Abstract:
A master subscriber station is provided that supports a relatively low cost slave subscriber station. The slave subscriber station relies upon the master subscriber station for certain functions and therefore can be implemented as a lower cost design. Duplication of functionality is therefore minimized, resulting in lower complexity and lower overall costs. The master subscriber station has the ability to process multiple video streams such that one or more of the additional streams can be sent via coaxial cabling to a slave subscriber station, as well as receive and process user control signals from the slave subscriber station. The slave subscriber has the ability to receive and demodulate video and audio streams sent by the master subscriber station, as well as send user control commands back to the master subscriber station over the same coaxial cable that delivers the video streams to the slave subscriber station.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the distribution of video and interactive services. More particularly, this invention relates to subscriber stations for receiving digital video programs and supporting interactive services. 
     2. Description of the Background Art 
     Distribution of digital video programs via cable television systems or direct broadcast satellite (DBS) is becoming increasingly popular. In addition, distribution of interactive services such as video on-demand services and Internet (including World Wide Web) access is also becoming increasingly popular. 
     Many homes now have a subscriber station or terminal that provides access to digital video programming. The subscriber station is typically in the form of a stand-alone set-top box. The stand-alone set-top box receives signals over a cable system, a direct broadcast system, or other distribution system. Of particular significance, the stand-alone set-top box supports only a single television (or monitor or other display-device). If a user wants to support another television (or monitor or other display device), then a second stand-alone set-top box is typically required. Moreover, if the user wants to support a third television (or monitor or other display device), then a third stand-alone set-top box is typically required. And so on for additional televisions. The need for multiple identical stand-alone set-top boxes to support multiple televisions is significant extra cost to the user and/or the service provider. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described disadvantages by providing a master subscriber station that supports a relatively low cost slave subscriber station. The slave subscriber station relies upon the master subscriber station for certain functions and therefore can be implemented as a lower cost design. Duplication of functionality is therefore minimized, resulting in lower complexity and lower overall costs. 
     One distinction of the master subscriber station over a conventional stand-alone set-top box is the master subscriber station&#39;s ability to process multiple video streams such that one or more of the additional streams can be sent via coaxial cabling to a slave subscriber station. Another distinction is the master subscriber station&#39;s ability to receive and process control signals from the slave subscriber station. In a preferred embodiment of the present invention, the master subscriber station can also function as a stand-alone unit with minimal modifications. 
     One distinction of the slave subscriber station over a conventional stand-alone set-top box is the slave subscriber station&#39;s ability to receive and demodulate video and audio streams sent by the master subscriber station. Another distinction is the slave subscriber station&#39;s ability to send remote control commands back to the master subscriber station over the same coaxial cable that delivers the video streams to the slave subscriber station. For example, in a video-on-demand (VOD) application, the slave subscriber station may send a command indicating a movie selection to the master subscriber station. As another example, in a digital video broadcast application, the slave subscriber station may send a command indicating that a broadcast channel selection to the master subscriber station. 
     In a preferred embodiment of the present invention, the slave subscriber station includes a relatively low-speed RF (radio frequency) modem (modulator/demodulator) to communicate with the master unit. This allows the slave subscriber station to be of substantially reduced cost and complexity because a substantial portion of the cost and complexity of a conventional stand alone set-top box lies in the RF tuner, IF (intermediate frequency) amplifier, and QAM demodulator circuitry. Also, in a preferred embodiment of the present invention, the slave subscriber station is housed in a small, inexpensive enclosure and uses a wall transformer to supply power. 
     In a preferred embodiment of the present invention; advantage is taken of the fact that the digital bit rate of a single video program (e.g., a movie) which is compressed according the MPEG (motion picture expert group) standard can be as low as 1.544 megabits per second (Mb/s), whereas the digital bit rate capacity of a standard 6 megahertz (MHz) analog CATV channel can be as high as 27 Mb/s for 64 QAM (quadrature amplitude modulation) and as high as 38.8 Mb/s for 256 QAM. The relatively large digital bandwidth of the standard analog CATV channel allows several digital MPEG channels to be packaged and transmitted within a single standard analog. CATV channel. 
     Unlike a conventional stand-alone digital set-top box which selects and demultiplexes only one MPEG digital channel at any one time, the master subscriber station is able to select and demultiplex multiple MPEG digital channels at any one time. One digital channel may be displayed on a television coupled to the master subscriber station, and a different (or the same) digital channel may be displayed on each of the slave subscriber stations. Furthermore, a second master subscriber station (possibly in a different residence in the same neighborhood) tuned to the same 6 MHz CATV channel as the first master-set top box mentioned above would be able to select and demultiplex different (or the same) digital channels as the first master subscriber station. 
     These and other features and advantages of the present invention may be better understood by considering the following detailed description of a preferred embodiment of the invention. In the course of this description, reference will frequently be made to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram including master and slave (remote) subscriber stations (set-top boxes) according to a first embodiment of the present invention. 
     FIG. 2 is a schematic diagram including master and slave (remote) subscriber stations (set-top boxes) according to a second embodiment of the present invention. 
     FIG. 3 is a schematic diagram of a video-on-demand (VOD) service including a VOD server and a network manager within a head-end of a cable distribution system in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a schematic diagram of a portion of a cable distribution system from multi-drop couplers to TV receivers in accordance with a preferred embodiment of the present invention. 
     FIG. 5 is a schematic diagram illustrating the distribution of multiple video programs from a VOD server to a residence in accordance with a preferred embodiment of the present invention. 
     FIG. 6A is a flow chart illustrating the basic method of operation of the master subscriber station in FIG.  1 . 
     FIG. 6B is a flow chart illustrating the basic method of operation of the slave subscriber station in FIG.  1 . 
     FIG. 7A is a flow chart illustrating the basic method of operation of the master subscriber station in FIG.  2 . 
     FIG. 7B is a flow chart illustrating the basic method of operation of the slave subscriber station in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 1 is a schematic diagram including a master subscriber station (set-top box)  102  and a slave (remote) subscriber station (set-top box).  104  according to a first embodiment of the present invention. The master  102  and slave  104  subscriber stations are both coupled to a power splitter  106  which in turn is coupled to a CATV (community antenna television or cable television) cable. The subscriber stations  102  and  104  may be set-top boxes, or they may alternatively be integrated with a television, a computer system, or other equipment. 
     In one preferred embodiment of the present invention, the CATV cable enters into a residence which includes the master  102  and one or more slave  104  subscriber stations. For example, the master subscriber station  102  may be in a family room and slave subscriber stations  104  may be in bedrooms or other rooms. In another preferred embodiment of the present invention, the CATV cable enters into a business which includes the master  102  and, one or more slave  104  subscriber stations. For example, the business may be a hotel with a slave subscriber stations  104  in the rooms or suites of the hotel. 
     According to a preferred embodiment of the present invention, the master subscriber station  102  is backward compatible with analog broadcasts in that it may operate in either an analog mode or a digital mode. In digital mode, the master subscriber station  102  is set-up by the cable head-end  302  to receive a specified 6 MHz QAM digitally modulated RF carrier containing the digital video programs destined to subscriber stations ( 102  and/or  104 ) fed by the head-end  302 . In a VOD application, the digital video programs may be movies in compressed digital format. In a digital video broadcast application, the digital video programs may be broadcast programs in compressed digital format. 
     According to a preferred embodiment of the present invention, the set-up of the master subscriber station  102  by the cable head-end  302  occurs via an out-of-band control channel. Control commands sent via the out-of-band control channel instructs the RF tuner in the master subscriber station  102  to tune to the CATV channel containing the desired digital video. 
     According to a preferred embodiment of the present invention, since several digital video streams may be contained in a single 6 MHz CATV channel, the digital video streams are differentiated and identified by program identifier (PID) tags located in the header of the digital MPEG packets carrying the video. The master subscriber station  102  is informed (also via the out-of-band control channel) which PID tag or tags identify video programs to be displayed on the television attached to the, master unit  102  and/or on the televisions attached to the slave subscriber stations  104  which are under the control of the master unit  102 . In this way, each master subscriber station  102  can ignore superfluous data and process only the video and audio streams intended for viewing within the residence or business. 
     In the embodiment shown in FIG. 1, the master subscriber station  102  is capable of demultiplexing a second encoded digital video and audio stream and sending the encoded stream to the slave subscriber station  104  via a digitally encoded carrier. The slave subscriber station  104  converts the signal from the digitally encoded carrier back into the encoded digital stream and then decodes the encoded digital stream to generate baseband video and audio signals for display on the television attached to the slave subscriber station. 
     Master&#39;s Front End 
     Turning in detail to FIG. 1, the master subscriber station  102  receives both digital and analog RF carriers at the “F” connector of the conventional channel ¾ RF modulator  112 . Preferably, the channel ¾ modulator  112  also serves as a RF bypass device so that the analog carrier may be forwarded to the associated television when the master subscriber station  102  is not being used. 
     When the master subscriber station  102  is being used, in either analog or digital mode, the broadband RF signal at the “F” connector is routed to a diplexer  114 . The diplexer  114  splits high frequencies and low frequencies into two separate ports while maintaining a constant input impedance. The high frequency port of the diplexer  114  feeds a 1:2 power splitter  116 . The outputs of the power splitter  116  feeds two RF tuners, the QPSK (quadrature phase-shift keying) “out-of-band” tuner  118  and the combination QAM “in-band” and analog tuner  120 . 
     The output of the QAM in-band tuner  120  is filtered by a 6 MHz SAW (surface acoustic wave) filter  122  which rejects all frequencies except in the range from 41 to 47 MHz. The output of the SAW filter  122  feeds an AGC (automatic gain control) amplifier  124  where the IF (intermediate frequency) signal is amplified and down converted to a 5 MHz baseband signal. The AGC amplifier  124 , as the name implies, is also the point in the system where the signal level is maintained at a constant. The output of the AGC amplifier  124  feeds an A/D (analog-to-digital) converter  126  with a constant signal level. The digital output of the A/D converter  126 , which is byte-wide at this point, is fed into a QAM/QPSK demodulator  128  which converts the digitized QAM carrier into an error corrected 27 MHz digital bit stream. The QAM/QPSK demodulator  128  also provides the gain control voltage that is fed back to both the QAM and QPSK AGC amplifiers  124 . 
     Similar to the output of the QAM in-band tuner  120 , the output of the QPSK out-of-band tuner  118  feeds a SAW filter  122 , which in turn feeds an AGC amplifier  124 , which inturn feeds an A/D converter  126 , which in turn feeds the same QAM/QPSK demodulator  128 . 
     One may consider the QAM/QPSK demodulator  128  as acting as an interface between the analog and digital portions of the master subscriber station  102 . The 27 Mb/s broadband digital signal from the QAM/QPSK demodulator  128  is fed into a FPGA (field programmable gate array)  130 . 
     The master subscriber station  102  also includes an IR (infrared) detector  138  for receiving infrared signals generated by a remote or user control unit for the master subscriber station  102 . A video processor  140  with a RISC (reduced instruction set computer) core and associated temporary storage memory  142  and longer-term non-volatile memory  144  operate in a conventional manner to generate video and audio signals for use by a television. The video processor  140  preferably takes the form of a conventional integrated MPEG video processor programmed to generate analog video and analog signals from MPEG encoded video and audio data packets. The video processor  140  is also programmed to process commands from the infrared detector  138  and to respond in an appropriate fashion. The analog video and audio signals generated by the video processor  140  are modulated by RF modulator  112  onto an RF carrier and sent to the television. Alternatively, or in addition, the analog video and audio signals generated by the video processor  140  may be provided as separate baseband outputs for the television. In addition, the RISC processor embedded in the video processor  140  supplies upstream signals in BPSK/QPSK (binary phase-shift keying/quadrature phase-shift keying) form to upstream transmitter  150 . 
     The above detailed description covers the “front-end” circuitry of the master subscriber station  102  which is common to the two embodiments shown in FIGS. 1 and 2. The “back-end” circuitry and the method of operation thereof is different between the two embodiments shown in FIGS. 1 and 2. 
     Master&#39;s Back End in First Embodiment 
     In FIG. 1, the FPGA  130  is programmed to filter or select the specific video and audio MPEG packets destined for the slave subscriber station  104 . The FPGA  130  also sends control data packets destined for the slave subscriber station  104  alongside the video and audio MPEG packets destined for the slave subscriber station  104 . By combining the control data packets with the video and audio data packets destined for the slave subscriber station  104 , the need for an additional out-of-band carrier between the two boxes is eliminated. In addition, the FPGA  130  processes the user control data coming from the slave subscriber station  104  via a RF (radio frequency) modem  132  on a daughter board  134  mounted within the master subscriber station  102 . 
     For each slave subscriber station  104 , there corresponds such a daughter board  134  in the master subscriber station  102 . The daughter board  134  may be implemented on a separate printed circuit board, or alternatively the chips of the daughter board  134  may be integrated onto the mother board of the master subscriber station  102 . 
     The selected control, video, and audio packets which are output from the FPGA  130  as bursts of 27 Mb/s are sent to a FIFO (first in first out) buffer  136  on the daughter board  134 . The function of the FIFO  136  is to take the bursty packets and, generate a lower but constant bit rate. The output frequency of the FIFO  136  is determined by the combination of control, video, audio, and null (or “stuffing”) packets. The null packets are used to tune the clocking frequency and therefore the output frequency of the FIFO  136  to allow the use of a convenient clock frequency already available in the master subscriber station  102 . The output from the FIFO  136  drives the input to the RF modem  132  which converts the digital bit stream to a digitally modulated RF carrier signal. 
     The digitally modulated RF carrier signal output by the RF modem  132  is sent downstream to the slave subscriber station  104  via coaxial cabling. In addition, the RF modem  132  in the master subscriber station  102  is capable of receiving an upstream signal transmitted from the slave subscriber station  104 . The upstream signal from the slave subscriber station  104  is a low bit rate signal which is an RF modulated version of the control commands that are typically received from the IR remote control associated the slave subscriber station  104 . The carrier frequencies for both the upstream and downstream communications between the master subscriber station  102  and the slave subscriber station  104  are either below 5 MHz, between 40 and 50 MHz, or above several hundred MHz, preferably above the highest frequency digital carriers to prevent interference with CATV and digital programming signals. 
     Slave in First Embodiment 
     In FIG. 1, the slave subscriber station  104  is simpler and less costly than the master subscriber station  102  because most of the RF signal processing and the in-band and out-of-band signaling takes place in the master subscriber station  102 . The slave subscriber station  104  performs primarily the MPEG video and audio decoding function. As shown in FIG. 1, the slave subscriber station  104  includes a RF modem  108 , a conventional channel ¾ modulator  146 , a video processor  140 , associated temporary  142  and longer-term  144  memories, an infrared detector  138 , and power conditioning electronics  148  to accommodate low cost wall transformer operation. 
     The conventional channel ¾ RF modulator  146  receives both digital and analog RF carriers. Preferably, the channel ¾ modulator  146  also serves as a RF bypass device so that the analog carrier may be forwarded to the associated television when the slave subscriber station is not being used. 
     When the slave subscriber station  104  is being used, the RF modem  108  in the slave subscriber station  104  receives and demodulates a 4 to 5 Mb/s FSK (frequency-shift keying) or PSK (phase-shift keying) modulated bit stream which carries the control, video, and audio packets from the master subscriber station  102 . The RF modem  108  also sends the low speed user control signals from the IR detector  138  in the slave subscriber station  104  to the master subscriber station  102  using a separate digitally modulated carrier. 
     The slave subscriber station  104  in FIG. 1 preferably provides conventional baseband video and stereo outputs, S-Video outputs, and a channel ¾ RF output. The power for the slave subscriber station  104  is preferably supplied by the wall-type transformer  110 , and power conditioning and voltage regulation  148  is performed on the main circuit board of the slave subscriber station  104 . 
     Second Embodiment 
     FIG. 2 is a schematic diagram including master and slave (remote) subscriber stations (set-top boxes) according to a second embodiment of the present invention. Similar to the first embodiment, the second embodiment uses the RF front-end of the master subscriber station  202  to process the digitally modulated carrier containing the 27 MHz digital video bit stream. However, in contrast to the first embodiment, the digital-to-analog (D/A) conversion and the MPEG decoding for the slave subscriber station  204  is performed on the daughter board  206  inside the master subscriber station  202 . 
     One advantage of the embodiment shown in FIG. 2 is that the complexity and cost of the slave subscriber station  204  is reduced. A further advantage of the embodiment shown in FIG. 2 is that copy protection is stronger since the signal between the master  202  and remote  204  subscriber stations is analog instead of digital. This makes it more difficult to copy the digital signal. The method of transmission between the master  202  and remote  204  subscriber stations can be either amplitude modulation (AM) or preferably frequency modulation (FM) in order to get the best signal quality and noise immunity. 
     Master&#39;s Back End in Second Embodiment 
     In FIG. 2, the master subscriber station  202  is capable of demultiplexing and decoding a second encoded digital video and audio stream to generate baseband video and audio signals. The master subscriber station  202  sends these baseband video and audio signals to the slave subscriber station  204 . 
     The daughter card  206  within the master subscriber station  202  contains a video processor  140 , associated memory ( 142  and  144 ), and an RF modem  208 . Alternatively, the chips from the daughter board  206  may be integrated into the main board of the master subscriber station  202 . 
     The FPGA  210  sends digitally modulated signals carrying the control, video, and audio data to the video processor  140  located on the daughter card  206 . Preferably, the video processor  140  selects only the control, video, and audio data packets which are tagged for delivery to the slave subscriber station  204  and ignores packets with other identification tags (packet identifiers or PIDs). This function is called PID filtering. 
     In addition to PID filtering, the video processor  140  on the daughter card  206  performs the following functions: MPEG decoding, video memory management, NTSC encoding, downstream out-of-band signal processing, remote-to-master signal processing, and on-screen display. 
     The RF modem  208  on the daughter card  206  receives baseband video and, audio signals from the processor  140  and transmits these video and audio signals to the slave subscriber station  204  via a modulated carrier. The modulated carrier is typically an amplitude modulation (AM) or frequency modulation (FM) carrier. The RF modem  208  also receives and demodulates a carrier that originates from the slave subscriber station  204  which contains signals from the infrared remote for the slave subscriber station  204 . 
     The bandwidth required for the upstream transmission from the slave subscriber station  204  to the master subscriber station  202  is only that necessary to relay the user control signals which typically have a bandwidth of a few kilohertz (KHz). The downstream bandwidth required to transport the video and audio carriers from the master subscriber station  202  to the slave subscriber station  204  depends on the type of modulation chosen, but the video and audio information bandwidth combined will typically require about 6 MHz total. This number takes into account the additional bandwidth required to accommodate the roll-off from the 4.2 MHz video signal and the two audio carriers. 
     Slave in Second Embodiment 
     In accordance with FIG. 2, the slave subscriber station includes a RF modem  216 , a diplexer  214 , a channel ¾ RF modulator  146 , a voltage regulator  148 , and an infrared detector  138 . 
     A notch filter  212  as shown in the second embodiment may also be used between the CATV cable and the power splitter  106 . The notch filter  212  is able to reduce port-to-port isolation (for example, from about 20-30 dB to about 8-10 dB) between the set-top boxes such that they can communicate to each other at the frequency to which the notch filter  212  is tuned. At the same time, the notch filter  212  is able to substantially prevent the communication signals from escaping from the residence. Note that such a notch filter  212  may also be used to advantage in the first embodiment. 
     FIG. 3 is a schematic diagram of a video-on-demand (VOD) service including a VOD server  312  and a network manager  314  within a head-end  302  of a cable distribution system in accordance with a preferred embodiment of the present invention. The VOD service illustrated in FIG. 3 is incorporated into a conventional hybrid fiber coax (HFC) distribution system and includes a VOD system  304  within a cable head-end  302 , distribution nodes  306 , and multi-drop couplers  308  to various end user systems (residences or businesses)  310 . 
     The couplings between the headend  302  and the nodes  306  are via optical fiber (and laser transmitters  312  and laser receivers  314 ) while the couplings between the nodes  306  and the end-user systems  310  are via coaxial cable. The coaxial cable from the node  306  to the multi-drop couplers is typically capable of carrying downstream 50 to 750 MHz of video programming and 70 to 130 MHz of set-top data to the end-user system  310 , and 15-30 MHz of upstream data from the end-user system  310 . 
     As illustrated in FIG. 3, the video server  312  may provide, for example, two movies  316 A and  316 B. The first movie  316 A may have been requested by and destined for delivery to a master subscriber station (master STB) of a particular end-user system  310 . The second movie  316 B may have been requested by and destined for delivery to a slave subscriber station (slave STB) under control of the master subscriber station. 
     FIG. 4 is a schematic diagram of a portion of a cable distribution system from multi-drop couplers  308  to TV receivers  402  within an end-user system (residence or business)  310  in accordance with a preferred embodiment of the present invention. Like the coaxial cable between the nodes  306  and the multi-drop couplers  308 , the cable drop from the couplers  308  to the end-user systems  310  is typically capable of carrying downstream 50 to 750 MHz of video programming and 70 to 130 MHz of set-top data and 15-30 MHz of upstream data. 
     FIG. 5 is a schematic diagram illustrating the distribution of multiple video programs from a VOD server  312  to an end-user system (residence or business)  310  in accordance with a preferred embodiment of the present invention. The example illustrated in FIG. 5 shows eight different video programs (# 1 -# 8 ) destined for delivery to seven different residences (A-G). 
     In particular, two video programs (# 1  and # 2 ) are destined for House A. As shown in FIG. 5, House A receives all eight video programs (# 1 -# 8 ), but only video programs # 1  and # 2  are identified by their PIDs as being destined for House A. Therefore, the master subscriber station  102  in House A ignores all but those two video programs. As shown in FIG. 5, video program # 1  is decoded by the master subscriber station  102  and displayed on the TV receiver  402  associated with the master subscriber station  102 . Meanwhile, video program # 2  is decoded by the slave subscriber station  104  and displayed on the TV receiver  402  associated with the slave subscriber station  104 . 
     FIG. 6A is a flow chart illustrating the basic method of operation of the master subscriber station  102  in FIG.  1 . 
     The process begins with the step  602  of receiving a first digitally modulated signal carrying a plurality of digital video programs (for example, programs # 1 - 8  shown in FIG. 5) from the distribution system. The next step  604  involves demodulating (by QAM demodulator  128 ) the first digitally modulated signal to obtain the plurality of digital video programs. In the following step  606 , the logic device  130  selects a first digital video program destined for the master unit  102  and a second digital video program destined for the slave unit  104 . 
     In a subsequent step  608 , the video processor  140  processes the first digital video program for display on a display device associated with the master subscriber station  104 . The processing of step  608  involves generating an analog baseband signal for the first program and generating an analog display signal carrying the baseband signal. In step  609  the analog display signal is sent to the master unit&#39;s display and the first program is displayed thereon. 
     In parallel with steps  608  and  609 , the master subscriber station  102  also performs steps  610 ,  612 , and  614 . In step  610 , packets containing the second digital video program are buffered by the FIFO buffer  136  and sent at a relatively constant bit rate to the RF modem  132 . In step  612 , the RF modem  132  generates a second digitally modulated signal which carries the second digital video program. Next, in step  614 , the RF modem  132  transmits the second digitally modulated signal to the slave subscriber station  104 . 
     FIG. 6B is a flow chart illustrating the basic method of operation of the slave subscriber station  104  in FIG.  1 . The process in FIG. 6B is a continuation of the process of FIG.  6 A. 
     The process continues in step  616  when the slave subscriber station  104  receives the second digitally modulated signal from the master subscriber station  102 . In step  618 , the RF modem  108  of the slave subscriber station  104  demodulates the second digitally modulated signal to derive a second digital video program. Next, in step  620 , the video processor  140  processes the second digital video program for display on a display device associated with the slave subscriber station  104 . The processing of step  620  involves generating an analog baseband signal for the second program and generating an analog display signal carrying the baseband signal. Finally, in step  622 , the analog display signal is sent to the slave unit&#39;s display and the second program is displayed thereon. 
     FIG. 7A is a flow chart illustrating the basic method of operation of the master subscriber station  202  in FIG.  2 . FIG. 7A has steps  602 ,  604 ,  606 ,  608 , and  609  in common with FIG.  6 A. 
     In parallel with steps.  608  and  609 , the master subscriber station  202  also performs steps  702 ,  704 , and  706 . In step  702 , the second digital video program is processed by the video processor  140  on the daughter card  206  to generate an analog baseband signal. In step  704 , the RF modem  208  generates an analog modulated signal which carries the baseband signal. Next, in step  706 , the analog modulated signal is transmitted by the RF modem  208  to the slave subscriber station  204 . 
     FIG. 7B is a flow chart illustrating the basic method of operation of the slave subscriber station  204  in FIG.  2 . The process in FIG. 7B is a continuation of the process of FIG.  7 A. 
     The process continues in step  708  when the slave subscriber station  204  receives the analog modulated signal from the master subscriber station  202 . In step  710 , the RF modem  216  of the slave subscriber station  204  demodulates the analog modulated signal to derive an analog baseband signal for the second program that is sent to the channel ¾ modulator  146 . Next, in step  712 , the channel ¾ modulator  146  generates an analog display signal carrying the baseband signal for the second program. Finally, in step  714 , the analog display signal is sent to the slave unit&#39;s display, and the second program is displayed thereon. 
     It is to be understood that the specific mechanisms and techniques which have been described are merely illustrative of one application of the principles of the invention. Numerous additional modifications may be made to the methods and apparatus described without departing from the true spirit of the invention. For example, the communication system between the master and slave set-top boxes may be implemented via a 900 MHz link or via the power lines in a building, instead of via a cable link.