GB2140601A - A television channel detecting arrangement for detecting to which channel a television set is tuned - Google Patents

Abstract

A television channel detecting arrangement, for detecting to which channel a television set is tuned, comprises means (14) for receiving a signal from a local oscillator (10) of the television set (12); tuning and detection means (16, 18, 20) which are such that, when the tuning means is tuned to the frequency of the signal from the local oscillator, a voltage is generated at the detection means; and controlling means (21, 25, 24, 22, 26) which uses stored binary numbers to vary over a range the frequency to which the tuning means is tuned. <IMAGE>

Description

SPECIFICATION Atelevision channel detecting arrangement for detecting to which channel a television set is tuned According to one aspect of the present invention, there is provided a television channel detecting arrangement, for detecting to which channel a television set is tuned, comprising: a) means for receiving a signal from a local oscillator of the television set; b) tuning and detection means which are such that, when the tuning means is tuned to the frequency of the signal from the local oscillator, a voltage is generated atthe detection means; and c) controlling means which uses stored binary numbers to vary over a range the frequency to which the tuning means is tuned.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block diagram of a television monitoring system; Figure 2 is a blockdiagram of a television channel detection unit; Figure 3 shows a practical embodiment of part of the detection unitshown in Figure 2; Figures 4a and 4b show a practical embodiment of another part of the detection unit shown in Figure2; Figure 5 is a block diagram of a people monitoring unit; Figure 6 shows a practical embodiment of the people monitoring unit of FigureS; Figure 7 shows an embodiment of a remote handset for use in the people monitoring unit of Figure 5;; Figure8 is a blockdiagram of an embodiment of a mains transmission unit; Figures9ato 9fare graphs illustrating the operation ofthe mainstransmission unit of Figure 8; Figure 10 is a block diagram of a meter which records information from a mains supply line, and transmits the information at night byway of a public telephone network; Figure 11 is a major side view of a removable semiconductor data module, Figures 12 and 13 being views inthe direction of arrows Aand B in Figure 11 respectively; Figure 14 is a block diagram of circuitry in the module; and Figure 15 is a block diagram of a meterfor usewith the module.

Figure 1 shows a block diagram of a television monitoring system comprising a television channel detection unit 1; a people monitoring unit 2; a mains transmission unit 3; and a central receiving station 4.

Thetelevision channel detection unit 1 will now be described in detail with reference to Figures 2 to 4.

The unit 1 is designed to sense ultra or very high frequency radiation from a tuner 10 in a domestic television receiver 12 and so determine if the channel to which the television receiver is tuned is one of a multiplicity ofchannels which have been preset into the detection unit 1. A different binary coded word is producedforeachchannel detected.Apick-upprobe 14 is in the vicinity of a local oscillator ofthetelevision receiver 12 to be monitored. The inductively coupled signal is fed into a modified variable capacitance diode tuned tuner 16. A standard television tuner could be used, provided that the frequency range is extended to cover the range of the local oscillator frequency radiated from the TV receiver.The signal from the tuned tuner is amplified using a conventional l.F. amplifier and surface acousticwave (S.A.W.}filter 18,for example one made by Mullard or Plessey. A d.c.

voltage is produced from a detector 20 when the tuner 1 6is tuned to the radiated frequencyoftheTV receiver 12. The unit 1 is programmed to lookforpreset frequencies by applying different tuning voltages to variable capacitance diodes within the tuner 16. The output ofthe detector 20 is connected to a low frequency oscillator 21 and an analogue tuning voltage is generated from a binary number using digital to analogue conversion. Binary numbers are stored in a non-volatile store memory chip 24 and each number is addressed in sequence from an address counter 25. The output ofthe memory 24 is connected to a digital to pulse width converter 22. The output markto space ratio of the converter is therefore a function of the addressed binary number.The resulting repetitive pulse train is averaged in an integrating amplifier 26to produce a d.c. tuning voltagewhich is proportional to the stored binary number. The tuner 16 can therefore be tuned by varying the binary number in the memory 24.

To set up the detection unit to receive different frequencies, an external plug-in unit is used. This external unit enables a particular store address to be selected and the memory 24 contents to be incremented or decremented to tune to the required frequency. The procedure is repeated for all the required frequencies.

In operation,the memory 24 is addressed in sequencefrom the address counter 25 until a voltage is detected. The address counter 25 is then halted and the tuner 16 is locked to the detected frequency. The binary store address number is used to identify the detected television channel number.

To preserve the memory 24 contents when power to the detection unit is switched off, either a battery powered random access memory (RAM) or an electrically alterable read only memory (EAROM) can be used. The address numbers that represent the detected television channels are outputted to the mains transmission unit 3.

Figure 3 shows a practical implementation ofthe tuner 16 (byway of example one made byThomson CSF of type MTS 200) and the amplifier and S.A.W.

filter 18 (by way of example one of type SW153A) and detector 20, and a practical implementation of another part ofthe detection unit 1 is shown in Figures 4a and 4b. The function of the address counter 25 and digital to pulse width converter 22 is achieved in one integrated circuit lC2 oftype AV-3-821 1 made by General Instruments.The memory 24 is an electrically alterable read only memory (EAROM) type ER1400 IC1 and the integrating amplifier is designed around integrated circuit IC3. The integrated circuit IC2 also provides band switching information for tuner 16 to have multiband operation,tuner 16 normallybeing in a condition for Band A operation unless + 12 volts is appliedtoeitherofthelinesmarked BandUHFand Band Bforitto besetto the corresponding one of these conditions.

Advantages ofthe above-described unit 1 are that only known required frequencies are looked for; and there is no direct electrically conductive connection between the unit and the television set.

Figures 5to 7 illustrate an embodimentofthe people monitoring unit 2. In orderto monitorthe viewing habits of people within a particular room, a push-button system is employed. Each person, who will at some time view the television set, is allocated a number. Inthe unitto be described the numberof users is limited to eight.

As shown in Figure 5, buttons 30 are housed in a self contained battery powered handset 32 placed in a convenient position within the room. When a person starts to view the television receiver, the button 30 assigned to that person is momentarily pressed. An infra-red 34, or ultrasonic 36,transmitteremits signals which are received by an infra-red 34a, or ultrasonic 36a, detector in a remote receiver unit 33. A data link is thereby established between the handset 32 and the remote receiver unit 33 and acodeuniquetothe number of the depressed button is received, decoded in decoder 38 and stored in one of eight bistables, e.g.

eig ht D fli p-flops 40. The output of that bistable is displayed as an identical number on a vacuum fluorescent display 42. When the viewer ceases to view, the same button is momentarily depressed and the appropriate bistable 40 in the receiver unit 33 is reset via the data link34, 34a or 36, 36a and the displayed number is cleared. The outputs ofthe eight bistables 40, which represent the people status, are connected to the mains transmission unit3 and are sent as part of a 16 bitword to the central recorder 4.

The facility of choosing between an infra-red or an ultrasonic data transfer between the handset 32 and the receiver unit 33 has been incorporated so thatthe option exists to select a mode which does not interfere with any existing remote control system which may already be in use bythe viewer.

Otherfeatures are also included in the receiver unit to remind viewers to update or check the input data and to reduce eroneous operation. These features include the features that (a) all the eight display digits will flash if the television receiver is on and no people viewing data is entered; (b) a reminder is activated every 10 minutes and the displayed digits are flashed for about 10 seconds; (c) all people inputs are inhibited if the television receiver is switched off; and (d) switches 44 are provided within the unit so that any ofthe eignt people inputs can be masked out.

The receiver unit 33 can be integral with the mains transmission unit 3 or can be as an add-on unit connected via a multi-way cable.

Figure 6 shows a practical embodiment of a people detection unit and display board in the infra-red mode, as selected by switch S1 ,the coded signal being detected by the sensor D3 and amplified by transistOr TR2 which also sets the d.c. bias. Resistors R20, R21 and diode D4 prevent overload under conditions of high inputsignal. The signal is a.c. coupled from the collector of TR2via C9to the integrated amplifier IC5.

The amplified signal on pimp 3 of IC5 is stretched, by the network Dl, R1 and C4, and DC shifted by transistor TRl so as to be compatible with the pulse position modulation decoderiCi I. VRl, and andC8lsetthe internal time referenceforthe decoderlCi f The binary coded signals which are present on A, lD,C, D (lCl 1), when any of the push buttons on the handset are depressed, are decoded by Icy 7 into eight individual signals, These signals can be masked by the switches S2a-S2h.

The eight bistables 1C12-lCI6 are used to storethe people status. The integrated circuits IC20 and IC40 generate multiplexed signals forthevacuum fluorescent display, the high voltage drives being provided bytransistorsTR3to TR17.

The counter IC10and IC9 (connectedastwo bistables) provides timing logic and divide a 3 Hz 32 clock to generate a flashing reminder for-seconds 3 after a delay of 682 seconds. Also a continuous 128 flashing ofthe display occurs after second if the 3 television receiver is on and no people are set into any ofthe eight bistables as detected by the 8-input AND gate lCl4.

A light dependent resistor LDR1 sets the intensity of the displayto allowforvarying ambient light conditions.

When the ultrasonic mode is selected by 51 ,the signal is amplified as before exceptthatthe network C1, C2, C3, R2, R3, R4forms a twin-tee filter network tuned to the resonant frequency ofthe ultrasonic transducer Xl.

In Figure 6, integrated circuits IC1 and IC3 are of type CD401 1 B; IC2 and IC17 are oftype CD 4028B; IC4 is oftype CD4543B; IC5 is of type TDA4050B; lC6 and IC8 one of type CD4071 B; IC7 is oftype CD4025B; IC9 is of type CD4001 B; lCl 0 is of type CD4040B; IC1 1 isof type ML926; IC 12, IC13, IC1 and Icy 6 are of type CD4013B; and IC14 is of type CD4068B.

The design of the remote handset shown in Figure 7 is centred around the integrated circuit ICIOl, type SL490, which produces a different pulse position modulated signal when any ofthe push buttons, PB1-PB8, are depressed. Resistors VR1, Rl02 and capacitor C102 set the pulse train clockfrequency.

In the infra-red mode, selected by 51 a and b, the coded pulse train is a.c. coupled via a capacitor C1 05 to transistors TR103, Tor104 and TR105, forming a cascaded amplifier. The powertransistorTR105 provides high current pulses to drive the infra-red emitting diodes LED1 and LED2.

In the ultrasonic mode IC101 also produces a 40kHz pulsed carrier, inhibited in the infra-red mode by switch Slb and set by VR2, R104 and C104.This carrier is amplified in a push-pull amplifierformed by TR101 and TR102 to drivethetransducerXl.The drive to the base ofTRl is is short-circuited by switch Slob.

One embodiment ofthe transmission unit 3 will now be described with reference to Figures 8 and 9.

This unit 3 is designed to accept data from the people monitoring unit 2 and from the television channel detection unit 1 and to transmit the data via an existing domestic house wiring system to a central receiving station 4.

As shown in Figure 8, a sine-wave output on the secondary of a mains transformerT1 which is connected to a power supply 53 is connectedto the input of a zero-crossing detector 50 and a voltage transition is generated each time the input waveform passes through zero. This transition is used as a reference to phase-lock a voltage controlled oscillator 52 ata predetermined carrierfrequency, e.g. 51.2kHz.

This frequency is divided by binary dividers in a 14 stage binary divider 54 and the output, 50 Hz, is used as an error signal for the phase-locked oscillator 52.

Thus, all the outputs from the binary dividers 54 are phase-locked to the mains supply at 50 Hz. Outputs of the binary dividers at 200 he and 100 Hz are decoded in a time slot generator 56 to gate the carrier frequency, in this case 51.2 kHz, into a particulartime slot selected by switch 551. In this particular applica- tionthe data is sent as a 16 bit word, preceded by 16 bits (i.e. 16 mains half cycles) when no carrier is sent.

This enablesthe receiver4to detectthe start of the 16 bit data word, the first bit ofwhich is always present.

The data from the people monitoring unit 2 and television channel detection unit 1 is parallel-loaded into a shift register 58 during the 16 blank half cycles and is sent out in serial form at a rate of, for example, one bit per 10 mS. The outputfrom the time slot generator 56 is a 2.5 mS long burst of 51.2 kHzcarrier which is gated on or off depending on the data stored in the shift register 58. The data word is repeated as long asthe system is switched on.

The gated carrier is amplified in poweramplifier60 and isolated from the mains supply by a tuned transformerT2. A band pass filter 62 is included to remove any harmonics which could cause radio interference.

In this particularapplication,the mainstransmission signal is inhibited when the television receiver is switched off, by way of input 64to the time slot generator 56.

The carrierfrequency (in this example 51.2 kHz) need not be a multiple of 50 Hz, and need not necessarily be phase-locked to the mains supply frequency. This system has been described with reference to one of fourtransmitters which all use the same carrier frequency; however, differentfrequencies could be used for each transmitter but this would complicate the receiver inputfilter design.

Each transmitter sends data ion a unique time slot, referenced to the zero crossing point in the mains supplywaveform. Thus only one transmitter is on at any given time, and as each transmitter is time-locked to the mains supply waveform, the receiving station 4 knows when to sample the mains supply to detect data from a particulartransmitter3.

The signal can be sent th rough the mains wiring by using anytwo conductors from thethree that may be available, i.e. (1) line and neutral, (2) line and earth, (3) neutral and earth.

A differentfrequency could be sent by a transmitter when not sending a digital :1;, which would mean that one of two frequencies was always present at the receiver. This would result in a reduced error count when interfering signals were present, and would enable a system of automatic level control to be used at the receiver to compensate for signal level variations due to load condition changes on the mains supply. However, such an arrangement would be more complex and therefore more expensive than that hereinabove described.

Figures9a-dshowtypical data received from each offourtransmitters; Figure 9e shows the 50 Hz mains signal with a superimposed 51.2 kHzsignal; and Figure 9fshowsthe 51.2 kHz signal at the output of a receiver input filter.

Advantages ofthe unit 3 are that television sets can be moved from pointto point by simply plugging into any mains socket without any modification of the system; only a two wire system is used; radio frequency interferenced is reduced to a minimum; all transmissions are synchronized to the mains supply; and where there is a plurality of such units 3 in different households, a single frequency is used for all units, and all units are asynchronous.

It is possible to have a meter which records information from a mains supply line, in a similar manner to that which has been described, and then transmits the information to a central computer by way of a public switched telephone network. In view of the fact that the load on the public telephone network is likely to be reduced at night, such transmission usually occurs at night. Such a meter will now be described with reference to Figure 10.

The meter is of a double insulated construction and is connected to a mains supply byway of a two core mains cable 70. The mains supply isfirst passed through a protectivefuse71 and an interference suppression filter 72, before feeding the primary of a mainstransformer73 and the primary of a 51 kHz tuned transformer 74 through a 50 Hz blocking capacitor 75. The mains transformer 73 provides the power required by a vacuum fluorescent clock display and driver electronics 76. It also provides powerto a battery charger77 which maintains a battery 78 in a fully charged condition when the mains supply is present. Display electronics, in the form of an ambient light level compensator 79, varies the brilliance of the display 76 in response to changes of the ambient light level. The zero-crossings of the mains transformer 73 secondary voltage are sensed in a zero-crossing detector 89 and fed to a computer system 80 to provide a reference signal related to the mains supply zero-crossings.

The signal which passes through the 51 kHztuned transformer 74 is fed to a comparator with hysteresis 81 the output of which clocks a divider circuit 82.

Should a 51 kHz signal be present on the mains wiring at a level in excess of about 60 mV peakto peak, the divider outputtoggles at 51 kHz; otherwise, the divider output is static, apartfrom occasional state changes caused by noise on the mains supply. The computer system 80 counts the number of state changes ofthe divider 82 output during certain intervals oftime defined bytheir relation to the mains zero-crossings. Should the number of state changes in such an interval exceed a preset threshold, the 51 kHzsignalisdeemedto be present on the mains wiring during that interval.

The battery 78 is float-charged from the mains, and powers all the electronic circuits apartfrom the display and driver electronics 76. It is protected against accidental short-circuiting by a fuse. The meter can maintain recorded information, keep track ofthe passage oftime, attach and detach itselffrom the telephone line atthe appointed times and answer calls from the centraI computer when-so attached without mains power. The computer system 80 is normally switched off, when the mains supply is absent, to conserve the battery charge. A crystal controlled pulse generator 83; a power control latch 84; a seconds counting latch 85; and a CMOS memory 86 are, however, powered at all times.

The pulse generator 83 sets both latches 84,85 at one second intervals and supplies a reference frequencyto clockthe computer system 80. The power control latch 84 is also set when ring current is detected on thetelephone line87. When the power control latch 84 is set, avoltage regulator and delay generator88 is enabled and the computer system 80 is powered from its regulated output. The delay generator ensures that the computer system 80 is not released until the circuits have had time to stabilise afterthey have been switched on. The computer system 80 resets the power control latch 84 when the computer system 80 requires to turn itself off. The seconds counting latch 85 is reset by the computer system 80 whenever it is found to be set and the internal computertime is advanced by one second.

Should the computerprogram fail due to some transient electrical disturbance, the seconds counting latch 85 will no longer be reset regularly. This condition is detected by an auto-restarttime 102 and thacomputersystem is powered off and restarted in the normal manner, thus saving the battery from damage due to deep discharge and allowing the meterto resume its normal operation. The CMOS memory 86 retains stored information when power is removed from the computer system.

A latching relay 90 in the meter is operated by a pair of power drivers 91 feeding separate coils in the relay 90. These power drivers 91 are driven directly by the computer system 80. If the detach driver is momentarily activated, the telephone instrument 92 is connected to thetelephone line 87 by the latching relay 90 and the telephone system operates in the normal manner. If the attach driver is momentarily activated the telephone instrument is disconnected from the line 87, its input is short-circuited and a meter ring detector 93 is connected across the line.

When ring current is present on thetelephone line 87 in this condition, the computer system 80 is powered up, if it is not already powered, and a signal from the ring detector 93 informs the computer system 80 that ring current is present. Ring current does not pass through the telephone instrument 92 and its bell does not ring.

The computer system 80 validates the presence of ring current for 800ms and then turns on a power driver 81 to operate a line seize relay 94. This relay 94 disconnects the ring detector 93 from the telephone line 87 and connects a line holding inductor and an a.c. coupled signal transformer 95 to the line. Carrier signals present on thetelephone line are coupled through the signal transformer95to an active line hybrid 96 which amplifies the received signal and separates it from the transmitted signal. The ampli fiedsignal is passed through a receive filter 97 which removes out-of band interference and is then squared up buy a limiting amplifier 98. The computer system 80 then directly demodulates the resultant signal.

The modulated signal which is transmitted to the telephone line is generated by the computer system 80 as a sequence oftimer output pulses corresponding to the zero-crossings ofthe outgoing signal.

These timer output pulses toggle a divider circuit 99 and the resultant output isfed to atransmitfilter 100 through a variable level generator 101. The computer system controls the output of the level generator 101 to compensateforthe variation in gain ofthetransmit filter 100 between a 2100 Hz echo suppression tone and the transmit carrier frequency. The transmitfilter 100 suppresses the harmonics in the level generator 101 output. The filter output 100 isfed through a 600 ohm matching resistor in the active line hybrid 96to the signal transformer95 which couples itto the telephone line 87.

In a system for monitoring the viewing habits of a plurality of households, each ofthe households would be installed with such a meter. The system is such that an existing telephone line of each household is used by the system without significantly diminishing the household's enjoyment of its telephone service. This is achieved by several means, the chief ofwhich is by operating only in a period in the early part of the morning. The telephone instrument of each household operates normally outside of half-hour intervals in this part ofthe morning, in which the meter is connected to the telephone line. In addition, once a meter has been successfully interrogated it detaches itself from the telephone line for the rest ofthe night.Forthose households who are accorded some degree of prioritythis will typically mean thatthey lose the full use oftheirtelephonefor only a few minutes each night.

Should someone attemptto call a household whilst its meter is connected to the telephone line, the meter will answerthecall with a continuoustoneto indicate that the telephone call has been answered by a machine. When the call is terminated, or after about 25 seconds, the meter will detach itselffrom the telephone line until the next half-hourtime slot If a second attempt is made to call the household, within half an hourofthefirst call, the call will be routed to the telephone instrument in the normal manner.

Should a member of a household wish to make an outgoing call whilstthe meter is connected to the telephone line, he must unplug the meter's telephone cord from the wall socket. Avariant of the meter system allowsforthe automatic handoverto the telephone instrument when the handset is raised to make an outgoing call.

Another important feature of the system is that the telephone calls are originated bythe central compu tersystem. In addition, the origination of calls centrally from the central computer ratherthan the meters allows the system to progress calls as quickly as possible. More meters can be interrogated per central telephone line and thetime that each meter spends on the telephone line is minimised.

There is a "holiday" button on the rear of each meter. In essence this button is used to indicate to the data collection system that a potential viewer is away on holiday. This is accomplished bytheviewer pressing the "holiday" button before departing on holiday. A display of AM 0:00 indicates thatthe button has ben sensed by the meter. In this condition the meterwill reportto the central computerthat "holiday" status is true. When the viewer returns from holiday, in responsetohistuningonthe television set, the meter displays time in the normal mannerandwill reportto the central computerthat "holiday" status is false.

The system is designed to collect audience data from the remote meters. It comprises a central mast computer (with a standby), associated communications equipment and optionally one or more slave computers connected to the master computer via private lines. Meters in households are interrogated (polled) over the public switched telephone network from the central computer and the slave computers.

The central computer is connected to several 300 baud modem/dialler pairs for meter polling, several 1200 baud modem/dialler pairs for communication with slaves,a disc drivefordata and program storage, a tape drivefordata backup and data interchange with an IBM system, a printer and a visual display unit console. The slave computers are connectedto several 300 baud modem/dialler pairs for meter polling and a 1200 baud modem for communication with the central computer. Polling is initiated by command to the central computer.

Additional commands may be issued to obtain reports from the system and to produce data tapes. A directory containing information on the meters to be polled is passed before each run from an IBM (RTM) computerto the central computer. After overnight data collection, data tapes are produced on com mand, aswell as an updateddirectorytape.These tapes are returned to the IBM (RTM) computerfor processing. Polling occurs overnight, the central computerallocatingworktoand receiving datafrom the slave computers. The slave computers are dialled at the beginning of data collection and remain in contactwith the central computer until data collection has ceased. The time available for polling is divided into eight half-hourtime slots as mentioned above.

Meters are divided into two classes, designated even and odd. Even meters are connected to the telephone line during even time slots. Odd meters are similarly connected during odd time slots. Once a meter has been successfully polled, it will not reconnect itself to thetelephone line during the remainderofthe night.

All data interchange overtelephone lines is error checked. When errors are detected, recovery procedures ensure that any detected corrupt information results in attempts to correctly retransmitthat information.This applies to transmissions between the central computer and the slaves and between the meter and the central or slave computer. Data collection by the central computer is stored immediately on disc and tape. Data collected by a slave from a meter is retained in the memory ofthe slave until that meter is disconnected from thetelephone line. The data is then transmitted to the central computer where it is stored on disc and tape.

Instead of having a storage system which is interrogated by way of a telephone line, it is possible to store the data received from the transmission means in a removable semiconductor data module.

Such a system then comprises a base station computer system complete with module reader, a number of meters and a larger number of removable Sata modules which circulate between the base station computer and the meters. The data modules arry time information from the base station to the meters and return time stamped viewing statements.

The module reader is an intelligent subsystem Which interfaces the data modules to the base station :omputersystem by means of a serial communicalions link. The module reader provides the means for the base station computer system to read the :ontents ofthe data module and to correct the time in :he data modules. At the same time, checks are made zn the operation ofthe data module and a visual ndication is provided to the operator ofthe oper zonal status ofthe module.

The data module consists of a printed circuit board ontaining a random access memory, a calendar lock, a backup battery and a means by which data nay be written to and read from the random access nemory and the calendar clockthrough two electrical contacts. The whole circuit board is encapsulated in a volyurethane foam plastics housing with two electric31 contacts protruding from recesses on opposite ides of the module housing.

Referring to Figures 11,12 and 13, the shape of the jata module 110 is roughlythatofa rectangular 3arallelepiped with sides of about l4mm, 70mm and 108mm respectively. Reference numeral 111 denotes the printed circuit board. Slots are provided in the rear panels ofthe meters and in the front panel of the nodule reader, through which a module can be nserted. These slots are only able to accept the ;mallestfaces ofthe module although they may do so in four different orientations. The electrical contacts 112,113 to the module are placed in the centre ofthe middle sized faces ofthe module. This mechanical 3rrangement,togetherwith the polarity insensitive natu re of the module interface, allows the module to Function equivalently with the module inserted into a meter or a module reader in all of its four possible orientations. The module housing departs from that of a rectangular parallelepiped in thefollowing ways.

First,the corners and edges are rounded to minimise damage during transport. Secondly,the largestfaces of the module are tapered to facilitate the insertion of the module into the module receptacle of either a meter or a module reader. Finally, in the centres ofthe middle sized faces there are rounded channels 114, 115 running perpendicularto the largestfaces of the module,the contacts 112, 113 protruding from the surfaces ofthese channels. Reference numerals 116 denote location pads.

The module receptacle within a meter or a module reader makes electrical contact with a module by means oftwo spring loaded contacts. These contacts bearonthe rounded channels in the sides ofthe module and serveto pull the module into the receptacle in the final few millimetres of its insertion.

This provides a positive feeling that a module has been fully inserted. This inward force also serves to retain the module in the receptacle should a meter be moved with a module in place.

Referring to Figure 14, a data module communicates with a meter over a two wire interface 117. The meter generates a pulse width modulated 32 kHz pulse train. The leading edge of each pulse serves to clock data in the module and to latch received data from the module. In the quiescent state, the meter generates a 25% dutycycle pulse train on the interface 117.

This pulse train is rectified by a bridge rectifier 118 in the module and fed to an energy storage capacitor 119. The energy storage capacitor powers a voltage regulator 120 which feeds power to the logic circuits in the module. A battery 121 provides powerto a random access memory 122 and a calendar clock 123, to maintain recorded data and time information when the module is removed from the meter.

The battery 121 is trickle charged from the voltage regulator 120whenthe moduleisinserted into a meter with mains power applied. A power switch 124 directs power from the voltage regulator 120 to the random access memory 122 and the calendar clock 123 when the logic circuits are powered.

When the module is not installed in a meterora module reader, the energy storage capacitor 119 is discharged and the logic circuits are not powered.

When the module is installed in a meter, orwhen mains power is applied to a meter, the energy storage capacitor 119 begins to charge and power is fed to the logic circuits in the module. A voltage detector with hysteresis 125 holds the logic circuits in a quiescent state until the voltage on the energy storage capacitor 119 has risen to a level at which the correct operation of all the circuits within the module can be guaranteed.

A second upper half bridge 126 feeds the pulses on the interface 117 to a clock recovery comparator 127 which separates the input pulses from the steady voltage level on the interface 117. The clock recovery comparator 127 triggers a data recovery monostable 128 and a timeout monostable 129. It also clocks a command shift register 130, a write data shift register 131 and a read data shift register 132.

The data recovery monostable 128 has an output pulse width of approximately one half ofthe input pulse period. At the trailing edge of the data recovery monostable 128 pulse, a data recovery latch 133 samplesthe output ofthe clock recovery comparator 127.

The meter and the module reader send commands and data to the removable module by pulse-width modulating the input pulse train. Each period of 30.511swill be referred to as a bit cell with the start of each bitcell considered to betheleading edgeofthe input pulse train. The quiescent state of a 25% duty cycle pulse, i.e. a pulse with a nominal length of 7.6ps, will be referred to as a zero bit. A 75% duty cycle pulse, i.e. a pulse with a nominal length of22.9pswilt be referred to as a one bit In addition, an interruption to the pulse train, i.e. a bit cell in which no pulse is present, will be referred to as a missing clock pulse.A write command to the data module consists of a start (one) bit, a zero bit, 14 bits of write address, 8 bits of write data and a missing clock pulse. The pulse-width modulateddatastream is recovered bythe data recovery latch t3laTheoutput ofthe data recovery latch 133 is shifted intothecommand shift register 130 and the write datashift register 131 on the leading edge of each input pulsewhen the start bit reaches the 24th stage ofthecommand shift register 130, command decode logic 13'4enablesthe data output ofthe write data shift register 131 and clocks the write data into the read data shift register 132 and into either a location in the random access memory 122 or a register in the calendarclock 123 depending upon the write address. At the same time, the first 8 stages ofthe command shift register 130 are reset The output ofthe read data shift register 132 controls a current sink 135 which loads the interface 117 when the input pulse is absent. The meters and the module readers feed the interface 117 with a voltage lowerthan that of the input pulse when the input pulse is absent. Thisvoltage isfedfrom a high impedance source and the loading caused bythe module current sink is detected by a voltage compa rator.

Thetimeoutmonostable 129 has an output pulse of approximately one and a halftimesthe input pulse period. In the quiescent state,25% duty cycle input and during the 24 bits ofthe command transferthis monostable is retriggered sufficientlyfrequentlythat it never times out. However, it does time out afterthe 24 bits of command have been transferred because of the missing clock pulse and in so doing resets the commandshift register 130. If a one should be shifted into the 25th stage ofthe command shift register 130, stages 9 to 24 ofthe shift register are resetto guard against false command decoding.

If during a write command aclock pulse is removed,for instance because of some intermittent electrical contact occasioned by the removal ofthe module from a meterwhilstthe record is being updated, the write operation is aborted and no data is inadvertently corrupted in the module. Similarly, protection is provided against pulse removal during a read command.

Following a write command with its associated missing clock pulse, successive input pulses shift data through the read data shift register 132. This data modulates the current sink 135, and so returns to the meter or module reader a record of what was written to the module.

A read command to the data module consists of a start (one) bitfollowed by a one bit, 14 bits of read address, a missing clock pulse and 8zero bitsto shift outthe read data. When the start bit reachesthe24th stage ofthe command shift register 130,thecom mand decode memory logic 134 enables data from either a location in the memory 122 or from a register in the calendar clock 123 depending upon the read address. This data is loaded into the read data shift register 132 and the first 8 stages of the command shift register are reset.

The missing clock pulse ofthe read command causesthetimeoutmonostable 129to resetthe command shift register 130. Successive input pulses shift data through the read data shift register 132 which modulatesthe current sink 135 and sotrans- mits data back across the interface 117. A zero at the output ofthe read data shift register 132 causesthe current sink 135 to turn on and so increasesthe loading on the interface 117. Aone at the output ofthe read data shift register 132 causes the current sink 135 to turn off and the loading on the interface 117 is removed. The current sink 135 is turned off during input pulses to reduce power dissipation.

The serial input to the read data shift register 132 is strapped to zero so that in the quiescent state, 25% duty cycle at the interface 117, the current sink 135 is turned on after each input pulse. This loading of the interface 117 inthe quiescent condition is used to detectthe presence ofthe module in a meter or a module reader.

Each meter is of double insulated construction and, referring to Figure 15, is connectedto a mains supply by way of a two core mains cable 136. The mains supply is first passed through a protective fuse 137 and an interference suppression filter 138, before feeding the primary of a mains transformer 139 and through a blocking capacitor 140 the primary of a 51 kHz tuned transformer 141. The mains transformer 139 provides the power required by a vacuum fluorescent clock display 142. It also provides power to a Svolt regulator 143 and a 12 volt regulator 144.

Display electronics, in the form of an ambient light level compensator 145, varies the brilliance of the display142 in response to changes of the ambient light level. The zero-crossings ofthe mains transformer 139 secondary voltage are sensed by a zerocrossing detector 146 and fed to a computer system 147to provide a reference signal related to the mains supply zero-crossings.

The signal which passesthrough the 51 kHzturned transformer 141 is fed to a comparator with hysteresis 148, the output of which clocks a divider circuit 149. Should a 51 kHz signal be present on the mains wiring at a level in excess of about 60 mV peak to peak,the divider 149 outputtoggles at 51 kHz.

Otherwise, the divider output is static apart from occasional state changes caused by noise on the mains supply. The computer system 147 counts the number of state changes ofthe divider output during certain intervals oftime defined bytheir relation to the mains zero-crossings. Should the number of state changes in such an interval exceed a preset threshold, the 51 kHz signal is deemed to be present on the mainswiringduringthatinterval.

When mains power is applied to the meter, a voltage comparator 150 holds the computer system 147 in a reset condition until the output of the 5 volt regulator 143 can be guaranteed. The computer system 147 generates pulses of variable duty cycle by means of a pulse gating circuit 151.The pulse gating circuit 151 turns on a power switch 152 which applies 12 volt pulsesfrom the 12 volt regulator 144to the module interface 117. A resistive divider 153 feeds 6 volts to the interface 117 at high impedance.

When the power switch 152 isturned off, a comparator 154 detectsthe loading of the interface 117 caused by the module's current sink circuit 135.

The outputfrom the comparator 154 is latched by flip-flop 155 on the leading edge ofthe 12 volt power pulse.

The pulse gating circuit 151 is clocked by a divider circuit 156 running from the computer system's clock.

The computer system 147 synchronises itself to the divider circuit 156 output and controls the pulse gating circuit 151. The pulse gating circuit 151 generates a 32 kHz pulse-width modulated pulse train which is fed to the data module when it is inserted into the receptacle in the rear ofthe meter. The computer system 147 can generate a pulse duty cycle of 25% or75% by means of the pulse gating circuit 151. In addition itcan suppressthe pulse output altogether to generate missing clock pulses.

The computer system 147detects the presence or absenceofa data module by detecting the module's loading ofthe interface 117 when a 25% duty cycle pulse train is applied to the interface 117. the module is absent, the computer system 147 displays OFF on the vacuum fluorescentclock display 142 by means of displaydriverelectronics 157.1f a module is present, the computer system 147 validates the information fields within the module, reads the time from the module and displays the time on the vacuum fluorescent clock display 142. It then proceeds to record time-stamped channel and people statements in the moduleastheyare receivedfrom the mains supply.

Claims (8)

1. Atelevision channel detecting arrangement, for detecting to which channel a television set is tuned, comprising: a) means for receiving a signal from a local oscillator of the television set; b) tuning and detection means which are such that, when thetuning means is tuned to the frequency ofthe signal from the local oscillator, a voltage is generated atthe detection means; and c) controlling meanswhich uses stored binary numbersto vary over a range the frequencyto which the tuning means is tuned.

2. An arrangement as claimed in claim 1, wherein the controlling means comprises storage means in which the binary numbers are stored and digital to analogue conversion means which receives in digital form a number from the storage means and which outputs an analogue voltage corresponding to that number.

3. An arrangement as claimed in claim 2, wherein the digital to analogue conversion means comprises a digital to pulse width convertor, the output of which has a markto space ratio related to that binary number, and integration means for producing said analogue voltage from the output of the digitalto pulse width convertor.

4. An arrangement as claimed in claim 2 or 3, wherein the controlling means furthercomprises an address counterfor sequentially addressing each binary number stored in the storage means.

5. An arrangement as claimed in any preceding claim, wherein thetuning means comprises a tuner including variable capacitance diodes which receives voltage from the controlling meanstovarythe capacitance of the diodes.

6. An arrangement as claimed in any preceding claim,wherein an output ofthetuning and detection means is connected to a local frequency oscillator, which in turn is connected to the controlling meansto inhibitthe operation of the controlling means when a voltage is generated at the detection means.

7. Anarrangementasclaimed in claim 4 or claim 4 and claim 5 or 6, wherein the address of each stored binary number is used to identify the detected television channel.

8. An arrangement as claimed in claim 1,substan- tially as hereinbefore described with reference to, and as shown in, Figures 2 to 4 ofthe accompanying drawings.