Conditional access television sound

A digitally encrypted audio signal may be transmitted with an analog television signal. Access to the analog television video signal may be controlled by using graphics overlay techniques. The use of digital encryption techniques on the audio channel prevents improper interception.

BACKGROUND

This invention relates generally to systems for conditional access to television programming.

In a conditional access television system, access to the television programming is limited to certain viewers who pay to receive the programming. In some cases, the audio and video portions of the broadcast are digitally encrypted at the broadcast head end and decrypted at the receiver end.

In other cases, analog broadcast systems may be utilized. In analog systems, a graphics overlay may be provided at the head end over the analog television programming to make it difficult for one to watch the programming. Conventionally, the overlay provides distortions and horizontal lines that disrupt the picture so that very little content can be discerned.

Analog conditional access television systems generally do not distort the audio. In some cases, the audio may have considerable value. Thus, it would be desirable to also distort the accompanying audio information so that only authorized receivers may enjoy the audio.

Human perception of sound is much more precise than human visual perception. Artifacts at an acceptable level for the video portion of an analog transmission may be far too large for the audio portion. Moreover, analog television provides an audio channel as a frequency modulated (FM) subcarrier with only 0.75 MHz bandwidth.

There are numerous sources of noise and distortion in television broadcasts. These imperfections may disrupt the clean decoding of the audio signal. The most serious artifact is ghosting or multi-path transmission. Ghosting results at the receiver when the primary, first arriving signal is then followed by several delayed, weaker signals. These weaker signals may have been reflected off of intervening obstructions such as buildings, mountains or other structures. The primary, first arriving signal and the weaker signals sum together at the radio frequency input to the television decoder.

Each time a ghost signal is added to the television signal mix, the implicit value of an encoded signal bit is changed. Thus, naïve decoding of the audio signal produces large artifacts due to the ghost signals.

Even in cable distribution systems, improper signal splitters, bad cable terminations and other defects produce echoes that have the same effect as multi-path transmission over the air. As a result, these types of distortions may be prevalent in both wireless and wired transmissions.

Thus, there is a need for better ways to provide an analog conditional access television system that adequately protects the audio portion of the television transmission.

DETAILED DESCRIPTION

Referring toFIG. 1, a conditional access analog television broadcast transmitter (e.g., TV broadcaster)10may transmit, from an antenna12, subcarriers modulated by analog video information18and digital audio information20. The analog video information18may be in a format conventionally used for wired or wireless television broadcasts. An example of a wireless television broadcast is an airwave broadcast and an example of a wired broadcast is a cable system television broadcast.

In accordance with one embodiment of the present invention, the audio information20is digitally encoded, compressed and then broadcast on a subcarrier with the analog video information18broadcast on an accompanying subcarrier. The transmitted television information (audio and video) may be received by antennas or other video input devices14associated with conditional access television receivers (e.g., analog TV receiver)16. A large number of television receivers16may be coupled to the audio/video transmission through a conventional antennas or through cable distribution system.

Referring toFIG. 2, the signal processing of the transmitter10begins by receiving an audio input signal, a video input signal and a clock input signal. The clock signal is utilized to generate an audio subcarrier and a video subcarrier in the carrier generation block104. The audio input signal is filtered at filter block100and then processed in audio processing block102.

At the same time, the video input signal is filtered in the filter block106and modulated in the video modulator block108using the video subcarrier to produce an analog video signal18. The analog video signal18and the digital audio signal20are added together in an adder110. After the signals are filtered at the filter block112, they may be broadcast from the antenna12.

On the receiver16end, the signal from the antenna or other video source14is subjected to video detection in the video detector block114inFIG. 3. The video detector114separates the audio signal20from the video signal18. The video signal is subject to further video processing in the video processing block116.

A variety of techniques may be used to obscure the analog video signal18to prevent interception and viewing by unauthorized persons. In one embodiment, shown inFIG. 4, the video frames202to be broadcast by the transmitter10are received by an obscuration block220. The block220further receives a graphics pattern204from a graphics pattern generator230. The block220adds the graphics pattern204to the frame202, to produce an obscured frame210. A plurality of obscured frames210may comprise an obscured analog video signal212.

The graphics pattern generator230further produces a pattern identifier206, to be received by an encryption block232. In one embodiment, a new pattern identifier is created for each graphics overlay pattern that is generated inside the transmitter10. The encryption block232encrypts the pattern identifier206, to produce an encrypted pattern identifier214.

Encrypted pattern identifier214may then be transmitted with the obscured analog video signal212. Accordingly, adder234combines the signals212and214to produce an obscured analog video signal18that includes encrypted pattern information. In one embodiment of the present invention, the encrypted pattern identifier214is transmitted on the vertical blanking interval (VBI) of the obscured analog video signal18.

The receiver16includes a graphics subsystem34with a graphics controller320and a frame buffer312. The graphics subsystem34may receive the obscured analog video signal18from the transmitter10. The graphics controller320sends each frame of the analog video signal18to a frame buffer312. The graphics controller320extracts the encrypted pattern identifier214from the incoming signal18and sends the signal to a system memory26.

In system memory26, decryption340, followed by a pattern construction (e.g., construct pattern block)342, are performed on the pattern identifier214, in one embodiment of the invention. These operations produce a new graphics overlay pattern344, to be added to the obscured video frame18in the frame buffer312. An unobscured analog video signal22may then be sent to a display such as a television set (not shown).

Each graphics pattern204may be associated with a pattern identifier206. The pattern identifier206may be used by one or more receivers16to recover as much as the original signal208as possible.

Each frame202is coupled with a graphics pattern204to produce an obscured frame210. The pattern identifier206associated with the graphics pattern204is added to the obscured frame210, to produce an obscured and encoded signal18, prior to transmission. The transmitter10may then send the obscured signal18to a receiver16. Without removing the obscuration from the signal18, an interceptor of the signal can only view an image that is confusing and frustrating.

In the receiver16, obscuration removal software directs the graphics controller320to send the video frame210to the frame buffer312and the encrypted pattern identifier214to the system memory26. The obscuration removal software encrypts the pattern identifier214residing in the system memory26. From the decrypted pattern identifier, the software may further construct a graphics pattern that is complementary to the graphics pattern204created at the transmitter10.

The obscuration removal software may direct the graphics controller320to add the complementary graphics pattern to the frames in the frame buffer312. A combination of the complementary graphics pattern and the contents of the frame buffer312substantially removes the obscuration from the analog video signal18.

Referring toFIG. 5, the broadcast transmitter10generates a digital audio signal20from an analog audio signal received by analog to digital converter42. The converter42converts the audio signal into a digital form so it can be compressed in a compressor shown as the compress block44. Any of a wide variety of compression techniques may be utilized including those in accordance with The Motion Picture Experts Group compression standards MPEG-1, layer 3, International Organization for Standardisation (Geneva Switzerland) ISO/TEC 11172-3 (1993) (commonly called “MP3”). In one embodiment of the present invention, a bit stream of a 100 Kbits per second may result.

The compressed audio stream may be compressed sufficiently to be useful with the bandwidth available for the audio component of the television broadcast. The compressed audio stream may be encrypted in an encryption unit shown as the encrypt block45. Any conventional digital encryption technique may be utilized.

Next, the encrypted, compressed bit stream is modulated in a modulator shown as the modulate block46. The modulator46modulates the frequency-modulated subcarrier produced by the television transmitter10. The modulator46may also band limit the signal to avoid interference between the video and audio components. A band limited digital audio signal results that may only be perceived, if intercepted, as white noise.

Likewise, a video signal modulator (not shown) may band limit the video signal for the same reason. The analog video signal18may be modulated using conventional analog amplitude modulation techniques on the video carrier and transmitted with the audio signal20modulated onto the audio subcarrier.

The modulated audio signal may be converted into a plurality of frequency division subchannels. InFIG. 5, four such subchannels are shown for illustration purposes. Each of the subchannels is then subjected to an inverse fast Fourier transform (IFFT) in the unit48to implement Orthogonal Frequency Division Multiplexing in one embodiment.

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission technique. It separates the available spectrum into many subcarriers. Each subcarrier may be modulated by a low rate data stream. OFDM may use the spectrum efficiently by spacing the channels more closely than is possible with other techniques. This may be achieved by making all the subcarriers orthogonal to one another, preventing interference between the closely spaced subcarriers.

Conventionally, the minimum required bandwidth for speech is only three kilohertz. The allocated bandwidth in practical systems may be greater than the minimum to prevent subchannels from interfering with one another. The resulting added bandwidth enables signals from neighboring channels to be filtered out and allows for drift in the center frequency of the transmitter and receiver. As much as fifty percent of the total spectrum may be wasted through the use of extra spacing between channels.

OFDM splits the available bandwidth into many narrow channels (for example from one hundred to eight thousand). The subcarriers for each channel are made orthogonal to one another. As a result, they may be spaced very closely together. The orthogonality of the carriers means that each carrier has an integer number of cycles over a symbol period. As a result, the spectrum of each carrier has a null at the center frequency of each of the other subcarriers. This reduces the interference between subcarriers, allowing them to be spaced closely together.

Each subcarrier in an OFDM signal has a very narrow bandwidth resulting in a low symbol rate. This results in a signal having a high tolerance to multi-path transmissions. The delay spread from multi-path transmissions must be very long to cause significant signal interference in a properly designed OFDM system.

The OFDM signal is generated by first choosing the spectrum required, based on the input data and the modulation scheme. Each subcarrier is assigned some data to transmit. The required amplitude and phase of the subcarrier is then calculated based on the modulation scheme. Typical modulation schemes include differential Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM). The needed spectrum is then converted back to the time domain using an inverse Fourier transform, such as an inverse fast Fourier transform (IFFT).

A fast Fourier transform transforms a cyclic time domain signal into its equivalent in the frequency domain. This may be done by finding the equivalent waveform, generated by the sum of orthogonal sinusoidal components. The amplitude and phase of the components represent the frequency spectrum of the time domain signal. The IFFT unit48performs the reverse process, transforming the amplitude and phase of each component into a time domain signal. Each data point in the frequency spectrum used for an FFT or an IFFT is called a bin.

The orthogonal carrier used in the OFDM signal can be easily generated by setting the amplitude and phase of each bin and then performing the IFFT. Since each bin of an IFFT corresponds to the amplitude and phase of a set of orthogonal sinusoidal signals, a reverse process guarantees that the carriers generated are orthogonal.

The inverse Fourier fast transform signal is then converted from a parallel to a serial format and from a digital to an analog format, in the digital to analog converter50, resulting in a digital audio signal20. The signal20is broadcast by the transmitter10shown inFIG. 4on a subcarrier.

Referring toFIG. 6, the receiver16may include a processor22coupled to a north bridge24in one embodiment. The north bridge24is coupled between a bus28and system memory26. The bus28couples a south bridge30and a graphics subsystem34. The graphics subsystem34includes a frame buffer36.

The graphics subsystem34is coupled to a TV tuner/capture card38. The card38is coupled to an audio processing section40which receives both the audio20and video signals18from an antenna or other video input device14shown inFIG. 1. The TV tuner/capture card38is also coupled to e.g., an audio block32, such as an audio coder/decoder (CODEC) which is in turn coupled to the south bridge30.

Turning next toFIG. 7, the audio processing section40takes the audio signal20and converts it into a digital form in analog to digital converter52. The converter52also takes the serial bit stream and converts it into a set of parallel bit streams. The parallel bit streams are fast Fourier transformed in the FFT54and passed on to a demodulator such as the demodulate block56.

The demodulator56reverses the modulation previously accomplished by the modulator46. The demodulated serial bit stream signal is then decrypted in the decrypt unit57and decompressed in the decompress unit58. Finally, the audio signal is converted back into an analog form by a digital-to-analog converter60and added to the video signal22shown inFIG. 4by an adder. The resulting signal is passed on to the TV tuner/capture card38.

In high data rate communications, if the symbol period becomes smaller than the delay spread of the channel, inter-symbol interference (ISI) occurs. In multi-carrier systems, a number of data symbols are transmitted as different subcarriers in parallel, thereby increasing the symbol length. To increase the bandwidth efficiency, an orthogonal multi-carrier scheme is used in which the sub-bands are overlapping. Each sub-band only covers a smaller part of the total available frequency band. As a result, channel equalization becomes much simpler than in a single carrier system.

To reduce the inter-symbol interference, the tail of the OFDM symbol may be copied and used as a preamble. In effect, the tail serves as a cyclically extended guard interval that may be called a cyclic prefix. Referring toFIG. 8, the symbol68may be bordered by a cyclic prefix66. The cyclic prefix66may be formed by taking a portion of the symbol68and using it to form the cyclic prefix. The use of the cyclic prefix as a guard interval may simplify the channel equalization and the demodulation. It also may maintain carrier synchronization in the receiver.

In general, the guard interval allows time for multi-path signals from a previous signal to die away before the information from the current signal is gathered. Thus, the end of the symbol waveform may be placed at the start of the symbol as a guard interval, in one embodiment of the invention. This effectively extends the length of the symbol while maintaining the orthogonality of the waveform.

In a first embodiment of the present invention, an analog audio signal may be generated that can directly modulate the conventional FM subcarrier of a television transmitter10. The synthesized analog signal is limited to fifteen kilohertz of bandwidth. In a second embodiment, a waveform may be directly synthesized to replace the normal frequency modulated subcarrier. Then 0.75 megahertz of bandwidth is available.

For regional broadcasting, one may assume a reflection time of two hundred and fifty microseconds. A guard interval66time equal to the worst case multi-path delay time may be set at two hundred and fifty microseconds, for example. The guard interval66may be less than about one quarter of the symbol68duration. Thus, a one-millisecond symbol68time gives ample time for the echoes to die out allowing an accurate determination of the transmitted symbol68.

To allow a large amount of data to be transmitted, the subcarriers are closely spaced to each other's frequency. The optimal frequency spacing is one over the period of the symbol68, which equals one kilohertz in one embodiment.

Thus, in the first embodiment, which uses a conventional FM audio subcarrier, about fifteen carriers may be utilized with an aggregate baud rate of fifteen kilobaud. The baud rate combined with the need to transmit data at one hundred kilobits per second determines that each signal carries about six bits of information.

In the second embodiment, seven hundred and fifty carriers may be utilized at a rate of seven hundred and fifty kilobaud. Very simple binary keying with each symbol carrying one bit of information may be used. There are well-developed techniques for producing multi-level, multi-bit symbols.