Abstract:
A novel method and apparatus for scrambling a television signal includes using a synchronization separator to separate a synchronization signal from an input audio-visual signal. The synchronization signal is then used to regulate an oscillator producing an audio carrier signal. In this manner, the audio-visual signal is transmitted without the component synchronization signal rendering it useless to a conventional television receiver. The descrambler and descrambling method of the invention involve removing the audio carrier signal from the scrambled signal and using the audio carrier signal to generate the color sub-carrier signal and a vertical time frame reference signal necessary to synchronize the transmitted audio visual signal for display.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of audio-visual signal transmission and reception. More particularly, the present invention relates to a method and apparatus for scrambling an audio-visual signal during transmission in order to control reception and use of the signal. 
     BACKGROUND OF THE INVENTION 
     Cable and pay-per-view television programming are extremely popular and widely available in modern society. Those firms providing such programming are frequently lucrative and successful. This success comes from the ability to prevent those who have not purchased the service from receiving and using the audio-visual signals transmitted by the service provider. 
     In general, it is common for a cable company or other service provider to scramble its signals to prevent their use by unauthorized parties. This scrambling is intended to make the signals unintelligible to typical television sets which are not authorized to receive and use the scrambled signal. When the signal is scrambled, a conventional television will not recognize the signal, or at best, display a distorted image from the signal. 
     Cable companies which provide a number of channels over a cable connection may only scramble certain channels so that cable subscribers can be required to pay additional fees for receiving these “premium” channels. The premium channels are typically movie or adult channels. 
     Those subscribers who have paid the service provider for reception of the scrambled signal will be given equipment for use with their television sets which unscrambles the signal. With this equipment, the authorized user can unscramble and receive the service provider&#39;s signals and view the programming contained in the transmission. 
     A well-known conventional method of scrambling a signal to prevent its unauthorized reception is to reverse the spectral positions of the audio and video portions of the signal. Typically, an audio-visual signal will have an audio carrier signal at a first frequency and a video carrier signal at a second frequency. The carrier signals are modulated to carry the respective video and audio data of the transmitted programming. 
     If, however, the audio carrier signal is transmitted at the second (video carrier) frequency, and the video carrier signal is transmitted at the first (audio carrier) frequency, the signal is scrambled. A television tuner will be attempting to receive the audio signal at the first frequency and the video signal at the second frequency, and will be unable to receive the reversed signal properly. 
     Many other methods of scrambling audio-visual signals during transmission are known in the art. However, all are subject to various deficiencies. For example, many scrambling methods are only partially effective. Thus, while the scrambled signal is not received clearly, the image and audio may still be intelligible. 
     Accordingly, there is a need in the art for an improved and more effective means and method of scrambling an audio-visual signal. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide a method and apparatus for scrambling an audio-visual signal and for receiving and unscrambling the scrambled signal that is highly effective. 
     Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims. 
     To achieve these stated and other objects, the present invention may be embodied as a method of transmitting a scrambled electronic audio-visual signal by generating a digital audio signal component of the audio-visual signal having a time base which is common to a synchronization signal of a video signal component of the audio-visual signal and a color sub-carrier signal of the video signal. Prior to generating this audio signal, the method of the present invention includes removing the synchronization signal from the video signal. The color sub-carrier signal is also preferably removed prior to broadcast of the scrambled audio-visual signal. 
     In conjunction with the creation of the common time base of the audio signal, the method of the present invention includes inserting a fixed digital message into a vertical interval of the video signal which is used to reintegrate the synchronization signal with the video signal. 
     When the scrambled audio-visual signal is received, the method of the present invention includes reconstructing the synchronization signal and the color sub-carrier signal from the common time base of the audio signal. Specifically, the steps of this inventive method may be broadcasting the video signal and the audio signal in composite; receiving the composite signal; obtaining the common time base from the audio signal; and reconstructing the color sub-carrier and the synchronization signal from the common time base. 
     Following reconstruction of the signal components necessary to properly display the audio visual signal, the method includes reintegrating the reconstructed synchronization signal and the reconstructed color sub-carrier signal with a video signal component of the received composite signal. 
     The present invention also encompasses the circuitry necessary to apply the foregoing method. Specifically, the present invention encompasses an audio-visual signal scrambler having: a synchronization separator for receiving a clear video signal and removing therefrom a synchronization signal; and an oscillator and phase locked loop circuit for receiving the synchronization signal and using the synchronization signal to generate a time base common to the synchronization signal, a color sub-carrier signal and an audio signal of the audiovisual signal. 
     The scrambler of the present invention also includes an audio encoder for generating the audio signal by encoding input audio signals to an audio carrier signal having the common time base; and a video encoder for inserting a fixed digital message into a vertical interval of the video signal which is used to reintegrate the synchronization signal with the video signal. 
     The oscillator and phase locked loop circuit includes a first reference oscillator regulated by the synchronization signal; a second reference oscillator; and a phase comparator for comparing signals from the first and second reference oscillators. The second reference oscillator is regulated by a signal output by the phase comparator and low pass filtered through a low pass filter. 
     A first divider circuit is provided for dividing the output of the first oscillator to generate a reference signal on which the fixed message is based; and a second divider circuit is provided for dividing the output of the first oscillator before the output of the first oscillator is input to the phase comparator. 
     The present invention also encompasses a descrambler for descrambling a scrambled audio-visual signal. Such a descrambler includes a tuner for receiving the scrambled audio-visual signal; a QPSK demodulator for removing a common time base from an audio carrier signal of the audio-visual signal; a clock and timing recovery circuit for receiving the common time base and generating therefrom a vertical time frame reference signal and a color sub-carrier signal phase locked to the audio carrier signal; and a video decoder connected to a synchronization and color burst reconstruction circuit for unscrambling a video signal component of the audiovisual signal using the color sub-carrier signal and the vertical time frame reference signal. 
     Preferably, the descrambler of the present invention also includes a video demodulator connected between the tuner and the video decoder; and an audio decoder for decoding an audio component of the audio-visual signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate the present invention and are a part of the specification. Together with the following description, the drawings demonstrate and explain the principles of the present invention. 
     FIG. 1 is block diagram showing a signal encoder for scrambling an audio-visual signal according to the principles of the present invention. 
     FIG. 2 is a block diagram showing a signal decoder for unscrambling an audio-visual signal according to the principles of the present invention. 
     FIG. 3 is a block diagram illustrating the oscillators and phase locked loop circuit of FIG.  1 . 
     FIG. 4 is a block diagram illustrating a portion of the decoder of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Under the principles of the present invention, a composite television signal is modified to inhibit clear and stable reception by unauthorized standard television receivers or decoders. All horizontal and vertical synchronizing portions of the composite signal, and all the color sub-carrier bursts are replaced with a nonstandard signal format. This non-standard portion of the signal format may be a constant luminance level, or may be some form of in-band auxiliary data. 
     Also, the standard FM audio sub-carrier is replaced with a digital QPSK (quadrature phase shift keyed) audio sub-carrier. In order to convey a time reference for horizontal and vertical line position, a digital frame rate bit pattern is inserted in one line of the vertical interval. 
     At the encoder, the composite video time base and the digital audio QPSK sub-carrier time base are derived from a single clock source with a frequency F S . The composite video time base and the digital audio QPSK sub-carrier time base include vertical and horizontal synchronizing information, the color sub-carrier frequency, and all timing information required for the digital QPSK audio transmission. 
     The reference frequency F S  is N times the color sub-carrier and is also directly related to the time base for the digital audio sub-carrier encoder and decoder. The digital audio clock source crystal oscillator produces a frequency F A  which is locked to the frequency F S  through a phase-locked loop. The signal F S  is divided by M and is compared to F A  divided by P. In this way, the common time base is inherent in the digital audio QPSK sub-carrier. All timing for the digital audio sub-carrier system is derived from the clock signal F A , which is locked to F S . 
     At the decoder, the frequency F A  is recovered from the received QPSK signal. A clock signal with a frequency F 1  is derived from F A  by dividing by P. The time base clock signal of frequency F S  is recovered using F 1  by means of a phase locked loop. The color sub-carrier is derived from F S  by dividing by N. The fixed reference pattern in the vertical interval is decoded in order to facilitate reconstruction of all video synchronizing pulses and color sub-carrier bursts. 
     Using the drawings, the preferred embodiments of the present invention will now be explained. 
     FIG. 1 illustrates an encoder for scrambling a television signal according to the principles of the present invention. A clear, unscrambled video signal  103  and left and right audio signals  107  and  108  are input to the encoder. The video signal  103  is processed by a synchronization separator circuit  101  which removes a synchronization component from the signal which provides all the synchronization information necessary for a television receiver to utilize the signal. The video signal, stripped of the synchronization signal, is then provided to a video encoder  102 . 
     The synchronization signal removed from the video signal  101  is provided to a circuit  105  which includes reference oscillators and a phase locked loop circuit. Using the signal from the synchronization separator  101 , this circuit  105  provides a reference signal with frequency F S  to the video encoder  102 . 
     The encoder  102  encodes the video signal by removing the color sub-carrier signal. The encoder  102  also uses the reference signal F S  to insert a fixed digital message (a serial bit pattern of 24 to 30 bits) into the vertical interval of the video signal. This fixed digital message establishes a known point in time in the video frame which can be used to restore the video synchronization information to the video signal following reconstruction of the synchronization information by a decoder receiving the broadcast of the scrambled television signal. The encoder  102  then outputs an encoded video signal  104 . 
     The oscillator and phase locked loop circuit  105  also outputs a second reference signal with frequency F A . The signal F A  is provided to an audio encoder  106 . The audio encoder  106  also receives the left and right audio signals  107 ,  108  and uses the signal F A  to synchronize the time base of the audio bit stream with a reference oscillator  301  (FIG. 3) which is synchronized with the synchronization signal from the separator  101 . 
     As shown in FIG. 1, the audio encoder  106  includes a pair of analog-to-digital (A/D) converters  111  and  112  which respectively receive the left and right audio signals  107  and  108 . After conversion to a digital signal format, the left and right audio signals  107  and  108  are output respectively by the A/D converters  111  and  112  to a Dolby AC-1 audio encoder  113 . 
     The output of the audio encoder  113  is input to a digital encryption and formatting circuit  114 . The encryption and formatting circuit  114  also receives the signal F A  as the audio sub-carrier signal from the oscillators and phase locked loop circuit  105 , and encodes the audio data to the new sub-carrier signal F A . 
     The output from the digital encryption and formatting circuit  114  is then provided to a QPSK modulator  115  which replaces the standard FM audio sub-carrier with a digital QPSK audio sub-carrier. The modulator  115  then outputs an encoded audio signal  116  with a QPSK audio sub-carrier based on F A . Consequently, the time base of the encoded QPSK audio signal is common to the synchronization signal output by the separator  101  and to the color sub-carrier signal. 
     The encoded audio signal  116  is sent from the audio encoder  106  to an adder  109 . The encoded video signal  104  is also provided to the adder  109 . The adder  109  then generates a composite encoded signal  110  for transmission. The signal  110  is an intermediate frequency (IF) signal which is subsequently converted to a radio frequency (RF) signal for broadcasting. 
     In its encoded form, unauthorized television receivers will be unable to make use of the broadcast signal  204 . The signal does not contain the standard audio carrier frequency or the standard color sub-carrier frequency. Moreover, no synchronization data necessary for an unaided television receiver to intelligibly display the images carried by the signal is provided. Rather, the color sub-carrier, the composite synchronization signal and the time base of the QPSK audio signal are all synchronized with the reference oscillator  301  of the oscillator and phase locked loop circuit  105 . This common time base provided by the reference oscillator  301  is later extracted from the QPSK sub-carrier and used to reconstruct the information necessary to properly decode and display the composite television signal. 
     FIG. 2 illustrates a decoder according to the principles of the present invention for decoding a broadcast television signal encoded by the encoder of FIG.  1 . The scrambled RF signal  204  is received by a tuner  201  of a television receiver which is tuned to received the encoded signal  204 . 
     The tuner  201  provides the encoded signal to a video demodulator  202  and a QPSK demodulator  207 . The QPSK demodulator  207  recovers the common time base to which the composite sync signal, color sub-carrier and digital audio bit stream are all synchronized. This common time base gives rise to the reference signal F A . Then the QPSK demodulator  207  provides the audio portion of the signal to an audio decoder  209  and the reference signal F A  to a clock and timing recovery circuit  208 . 
     Using the reference signal F A  from the QPSK demodulator  207  and the output of the video demodulator  202 , the clock and timing recovery circuit reconstructs the synchronization information needed to accurately display the scrambled television signal. Specifically, the clock timing and recovery circuit  208  provides an audio recovery signal to the audio decoder  209  which directs the audio decoder  209  to the proper audio carrier signal frequency so that the decoder  209  can decode the audio signal provided by the QPSK demodulator  207  and provide a clear audio signal  210  for use by a television receiver. 
     The clock timing and recovery circuit  208  also reconstructs reference signal F S  and a vertical time (frame) reference signal. These two signals are provided to the synchronization and color burst reconstruction circuit  205 . The circuit  205  exchanges data with the video decoder  203  to generate a clear video signal  206  which can be accurately displayed by a television receiver. 
     FIG. 3 details the oscillators and phase locked loop circuit  105  of FIG.  1 . As shown in FIG. 3, the signal  313  from the synchronization separator  101  is provided to a phase comparator  315 . The phase comparator  315  compares the phase of the signal  313  with the phase of a reference signal from a reference oscillator  301 . In the specific example illustrated in FIG. 3, the reference oscillator  301  produces a signal at 46.534085×10 6  Hz. 
     The results of the comparison are output through a low pass filter  314  to the reference oscillator  301  to lock the phase of the reference oscillator to the phase of the signal  313  from the synchronization separator. The reference signal from the reference oscillator  301  is then provided to a divider  302  which divides the reference signal by a divisor N. In the illustrated example, the divider  302  divides the signal by 13 to produce a signal  303  at 3.579545×10 6  Hz. This signal is the reference signal F S  and is by the encoder  102  to mark the video signal with a known point in time so that the synchronization information removed by the separator  101  can be reintegrated properly to with the video-signal by the decoder  203  and reconstruction circuit  205 . 
     The reference signal from the oscillator  301  is also provided to another divider which divides the frequency by a divisor of M. In the specific example illustrated, M=3487 and the division is accomplished by a series of two dividers, a first divider  304  dividing by M 1 , where M 1 =11, and a second divider  305  dividing by M 2 , where M 2 =317 (11×317=3487). The resulting reference signal has frequency F 1 , in the example 13.3450201 MHz. The F 1  signal is provided to a phase comparator  306  of a phase locked loop. 
     A second reference oscillator  307  in circuit  105  provides a reference signal which, in the illustrated example, has a frequency of 15.266703×10 6  Hz. This signal is the basis for the reference signal with frequency F A  described above. 
     The output of the oscillator  307  is divided by P. In the illustrated example, P=1144 and the division is accomplished by a series of two dividers. A first divider  312  divides the reference signal from oscillator  307  by P 1 , where P 1 =2, and a second divider  312  divides the resulting signal by P 2 , where P 2 =572 (2×572=1144). The resulting signal again has frequency F 1 , which in the example is 13.3450201 MHz. The F 1  signal from divider  312  is also provided to the phase comparator  306 . 
     The phase comparator  306  compares the F 1  signals provided respectively from dividers  305  and  312  and outputs a signal representing the result of the comparison to a low pass filter  310 . The low pass filter filters the signal and provides a resulting signal V c  to the reference oscillator  307  to regulate the oscillator  307 , thereby completing the phase locked loop. 
     The reference signal output by oscillator  307  is also provided to a divider  308  which, in the illustrated example, divides the signal by 2. The resulting signal is the reference signal with frequency F A  and is output to the audio encoder  106  as a digital audio QPSK carrier signal. In the illustrated example, F A =7.6333515×10 6  Hz. 
     Consequently, a common time base between the audio and color sub-carriers is inherent in the reference signal F A  which is locked to the reference signal F S . All the timing for the digital audio sub-carrier system is derived from the reference signal F A . 
     Because the oscillator  301  is regulated by the synchronization signal separated from the clear video signal  103  by the separator  101 , and because the phase of oscillator  307  is locked to that of oscillator  301 , the output of oscillator  307 , reference signal F A  used as the audio carrier signal, can be used by the decoder of FIG. 2 to reconstruct the synchronization data and color sub-carrier signal necessary to properly display the encoded video signal  104 . 
     FIG. 4 illustrates the details of the clock and timing recovery circuit  208  of FIG.  2 . As shown in FIG. 2, the QPSK demodulator  207  recovers the audio carrier signal F A  from the scrambled signal  204  and provides that signal to the clock and timing recovery circuit  208 . Within the clock and timing recovery circuit  208 , the signal F A  from the QPSK demodulator  207  is provided to a QPSK clock recovery circuit  401 , which divides the signal with frequency F A  by 2 to generate a reference signal with frequency F 1 . The F 1  signal is input to a phase comparator  402 . 
     A reference oscillator  404  identical to the oscillator  301  in FIG. 3 is also provided within in the circuit  208 . The oscillator  404  outputs a reference signal which, in the illustrated example is again 46.534085×10 6  Hz. The output of oscillator  404  is divided by divider  407 . In the illustrated example, divider  407  divides the frequency of the reference signal from oscillator  404  by N, where N=13. The resulting signal  408  is the color sub-carrier signal with frequency F S , e.g. 3.579545×10 6  Hz, which is provided to the synchronization and color burst reconstruction circuit  205  for descrambling the video signal. 
     To accurately generate the reference signal F S , the output from the reference oscillator  404  is also provided through a series of two dividers to the phase comparator  402 . The two dividers divide the signal frequency by M. In the illustrated example, M=3487, and the division is accomplished by the first divider  304  dividing by M 1 , where M 1 =11, and a second divider  305  dividing by M 2 , where M 2 =317 (11×317=3487). The resulting reference signal has frequency F 1 , in the example 13.3450201 KHz. The F 1  signal from divider  406  is provided to the phase comparator  402 . 
     The comparator  402  compares the F 1  signal from divider  406  with the F 1  signal from circuit  401 . The signal resulting from the comparison is low pass filtered by filter  403 . The result V C  is used to regulate the oscillator  404 , locking the generated color sub-carrier of frequency F S  to the F A  signal recovered by the QPSK demodulator  207 . 
     The preceding description has been presented only to illustrate and describe the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. 
     The preferred embodiment was chosen and described in order to best explain the principles of the invention and its practical application. The preceding description is intended to enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.