WO1990004306A1

WO1990004306A1 - Signal transmission

Info

Publication number: WO1990004306A1
Application number: PCT/GB1989/001220
Authority: WO
Inventors: Herrold George Anglin
Original assignee: International Media Communications Plc
Priority date: 1988-10-13
Filing date: 1989-10-13
Publication date: 1990-04-19
Also published as: AU4404689A

Abstract

The specification discloses a system for the transmission of video signals between first and second locations, by use of mains electrical circuitry. To this end, the system comprises first processing means for receiving and processing the video signal, first coupling means for introducing the processed video signal into electrical mains circuitry at the first location, second coupling means for extracting the processed video signal from the mains at the second location, and second processing means for processing the signal thus obtained. Both digital and analogue systems are disclosed.

Description

SIGNAL TRANSMISSION

This invention relates to the transmission of signals between two or more locations by use of electrical mains circuitry.

For many years electronics engineers have worked on the transmission of signals through electrical mains circuitry. This development effort has led to devices such as intercoms in which audio signals from a microphone are encoded at one location, transmitted as a carrier signal through the mains and decoded at another location for driving a speaker.

Until now, and despite this ^"considerable development activity, attempts to transmit signals through the mains have been hampered by interference arising from fluctuations in the characteristics of the mains. These fluctuations arise in day-to-day use of the mains circuitry as different loads are connected, used, and disconnected. Video signals- in particular suffer greatly from this interference in view of their complexity and large bandwidth.

The principal aim of this invention is to enable the transmission of video signals through electrical mains circuitry, without encountering unacceptable levels of interference.

With this aim in view and in accordance with one aspect of this invention, we propose a video-signal transmission system comprising first processing means for receiving and processing a video signal, first coupling means for introducing the processed video signal into electrical mains circuitry at a first location, second coupling means for extracting the processed video signal from the electrical mains circuitry at a second location, and further processing means for processing the signal thus obtained.

In another aspect of this invention, we propose a method for transmitting a video signal through electrical mains circuitry, comprising the steps of receiving and processing the video signal in a first processing operation, introducing the processed video signal into the electrical mains circuitry at a first location, extracting the processed video signal from the electrical mains circuitry at a second location, and processing the signal thus obtained, in a second processing operation.

By means of the invention, video signals are available at all locations of the building that are served by mains circuitry. This means that for example a video recorder or a video camera located in one room can be connected for viewing to a television monitor located in another room, or to a plurality of television monitors located . in various rooms throughout the building. With suitable processing operations, it is possible to transmit simultaneously signals other than video signals, such as audio signals or data signals.

In one way of carrying the invention into effect, the video signals are amplitude modulated onto a carrier wave of 35 to 40 MHz and pass through at least one tuned amplifier before being introduced into the mains. Once extracted from the mains, the video signals are heavily filtered, amplified with precise gain control and up-converted to UHF frequency levels suitable for connection to a TV monitor.

In another embodiment, the video signals are digitised and are introduced into the mains in digital form at a clock rate of approximately 40 Mhz. The digital video signals are then extracted from the mains at a remote location and are converted to analogue form before being up-converted to UHF frequency levels. This arrangement gives improved resistance to interference during transmission.

It is advantageous for the red, green and blue video signals to be separated before analogue to digital conversion so that digital representations of the respective signals are produced. The three digital signals are then multiplexed for transmission. This arrangement allows parallel processing of the respective signals, which simplifies the logic circuitry required. Once received at a remote location, the digital video signal is demultiplexed to produce separate red, green and blue digital video signals. These three digital signals are converted separately to analogue form and are then combined to reconstruct a composite analogue video signal. Any associated audio signal may also be transmitted in digital form, being extracted from the transmitted digital signal upon reception and then being converted to analogue form for subsequent combination with the analogue video signal. Conveniently, the digital video and audio signals are interlaced for transmission together with a clock pulse. It is preferred that a blank period ' is left in each transmission cycle to allow for transmission of other digital signals in either direction through the mains.

Conveniently, the video signals can be accessed by connecting coupling means to an existing electrical mains power socket. This obviates the need for, and expense of, separate wiring. Thus, there is no need for costly installation of wiring and, possibly, consequential redecoration.

In an advantageous embodiment of this invention, means are provided for relaying remote-control signals between a remote-control unit and the television or other device that the unit controls. These means may include a signal receiver and a signal transmitter connected to one another through the electrical mains circuitry via coding and decoding devices. In order that the invention may be more readily understood reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a circuit diagram of an encoder/amplifier circuit according to one aspect of this invention;

Figure 2 is a circuit diagram of a decoder/converter circuit according to another aspect of this invention;

Figure 3 is a layout diagram of an encoder/transmitter circuit according to a preferred arrangement of the invention, and;

Figure 4 is a layout diagram of a decoder circuit according to a preferred arrangement of the invention.

Referring to Figure 1, an encoder/amplifier circuit comprises an ^'encoder portion, shown on the left side of the diagram, and an amplifier portion, shown on the right side of the diagram. The encoder portion of the circuit shown in Figure 1 comprises a TDA5660 integrated circuit 10 which receives baseband video and audio signal inputs at pins 1 and 10 respectively. Typically, the video signals will have a bandwidth of between 0 and 5.5MHz with a level of 1.0V peak-to-peak, and the audio signals will have a bandwidth of 50Hz to 12kHz with a level of 0.5V RMS. Both the video signals and the audio signals are fed from respective connection points as shown through blocking capacitors to ensure that the TDA5660 i.e. 10 is isolated from potentially damaging 'spike' inputs.

The TDA5660 i.e. 10 modulates the video signals onto a carrier wave whose frequency is high, preferably about

35MHz to 40MHz. The significance of this high frequency is that the majority of interference encountered is of a substantially lower frequency and therefore can be separated in subsequent filtering operations with relative ease. Furthermore, the high frequency allows sufficient bandwidth for the transmission of complex video information. The audio signal may be modulated at say 6MHz below the video signal. An advantageous feature of the TDA5660 i.e. is that the audio signals can be frequency modulated for high quality whilst the video signals can be amplitude modulated. Amplitude modulation of video signals is convenient because video equipment generally works on an AM basis and therefore, by using AM for the video signals, there is no need for FM-AM conversion.

The modulated output from the TDA 5660 i.e. 10 appears at pin 15 and is fed into a balance circuit 12 which isolates the TDA 5660 i.e. from the remaining circuitry. After passing through the balance circuit 12, the modulated signal is filtered through a series of three chokes 14 to eliminate any spurious high-frequency signals that may be introduced by the TDA5660 i.e.. The signal then enters the amplification stage shown on the right of the diagram.

The amplifier portion comprises two amplifier circuits 16,18 connected in series, which are delineated by dotted lines in the diagram for clarity. The amplifier circuits 16,18 are tuned so as to provide greatest amplification for the desired frequency i.e. the frequency of the video/audio signals. Thus, the amplifiers act to enhance the quality of the signal by reducing further the effect of spurious signals.

In the circuit illustrated, the first amplifier circuit 16 may be tuned by adjustment of a variable capacitor 20 and a variable inductor 22, and the second amplifier may be tuned by adjustment of a variable inductor 24. The settings of these components may be varied by a technician as part of a setting-up process.

After amplification, the signal passes through a current damping matching stage 26 and is output to a matching circuit for matching to the mains.

The matching circuit comprises a toroidal coil 28

^~about which input and output coils are wound. Bifilar winding is preferred so as to ensure close coupling between the input and the output. The core of the coil 28 is of a ferrous metal whose characteristics are selected to tailor the matching circuit to suit the frequency being used. The matching circuit is connected to the live and neutral lines of the mains circuitry through high-value capacitors 30. These capacitors 30 serve to isolate the matching circuit from the low-frequency mains voltage whilst allowing passage of the high-frequency video/audio information. The capacitors thereby ensure that the coil 28 is not driven into saturation by the large-amplitude mains voltage.

Once introduced into the electrical mains supply of a building, the video/audio signals are available at any electrical power socket forming part of the building's mains system. At one such socket, the video/audio signals may be received by a decoder/converter unit such as that shown in Figure 2 of the drawings.

Referring now to Figure 2, the decoder/converter unit is coupled to the mains electricity supply through capacitors 32 which block the large-amplitude mains voltage in much the same way as the capacitors 30 referred to above. Similarly, the decoder/converter unit is coupled to the mains using a toroidal-coil matching circuit 34 whose characteristics are tailored to suit the frequency being used. After passing through the matching circuit 34, the video/audio signal is firstly passed through a capacitor 36 which isolates the decoder/converter from potentially-damaging 'spike^■ inputs. Spike protection is enhanced by a Zener diode 38 which 'clamps' the peak spike voltage. Thereafter, the signal enters a band-pass filter circuit 40 which reduces any spurious signals from outside a desired frequency band (about 30Mhz to 60MHz) .

Upon emerging from the band-pass filter, the video/audio signal undergoes initial amplification by a field-effect transistor (FET) 42. The FET 42 is selected for its low input impedance and high output impedance, together with its high sensitivity and ready controllability. In this application, the FET 42 firstly amplifies the signal (which may be very weak in view of losses during passage through the mains) , and secondly ensures gain stability through a feedback loop, as will be explained.

The next stage after the FET 42 is a second amplifier stage 44 which drives a saw filter 46. The saw filter

46 sets a distinct bandwidth, typically a 24dB cut-off on either side of a centre frequency of 48Mhz. The saw filter 46 thus reduces still further spurious signals from outside the desired frequency range.

The saw filter output passes through a further amplification stage 48 comprising two transistors in series which drive an NE592 integrated circuit 50. The NE592 i.e. is an amplifier which amplifies an input of the order of microvolts to an output of the order of millivolts, with minimal addition of noise.

The NE592 output is passed through a 2-:l step-down transformer 52 to attain a level of approximately lmV, as required by UHF television inputs. The signal is then fed to an up-converter 54 which increases the signal frequency to UHF levels, about 450MHz to 650MHz, for connection to a television monitor or the like at a UHF connection point marked 'U' in Figure 2. It is preferred that the up-converter 54 is a stripline-type device as shown, for simplicity and cheapness.

Referring back to the FET 42, it will be recalled that the FET is used to control the gain of the decoder circuit by use of a feedback loop. The feedback loop starts at point P (the input to the up-convertor) and is designed to ensure that the voltage level at that point remains around the desired l V level.

The componentry of the feedback loop, indicated generally by the reference numeral 56, converts the voltage at point P into a d.c. voltage level by a process of rectification. This voltage level is used to control a BC184 transistor 58 as a voltage amplifier, the resulting amplified voltage level being passed back to the FET 42 through a feedback link 60. The ^" amplified voltage level controls the biasing of the FET 42, and thus controls the gain of. the decoder circuit.

The feedback loop 56 is arranged such that if the voltage level at point P rises above a desired value (in this case about lmV) , the FET 42 will be biased in such a way as to reduce the gain of the decoder circuit, and vice versa if the voltage level at point P falls below the desired value.

Figures 3 and 4 illustrate a preferred arrangement of the invention in which signal transmission is based upon digital circuitry. Starting with Figure 3, this shows an encoder/transmitter circuit in which video and audio signals are input separately as. before. The composite video signal firstly passes through a video buffer 62 which ensures that a stable signal is supplied to subsequent circuitry. The video signal then undergoes processing which begins with a sync separator 64. The sync separator 64 extracts the sync pulse from the video signal, and this pulse is used to drive a timing generator 66 in conjunction with a clock pulse input derived from a clock 68.

Following sync separation, the composite analogue video signal passes into a colour processor 70 which splits the composite signal into separate red, green and blue analogue signals. These three analogue signals are then each converted into digital form by a triple 4-bit flash-type analogue to digital converter (ADC) 72. After conversion, the three (now digital) video signals are latched in a 12-bit register 74, before being passed to a 12-bit multiplexer 76 under control of the timing generator 66.

If desired for higher quality, analogue-to-digital conversion may be carried out with an ADC of more than four bits (say eight bits), in which case the register 74 and multiplexer 76 can be of greater than twelve bits.

Incoming analogue audio signals are firstly fed through a 12kHz low-pass filter 78 to filter out high-frequency signal components, and are then converted to digital form by a single 8-bit ADC 80. The resulting digital audio signal is then passed to an 8-bit multiplexer 82 under control of the timing generator 66.

The output from the respective multiplexers 76,82 is supplied to a formatting circuit 84 under control of the timing generator 66. The formatting circuit 84 also receives a data clock pulse directly from the timing generator 66.

The timing generator 66 is arranged such that the respective multiplexers 76,82 are triggered to release their data to the formatting circuit 84 in turn, whereby the audio and video signals are interlaced together with the data clock pulse for subsequent transmission. In an advantageous embodiment, a 'blank' period is left in each cycle so as to leave room for any other information that it may be desired to transmit in either direction through the mains. An example of this 'other' information is a remote-control signal, as will be explained below.

The digital signal that emerges from the formatting circuit 84 passes through a buffer 86 and is then fed into a mains electrical system via a matching circuit 88. The matching circuit 88 is essentially identical to that described above in relation to Figure 1, and therefore is not illustrated or described further here.

Referring now to Figure 4, this shows a decoder circuit which is coupled to the mains electrical system in similar manner to the circuit illustrated in Figure 2 and described in detail above. Thus, the digital signal is extracted from the mains by a matching circuit 90 and then passes through a transient suppressor 92 and a filter circuit 94. These components are analogous to the matching circuit 34, capacitor 36, Zener diode 38 and band-pass filter circuit 40 of Figure 2, and therefore need not be described further. After filtering, processing of the digital signal begins with recovery of the data clock pulse in a pulse separator 96. The data clock pulse is synchronised with a pulse derived from a- clock 98 by means of a phase-locked loop (PLL) 100. The PLL 100 produces a clock output which drives a timing generator 102.

Once the data clock pulse has been recovered, the digital signal is fed into a separator circuit 104, which separates the digital video component from the digital audio component under control of the timing generator 102. Thereafter, the digital video component is de-multiplexed in a 12-bit demultiplexer 106 and the respective bits associated with the red, green and blue signals are separated and latched in respective red, green and blue 4-bit latches 108, 110 and 112. The contents of the latches 108, 110 and 112 are then passed under control of the timing generator 102 through respective 4-bit digital-to-analogue converters (DAC's) 114, 116 and 118. The separate red, green and blue analogue signals which result are then combined in a colour encoder 120 to reconstruct a composite analogue video signal. After leaving the separator circuit 104, the digital audio signal is in serial form and is then de-multipexed by an 8-bit demultiplexer 122. This converts the digital audio signal from serial form to parallel form, whereupon the digital audio signal is converted to analogue form by an 8-bit DAC 124. The resulting analogue audio signal is then combined with the composite video signal in a modulator 126.

Once the analogue audio and video signals have been reconstructed and combined as described above, the combined signal is output to an up-converter 128. The up-converter 128 can be of similar construction to that shown in Figure 2 under reference numeral 54, and therefore need not be described again.

As will be clear, a major benefit of this invention is that a video film or other programme may be viewed on a monitor remote from the video " source. In particular, a video source such as a video recorder situated in one room of a building may be connected for viewing to a television situated in another room. This, for example, enables a viewer to watch a programme originating from a video recorder situated in a lounge, whilst remaining in his or her bedroom. Unfortunately, problems may well arise if the viewer wishes to operate the controls of the video recorder from the bedroom using a remote control unit. This is because a remote control unit is likely to be ineffective due to the blocking effect of walls between the viewer and the video recorder, unless of course the remote control is connected to the video recorder by a dedicated, wire link.

In the present invention, provision may be made for a remote-control link whereby a viewer in one room can control a video recorder or other video source situated in another room, by using the standard remote-control unit. This link comprises a receiver unit positioned to receive control signals from the remote-control unit. Conveniently, the receiver unit is placed on top of, or otherwise adjacent to, the viewer's television monitor so as to receive control signals when the remote-control unit is pointed towards the monitor.

Once received by the receiver unit, the control signals are encoded by suitable circuitry, amplitude modulated onto a carrier of say IMhz - 2Mhz and then fed into the mains. Alternatively, in a digital system, the control signals could be transmitted in digital form during the aforementioned 'blank' period in each transmission cycle. The control signals are then available at the electrical power socket into which the video source is connected, and can be obtained using suitable decoder circuitry. It is envisaged that the encoder and decoder circuitry can be coupled to the mains in much the same way as previously described i.e. by use of toroidal-coil matching circuits. However, the characteristics of the matching circuits are advantageously different to those of the matching circuits used for the video/ audio signals so as to suit any different carrier frequency being used.

Once obtained from the mains, the control signals are converted into a form corresponding to that produced by the remote-control unit e.g. infra red pulses, and are re-radiated by a repeater transmitter for reception by the video^{- "}source. The control signals radiated in this way are, in essence, copies of the signals produced by the remote-control unit and therefore act to control the video source in the way intended by the viewer. In an alternative arrangement, the control signals are not re-radiated by a repeater transmitter but are instead fed into the video source to act directly upon the remote-control circuitry therein.

As will be clear to those skilled in the art, this invention overcomes the aforementioned remote-control difficulties without recourse to a dedicated wire link, and enables a standard remote-control unit to be used in a way which, to a user, is quite conventional.

It is envisaged that the encoder/amplifier circuit, the decoder/converter circuit, and the remote-control receiver and transmitter units can be housed in unobtrusive housings for addition to an existing home video system. Alternatively, the various components may be incorporated into video equipment at the design stage so as to provide a fully-integrated system. Such an arrangement would be particularly simple where the system is digital, particularly if the various logic components were condensed onto one or two specialised integrated circuits suitable for fitment to various manufacturer's equipment. Many variations are possible without departing from the scope of the present invention. For instance, the carrier wave can be of any suitable frequency, the upper limit being dictated by inductive losses in the mains wiring. The lower limit is dictated by interference with radio sources etc. below about lOMhz, and by bandwidth considerations.

Claims

1. A video-signal transmission system comprising first processing means for receiving and processing a video signal, first coupling means for introducing the processed video signal into electrical mains circuitry at a first location, second coupling means for extracting the processed video signal from the electrical mains circuitry at a second location, and second processing means for processing the signal thus obtained.

2. A video-signal transmission system according to claim 1, wherein the first processing means comprises a digital encoder and the second processing means comprises a digital decoder.

3. A video-signal transmission system according to claim 2, wherei -^_he first processing means comprises means for separating the video signal into a plurality of analogue colour components, and means for converting each of the analogue colour components into a corresponding digital colour signal.

. A video-signal transmission system according to claim 3, wherein the first processing means comprises means for multiplexing the digital colour signals to form a combined digital video signal.

5. A video-signal transmission system according to claim 3 or claim 4, having formatting means arranged to interlace the digital colour signals or the combined digital video signal with a data clock pulse to form a transmission cycle.

6. A video-signal transmission system according to claim 5, wherein the first processing means comprises means for converting an analogue audio signal into a corresponding digital audio signal, and wherein the formatting means is arranged to interlace the digital audio signal into the transmission cycle.

7. A video-signal transmission system according to claim 5 or claim 6, wherein the formatting means is arranged to --interlace a blank period into the transmission cycle.

8. A video-signal transmission system according to any of claims 5 to 7, wherein the second processing means comprises separator means to separate the components of the transmission cycle.

9. A video-signal transmission system according to claim 8, wherein the second processing means includes means for demultiplexing the digital video signal, and a plurality of latch means for separately latching the respective digital colour signals.

10. A video-signal transmission system according to claim 9, wherein each of the latch means has associated means for converting the respective digital colour signal into a corresponding analogue colour signal.

11. A video-signal transmission system according to claim 10, wherein the second processing means includes encoder means for combining the analogue colour signals to reconstruct the video signal in analogue composite form.

12. A video-signal transmission system according to claim 11, comprising means for converting the digital audio signal into a corresponding analogue audio signal, and means for modulating the analogue audio signal together with the analogue video signal.

13. A video-signal transmission system according to claim 1, wherein the first processing means comprises means for modulating the video signal onto a carrier wave and amplifier means for amplifying the modulated video signal.

14. A video-signal transmission system according to claim 13, wherein the amplifier means, comprises a series of tuned amplifiers.

15. A video-signal transmission system according to claim 1, wherein the first and second coupling means each comprise a toroidal coil.

16. A video-signal transmission system according to claim 1, wherein the second processing means comprises amplifier means including a field-effect transistor.

17. A video-signal transmission system according to any preceding claim, wherein the second processing means includes an up-converter.

18. A video-signal transmission system according to any preceding claim, having a transient suppressor and a filter between the second coupling means and the second processing means.

19. A video-signal transmission system according to any preceding claim, having...- receiver means and encoder means for receiving and encoding a remote-control signal, means for introducing the coded remote control signal to the mains at the second location, means for extracting the coded remote control signal from the mains at the first location, ^"and decoder means for decoding the coded remote control signal after extraction.

20. A video-signal transmission system according to claim 19, having transmitter means, associated with the decoder means, for transmitting a remote-control signal corresponding to the remote-control signal received by the receiver means.

21. A video-signal transmission system according to claim 19 or claim 20, when appendant to claim 7, wherein the encoder means is digital and is arranged to transmit the coded remote control signal through the mains during the blank period in each transmission cycle.

22. A method for transmitting a video signal through electrical mains circuitry, comprising the steps of processing the video signal in a first processing operation, introducing the processed video signal into the mains at a first location, extracting the processed video signal from the mains at a second location, and processing the signal thus obtained, in a second processing operation.

-23. A method according to claim 22, wherein the first processing operation comprises the step of converting the video signal from analogue to digital form at a high clock frequency.

24. A method according to claim 23, wherein the clock frequency is between 25MHz and 50MHz.

25. A method according to any of claims 22 to 24, wherein the first processing operation comprises the step of separating the video signal into a plurality of analogue colour components and then converting each of the analogue colour components into a corresponding digital colour signal.

26. A method according to claim 25, comprising the step of multiplexing the digital colour signals to form a combined digital video signal.

27. A method according to claim 25 or claim 26, comprising the step of interlacing the digital colour signals or the combined digital video signal with a data clock pulse to form a transmission cycle.

28. A method according to claim 27, comprising the steps of converting an analogue audio signal into a corresponding digital audio signal, and then interlacing the digital audio signal into the transmission cycle.

29. A method according to claim 27 or claim 28, comprising the step of incorporating a blank period into the transmission cycle.

- SO - SO. A method according to any of claims 27 to 29, wherein the second processing operation comprises the step of separating the components of the transmission cycle.

31. A method according to claim 30, comprising the steps of demultiplexing the digital video signal, separately latching the respective digital colour signals, converting each respective digital colour signal into a corresponding analogue colour signal, and combining the respective analogue colour signals to reconstruct the video signal in analogue composite form.

32. A method according to claim 31, comprising the steps of converting the digital audio signal into a corresponding analogue audio signal, and then modulating the analogue audio signal together with the analogue video signal.

33. A method according to claim 22, wherein the first processing operation comprises the steps of modulating the video signal onto a carrier wave of high frequency and then amplifying the modulated video signal.

34. A method according to claim 22, comprising the steps of receiving and encoding a remote-control signal, introducing the coded remote-control signal into the mains at the remote location, extracting the coded remote-control signal from the mains at the base location, and decoding the coded remote-control signal after extraction.

35. A method according to claim 34, comprising the further step of transmitting a remote-control signal corresponding to the remote-control signal that is received.

36. A method according to claim 34 or claim 35, when appendant to claim 29, comprising the step of transmitting the coded remote control signal in digital form during the blank period in each transmission cycle.