referring to fig1 ., the system 10 of the invention includes a central station 20 and a plurality of subscriber units 30 , each associated with a subscriber tv set 40 . conventional tape or film tv transmitting apparatus 50 is also provided in association with the central station . the usual matv / catv cable 60 is used to interconnect the various parts of the system through directional taps 64 and appropriate tv splitters and the like , as is well known . each subscriber unit 30 includes a digital data encoder and a transmitter for transmitting a digital word , including the address of each tv set and its status , to the central station 20 where the digital word is read and recognized and processed by the central station circuitry . each subscriber unit , while essentially identical to all others , includes circuit means to permit its transmitter to transmit information in the form of a digital word at a sufficiently different time than every other transmitter so that interference between subscriber unit transmissions is minimied . as a further precaution , the central station equipment is normally programmed not to bill a subscriber or to perform some other function before it receives a selected number of transmissions of identical information from a subscriber unit . the following description of specific features of the invention are related , for purposes of illustration , to a motel pay tv system wherein the subscriber units 30 are found in the motel rooms and are called room units , and a tv program transmitter 50 and central station 20 are found in the motel office area . a typical room unit 30 is shown in more detail in fig2 and includes a midband converter 70 coupled to the motel matv cable 60 and adapted to be tuned to receive the tv transmissions from one or more tv transmitters 50 on selected channels . the converter 70 has its output coupled into the tv set 40 , as is well known in the art , to couple the received signals thereto . the room circuit also includes a digital encoding circuit 100 of any suitable type which is coupled to the midband converter so that a digital bit or bits can be entered therein representing the channel to which the converter is tuned . in addition , the address of the room tv set is entered in the encoding unit 100 by means represented by block 102 . other data could also be entered in encoder 100 , as is well known in the art . in one arrangement , a 16 - bit code was used including three synch bits , 10 bits for address , and three separate bits to represent the channel selected or other data . the room unit 30 also includes a digital transmitter 110 coupled to the output of the encoder 100 . the transmitter is coupled to the matv cable 60 through a directional tap 64 and then to the central station where it is processed in a manner to be described . room unit 30 also includes a power supply 80 and a power pulser or switching circuit 90 , to which the power supply is connected along with a noise generator 91 . typically , the power pulser includes , as shown in fig3 a capacitor 92 coupled through a resistive path 93 to the power supply and through a second path to the noise generator 91 . the power pulser also includes circuit means 94 with functions as a switch which is closed when the capacitor 92 is charged to a predetermined level and through which power is coupled to the transmitter 110 , the digital encoder 100 , and to any other parts of the system which require power during the time of operation of the transmitter . this is the transmission time for a digital word . referring to fig4 the central station 20 of the system 10 includes a receiver 31 , a digital decoder 32 , a computer interface module 33 which is essentially a serial - to - parallel converter for the digital word , a computer 34 , and a readout such as a teletype 35 . in operation of the system of the invention 10 , in each room unit 30 , the data encoder 100 is set by the converter 70 to generate digital bits representing the pay channel to which the room tv set is tuned , if it is so tuned , and it is set by circuit 102 to include a digit or digits representing the room address . with the power supply 80 and noise generator 91 feeding charge to the capacitor 92 , the capacitor gradually charges , and , when it reaches a predetermined level determined by the various parameters of the power pulser circuit 90 , the pulser switches on and couples power to the transmitter 110 and the digital encoder 100 , and the appropriate digital word is transmitted , in serial form , to the central station on cable 60 . as determined by the components of the room unit , the transmitter 110 is triggered , typically every two to five seconds , or so , to transmit the digital word to the central station . the information from each room unit is received , decoded , changed from serial to parallel , for example , and stored by the computer . the computer can be programmed so that , if the pay tv information from a room is received steadily for , say ten minutes , then an entry is made to bill the room occupant , or it is printed out on the teletype , or some other operation can be performed . referring to fig5 a system 10 &# 39 ; embodying a modification of the invention includes a room unit 140 associated with each room tv set 40 , and , in this case , the tv programs are transmitted directly to the tv sets in conventional fashion , and not through a converter , as in system 10 . thus , the system 10 &# 39 ; includes , at the central station , tv transmitter 50 coupled through a modulator 221 , which serves to place the video signal on a selected tv channel frequency , an amplifier 230 , and any other required circuits to the cable 60 which feeds the tv progrms to all of the room tv sets . at the central station , the tv transmitter 50 is also coupled through a conventional synch detector circuit 170 which detects the horizontal synch signals generated at the transmitter 50 and couples them through a suitable delay circuit 180 , if required , to compensate for delays in transmisson from the room units to the central station . the delay circuit is coupled to a width control circuit 183 which is used to adjust the width or time duration of the signals received from the synch detector , and the output of the width control circuit is coupled to one input of a coincidence detector circuit 186 ( the other input of which comes from the room units 140 ), the output of which is coupled through a computer interface 190 , which is a decoder circuit , for converting digital words from serial to parallel form , to a computer 200 . the output of the computer is coupled to a suitable display or recording device such as a teletype machine 210 . the cable 60 is connected to the input of a receiver 220 , and the output of the receiver is coupled to the coincidence detector 186 to provide the second input thereto which is to be compared with the input from the synch detector 170 . referring now to the room circuit 140 , which is found in each motel room , such circuit is shown in block form in fig6 and includes an electrostatic pickup or sensor 240 in the form of a wire , plate , pickup coil , or the like operatively coupled to the tv set in any suitable manner , for example , by being positioned inside or outside the tv cabinet . the sensor is not connected directly into the tv circuit . the sensor 240 picks up the horizontal synch signals of the received picture signal , and the associated circuit elements are arranged to respond only to the synch pulses . the sensor 240 is connected to an amplifier - peak detector 242 having a high input impedance and means for developing clean , sharp pulses from the synch pulses which are picked up by the sensor . the output of amplifier 242 is connected to a summing circuit 244 , along with the output of a noise generator 250 and a ramp or sawtooth signal generator circuit 260 . the circuit 260 developes a slowly rising ramp signal which repeats at regular intervals , e . g ., every two seconds , and the noise generator develops a completely random , slowly varying signal . the output of the summing amplifier 244 is connected to a power pulser 266 , generally of the type described above , which pulses on the system power supply 270 in response to a selected output level from the summing amplifier , whereby power is connected to the other circuit elements as required during an operating cycle . the room circuit 140 also includes a digital data encoder and data selector 280 which is driven by a binary - coded decimal counter 290 which is operated by a clock 284 . digital data bits representing room address and the like are set into the data selector by any suitable means well known in the art and represented by block 294 . typically , as above , the data sorce 294 includes sixteen status indicator positions which can be set into , or can be scanned by , the data encoder 280 . the output of data encoder 280 is fed into a pulse code modulated transmitter 298 which is connected to the cable 60 , and thus can transmit a signal , comprising digital bits , back to the receiver 220 at the central station 30 , as described above with respect to system 10 . system 10 &# 39 ; shown in fig5 and 6 operates as follows . the picture signal received at the room tv set 40 includes horizontal synch pulses which are picked up by the sensor 240 and coupled into the amplifier 42 and then into the summing amplifier 244 . the output of the noise generator 250 , a slowly varying random signal , and the output of the ramp generator 260 are also fed into the summing amplifier 244 which is arranged to switch on the power pulser 266 when the input signals combine to reach a predetermined voltage level . the is level is reached and the circuit fires when one of the sharp synch pulses is added to the other two voltage waves . when the output of the summing amplifier 244 operates the pulsing circuit 66 , the power supply 270 provides power for the various operating portions of the circuit as required . when the power is turned on , the clock 284 and counter 290 combine to scan the data encoder 280 and to generate a digital word comprising a series of pulses or binary - coded decimal bits which include information relating to the channel being watched on the tv set , the room address and the like , and this digital word is transmitted by transmitter 298 over cable 60 to the circuit elements at the central station module . it is noted that the instant at which this transmission takes place is determined by a synch pulse being added to the ramp signal and the noise signal . in one embodiment of the invention , the transmission time for a digital word was 160 microseconds . at the central station module , the signal sent by the transmitter 298 is fed into the receiver 220 and into the coincidence detector 186 where it coincides with the synch pulse transmitted thereto from circuit 170 . the leading edge of the received digital word is compared with the synch signals received from the program transmitter 50 to check for time coincidence . it is to be understood that the system may be arranged to check phase coincidence , or the two signals may be related in other suitable manner . in coincidence exists , the coincidence detector provides an output signal which is registered in the computer 200 . this means that the occupant of that room is viewing the tv transmission , and he will be billed for this viewing by suitable computer programming . it is noted that the width of the synch pulses provided by width control circuit 183 determines the time period over which the two signals must overlap to represent coincidence and to provide an output from coincidence circuit 186 . other disposition can be made of the information thus received . again , as described above , the computer program can be arranged so that billing or other function is performed only after a plurality of identical transmissions are received from any one room unit to indicate that a particular program is being watched . those skilled in the art will be able to program a computer to enter information relating to whether a pay program is watched by a room unit , and then to use that information in some way for automatic billing . in one suitable arrangement , each digital word out of the interface circuit 190 which contains a signal bit which signifies that a pay channel is being watched is entered in a first counter 301 , there being one such first counter for each room unit . each first counter has an eight - count capacity , and , when eight counts have been received , it means that the room unit has selected a pay tv channel . when a first counter is filled , a second counter 303 ( one for each room unit ) is enabled through suitable logic circuits 302 , and , as long as the same digital words are received , a count is entered therein every thirty seconds for a total of ten minutes , and , at the end of ten minutes , the second counter provides an output signal to billing apparatus 305 which causes the room in question to be billed for the program being watched . with this arrangement , a room is permitted ten minutes of free watching before billing takes place . it is clear that , if system 10 &# 39 ; is used to monitor more than one program channel , then the horizontal synchronizing signals of each program channel must occur at different times than those of every other channel . those skilled in the art can readily provide the various components of the systems described above , such components being well known and readily available . for example , the midband converter 70 , is a circuit module which is well known and is widely used in cable television to convert signals of various frequencies to frequencies usable by commercial television sets . a typical cable tv system using a converter is shown in u . s . p . at no . 3 , 769 , 579 and in u . s . p . at no . 3 , 665 , 311 ( element 28 ), and converters are sold by ael communications corporation and oak industries , inc . another system element , the digital data encoder , is also well known and readily available . in one system , the data encoder consisted of 1 two 74150 integrated circuit modules , made by texas instruments , signetics , and others and shown , for example , in signetics &# 39 ; digital linear mos data book , 1974 , and texas instruments ` the ttl data book for design engineers 1973 , and 2 a type 7493 divide - by - 16 counter made by texas instruments , signetics , national semiconductor , and fairchild . such encoders , also known as multiplexers , are also shown and described in howard w . sams &# 39 ; publication 20967 , transistor - transistor logic , 1973 , on pages 66 and 67 , and in u . s . pat . no . 3 , 786 , 424 .