This application is a continuation in part of application Ser. No. 08/939,368 filed Sep. 29, 1997, now U.S. Pat. No. 6,021,307, which is a continuation in part of application Ser. No. 08/644,838 filed May 10, 1996, now abandoned, which is a continuation in part of application Ser. No. 08/279,424 filed Jul. 25, 1994, now abandoned, and application Ser. No. 08/255,649 filed Jun. 8, 1994, now abandoned. These four patent applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to information distribution, and more particularly to distributing information using a broadcast channel and a bi-directional communication channel. 
     BACKGROUND OF THE INVENTION 
     Recent advancements in modem and computer technology allow large amount of digital data to be transmitted electronically. A number of information providers (such as newspaper and magazine publishers) and on-line information distributors have formed partnerships to deliver newspaper and other information on-line. In this system, a subscriber uses a computer and a modem to connect, through a regular phone line, to the computer of an on-line information provider. The subscriber can retrieve information, including newspaper articles, stored in the computer of the information provider. 
     On-line delivery of newspaper has many advantages. For example, the information can be updated throughout the day while the printed version is printed only once or twice a day. Further, it is possible to do text-based searches on the information. However, it is found that on-line deliver of newspaper and other information is slow. For example, a subscriber has to wait many seconds for a newspaper article to be delivered. The quality of the electronic newspaper is low. For example, in order to reduce storage and communication requirements, graphic images appeared in the printed version are not universally supplied in the on-line version of newspaper. One of the reasons for such poor performance is the limited bandwidth of communication channels used by on-line information distributors. Another reason is that information is centrally processed by the computer at the site of the information distributor, with the result that each subscriber only gets a small slice of the time of the computer. 
     SUMMARY OF THE INVENTION 
     The present invention uses two channels to deliver digital information: a broadcast channel and a bidirectional channel. The broadcast channel is used to deliver the bulb of the digital information to subscribers. The amount of information delivered is preferably sufficient to satisfy the needs of a large number of subscribers so that they do not have to obtain additional information using the bi-directional channel. The broadcasted information is stored on fast storage media located at subscriber sites. As a result, search and retrieval of the broadcasted information is quick. Further, the broadcasted information is processed locally using a dedicated on-site processor instead of relying on the computers of the information distributors. As a result, the load on the computers of the information distributors is reduced. If the subscribers desire to receive additional information relating to the broadcasted information, the bi-directional communication channel is used to transmit the request and the requested information. 
     The distribution costs of broadcast channels are typically much lower than that of a bi-directional communication channel. Consequently, the major portion of information is delivered using low cost distribution channels. For a large number of subscribers, the broadcasted information will provide all the information they normally need. Thus, expensive bi-directional communication channels are used only occasionally. 
     These and other features and advantages of the present invention will be fully understood by referring to the following detailed description in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing showing an information distribution system of the present invention. 
     FIG. 2A shows a newspaper article as displayed on a monitor of the information distribution system shown in FIG.  1 . 
     FIG. 2B shows the contents of the broadcast information that corresponds to the newspaper article of FIG.  2 A. 
     FIG. 3 shows another embodiment of the information distribution system of the present invention. 
     FIG. 4 is a schematic drawing showing another information distribution system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram of an information distribution system  100  in accordance with the present invention. In this embodiment, system  100  is designed to electronically distribute newspaper. It should be pointed out that system  100  can also be used advantageously to distribute other types of information. System  100  contains a plurality of subscriber units (such as units  102  and  104 ) each connected to a bi-directional communication channel (e.g., telephone connections  106  and  108  coupled to units  102  and  104 , respectively) and a satellite transponder  110  for broadcasting digital data to these subscriber units. Telephone connections  106  and  108  (which could be line-based or wireless) are coupled to a central database  109 . In system  100 , satellite transponder  110  is used to broadcast the content of a newspaper to the subscriber units while telephone connections  106  and  108  are used to provide additional information (stored in central database  109 ) to subscriber units  102  and  104 , respectively, on a demand basis. 
     The structures of these subscriber units are substantially identical; consequently, only one of these units, such as unit  102 , is described in detail. Unit  102  contains an antenna  116  for receiving broadcast signals from satellite transponder  110 , a signal/data processor  118  for performing signal and data processing functions, a monitor  120  for displaying the electronic newspaper, and an input device  122  (such as a keyboard and/or a mouse). 
     Signal/data processor  118  contains a transponder interface  132  for processing transponder signal received from antenna  116 . Transponder interface  132  typically contains a low noise receiver for receiving high frequency (e.g., C or Ku band) transponder signal and a “universal data interface” for converting the transponder signal to digital data. The retrieved data is stored in nonvolatile storage  134 , such as a hard disk or solid state flash memory. Preferably, satellite transponder  110  broadcasts the newspaper data at predetermined times. Thus, a real-time clock  136  is preferably used to turn on interface  132  at the predetermined times. Processor  118  contains a microcomputer  140  that coordinates the operation of clock  136 , nonvolatile storage  134 , and interface  132 . Processor  118  also contains a communication interface  142  for sending and receiving digital data from central database  109  through telephone connection  106 . 
     The time for broadcast is preferably chosen when communication load of transponder  110  is at a low level (e.g., around mid-night). As a result, the cost of information delivery is low. Alternatively, the time of broadcasting is chosen by transponder  110  because it knows when communication load is light. In this case, transponder  110  first sends a signal to signal/data processor  118  for alerting processor  118  to receive and process the newspaper information. 
     A user can use the input device  122  and monitor  120  to read the content of the electronic newspaper stored in nonvolatile storage  134 . In this embodiment, the complete content of the newspaper is stored in nonvolatile storage  134 . The term “complete content” means that the user is able to read the newspaper without relying on information stored in central database  109  (although other embodiments may deliver less than the complete content). In this aspect, system  100  functions in a similar way as the distribution of a conventional printed newspaper. However, the digital data of the electronic newspaper delivered by satellite transponder  110  preferably contains linkage reference that allows fast retrieval of additional information from central database  109 . 
     If the newspaper information received from satellite transponder  110  is sufficient to satisfy the needs of a user, signal/data processor  118  will not activate telephone connection  106 . However, if the user wishes to receive additional information relating to an item mentioned in the electronic newspaper (e.g., by selecting at the item using the input device), process  118  will retrieve the information stored in central database  109  using the embedded linkage reference. 
     In system  100  of the present invention, the complete content of the electronic newspaper (including graphics and other multimedia contents, if delivered) is stored in nonvolatile storage  134 , which has fast access time. Further, a dedicated processor (i.e., microcomputer  140 ) is used to process newspaper information. On the other hand, prior art on-line newspaper distribution systems rely on modem to deliver the content of the newspaper stored in a central site. Further, the processor in the central site has to serve many users in delivering the newspaper. As a result, system  100  has superior performance compared to the prior art on-line newspaper delivery systems. 
     If it is desirable to limit circulation of the newspaper to a certain class of subscribers only (e.g., paid subscribers), the data transmitted by transponder  110  could be encrypted. As a result, only subscribers who have a decryption key are able to read the newspaper. In the case, microcomputer  140  also performs decryption functions. 
     FIG. 2A shows an example of a portion of a newspaper article as seen on monitor  120 . In FIG. 2A, the terms that a user may obtain additional information are underlined (or highlighted in other ways, such as setting in different colors, depending on the choice of the publisher). If desired, the user may select these terms using a pointing device, such as a mouse, and signal/data processor  118  will obtain the additional information from central database  109 . 
     FIG. 2B shows the same portion in FIG. 2A as transmitted by transponder  110  (for simplicity, the embedded formatting codes, such as center, bold, etc., are not shown). Each of the terms underlined in FIG. 2A are enclosed by a special symbol (e.g., the “” symbol) and followed by a linkage reference enclosed by another special symbol (e.g., the “” symbol). These symbols are invisible to the users and are recognizable only by microcomputer  140 . 
     When an underlined term in FIG. 2A is selected by a user, microcomputer  140  extracts the linkage reference and transmits it to central database  109 . The linkage reference allows central database  109  to retrieve the necessary information quickly without doing extensive searches. As a result, the response time of system  100  is fast. The retrieved information can itself contains linkage references and can be searched. 
     If the speed of searching and retrieving data by central database  109  is fast, it may not be necessary to include linkage reference in the information broadcasted by transponder  110 . In this case, the user selects (e.g., using the mouse) words and terms he/she is interested in. Signal/data processor  118  transmits the selected items to central database  109 , which searches for matches in its database. Matched information is sent to subscriber unit  102  for processing. 
     The bi-directional channel also allows updating of the broadcasted information. There is typically a time difference between the broadcast and display of information. New information gathered during this time difference can be stored in central database  109  and later transmitted to signal/data processor  118 . 
     In this embodiment of the present invention, satellite transponder  110  is used as the vehicle to electronically broadcast newspaper. However, other broadcast distribution methods can be used. In the present invention, broadcast is defined as one-to-many distribution of information. The broadcast distribution channels do not have to be electrical. For example, the present invention allows the distribution of compact disc read-only memories (CDROMs) encoded with digital information to the subscriber sites. In the case of electrical broadcast communication channels, both wired and wireless can be used. Preferably, unidirectional channels are used for broadcast because of their low cost; however, the present invention does not preclude the use of bidirectional communication channels (such as telephone lines) as means for distributing broadcast (i.e., one to many) information. 
     FIG. 4 is a schematic diagram of another information distribution system of the present invention. Similar elements in FIGS. 1 and 4 have the same reference numerals. In FIG. 4, a plurality of CDs (such as CDs  152  and  154 ) encoded with the above described information are distributed to subscriber units  102  and  104 . Instead of antennas and transponder interfaces, subscriber units  102  and  104  contains CD readers (such as CD reader  156 ). 
     Current technology allows the size of antenna  116  to be as small as 2 feet. The costs of antenna  116  and transponder interface  132  are already low enough to be within the reach of small business or a typical household. The newspaper publisher has to pay for the use of the transponder. However, the costs are comparable to the printing and distribution costs of printed newspaper. It is anticipated that the costs of the newspaper distribution system in accordance with the present invention will be lowered as the number of subscribers increases. 
     FIG. 3 shows another embodiment of a newspaper distribution system  200  of the present invention. System  200  contains a satellite transponder  210 , an earth station  214 , and a plurality of subscriber units, such as units  222  and  224 . Transponder  210  functions in a similar way as transponder  110  of FIG.  1  and subscriber units  222  and  224  function in a similar way as subscriber units  102  and  104  of FIG.  1 . Earth station  214  receives digital data transmitted by transponder  210  using an antenna  216 . The data is distributed to subscriber units  222  and  224  via wired communication channel  228 , such as cable and optic fiber. Other earth stations could be placed in strategic locations throughout the country to serve their respective subscribers in a similar manner as earth station  214  and subscriber units  222  and  224 . As a result, a large geographic area can be served simultaneously by satellite transponder  210 . The advantage of this embodiment is that the equipment costs incurred by the subscriber units are low. 
     In some locations, it may not be desirable to use wired communication channel to link an earth station to subscribers. In such case, wireless communication channel could be used. FIG. 3 shows an earth station  234  that receives transponder signal from transponder  210  using an antenna  236 . Earth station  234  in turn broadcasts the digital data to its subscribers, such as subscriber units  242  and  244 . 
     In one embodiment of system  200 , teletext technology is used to link earth station  234  and subscriber units  242  and  244 . Thus, earth station  234  could be located adjacent to a television transmission station. The digital data received by earth station  234  can be integrated to the vertical blanking interval of a TV signal, which is broadcasted using an antenna  238 . Subscriber units  242  and  244  receive the signal using antennas  239 , and  240 , respectively. The digital data is then retrieved. Various improvements and refinements of the teletext technology are well known and can be incorporated into system  200 . 
     It should be obvious to a person skilled in the art that systems  100  and  200  are not limited to the distribution of newspaper. Further, electronic newspapers of the future may contains contents which are not available in the printed version, such as video and other multimedia compositions. Other information, such as magazines, graphic images, electronic mails, computer games, multimedia work, or interactive movie, could also be advantageously distribution using a system similar to systems  100  and  200 . For example, if it is desirable to distribute interactive movie, the non-interactive portion can be broadcasted while the interactive portion is delivered using a bi-directional channel. 
     There has thus shown and described a novel information distribution system. Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering this specification and the accompanying drawings. All such changes, modifications, variations, uses, and applications are covered by the scope of this invention which is limited only by the appended claims.
An information distribution system encodes a first set of digital data on a plurality of portable read-only storage devices. Additional information is stored in a database that is accessible by using a bi-directional channel. The first set of digital data contains a plurality of special displayable terms, a first non-displayable symbol, a plurality of linkage references, and a second non-displayable symbol. A user can select at least one special displayable term. The linking reference associated with the selected special displayable term is sent to the database via the bi-directional channel. The database uses the linking reference to search for information, and returns the resulting information to the user.