PASSIVE TRANSMISSION OF DATA OVER
CABLE TV SYSTEMS
BACKGROUND OF THE INVENTION 5
The present invention is related to data transmission and particularly transmission of data over cable TV systems.
The initially installed cable systems for the transmission of television signals were primarily one-way trans- 10 mission systems wherein the transmission of the TV signal was made through repeaters, periodically installed in the cable system, which amplified the signal. Transmission was one-way from the "head-end" to the subscribers. Two-way systems have now been specified '5 (required) and all systems installed after 1977 are "twoway capable", although not "two-way ready".
Two-way systems employ diplexers in the repeaters. The diplexer breaks up the service spectrum into two components, a low component of five magahertz to 20 thirty megahertz for return signaling, and a high component of fifty-four megahertz to four hundred megahertz, for transmission to the subscriber. With current two-way ready systems both the high band and the low band are amplified. Two-way transmission of data takes 25 place by "polling" with an outgoing signal, multiplexed in with the TV signals, transmitted to the subscriber on the high band and the return signal transmitted on the power amplified low band. With current two-way "polling" systems, responses or return signals from the vari- 30 ous subscribers are transmitted on a single time shared return channel.
A clear disadvantage of a polling system is that if one of the remote units in the transmitters locks up and it stays on, the entire system is jammed. No one else can 35 transmit. A more serious disadvantage arises from the use of amplification. If power is lost at any repeater in the system, the signal is lost and cannot be received or decoded by the "head-end" modem. This is an obvious problem with informational type data and particularly a 40 problem when such a system is employed as an alarm system to indicate fire, unauthorized entry or the like. It should also be obvious that, with amplification, it is only possible to transmit in one direction on the low band.
SUMMARY OF THE INVENTION 45
The present invention permits two-way transmission to take place on the low band of a two-way capable cable system without amplification. To allow two-way transmission to take place, jumpers may be installed in 50 place of reverse amplifiers. Signals generated at one end are received with a sensitive, low noise, "head-end" modem at the other end. Thus, the two-way data transmission is independent of power failures affecting repeaters in the cable system. 55
The invention has similar advantages when used only for remote transmissions such as in a security system. No interrogation is necessary. Only one-way transmission, incoming, is utilized but it is supervised and indicates loss of signal. By using multiple received channel- 60 s—each subscriber has his own unique carrier frequency—rather than a single time shared transmission channel, malfunction of one remote terminal does not affect the system. Since the signal is not amplified at the repeaters, power loss to the repeaters has no effect on a 65 one-way system.
Basically, the system is designed and operates in the following manner. Eight channels for digital data can be
allocated to and received from each subscriber. This data, for a security system, might be panic alarm, fire alarm, intrusion alarm, a channel to indicate whether or not the alarms are disarmed or in service, and other utility or supervisory transmissions such as whether the remote unit is in service, battery status, loop checks, and the like. The subscriber transmission channels are preferably transmitted on R.F. carrier frequencies with a ten kilohertz frequency separation thus permitting over two thousand five hundred carrier frequencies to be transmitted as transmission channels on the low band, each containing eight digital data channels of information. The carrier frequencies are received by a plurality of head-end modems each designed to receive a set of eight transmission channels (carrier frequencies). When received, each head-end modem utilizes a fold-over baseband demultiplexing technique to produce a set of eight standardized baseband frequencies corresponding to its unique set of carrier frequencies, so that audio range filters can be employed in the design rather than RF filters.
The demultiplexing technique is similar to a double side band demultiplexer with the frequency of the local oscillator offset from the middle frequency received so that no image is rejected. This produces a baseband output with the transmission channels interleaved at five kilohertz intervals.
Each transmission channel from each subscriber is preferably connected to a microprocessor based channel section to decode the eight digital data channels. Because of the differences in gain (loss) due to line loss, distance of the line, and the like, an automatic gain control (AGC) software routine is utilized which is digitally controlled by the microprocessor. The user at the central office terminal can also select one of three error correction algorithms which are also encoded in the microprocessor for processing the data.
The output of the head-end modem is connected to a display terminal and/or printer of conventional design using an RS232 scanner interface board. Using the RS232 scanner board the digital data channels from each subscriber are sequentially scanned. The eight bit address input to the modem from the scanner is decoded as follows. The five most significant bits are compared with the particular address of the head-end modem whose data is to be scanned. This address is encoded in the modem with a dip switch. The least three significant bits represent the baseband carrier frequencies one through eight for each head-end modem. Therefore, with a single scanner and thirty-two head-end modems each of eight digital data channels can be scanned for up to two hundred fifty-six subscribers. Additional subscribers can be served simply by adding additional scanners.
With the system elements disclosed an extremely dependable communications system is achieved which is of modular design. That is, an initial limited system can be installed to service a few customers and upgraded as more subscribers subscribe to the service. Except for the circuit components which determine the carrier frequency of subscribers' transmission channels, all modules are interchangeable with variables such as address and preferred error correction algorithms selectable for each subscriber with manual switches.
The overall design of the system yields an inexpensive communication system for enhanced services on TV cable systems that can operate with over one hun