch to close in the event said energy level is above a predetermined threshold.
2. A filter circuit as defined in claim 1, in which the detecting means is comprised of a first rectifier for receiving and rectifying the filtered upstream signal, a first integrator for receiving and integrating the rectified signal from the rectifier to accumulate its energy, and a level detector for receiving the integrated signal, comparing it with a controllable reference level, and for outputting a bilevel signal having its level dependent on whether or not the reference level is exceeded by the integrated signal, and further including means for applying the bilevel signal to the electronic switch to enable or inhibit its operation.
3. A filter circuit as defined in claim 2, wherein said detecting means further includes an operational amplifier interposed between the integrator and level detector, the integrated signal being applied to its noninverting input, and a second rectifier having its output connected to a second integrator and its input connected through a low value resistor to ground, the output of the second integrator being connected to the inverting input of the operational amplifier, the first and second rectifiers and the first and second integrators being formed of similar components.
4. A filter circuit as defined in claim 1 in which said at least one bandpass filter is narrowband.
5. A filter circuit as defined in claim 1 in which said at least one bandpass filter is sufficiently broadband so as to accommodate the passage of at least one video signal.
6. A filter circuit as defined in claim 1, further including means for receiving a control signal from upstream of said at least one bandpass filter, means for demodulating and decoding the control signal, and means for enabling or inhibiting the electronic switch by applying the decoded control signal thereto.
7. A filter circuit as defined in claim 1, further including means for receiving a control signal from upstream of said at least one bandpass filter, means for demodulating and decoding the control signal, and means for operating the electronic switch by applying the decoded signal thereto, the electronic switch including a first contact coupled to said upstream line and a second contact, an encoder containing at least one input connected to the second contact of said electronic switch, the output of the encoder being connected to a modulator for modulating a state signal received from the encoder under control of the control signal onto a carrier signal, the output of the modulator being connected to said upstream line.
8. A filter circuit as defined in claim 1 further comprising a high frequency band bandpass filter having a passband above the frequency range of said at least one bandpass filter, connected in parallel with said at least one bandpass filter, for passing downstream signals therearound.
9. A filter circuit as defined in claim 2 in which said at least one bandpass filter is narrowband.
10. A filter circuit as defined in claim 2 in which said at least one bandpass filter is sufficiently broadband so as to accommodate the passage of at least one video signal.
11. A filter circuit as defined in claim 3 in which said at least one bandpass filter is narrowband.
12. A filter circuit as defined in claim 3 in which said at least one bandpass filter is sufficiently broadband so as to accommodate the passage of at least one video signal.
13. A filter circuit as defined in claim 2, further including means for receiving a control signal from upstream of said at least one bandpass filter, means for demodulating and decoding the control signal, and means for enabling or inhibiting the electronic switch by applying the decoded control signal thereto.
14. A filter circuit as defined in claim 2, further including means for receiving a control signal from upstream of said at least one bandpass filter, means for demodulating and decoding the control signal, and means for operating the electronic switch by applying the coded control signal thereto, the electronic switch including a first contact coupled to said upstream line and a second contact, an encoder containing at least one input connected to the second contact of said electronic switch, the output of the encoder being connected to a modulator for modulating a state signal received from the encoder onto a carrier signal, the output of the modulator being connected to said upstream line.
15. A filter circuit as defined in claim 3, further including means for receiving a control signal from upstream of said at least one bandpass filter means for demodulating and decoding the control signal, and means for enabling or inhibiting the electronic switch by applying the decoded control signal thereto.
16. A filter circuit as defined in claim 3, further including means for receiving a control signal from upstream of said at least one bandpass filter, means for demodulating and decoding the control signal, and means for operating the electronic switch by applying the decoded control signal thereto, the electronic switch including a first contact coupled to said upstream line and a second contact, an encoder containing at least one input connected to the second contact of said electronic switch, the output of the encoder being connected to a modulator for modulating a state signal received from the encoder onto a carrier signal, the output of the modulator being connected to said upstream line.
17. A filter circuit as defined in claim 2 further comprising a high frequency band bandpass filter having a passband above the frequency range of said at least one bandpass filter, connected in parallel with said at least one bandpass filter, for passing downstream signals therearound.
18. A filter circuit as defined in claim 3 further comprising a high frequency band bandpass filter having a passband above the frequency range of said at least one bandpass filter, connected in parallel with said at least one bandpass filter, for passing downstream signals therearound.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000616759A CA1336340C (en) | 1993-10-29 | 1993-10-29 | Catv network with filters |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000616759A CA1336340C (en) | 1993-10-29 | 1993-10-29 | Catv network with filters |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000564762A Division CA1327238C (en) | 1988-04-21 | 1988-04-21 | Catv network with filters |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1336340C true CA1336340C (en) | 1995-07-18 |
Family
ID=4140948
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000616759A Expired - Fee Related CA1336340C (en) | 1993-10-29 | 1993-10-29 | Catv network with filters |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1336340C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1014721A1 (en) * | 1997-04-28 | 2000-06-28 | Kabushiki Kaisha Toshiba | Rf output device for catv |
-
1993
- 1993-10-29 CA CA000616759A patent/CA1336340C/en not_active Expired - Fee Related
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1014721A1 (en) * | 1997-04-28 | 2000-06-28 | Kabushiki Kaisha Toshiba | Rf output device for catv |
| EP1014721A4 (en) * | 1997-04-28 | 2005-06-29 | Toshiba Kk | Rf output device for catv |
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