Abstract:
Through a memory device and a control circuit on the transmitter side, temporal gaps are inserted into the data stream, said temporal gaps being removed again on the receiving side by an additional memory device. Consequently, the necessary continuous data stream is able to be reproduced for the transmission of the video signals. The gaps in the data stream of the base station and the data and the requests are evaluated by a user-side control circuit, from these, switch-over signals are generated, and the user devices are switched over. The base state switches the station to receive mode in the temporal gaps of the video signals or of the data stream. The present invention is applicable to the transmission of continuous signals using a backward channel via microwaves.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a device for video, sound and data transmission, and more particularly to a method and device which provides a two-way transmission of data between a base station and users. 
     RELATED TECHNOLOGY 
     In the field of transmission of interactive television applications, certain methods permit the transmission of asymmetrical data streams and also of some symmetrical data streams. ATM (asynchronous transfer mode) and MPEG signals (video, sound, and data signals, coded according to the standard specifications of the Motion Picture Expert Group) can be transmitted simultaneously in both directions on the basis of this principle. 
     At the present time, work is underway in many countries on interactive broadband systems, such systems permitting two-way transmission through a forward and a backward channel. A high bandwidth is needed for the transmission of video signals. Some broadband systems are wireless. It is customary to use a frequency range near the forward channel for the high-frequency backward channel. The closer together the forward channel and the backward channel are, the better the propagation characteristics of the two channels conform and the more likely it is that the same antenna can be used for both channels. 
     One such method is described, for example, in DAVIC 1.1, Specification Part 08, Lower Layer Protocols And Physical Interfaces, LMDS Specifications, Revision 1.1 3/1996. The current standardization proposals of DAVIC (Digital Audio-Visual Council) are based on the assumption of such a frequency-separation of both channels. FIG. 2 (prior art) illustrates the frequency allocation scheme in the case of a transmission between a STB (set-top box) and a bidirectional converter, which converts the transmitted and the received signal on the user side. 
     To provide decoupling, U.S. Pat. No. 4,747,160 and European Patent 0 282 347 propose using differing polarity of the field vectors. However, the disadvantage of this method is that it can only be used in the radio communication field and other methods have to be used for feeding the antenna systems. 
     Another drawback of a small distance between channels is that the transmitted signals can couple over to the receiving side. For that reason, some manufacturers favor using separated antennas. However, it should be possible to avoid the substantial outlay associated with this on the user side. 
     SUMMARY OF THE INVENTION 
     A goal of the present invention is to reduce costs on the user side, as well as for the base station. Therefore, an object of the present invention is to make double use of the antennas of the user and the base station side, to avoid using costly filters for separating the signal domains, and to provide a solution which can also be used for feeding pathways. 
     The present invention therefore provides a method for use in program transmission of digitalized TV-video, sound and data signals from a base station to users and back via a backward channel with time-division-multiplexing two-way operation and with HF modulation for wireless connection, characterized in that 
     on the transmitter side, following digitalization and prior to dispatching, temporal gaps (time intervals) are formed in the data stream; 
     during these gaps, the HF modulation is blanked, and the transmitting antenna is switched over to receive mode for the backward channel; 
     the same frequency band is used for the backward channel as for the program transmission; 
     the switching over of the users′ receiving antennas is controlled synchronously by the transmitter-side switch-over; and 
     a memory likewise controlled by said transmitter-side switch-over is read out and read in, respectively, on the transmitter and receiver side. 
     Advantageous further developments and improvements of the method include: (1) that an orthogonal multicarrier method is used for modulating the forward channel or the backward channel; (2) that a code-multiplexing method is used as modulation of the backward channel, for its simultaneous use by a plurality of users; (3) that gaps for the transmission of digital information and gaps for first-time calls (first user requests to send data) and user signals are formed by the base station and inserted into the data stream, and that the gaps for first-time calls and user signals are predefined in accordance with a time pattern as periodically recurring gaps, the period duration being fixed as a function of the volume of traffic and of the distance to the base station; and (4) that after the first-time call, each accepted user is assigned his or her own code, which in the event of a lasting interruption in the return call on the part of the user of a defined duration, which is preferably a function of the traffic density, is canceled and assigned elsewhere. The code multiplexing and the division of gaps into those for first time calls and those for transmission of digital information are similar to those known from the mobile telephone communications field. 
     The present invention also provides a device for transmission of digitized TV-video, sound and data signals, comprising transmitters or receivers with HF modulators or demodulators and antennas, which have available a backward channel for wireless connection, characterized in that assigned in each case to the base station and to the users are memory devices, converters, and switch-over devices for switching over the direction of the antennas as transmitting and receiving antennas, as well as a control device, which produces transmission gaps at the base station, and are switched in the case of the users as a function of the control of the base station and of predefined time periods. 
     Advantageous further developments of this device and improvements thereto include: (1) that the same oscillator is used in the converter of the base station and of the users, in each case for sending and receiving; (2) that a switching signal for activating the switch-over is fed via the HF feeder line to the converters; and (3) that in the case of intermediately switched cable distribution networks designed for one direction and comprising a head-end station at the receiving antenna and a plurality of users, the switching over of the antennas takes place in the head-end station and the storage with the user and the head-end station, and that the backward direction is bridged by means of additional wireless connection(s), preferably in the ISM band. 
     In a surprisingly simple manner, the present invention makes it possible to use the same HF channel to supply video signals, i.e. to use continuous data streams for the forward and backward directions. The somewhat contradictory goals of, on the one hand, providing continuous video data streams necessary for the forward path and, on the other hand, permitting larger gaps necessary in the backward channel data stream are overcome by interposing receiver-side memory devices. The method unites logical backward channels and logical forward channels in one physical channel (i.e. at a single transmission frequency). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be elucidated in the following on the basis of exemplary embodiments. The corresponding drawings show: 
     FIG. 1 schematically represented, one base station and one user station; 
     FIG. 2 the known frequency allocation scheme as per DAVIC; 
     FIG. 3 one field of application of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a base station  1 , a base station conversion unit  2 , a base station antenna  3 , a user antenna  5 , a user conversion unit  6  and a user receiver unit or set top box  7 . The conversion units  2 ,  6  up and down convert the HF signals transmitted between the antennae  3  and  5  in known fashion. 
     The present invention provides that a forward data stream, for example a television program emitted from the base station  1 , is provided with gaps, so that a user may send information or requests back to the base station through a return data stream interposed in the gaps. 
     To produce gaps in a data stream D 1 , which is to be emitted by the base station  1 , the data stream D 1  is fed into a FIFO memory  10 , as shown in FIG.  1 . The data stream D 1  is simultaneously directed into control circuit St 1 , represented by block  20 . As a function of the data and of a predefined time, control circuit St 1  generates control signals for the readout operation from the FIFO memory  10  and activation signals for modulator Mo, shown at block  30 , or demodulator De, shown at block  40 . (As a specific example of the control circuit generating control signals as a function of the data and of a predefined time, the control circuit St 1  can create the gaps periodically, and set the gap duration to a time based on the volume of traffic in data stream D 4  or D 1 . Of course, other combinations are possible such as creating gaps based on the volume of data in data stream D 1  and setting the gap duration to a fixed time.) The control circuit St 1  also generates activation signals for the corresponding conversion, upwards by means of an up converter UC, shown at block  50 , or downwards by means of a down converter DC, shown at block  60 . The signal flow in the HF channel and, of course, also in the feeding and receiving devices is stipulated by control circuit St 1 . The signals can be routed from the conversion units to the signal conditioning units in accordance with the same time regimen. 
     A switch  70  directs the data flow within the base station  1 . As shown in FIG. 1, the switch  70  is such that the main or forward data stream is sent through the modulator Mo to be up converted to the antenna  3  and sent to the user. When a signal from the user is to be received, the user signal is down converted and demodulated and stored in a memory  80 , which can then be read out as data stream D 4 . The switch  70  in this condition is switched downwards from the position shown in FIG.  1 . 
     At set top box  7 , the control circuits St 2 , shown at block  120 , are available to the users and are normally switched to a receive mode, as shown. The main data stream having the gaps is received through user antenna  5 , down converted through down converter  160 , demodulated through demodulator  140 , stored in memory  180 , and sent out as data stream D 2 . This data stream D 2  is, for example, a television program the same as that supplied by data stream D 1 , but slightly time delayed as a function of at least the transmission distance between the antennae and of the storage time in the memories. 
     Only when control circuit St 1  at the base station  1 : (a) produces a gap in the main data stream and switches over to receive, and the previously transmitted data stream permits a switch over, and (b) when a request from this user is provided, does the corresponding control circuit St 2  switch over the set top box  7  and user converter unit  6  to a send mode. (A request from the user may be determined, for example, by the user sending data through data stream D 3  into a FIFO memory  110  and St 2 .) When a gap in the main data stream arrives (or a switch over signal is received, as described below), the switch  170  is then switched downward from that shown in FIG. 1, and the data in FIFO memory  110  is read out through modulator  130 , up converter  150  and antenna  5  to the base station as the gap occurs in the main data stream. 
     The temporal gaps in the data stream of the base station are advantageously subdivided on the one hand into gaps for first time calls and user signals, which periodically recur, but whose time periods are defined as a function of the volume of traffic and of the distance to the base station, and on the other hand into gaps for digital information pertaining to the user. After a request signal has been transmitted by a user device in the gaps for first-time calls and user signals, and this request has been accepted by the base station, digitalized information can be transmitted by the user device in the gaps of the base station&#39;s data stream provided for that purpose. A similar transmission scheme is known from mobile communications, for example the GSM (Global System Mobile Communications) standard, which permit a time advance as a function of the user&#39;s distance to the base station. 
     A switch-over signal can be transmitted from control circuit St 1  to the converter unit, just as when polarization received from satellite systems is switched over, through a different voltage over the HF supply line. In the same way, the converters can be fed over this line. These advantageous ways of utilizing the feeder line for the transmission in both directions, for the transmission of the supply voltage and of the switch-over voltage can, of course, also be effected on the user side. Thus, it is the case when this method is applied that only one line to the converter unit is needed. 
     FIG. 3 depicts the advantageous use of the present invention in a single connection. Depicted is a connection from a video server  4  via base station  1 , converter or conversion unit  2  (up converter UC, down converter DC), a radio or microwave-communication field  9  and, again on the user side, to the converter or conversion unit  6  (up converter UC, down converter DC), and to the auxiliary device  7  (set-top box). 
     The connection shown allows the signal flow in both directions, the base station specifying the time regimen, i.e., the gaps. The converters comprise, on both sides of such a connection, units for converting to higher frequencies (up converters UC) and units for converting to lower frequencies (down converters DC). Illustrated on the user side is also a personal computer  15  with keyboard  16  and a video device  17 . 
     In cable distribution networks that are not designed for backward transmission, the backward channel can be so constituted that each subscriber has at its disposal in the ISM (in-service monitoring) band a transmitter which bridges the pathway to the head-end station of the cable distribution network. In the head-end station, a conversion to the previously described time slot process can take place for the (further) backward channel between the base station and the head-end station. On the way back between the subscriber and the head-end station, a frequency hopping method can be applied. 
     Analog television signals may likewise be transmitted in the forward and backward direction, the control circuits only being able to draw upon the pulse frames and external control signals for analysis purposes. The present method is not limited to HF channels. It can generally also be applied to the feeder lines to the converters and even to an adjoining cable. The control devices St 1  and St 2  can be microprocessors. 
     The following is a description of several of the abbreviations used herein: 
     ATM—asynchronous transfer mode, packet-assembled asynchronous transmission operation 
     BS—base station 
     D 1 —data stream, transmission data stream of the base station 
     D 2 —data stream, receiving data stream of the user 
     D 3 —data stream, transmitting data stream of the user 
     D 4 —data stream, receiving data stream of the base station 
     FS—television unit 
     St 1 —control circuit of the base station 
     St 2 —control circuit of the user 
     DC—down converter, converting to low frequency 
     De—demodulator 
     FIFO—first in first out, data storage unit 
     LMDS—local multipoint distribution service system, local microwave system (f=10 GHz-60 GHz) 
     MAC—medium access control, access control 
     MMDS—multichannel multipoint distribution service system, multipoint microwave distribution system (f=2-10 GHz) 
     Mo—modulator 
     NIU—network interface unit, transition unit to the network 
     PC—personal computer 
     Sp—memory device 
     STB—set-top box, auxiliary device at subscriber 
     Ta—keyboard 
     TDMA—time division multiple access, time division for simultaneous accessing of various subscribers 
     UC—up converter, converting to higher frequency 
     VOD—video-on-demand 
     VS—video server, computer station for supplying many TV programs, which are simultaneous, but vary in time 
     MPEG signals—video, sound and data signals, which are coded in accordance with the standard instructions of MPEG (Motion Picture Expert Group).