GB2050675A - Video Copying and Transmission Inhibitor - Google Patents

Abstract

A system for transmitting signals from video tapes incorporating means for producing modified field synchronising pulses, giving rise to a delay in the start of vertical drive pulses, whereby the correlation between line and field synchronising pulses is destroyed, and means for recording said modified synchronising pulses on a control track of a video tape recorder to serve thereon as a timing reference, so correlating the tape lateral speed and the scanner head rotational speed of said recorder. <IMAGE>

Description

SPECIFICATION Video Copying and Transmission Inhibitor This invention relates to means for inhibiting the unauthorised copying of electromagnetically transmitted video and audio information.

It is a problem to transmit signals, derived from video tape recordings, while preserving their exclusiveness free from piracy, which takes the form of copying by unauthorised parties, and it is therefore an objec of the present invention to provide a system of video tape recording, and the radio transmission of signals therefrom, exclusively to television operators who are provided with a key to the operation of the system.

Presently known "no copy" systems of video tape recording and radio transmission have involved sophisticated circuitry and a variety of modified and normal field synchronising pulses in composite video wave forms, but it is an object of the invention to dispense with such systems.

In particular, it is an object of the invention to incorporate in a video tape an inbuilt incoherence which is reflected in any authorised copy made therefrom after radio transmission.

According to the invention therefore, in one of its aspects, a system of transmitting signals from video tapes incorporates means for producing modified synchronising pulses, giving rise to a delay in the start of vertical drive pulses, whereby the correlation between line and field synchronising pulses is destroyed, said modified synchronising pulses being recorded on a control track of a video recorder, and employed as a timing reference, so correlating the tape lateral speed and the scanner head rotational speed of said recorder.

In one embodiment of the invention, a video recorder employs a braking type servo to control video scanner head rotation (i.e. the tape speed is fixed). During playback, as the first field of a prerecorded frame comes into position for playback on the scanner head, a control track pulse is read out at the control track head. This event is compared with a pulse generated in the scanner head that signals the position of the "A" video head. This pulse is generated at the field rate. If the head is early, head drum speed is too high, with the result that the pulse comparison increases the brake current to retard the head position. Late arrival will cause the opposite effect. In the record mode, separated vertical sync., which is obtained from the input video signal, serves as the timing reference for the servo system.

If control track pulses are delayed in real time by 0.5 line duration, it will be seen the head servo will decrease its rotational speed, i.e. pulses generated by the scanner head are always early with respect to control track pulses.

As applied to either a radio transmission system or a digital processing system, the incoming signal is either looped through to the output or is processed. Processing may be either (a) analogue or (b) digital.

Analogue Processing A sample of the video signal is fed to a synch.

separator system. This system comprises:~ 1) an inverting video amplifier, 2) a composite sync. separator, 3) a vertical sync. separator, 4) a monostable multivibrator adapted to supply vertical drive pulses.

Two outputs are derived from the sync.

separator, namely~ i) composite sync. and, ii) vertical drive.

The separated composite sync. is fed via a buffer stage to an output terminal. This signal is used for the correct timing reference pulses on the control track of the video recorder. For radio transmission composite sync. is not used.

Vertical drive pulse output from the sync.

separator is fed to a programmable divider.

Selected output from this divider chain is fed to a differentiator.

The function of the divider chain is eventually to insert the offset pulses on the following:~ 1) all fields, 2) all odd or all even fields, 3) in the case of NTSC a) all fields, and b) fields 1 or 2 only.

Output of the divider chain is fed to a differentiator in order to obtain a narrow trigger pulse (i.e. 0.5 H nominal).

The pulse width of the differentiated pulse is adjustable in order to obtain the exact delay required.

The monostable multivibrator is triggered from the trailing edge of the trigger pulse. That is to say, the delay is dependent on the width of the trigger pulse.

The output of the monostable multivibrator is a pulse whose duration is 1.5 H-2.5 H (adjustable). The pulse width determines the duration of the offset pulse. The output of the monostable multivibrator is fed to pulse inverting means and a pulse output amplifier, and finally via running resistors to the output.

The output level of the offset pulse is adjustable, and the level depends on the recorder format used, nominally 150 to 200 millivolts.

Actual running is accomplished at the input of the recorder input resistor or (for testing) at a 75 ohm terminating resistor.

Digital Processing A comprehensive description of digital processing will not be given here but persons skilled in the art will appreciate that the video signal is fed to a clamping amplifier, and hence to an analogue-to-digital converter. The video signal is also fed to a sync. separator genlock circuit. The genlock circuit locks the clock to the synching.

pulses.

A clock generator is provided at three times the sub-carrier frequency, synchronised by processed sync. Three tc and tc are required for the digital processing of the system.

Line store read and write generators process the microprocessed vertical pulses of the nominated amplitude, number of fields and duration of sync. and offset pulses are digitally controlled to the required accuracy.

Nomination of amplitude, number of fields, and video identity is programmable. Programmes are provided for each format. The digital-to-analogue converter converts the digital video information to the modified analogue video signal.

The luminance-chrominance equilizer equilises the difference in delay between the luminance and chrominance singals.

A video output amplifier provides a Sin x x signal of the proper amplitude for output distribution.

Radio Transmission A modified video signal as described above is fed to an l.F. modulator, channel converter and R.F. power amplifier. This part of the system pertains to radio transmission and also to closed circuit television, such as cablevision.

Processing may be either analogue or digital.

One embodiment of the invention will now be described herein with reference to the accompanying drawings, in which:~ Figure 1 shows schematically the positioning of an offset pulse, Figure 2 shows schematically the positioning of serration pulses with respect to a second synchronising pulse, and Figure 3 is a block diagram showing a method of generating, shaping and combining various signals required for the system.

Upon referring to the drawings it will be seen that an external drive is fed into a vertical drive input amplifier. The duration of the vertical drive is usually about 21 lines or 21 H (where H is the line duration). The required pulsewidth for the offset pulse is 2 H for PAL and 2.5 H for NTSC since the vertical interval is 2.5 H for PAL and 3 H for NTSC, and because the trailing edge of the offset pulse must coincide with the last vertical synchronising pulse it may be seen that the leading edge of the offset pulse must be delayed by the duration of one vertical sync. pulse, as shown in Figure 1.

This delay is accomplished by differentiating the vertical drive pulse. The time constant of the differentiation network is made variable, in order to obtain the required pulse width. The differentiated pulse is inverted in a NOR gate and fed to a trailing edge triggered monstable multivibrator.

The output pulse width is variable (with a typical range of 2.0-2.5 H) in order to adjust the exact pulse width. The pulse is amplified and inverted (i.e. made positive) and applied to the output of the video amplifier. Current summing into the video amplifier load resistor or input device then occurs.

The 2.5-3.0 H gate pulse is derived in a similar manner, except that the monostable multivibrator is triggered from the leading edge.

The gate has two input signals, i.e. comp. sync.

and the gate pulse. The output of the gate consist of four serration pulses only. These serration pulses are delayed so that the trailing edge coincides with the leading edge of the second vertical synchronising pulse as shown in Figure 2.

Their required duration is about 0.5 u/sec.

The delayed serration pulses are inverted, amplified and fed to the summing network at the output of the video amplifier.

Because of the nature of the modified vertical interval it is essential to employ backporch clamping. Line clamping pulses are obtained by delaying comp. sync. line pulses and by inhibiting the vertical sync. pulses during the vertical sync.

pulse period. The clamping gate is operated by the same pulse as that for deriving serration pulses. This gate will hold the blanking level constant.

Thus, it will be appreciated that during transmission, if the output of the video amplifier is fed into a T.V. transmitter, this will render recording of an off-air signal impossible.

The system as applied to a digital processor is identical with the analogue system above described. The microprocessor is programmed to modify the signal as before.

Claims (5)

Claims

1. A system of transmitting signals from video tapes incorporating means for producing modified field synchronising pulses, giving rise to a delay in the start of vertical drive pulses, whereby the correlation between line and field synchronising pulses is destroyed, and means for recording said modified synchronising pulses on a control track of a video tape recorder to serve thereon as a timing reference, so correlating the tape lateral speed and the scanner head rotational speed of said recorder.

2. A system as claimed in claim 1, wherein said recorder includes a braking servo which controls video scanner head rotation whereby, as the first field of a pre-recorded frame is positioned for playback at the scanner head, a control track pulse is read out at the scanner head, this event being compared with a pulse generated at the field rate in the scanner head to signal the position of said head, and means for producing an increased brake current to retard said head position in response to the comparison if the head drum speed is too high, or advance said head position if the head drum speed is too low.

3. A system as claimed in claim 1 or claim 2, wherein said vertical drive pulses are fed to an input amplifier via means which provide an offset whereby the trailing edge of an offset pulse coincides with the last vertical synchronising pulse, the leading edge of said offset pulse being delayed by the duration of one vertical synchronising pulse, said delay being accomplished by means which differentiate said vertical drive pulses.

4. A system as claimed in claim 3, wherein the time constant of said differentiating means is variable to provide differentiated pulses of required width, and wherein means are provided with invert said pulses in a NOR gate and feed them to a trailing edge triggered multivibrator, the pulse output of which is amplified and inverted before being applied to the output of a video amplifier in which current summing occurs.

5. A system of transmitting signals from video tapes, substantially as described herein with reference to the accompanying drawings.