GB2319683A

GB2319683A - Apparatus to decode an input video signal

Info

Publication number: GB2319683A
Application number: GB9624002A
Authority: GB
Inventors: Michael Bartholomew Sparks
Original assignee: NDS Ltd; Digi Media Vision Ltd
Current assignee: Digi Media Vision Ltd; Synamedia Ltd
Priority date: 1996-11-19
Filing date: 1996-11-19
Publication date: 1998-05-27
Also published as: GB9624002D0

Abstract

A video cassette recorder is a mechanical system and so has a different tolerance to video timing than a television set. To prevent unauthorised recording a deliberate dither is therefore included in the video timing in a way that a television can follow but a VCR cannot. To fingerprint an unauthorised recording, a more subtle timing dither is used which allows the VCR to record, but identifies the source of the recording. In each case the variation in video timing is achieved by applying a dither to the clock oscillator in the receiver or by applying dither to the pulse generation circuits which insert the blanking and synchronising pulses.

Description

Method and Apparatus to Decode an Input Signal The present invention relates to a method and apparatus to decode an input signal which carries encoded video information. The invention is of particular advantage in decoding an input signal in which the encoded information is digitally encoded and represents a digital video signal.

Copy control of video signals is important to programme makers to ensure they receive revenue from the distribution of their programmes. This is particularly important for newer digital broadcast services, where the ability to show programmes on a pay-per-view basis is a significant part of a broadcasters revenue. It is important that a viewer who has paid to view a programme cannot make an unauthorised record of the programme and pass it to others. Two copy control strategies are possible: copy protection in which the video signal is modified by taking advantage of the differences in operation between a video cassette recorder (VCR) and TV, so that whilst it is viewable on a TV set, it cannot be recorded on a video cassette recorder; and fingerprinting in which the video signal is modified in a more subtle way so that if it is copied, then the source of the copy can be identified.

A method for copy protection (also known as anti-taping) has already been proposed which relies on the presence in a VCR of an automatic gain control (AGC) function as part of its video input circuit, whilst a TV does not. The AGC normally uses the sync tip amplitude to measure the video signal.

Adding extra pulses to the video signal, which have the effect of making the sync pulses appear to be either too large or too small defeats the AGC and makes recording the programme difficult. However, it is a fairly simple process to remove these pulses and make the programme recordable. For a digital video system in which the output video signal is generated by a digitalto-analogue converter (DAC), the large pulses needed are a particular problem, requiring higher power supply voltages and more DAC bits.

Methods for fingerprinting operate in a similar way. Pulses representing digital information, such as the serial number of the equipment producing the video signal, are added to the video signal either in the blanking or active picture areas. The information does not need to be added very often, so it would not be apparent to the casual viewer. But again if the fingerprint is added in the blanking then it is readily removed.

It is an object of the present invention to provide an improved method and apparatus to give either copy protection or fingerprinting.

According to the present invention there is now provided a method of decoding an input signal canying encoded video information, the method comprising; deriving a clock signal referenced to the input signal, decoding the encoded information by reference to the clock signal to produce an output decoded signal, and applying a controlled timing fluctuation to the output signal relative to the input signal.

The fluctuation of the output signal may be effected by causing a fluctuation in the clock signal frequency relative to the input signal or it may be effected by causing a fluctuation in the timing of the output signal relative to the clock signal.

Further according to the present invention, there is provided apparatus for decoding an input signal carrying encoded video information, the apparatus comprising; a clock generator to generate a clock signal referenced to the input signal, a decoder to decode the encoded information by reference to the clock signal to produce an output decoded signal, and means to apply a controlled timing fluctuation to the output signal relative to the input signal.

The means to apply a controlled timing fluctuation may be means to cause a fluctuation in the clock signal frequency of the clock signal generator relative to the input signal or may be means to cause a fluctuation in the timing of the output signal relative to the clock signal.

The invention will now be described, by way of example, with reference to the accompanying drawing which shows a video signal receiver embodying the present invention.

An RFinput for a digital video signal is supplied to an RF demodulator 10 to generate an MPEG-2 Transport Stream which is itself supplied to an MPEG-2 demultiplexer 11. The video signal is transmitted in compressed format as a digital bitstream and the demultiplexed compressed video information is sent by the demultiplexer 11 to a video decoder 12.

The video decoder 12 includes a memory 13 which is used as output picture storage for the digital video signal which is produced by the decoder 12 from the compressed video input as part of the decoding process. The generation of the decoded video signal is controlled by reference to a clock signal. In the MPEG-2 receiver, the clock has a nominal frequency of 27 MHz and is locked to a similar clock at the transmitter by the programme clock reference (PCR) packets within the MPEG-2 Transport Stream.

The PCR packets are sent by the demultiplexer 11 to a comparator and filter circuit 14 which derives a control input for a video clock oscillator 15. The video decoder 12 decodes the compressed digital video signal by reference to the clock signal generated by the clock oscillator 15. The clock oscillator may be either a voltage controlled crystal oscillator (VCXO) or a numerically controlled oscillator (NCO).

The digital video signal from the decoder 12 is supplied to a PAUNTSC encoder and digital to analogue converter 16 which is used to generate the video output signal under the control of the video clock signal. Because the transmitted bitstream has information relating only to the active part of the picture, all the blanking and synchronising parts of the output video waveform are generated by digital blanking and sync pulse circuits within the converter 16.

A dither signal is generated in the circuit 14 in order to cause a fluctuation in or modulation of the control applied to the clock oscillator 15 which in tum will modulate the video clock frequency and hence the timing of the output video.

In the case of a numerically controlled oscillator, the clock frequency may be under software control so that adding the dither is a software task and has little impact on the cost of the receiver.

As an alternative to modulating the clock frequency, the pulse generation circuits in the block 16 can be made to produce a fluctuation or modulation of the duration and/or number of pulses. Changing the number of blanking and sync pulses would have the effect of producing a video signal of a different line standard, for instance by changing the number of lines per television field or frame. A combination of clock modulation and pulse variation can be used.

A VCR is a mechanical system, and has a different tolerance to video timing compared to a television. For copy protection, the deliberate dither applied to the video timing is applied in a way such that a television can follow the dither but a VCR cannot. Whereas the existing copy protection methods using pulse techniques are easy to defeat by removing the pulses, a time variation is more difficult to remove because the video signal must be stored and this is relatively expensive.

For fingerprinting, the variation of the timing is chosen such that the video signal can be recorded on a VCR but it can be detected and recognised.

Such fingerprinting is difficult to detect and remove since the timing variations are subtle and not readily seen with standard test equipment.

When the clock oscillator 15 is implemented using a numerically controlled oscillator, the phase comparator and filter function may be implemented in software and the control signal between the blocks becomes data written into a register. The oscillator frequency is therefore set by software and it is possible to vary the clock frequency (and therefore the video timing) by precise amounts under software control. This allows complex timing variations to be added which are difficult to remove transparently.

These methods and apparatus which have been described are particularly suited to MPEG-2 receivers since the signal is stored in a compressed form as part of the normal decoding process. This allows use of the storage inherent in the receiver in generating an output signal with a timing fluctuation with minimal added circuitry.

Claims

1. A method of decoding an input signal carrying encoded video information, the method comprising: deriving a clock signal referenced to the input signal, decoding the encoded information by reference to the clock signal to produce an output decoded signal, and applying a controlled timing fluctuation to the output signal relative to the input signal.

2. A method as claimed in claim 1, wherein the fluctuation of the output signal is effected by causing a fluctuation in the clock signal frequency.

3. A method as claimed in claim 1, wherein the fluctuation of the output signal is effected by causing a fluctuation of the timing of the output signal relative to the clock signal.

4. A method as claimed in any one of the preceding claims, which is applied to decoding an input signal which is a video signal transmitted in compressed format as a digital bitstream.

5. Apparatus for decoding an input signal carrying encoded video information, the apparatus comprising: a clock signal generator to generate a clock signal referenced to the input signal, a decoder to decode the encoded information by reference to the clock signal to produce an output decoded signal, and means to apply a controlled timing fluctuation to the output signal relative to the input signal.

6. Apparatus as claimed in claim 5, wherein the means to apply a controlled timing fluctuation is adapted to cause a fluctuation of the clock signal frequency from the clock signal generator relative to the input signal.

7. Apparatus as claimed in claim 5, wherein the means to apply a controlled timing fluctuation is adapted to cause a fluctuation of the output signal relative to the clock signal.

8. Apparatus as claimed in any one of the claims 5 to 7, adapted to receive and decode an input signal in the form of a video signal transmitted in compressed format as a digital bitstream.