GB2113940A - Data transmission system - Google Patents

Abstract

A data transmission system is arranged so as to prevent unauthorised reception of transmitted information. At a transmitter at least some of the television line periods are displaced in time so as to remove their regularity, and a recipient is able to reconstitute a coherent television picture, only with a receiver in which the regularity of the line periods is restored and correct line synchronising pulses generated so as to permit the display of the television picture on a conventional monitor. The time displacement is in accordance with a pseudo random sequence held at both the transmitter and the receiver. <IMAGE>

Description

SPECIFICATION Data transmission system This invention relates to data transmission systems and is particularly applicable to systems of this kind in which unauthorised reception of the transmitted information is to be prevented.

Although secure transmission systems are known, they are often unduly complex or cumbersome and may not be suitable for the transmission of certain signals which are generated in a particular format and have a very wide bandwidth.

The present invention seeks to provide an improved data transmission system.

According to a first aspect of this invention, a data transmission system includes a transmitter for transmitting a data signal from which periodic synchronising pulses have been removed; and a receiver for receiving said signals and including a stable frequency source from which periodic synchronising pulses are generated, and means for utilising the periodic synchronising pulses to enable the original data to be reconstituted.

Preferably the data represents television information, in which case the periodic synchronising pulses occur at the line frequency of the television system.

It is necessary to provide a stable frequency source at the transmitter which is as stable as the frequency source at the receiver so that the two stable frequency sources do not drift apart in frequency to any appreciable extent. Since no synchronising pulses are themselves transmitted, some other means is provided for initially synchronising the received video signal with the deflection waveforms generated within the monitor, after which the stable frequency source present at the receiver maintains correct synchronisation. The initial synchronisation can be achieved by transmitting a suitable timing signal, either as a single event when transmission commences or periodically depending on the actual stabilities of the frequency sources.

Preferably the transmitter periodically transmits a time reference signal to the receiver to enable synchronisation to be maintained accurately despite minor frequency changes which might occur. In this event the time reference signal is sent periodically at intervals which are long compared to the normal line period of a television signal, but even so the periodic transmission of this time reference signal greatly eases the stability requirements of the two stable frequency sources.

According to a second aspect of this invention, a television transmission system includes, at a transmitter: means for generating a television video signal representative of a viewed scene and consisting of a sequence of line periods occurring at regular predetermined intervals; means for displacing in time at least some of the line periods so as to remove the regularity of the predetermined intervals and to thereby produce a modified video signal; and means for transmitting said modified signal to a receiver which includes: means for receiving said modified signal; means for restoring the regularity of the line periods so as to permit the display on a monitor of a television picture corresponding to the scene represented by the originally generated video signal.

Means can be provided for inserting line synchronisation pulses into the restored video signal so that a conventional monitor using only internal timing can display the picture.

Alternatively the monitor itself can be directly controlled from a stable frequency source which is also used to restore the regularity of the line periods.

The time displacement imparted to the original video signal so as to modify it can be a constant value which is applied only to selected line periods so as to shift them in time relative to other line periods which may be time shifted by a different amount, which itself, could possibly be zero, or the magnitude of the time displacement can be variable and can be applied to line periodic so as to shift them to greater or lesser extents, or even a combination of these techniques could be used.

The nature of the time displacements is such as to prevent the display of a coherent television picture by the modified video signal. To enhance the security of the transmission system, preferably the time displacement is imparted to pseudo randomly selected lines or groups of lines so that the selection is a substantially unpredictable manner. In those cases where the magnitude of the time displacement is also varied, the magnitudes by which the different time periods are delayed can advantageously also be in a pseudo random manner, but this aspect may not always be necessary. Since variation of the magnitude of the time displacement would add significantly to the complexity of the system, its use may not always be justified or desirable.

The original video signal is restored at the receiver by the application of a process which is the reciprocal of that employed at the transmitter.

This means that each line period is delayed at the receiver by an interval which is the complement of the delay originally imposed on that line so that in the restored video signal its line periods are all delayed by the same total amount.

In practice the transmitter will usually radiate the video information as a broadcast signal, but this is not essential, and instead wire or cable distribution systems could be utilised for the transmission of the modified video signal.

The invention is further-described by way of example with reference to the accompanying drawings, in which Figure 1 shows a television system in accordance with this invention and Figure 2 is an explanatory diagram; Referring to the drawings, a television transmitting arrangement 1 includes a television camera 2, which televises a viewed scene and generates a video signal under the control of a camera control unit 3. The camera control unit generates the waveforms and synchronising pulses necessary for the generation of a stable video signal and it derives its reference waveforms from a very stable high frequency oscillator 4 via a timing circuit 41. The oscillator 4 is a particularly stable high frequency source which typically has a stability which is well in excess of one part in 106 over a 1 000C temperature range, but the stability could be very much higher than this.The video waveform present at the output of the camera 2 consists of a series of line periods which are .sepamted by what is usually termed line blanking periods. The blanking periods contain synchronisation pulses. As is known, a television picture consists of a large number, typically 625 television lines which together constitute one television frame, and to avoid objectionable visual flicker the frame is usally divided into two field periods. The output of the camera 2 at point a, is represented diagrammatically in Figure 2 at line a in which a number of lines ABC-J are shown separated by appropriate line blanking intervals.

As is shown in line a, the duration of each line period including the blanking interval (n) is 64 microseconds.

The output of the video camera 2 is fed simultaneously to two delay circuits 5 and 6. In the present example, delay circuit 5 produces a delay of 64 microseconds and delay circuit 6 produces a delay of 76 microseconds. The difference of 1 2 microseconds corresponds to the duration of a line blanking interval and the purpose of the two delay circuits 5 and 6 is to produce two video waveforms having this relative delay. Each delay circuit typically consists of a serial shift register with the total delay being determined by the number of individual storage bits and the clock rate at which the signal is fed through the shift registers. Timing signals which control the operation of the delay circuits 5 and 6 are generated by the timing circuit 41.

A switch 7 selects the output from one or other of the two delay circuits 5 or 6 under the control of a delay selector 8. The waveforms at the outputs df delay circuits 5 and 6 are illustrated at lines b and c of Figure 2 and it will be seen that one waveform is simply delayed by 12 microseconds relative to the other. The cross hatching illustrated on the waveforms simply indicatesthrough which of the two delays 5 and 6 the relative signal has been passed. The waveform generated by the delay selector 8 and which represents the operation of the switch 7 is illustrated at line d and it will be seen that it operates to select groups of three lines between switching events.This is purely diagrammatic, and in practice the number of sequential lines occurring between operation of the switch 7 will vary in a pseudo random manner, but its mean value is preferably well in excess of three and is typically between twenty and thirty.

Thus the selector 8 generates an output in accordance with the pseudo random sequence. At the output of the switch 7 is a modified video signal as illustrated in line e of Figure 2 and it will be seen that it consists of an interleaved sequence of television lines passed by the two delay circuits 5 and 6. This waveform e is passed to a transmitter unit 9, which radiates the signal using conventional radio frequency equipment. In practice, the transmitted signal may also include audio information as appropriate, but this aspect is conventional.

Although in practice the television signal is likely to be broadcast, i.e. transmitted by an omnidirectional antenna, this is clearly not necessary and if appropriate it can be transmitted as a directionally radiated beam.

The radiated signal is received at a receiver 10 tuned to the appropriate radio frequency carrier signal, and after demodulation at a demodulator 20 the video signal takes the form of the waveform diagrammatically shown in line fof Figure 2. This waveform is passed to the inputs of two further delay circuits 11 and 12 having characteristics which exactly correspond to delay circuits 5 and 6 respectively and which are controlled by timing circuit 21. The respective output waveforms are illustrated in lines g and h of Figure 2, and the required output is selected by means of a switch 13 which is controlled by a further delay selector 14 which operates in accordance with the same pseudo random sequence as selector 8.The switch 1 3 is operated in accordance with the waveform illustrated at line i of Figure 2, and it will be seen that the selection corresponds to the complement of the previous waveform of line d.

The delay selector 14 receives, via a timing circuit 21, a stable reference frequency from a stable slave oscillator 1 5 which operates at the same frequency as stable oscillator 4 and with a corresponding degree of stability. The operation of the switch 1 3 is to reconstitute the original video waveform and this is illustrated in lines of Figure 2. It will be seen that the line periods are now separated by a constant line blanking interval, which is equal to the 1 2 microseconds of period (n). It will be appreciated that the line blanking signals themselves are not necessarily present in these line blanking intervals, and in particular the television line fis followed by a delayed version of part of the same line.Nevertheless, the information content of the television lines is now correctly spaced in time, and the final video waveform is produced when the appropriate line blanking and line synchronisation signals are reinserted at a line puise combiner 1 8 which receives the necessary timing signals from the timing circuit 21 and inserts them in the video signal. The corresponding output waveform is as illustrated in line k of Figure 2, and it is this waveform which is presented to a conventional television monitor 1 6 for display.

Alternatively the monitor itself could be controlled by frequencies and waveforms derived from a display control unit 17, which in turn derives its reference frequency from the timing circuit 21. In this case the line pulse combiner 1 8 would not be necessary.

In a conventional manner, the monitor 1 6 identifies the line synchronising pulses and uses them to produce a correct registration of the line content of the video waveforms so as to enable the viewed scene to be sensibly reproduced. The monitor 1 6 would not be able to produce a coherent television picture directly from the waveform shown in line fof Figure 2, since the different television lines would be horizontally offset by an interval corresponding to the time difference between the two delay circuits 11 and 12.

It will be appreciated that the delay selector 1 4 operates in accordance with the same pseudo random sequence which is held in the delay selector 8. In choosing a suitable pseudo random sequence, it may be desirable to ensure that large blocks of modified lines having the same delay are not transmitted. It is, of course, necessary to synchronise the operation of the pseudo random sequence generator at the receiver with that of the transmitter and this would normally be achieved by transmitting an appropriate synchronising signal at the commencement of a transmission period. Once the two pseudo random sequences have been synchronised with each other they will stay in step provided that the two stable oscillators 4 and 1 5 do not drift in frequency relative to each other.To prevent this happening peripdic clock synchronising signals can be transmitted and in practice it would be sufficient to transmit these signals at intervals of every five frame periods or so-these signals could conveniently be a particular tone which is recognised by a narrow band filter located at the receiver 1 0. In Figure 1 the clock synchronising pulses are extracted within the demodulator 20 and fed to the stable slave oscillator 1 5 to hold it exactly in step with oscillator 4.

As the receiver contains a very stable oscillator 1 5, it is not, in fact, necessary for the line blanking periods to be transmitted, and the security of the system can be enhanced by removing all line synchronising information prior to transmission, in which case artificial video signals can be used to fill the gaps which would otherwise be present in the blanking periods of the true video waveform.

The artificial video signals would, of course, be automatically removed when the blanking pulses are re-inserted.

Claims (7)

1. A data transmission system including a transmitter for transmitting a data signal from which periodic synchronising pulses have been removed; and a receiver for receiving said signals and including a stable frequency source from which periodic synchronising pulses are generated, and means for utilising the periodic synchronising pulses to enable the original data to be reconstituted.

2. A system as claimed in claim 1 and wherein the transmitter periodically transmits a time reference signal to the receiver to enable synchronisation to be maintained accurately despite minor frequency changes which might occur.

3. A television transmission system including, at a transmitter: means for generating a television video signal representative of a viewed scene and consisting of a sequence of line periods occurring at regular predetermined intervals; means for displacing in time at least sortie of the line period is so as to remove the regularity of the predetermined intervals and to thereby produce a modified video signal; and means fdr transmitting said modified signal to a receiver which includes: means for receiving said modified signal: means for restoring the regularity of the line periods so as to permit the display on a monitor of a television picture corresponding to the scene represented by the originally generated video signal.

4. A system as claimed in claim 3 and wherein the time displacement imparted to the original video signal so as to modify it is a constant value which is applied only to selected line periods so as to shift them in time relative to other line periods.

5. A system as claimed in claim 4 and wherein the selected line periods are chosen in a pseudo random manner.

6. A system as claimed in claim 3 and wherein the magnitude of the time displacements is variable, and is applied to line periods so as to shift them to greater or lesser extents in a pseudo random manner.

7. A television transmission system substantially as illustrated in and described with reference to the accompanying drawings.

7. A system as claimed in any of claims 3 to 6 and wherein periodic clock synchronising signals derived from a stable frequency source at the transmitter are transmitted so as to permit synchronisation of a slave stable frequency source located at the receiver.

8. A system as claimed in claim 7 and wherein the periodic clock synchronising signals are transmitted to the receiver as particular tones.

9. A television transmission system substantially as illustrated in and described with reference to the accompanying drawings.

New claims or amendments to claims filed on 18 December 1981 Superseded claims Ali New or amended claims:- All CLAIMS

1. A television transmission system including, at a transmitter: means for generating a television video signal representative of a viewed scene and consisting of a sequence of line periods occurring at regular predetermined intervals; means for displacing in time at least some of the line periods so as to remove the regularity of the predetermined intervals and to thereby produce a modified video signal; and means for transmitting said modified signal to a receiver which includes: means for receiving said modified signal means for restoring the regularity of the line periods so as to permit the display on a monitor of a television picture corresponding to the scene represented by the originally generated video signal.

2. A system as claimed claim 1 and wherein the time displacement imparted to the original video signal so as to modify it is a constant value which is applied only to selected line periods so as to shift them in time relative to other line periods.

3. A system as claimed in claim 2 and wherein the selected line periods are chosen in a pseudo random manner.

4. A system as claimed in claim 1 and wherein the magnitude of the time displacements is variable, and is applied to line periods so as to shift them to greater or lesser extents in a pseudo random manner.

5. A system as claimed in any of claims 1 to 4 and wherein periodic clock synchronising signals derived from a stable frequency source at the transmitter are transmitted so as to permit synchronisation of a slave stable frequency source located at the receiver.

6. A system as claimed in claim 5 and wherein the periodic clock synchronising signals are transmitted to the receiver as particular tones.