Abstract:
A digital television receiver that decodes multiple programs concurrently to reduce the amount of time needed to display an adjacent channel in a channel surfing operation includes a single transport decoder and a single video/audio decoder. The transport decoder decodes multiple transport streams in a single channel. Each transport stream represents a respectively different minor channel transmitted in the major channel. When a channel change operation occurs, the audio/video decoder selects the next transport stream that is provided by the transport decoder. In another embodiment of the invention, the single video/audio decoder includes multiple buffers and concurrently decodes multiple program streams. When a program switch occurs, the program stream for a next buffer is provided to the video display. In yet another embodiment, the television receiver includes multiple tuners, each capable of receiving a respective channel. Each tuner is coupled to a respective transport decoder and each transport decoder is coupled to a respective decoder. In this embodiment of the invention, when a channel switch occurs, the video signal is immediately available at the output of one of the decoders and is displayed. At the same time, one of the tuners is tuned to the next channel in the sequence in anticipation of a subsequent channel switch operation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns digital television receivers and in particular a receiver including multiple program decoders and/or multiple tuners to allow programs from successive channels to be quickly displayed and selected. 
     Digital television programming is available from many sources, these include satellite broadcasts, digital cable systems and terrestrial broadcasts systems. A viewer connected to a digital cable system may, for example, be able to receive up to 500 channels. This is possible because the digital television signals are compressed to eliminate redundancies from line to line and frame to frame and, thus, the required bandwidth needed to transmit the signals is also reduced. 
     The compression technology used to achieve this efficiency, however, also makes the signals difficult to decode and display. A typical motion compensated predicted frame, for example, is encoded in many steps. First corresponding image data from a previous frame, as determined by a motion compensation processor, is subtracted from the image data in the current frame. Next, this difference data is encoded using a discrete cosine transform process. The discrete cosine transform coefficients are applied to a variable quantiser to reduce the number of bits used to represent each coefficient. Next, the quantized coefficients are processed through a variable length coder and run length coder to further reduce the number of bits needed to represent the video signal. The resulting bit-stream is packetized into variable-length program elementary stream (PES) packets which are further decomposed into fixed-length transport packets. The packetized bit-stream is further annotated with information needed to recover the bit-stream from the packets. Finally, forward error correction code information is added to the signal. The encoded signal is then modulated onto a radio frequency (RF) virtual side band carrier signal for terrestrial broadcast video systems or on to a quadrature amplitude modulated carrier for cable broadcast systems. 
     The decoding operations to reproduce the video signal must reverse the operation performed by the encoder in order to produce a decoded signal. A typical digital television receiver locks a tuner on to the pilot signal of the RF digital television signal then it demodulates the RF television signal to produce a baseband encoded television signal. Next, the forward error correction coding is processed to recover the transport packets. The transport packets are then processed to reproduce the PES packets and the PES packets are processed to reproduce the elementary bit-stream. The elementary bit-stream for the video signal is applied to a variable length decoder and run-length decoder which produce fixed length code values representing the frequency domain coefficients as well as other side information (e.g. motion vectors) needed to decode the video signal. Next, the fixed length coefficient values are applied to an inverse quantiser and to an inverse discrete cosine transform processor to reproduce pixel values. 
     If the frame being decoded is a motion compensated frame, the pixel values are differential pixel values that are referenced to a pixel values in a prior frame or field as indicated by a motion vector. This motion vector is received as part of the side information in the transmitted signal. The pixels represented by the motion vector are recovered and added to the differential pixels in order to reproduce the video signal. 
     Similar digital encoding techniques are used for the audio portion of the television signal. 
     The complex coding scheme used for digital television programming produces great coding efficiency but causes delay in the reproduction of a video signal. The time between when a television tuner is first tuned to a digital television channel and the time that the video information is displayed may be as long as 1 to 4 seconds. 
     Channel surfing is a process by which a viewer successively tunes a television receiver to one channel after another in order to find a program that he or she wishes to watch. On conventional analog television receivers, the time between the display of images for successive channels in a channel surfing operation is less than one second. The increase in the amount of time needed to display images from successive digital television programs may frustrate even the most patient channel surfer. 
     In order to over come the problem of channel surfing many cable systems provide electronic program guides (EPG). These guides allow a viewer to bring up a description of programming currently available on multiple channels, typically in a grid form. By visually scanning the grid a viewer may determine the programming on any of a number of channels and tune to a channel by selecting an item from the grid. It is difficult, however, to succinctly describe a television program in a grid format. Consequently many viewers perceive these guides as boring or of limited utility. 
     Another system which has been proposed is described in U.S. Pat. No. 5,532,748 entitled Hybrid Analog/Digital Television Transmission System. This patent describes a system that includes an analog video signal along with the digital television signal to produce a hybrid television signal for use during channel change operations. When a viewer using this system changes channels the low-resolution analog signal is displayed at first followed by the digital signal when the digital decoder has enough information. This system, however, does not allow digital systems to retain the high quality picture during channel surfing which users have come to expect from digital systems. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a digital television receiver that decodes multiple programs concurrently to reduce the amount of time needed to display an adjacent channel in a channel surfing operation. 
     One embodiment of the receiver employs a single transport decoder and a single video/audio decoder. The transport decoder concurrently decodes multiple transport streams in a single channel. Each transport stream represents a respectively different minor channel transmitted in the major channel. When a channel change operation occurs, the audio/video decoder selects the next transport stream that is provided by the transport decoder. 
     According to a second embodiment of the invention, the single video/audio decoder includes multiple buffers and concurrently decodes multiple program streams. When a program switch occurs, the program stream for a next buffer is provided to the video display. 
     According to another embodiment of the invention, the television receiver includes multiple tuners, each capable of receiving a respective channel. Each tuner is coupled to a transport decoder and each transport decoder is coupled to a respective video/audio decoder. In this embodiment of the invention, when a channel switch occurs, the video signal is immediately available at the output of one of the decoders and is displayed. At the same time, one of the tuners is tuned to the next channel in the sequence in anticipation of a subsequent channel switch operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a television receiver according to a first embodiment of the invention. 
     FIG. 2 is a block diagram of a television receiver in accordance with a second embodiment of the invention. 
     FIG. 3 is a block diagram of a television system in accordance with the third exemplary embodiment of the invention. 
     FIG. 4 is a block diagram of a television decoder system in accordance with a fourth embodiment of the invention. 
     FIG. 5 is a flow chart diagram that is useful for describing the embodiment of the invention shown FIGS. 3 and 4. 
     FIG. 6 is a flow chart diagram which is useful for describing the embodiments of the invention shown in FIGS. 1 through 4 or for a software implementation of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram of a first exemplary embodiment of the invention. The television receiver shown in FIG. 1 includes a single antenna  110  and a tuner  112 . Although the antenna and tuner are shown as receiving a terrestrially broadcast digital television (DTV) signal, it is contemplated that the tuner  112  may receive cable signals or signals from a satellite receiver (not shown). 
     The tuner  112  demodulates the DTV signal and applies the baseband DTV signal to a transport decoder  114 . In the exemplary embodiment of the invention the transport decoder  114  includes two channel buffers  116  and  118 . Each channel buffer includes a buffer memory holding respective audio, video and data channels for a respective television program. In addition to the channel buffers  116  and  118 , the transport decoder  114  may include additional channel buffers such as buffer  120  shown in phantom. 
     When the television receiver receives a multi-program channel, the transport decoder  114  applies signals from respective ones of the programs to the respective channel buffers  116 ,  118  and, optionally,  120 . Buffer  116  may, for example, hold the audio, video and data channels for the last minor channel in the multi-channel bit-stream, channel buffer  118  may hold audio, video and data programs for the first minor channel in the multi-channel stream and channel buffer  120  may hold the audio, video and data programs for the second minor channel. 
     The transport decoder  114  is controlled by a microprocessor  152  which is coupled to an infra red (IR) receiver  158  or to controls (not shown) on the front panel (not shown) of the television receiver. The IR receiver  158  receives command signals from a remote control device  160 . When the user presses a channel up button on the remote control device  160 , the IR receiver receives the command and provides it to the microprocessor  152 . Likewise, when the user presses the channel up button on the front panel, the microprocessor  152  receives the command directly. Microprocessor  152 , in response to the channel-up command, causes the tuner  112  to tune to the next higher frequency as indicated by the channel map  156  in a memory  154  coupled to the microprocessor  152 . If the new channel tuned by the tuner  112  is a multi-program channel, the transport decoder  114  immediately begins to decode two or three of the minor channels in the multi-program bit-stream. 
     Data for all two or three concurrently decoded minor channels is provided to an AC3 audio decoder  122  and an MPEG video decoder  134 . The exemplary AC3 decoder  122  includes two channel buffers  124  and  126  and an optional third channel buffer  128 . Each of these channel buffers holds a decoded audio signal. When the viewer selects a particular minor channel from the multi-program bit-stream, the AC3 decoder  122  is configured to provide the decoded audio signal for the selected channel from one of its buffers  124 ,  126  or  128  to the audio processor  130 . The audio processor controls the audio signal, for example, by equalization or volume control to produce an audio output at the speakers  132 . 
     Transport decoder  114  simultaneously provides the video and data components of the decoded television signals to the MPEG decoder  134 . MPEG decoder  134  stores the data received from the transport decoder into respective video buffering verifier (VBV) buffers  136  and  138 , and optionally  140 . These buffers provide a variable length bit-stream to the MPEG decoder for each of the respective minor channels in the multi-program channel. The MPEG decoder  134  is also coupled to respective reference and output buffers  142 ,  144  and optionally  146 . Each of these buffers holds reference frames used by the MPEG decoder to reproduce motion predictively encoded video fields or frames and an output buffer which holds the output video signal currently being provided to the video processor  148 . The video processor  148  processes the output signal to adjust, for example, color and tint and provides the video output signal to a display device  150 . 
     In the embodiment of the invention in which only two channel buffers are used by the transport decoder and AC3 decoder and only two VBV buffers and reference and output buffers are used by the MPEG decoder  134 , channel surfing is only facilitated among the minor channels of a multi-program channel and only in one direction. Thus, for example, a user may quickly surf through channels in a upward direction (increasing minor channel numbers) or in a downward direction (decreasing minor channel numbers) but may not switch between these during a given surfing operation without experiencing normal DTV decoding delays. 
     In operation, once the tuner  112  is tuned to a major channel that has minor channels, when the viewer presses the channel up button on the remote control  160  the microprocessor  152  causes the transport decoder  114  to reassign one of the channel buffers  116  and  118  to the next higher minor channel in the multi-program bit-stream. This causes the corresponding channel buffer in the  124  or  126  coupled to the AC3 decoder  122  to also switch to the next higher minor channel as well as the corresponding VBV buffer  136  or  138  and the corresponding reference and output buffer  142  and  144  coupled to the MPEG decoder  134 . The buffers that are switched are the ones that were providing the signal which was being reproduced before the channel change command was received. When the channel up button is pressed, the microprocessor  152  controls the MPEG decoder  134  and the AC3 decoder  122  to immediately provide the audio signal from the next higher channel. This causes a shift in the channel buffer being used. If the optional buffers  120 ,  140  and  146  are used, the system shown in FIG. 1 may facilitate minor channel surfing in both the upward (greater channel numbers) and downward (lesser channel numbers) directions. 
     The exemplary embodiment of the invention shown in FIG. 1 facilitates surfing among the minor channels in a major channel but does not affect surfing among major channels. 
     FIG. 2 is a block diagram of a second exemplary embodiment of the invention that allows viewers to surf both minor channels and major channels. The embodiment of the invention shown in FIG. 2 adds a second tuner  212  with its corresponding antenna  210 , a second transport decoder  214  with its corresponding channel buffers  216  and  218  and a multiplexer  220 . 
     Because the embodiment of the invention shown in FIG. 2 use only two tuners  112  and  212 , it supports channel surfing in only one direction, either to higher numbered channels or lower numbered channels. It is contemplated that the surfing direction may be preprogrammed or it may be selectable by the viewer either as a part of the set up process or dynamically while the viewer is watching television programs. 
     The embodiment of the invention shown in FIG. 2 may be used to aid is channel surfing in two different ways. The first method uses two transport decoders  114  and  214  to concurrently decode DTV programs received on successive RF channels by tuners  112  and  212  respectively. If the viewer is watching a program provided by tuner  112  and transport decoder  114  and then selects a program in the channel that is being received and decoded by tuner  212  and transport decoder  214 , the transport decoder  214  will have already decoded the bit-stream for that program. Accordingly, the microprocessor  152  controls the multiplexer  220  to provide the bit-stream to the AC3 decoder  122  and MPEG decoder  134 . In this embodiment of the invention, only the transport decoding delay is avoided during channel surfing. 
     A second method for using the embodiment of the invention shown in FIG. 2 decodes separate bit-streams in each of the transport decoders  114  and  214  and, responsive to the microprocessor  152 , provides both bit-streams to the AC3 decoder  122  and MPEG decoder  134  via the multiplexer  220 . The decoder  122  decodes the two bit-streams, storing data for one bit-stream in the channel buffer  124  and for the other bit-stream in channel buffer  126  of AC3 decoder  122 . The MPEG decoder  134  also concurrently decodes two bit-streams, storing the data for one program into VBV buffer  138  and reference and output buffers  142  while storing the data for the other program in the VBV buffers  136  and reference and output buffers  144 . According to this second method, the microprocessor  152  controls the multiplexer  220  to provide data for both channels to the AC3 decoder  122  and MPEG decoder  134  in an interleaved manner such that both signals may be concurrently decoded. 
     In an exemplary embodiment of the invention, the AC3 decoder  122  and MPEG decoder  134  interact with the microprocessor  152  using, for example, high-water mark and low-water mark signals from the respective channel buffers and VBV buffers. For example, the microprocessor  152  may condition the multiplexer  220  to provide data from channel buffer  118  coupled to transport decoder  114  to the AC3 decoder  122  and MPEG decoder  134  until the respective channel buffer  124  and VBV buffer  136  reach their respective high-water mark pointers. When the decoders  122  and  134  signal the microprocessor  152  that these high-water mark values have been reached, the microprocessor  152  conditions the multiplexer  220  to provide bit-stream data from channel buffer  218  coupled to transport decoder  214 . The AC3 decoder  122  is conditioned to store this data into channel buffer  126  and VBV buffer  136 . Once the input data is stored into the respective channel buffer  124  or  126  and VBV  138  or  136 , the decoder  122  and  134  may independently process the data to recover the output audio and video signals. The audio and video signals for the channel that is not selected are discarded while the signals for the selected channel are provided to the audio processor  130  and video processor  148 . 
     The embodiment of the invention shown in FIG. 2 may operate in the same way as the embodiment shown in FIG. 1 to allow a viewer to surf through minor channels in a single major channel. For this type of surfing, a single tuner  112  may be used with its transport decoder  114  to process both minor channels. In this instance, the other tuner,  212  and transport decoder  214  may be tuned to process the next major channel according to the chosen surf direction, so that switching among both major and minor channels may be accomplished quickly. 
     The embodiment of the invention shown in FIG. 2 includes optional channel buffer  128 , optional VBV buffer  140  and optional reference and output buffers  146 , all shown in phantom. These buffers may be used, for example, to aid in the switching operation of both upward and downward surfing operations, at least for minor channels. When the system is operated in the first method described above, in which only the transport decoder delay is eliminated, it may be desirable to use these optional buffers to continue to decode and display the current program while loading new program information into the other two buffers to display both the newly selected program and the next program in the determined surf direction. 
     The exemplary embodiment of the invention shown in FIG. 3 augments the embodiment shown in FIG. 2 by adding a third tuner,  312 , with an antenna  310  and transport decoder  314  having an associated channel buffer  316 . The three transport decoders  114 ,  214  and  314  are coupled to respective input ports of a three port multiplexer  320 . Also in the exemplary embodiment of the invention shown in FIG. 3 channel buffer  128 , VBV buffer  140  and reference and output buffers  146  are no longer optional. 
     The embodiment of the invention shown in FIG. 3 operates in a manner similar to that shown in FIG. 2 except it is not restricted to a single surf direction. Because it uses three tuners,  112 ,  212 , and  312 , the embodiment of the invention shown in FIG. 3 may provide image and audio data from one channel while simultaneously obtaining image and sound data from channels immediately above and immediately below the one channel in the channel map  156 . 
     As with the embodiment in FIG. 2, the embodiment shown in FIG. 3 may operate either to decode only the transport stream of the three channels or to provide the decoded transport stream of the three channels to the AC3 decoder  122  and MPEG decoder  134  concurrently. Using this second method, the channel buffers  124 ,  126  and  128  hold audio data from respectively different ones of the three channels. In the same way, the VBV buffers  136 ,  138  and  140  hold bit-stream data from each of the channels and the reference and output buffers  142 ,  144  and  146  hold decoded image data from the respective channels. 
     In the embodiment of the invention shown in FIG. 3, each transport decoder  114 ,  214  and  314  uses only one channel buffer  116 ,  216  and  316  respectively. Because the next surf channel is either immediately above or immediately below the current channel in the channel map  156 , only three adjacent channels need to be decoded in order to provide full surfing capability. Accordingly only three tuners and three transport decoders, each with one channel buffer, are needed to implement the surf function. When the channel surfing function is used to display minor channels of a major channel, at least two of the tuners  112 ,  212  and  312  may be tuned to the same major channel while respectively different minor channels are decoded by the respective transport decoders  114 ,  214  and  314 . 
     The embodiment of the invention shown in FIGS. 2 and 3 works best when the programs that are decoded and displayed are relatively low bandwidth signals (e.g. Main Profile Main Level or MP@ML). If, for example, all of the three programs being decoded by the system shown in FIG. 3 were main profile high level (MP@HL) signals, the rate at which data would be provided by the three signals may prevent the AC3 decoder  122  and MPEG decoder  134  from concurrently decoding the audio and video data from multiple channels with sufficient speed to provide the full channel surfing function. 
     FIG. 4 is a block diagram of a more robust embodiment of the invention that solves this problem. The embodiment of the invention shown in FIG. 4 includes three tuners,  112 ,  212  and  312 , each having a respective antenna,  110 ,  210  and  310  and a respective transport decoder  114 ,  214  and  314 . Each transport decoder has a channel buffer  120 ,  220  and  320  as well as two optional channel buffers,  116  and  118  for decoder  114 ;  216  and  218  for decoder  214  and  316  and  318  for decoder  314 . Each transport decoder is coupled to a respective audio and video decoding system. 
     For the sake of simplicity details of only the audio decoding system  410  and video decoding system  412  are shown in FIG.  4 . Audio decoding system  410  includes AC3 decoder  414  and one or more channel buffers  413 . Video decoding system  412  includes MPEG decoder  416 , one or more VBV buffers  418  and one or more sets of reference and output buffers  420 . The audio decoding systems  422  and  426  are identical to the system  410  and the video decoding  424  and  428  to the system  412 . 
     Each of the video and audio decoding systems provides a respective decoded video and audio signal to an input port of multiplexer  430 . Multiplexer  430  is controlled by microprocessor  152  to direct one of the audio output signals to the audio processor  130  and one of the video output signals to the video processor  148 . 
     In this embodiment of the invention one tuner, for example tuner  212 , is controlled by the microprocessor  152  to tune to the channel that is currently being displayed. The output signal of this tuner is applied to transport decoder  214  which generates video and audio data streams for the selected channel. The audio data stream is applied to the audio decoder  422  and the video data stream is applied to the video decoder  424 . The output signals of these decoders are applied to the audio processor  130  and video processor  148  through the multiplexer  430  under control of microprocessor  152 . While the signal captured by tuner  212  and transport decoder  214  is displayed, tuner  112 , transport decoder  114 , audio decoder  410  and video decoder  412  are decoding video and audio data streams for a channel immediately above the selected channel in the channel map  156 . Likewise, tuner  312 , transport decoder  314 , audio decoder  426  and video decoder  428  are capturing and decoding video and audio data signals for a channel immediately below the selected channel in the channel map  156 . 
     If, in this example, the viewer presses the channel up button on the remote control unit  160 , a signal is sent through the IR receiver  158  to microprocessor  152  causing the microprocessor  152  to switch the multiplexer  430  to provide the audio and video data output signals provided by audio decoder  410  and video decoder  412  to the audio processor  130  and video processor  148 , respectively. At the same time, microprocessor  152  causes tuner  312  to tune to the channel above the newly selected channel in the channel map  156 , so that the audio and video data streams for this channel may be decoded by transport decoder  314  and provided to audio decoder  426  and video decoder  428  respectively. 
     As shown in FIG. 4, each of the transport decoders includes a fixed channel buffer and two optional channel buffers. In addition, each audio decoder includes one or more channel buffers and each video decoder includes one or more VBV buffers and one or more sets of reference and output buffers. 
     If each of the transport decoders and audio decoders includes two channel buffers and each video includes two VBV buffers and two sets of reference and output buffers, the system shown in FIG. 4 may simultaneously decode up to six digital television signals, two minor channel signals for each of three major channels. A viewer using this system may surf between adjacent minor channels and one of the major channels or may surf between adjacent major channels. If each transport decoder and each audio decoder includes three channel buffers and each MPEG decoder includes three VBV buffers and three sets of reference and output buffers, the embodiment of the invention shown in FIG. 4 may simultaneously decode up to nine digital television programs. This configuration allows a viewer to surf both up and down by either minor channels or major channels. 
     FIG. 5 is a flow chart diagram that illustrates an exemplary process which may be used by the microprocessor  152  to control the apparatus shown in FIG.  4 . In this process, at step  508 , channel N is selected for display. At step  510 , the process determines if the channel up button on the remote control  160  has been pressed. If so, step  512  is executed which marks the tuner and decoder that are currently providing audio and video data for channel N−1 as being available, and increments the variable N by one. Consequently, video and audio data from the tuner and the decoder for the next channel in the channel map are provided to the audio processor  130  and video processor  148  as the selected channel N. 
     Next, at step  514 , the microprocessor  152  gets the channel frequency for channel N+1 from the channel map  156  and assigns the available tuner and decoder to get the television signal from channel N+1 and decode the television signal to provide audio and video data. 
     After step  514  or if, at step  510 , it is determined that the channel up button is not pressed, step  516  is executed to determine if the channel down button has been pressed. If this button has been pressed at step  516 , the process executes step  518  which marks the tuner and decoder for channel N+1 as being available and decrements N by one. This step routes the audio and video signals from the tuner and decoder handling the next lower television signal to be provided to the audio and video processors  130  and  148 . Next, at step  520 , the microprocessor  152  gets the channel frequency for channel N−1 from the channel map and assigns that frequency to the available tuner such that the available tuner and its corresponding decoder will provide the audio and video signals for channel N−1 in preparation for the user again pressing the channel down button. 
     After step  520 , or if at  516 , the process determines the channel down button has not been pressed, step  522  is executed which sends the decoded data from channel N to the audio processor  130  and video processor  148  respectively. 
     Although the embodiments of the invention described above have assumed that the decoding and surfing operations are performed using hardware elements, it is contemplated that the invention may be practiced entirely in software. FIG. 6 is a block diagram that illustrates the control of parallel processes decoding audio and video data for a plurality of parallel broadcast channels. An exemplary software embodiment may be implemented on a general-purpose computer running a multi-threaded operating system. Running in this operating system environment, are three parallel decoding processes, each of which decodes a respectivly different bit-stream into respective audio and video data. The control process selects audio data from one of these processes and provides it to a sound card (not shown) to reproduce the audio portion of the data. The control process also routes one set of decoded video data to a video display processor (not shown) and monitor (not shown) for display. The exemplary computer is also coupled to receive a plurality of data streams each representing a respective television program. These data streams may be provided, for example, from a stored set of data or from a wide band width data link (not shown). The process shown in FIG. 6 as well as the audio and video decoding processes may be implemented in software stored on a computer readable carrier such as a magnetic or optical disk or a radio frequency or audio frequency carrier wave. 
     The control process shown in FIG. 6 operates as follows. At step  608 , channel N is selected for display. At step  610 , the process determines that the viewer wishes to display the next higher channel in the channel map. At step  612 , the control process terminates the decoding process which is decoding channel N−1 and increments the variable N. At step  614 , the control process initiates a new process to decode data from channel N+1, retaining the process for the previously displayed channel as channel N−1. After step  614 , or if at step  610 , the process determines that the viewer does not want to view the next higher channel in the channel map, step  616  is executed. This step determines if the viewer wishes to view the next lower channel in the channel map. If so, step  618  is executed in which the process that is currently decoding channel N+1 is terminated and the variable N is decremented by one. Next, at step  620 , the control process initiates a new process to decode the data stream for channel N−1. After step  620 , or if at step  616 , the process determines that the viewer does not wish to view the next lower channel, step  622  is executed which sends decoded data from channel N to the audio and video processors. 
     Although the invention has been disclosed in terms of exemplary embodiments it is contemplated that it may be practiced as described above within the scope of the attached claims.