Abstract:
A data transmission system for sending a plurality of signals from a sender to at least one receiving terminal. The sender comprises a memory for storing the plurality of signals, and a controller for causing the stored plurality of signals to be sent in a predetermined order, and for changing the predetermined order when the updating of a signal takes place, so that an updated signal is sent with priority to the receiving terminals.

Description:
This application is a continuation of application Ser. No. 08/070,015 filed on Jun. 1, 1993 now abandoned which is a continuation application of application Ser. No. 07/610,401 filed on Nov. 7, 1990 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement of a data transmission system suitable for CATV (cable television) systems. 
     2. Related Art 
     In a CATV system, video signals such as signals from VTRs or ITVs and signals of commercial broadcasting programs are transmitted from a sender through a transmission line (for example, a coaxial cable) to terminals such as tuners and video monitors. 
     It has become the practice in recent CATV systems that in order to cope with the diversification of information, character information blocks produced on a computer are sent to terminals in the form of character codes, along with video signals such as animation signals of the NTSC system, such character information blocks being displayed in color on monitors at the terminals. Such character information blocks are usually transmitted to terminals by assigning appropriate frequency bands in an operating frequency range to the character information blocks and by effecting a frequency-shift-keying or phase-shift-keying modulation of the character information blocks. In order to inhibit a wide frequency band from being occupied, it is common that a plurality of character information blocks are sent serially through one frequency band. 
     In a CATV data transmission system of the prior art, a plurality of video sources such as VTR, ITV and received signals of commercial broadcasting programs are respectively connected to video modulators to which different carrier frequencies are assigned. The modulated video signals are sent through a transmission line to tuners in receiving terminals. Character information blocks produced on a computer are fed through an adapter to an FSK modulator and sent to the tuners along with the modulated video signals. These video signals and character information blocks are demodulated at the tuners and displayed on monitors at selected channels. 
     FIG. 1 shows a manner by which character information blocks are serially sent in a CATV system of the prior art. In this figure, each information block D1, D2 . . . or Di includes character information to be displayed on one picture and comprises identification data, code data and property data. If one character picture is of the size of (80 columns)×(20 rows), the amount of the code data is 1600 bytes and if each property is defined by one byte, the property data is also 1600 bytes. FIG. 1 indicates that a plurality number i of information blocks D1, D2, . . . , Di are sent in one cycle. 
     At each receiving side, an operator designates a picture number to display, on a monitor, a character information block having the identification data corresponding to the designated picture number. Since the information transmission is effected uni-directionally just as in broadcasting and the character information blocks are received at terminals regardless of the order in which the character informnation blocks are sent, the character information blocks are at all times sent cyclically. 
     FIG. 2 is a block diagram showing the construction of an adapter 100 used in a CATV system of the prior art. Character information blocks D1, D2, . . . , Di produced serially on a computer are fed through an input interface 102 to a buffer memory 104 in an adapter 100. The stored blocks are sequentially taken out of the buffer memory 104 under the control of a data transmission control unit 106 and sent through an output interface 108 to an FSK modulator. 
     As could be understood from the above description, the adapter serves to store character information blocks in the buffer memory 104 and cyclically sends the stored character information blocks at a constant period. The computer provides new character information blocks to the adapter only when the updating of character information blocks takes place. It is thus unnecessary for the computer to deal with the control of information transmission, which reduces a load on the computer. On the other hand, it should be noted that, even when the updating of information blocks takes place, the computer is requested to send, as character information blocks for the initial picture, the whole character information blocks for a complete picture including new character information blocks produced when the updating of information blocks takes place. 
     In a data transmission system of a prior art CATV system, since the order of character information blocks produced on a computer and sent toterminals is fixed even when some character information blocks are requested to be updated, it usually takes some time for new character information blocks to reach a terminal from a sender, which results in a significant delay in the time taken to update information blocks to be displayed at monitors. Such a problem becomes more serious as the number of character pictures increases. For example, it takes 100 milliseconds to transmit a character information block for one picture, and therefore it takes 100×100 milliseconds to transmit character information blocks for 100 pictures. 
     In order to solve this problem, Japanese Patent Public Disclosure No. 157488/87 proposes that each terminal has a means by which character information blocks received at the terminal are stored in a data memory while some information blocks are renewed with the latest ones and the character information blocks stored in the data memory and included in a designated picture are read out from the data memory to be displayed. This can shorten the time taken to display updated information blocks. 
     Since such a means as described above is provided at a terminal, however, each terminal must have a data extracting circuit, line buffers, data storing RAMs and the like, which makes the circuit construction complicated. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a data transmission system wherein the updating of information data is effected at a sending side within a minimum delayed time and each terminal has a simple circuit construction. 
     in order to achieve the above-described object, the present invention provides a data transmission system for sending a plurality of signals from a sender to at least one receiving terminal. The sender comprises: 
     means for storing the plurality of signals; 
     means for causing the stored plurality of signals to be sent in a predetermined order; and 
     means for changing the predetermined order when the updating of signal takes place, so that an updated signal is sent with priority to said receiving terminals. 
     According to the construction described above, the present invention is advantageous in that data transmission can be effected at higher speed and that the respective terminals can have a simpler structure. 
     Other objects and advantages of the invention will become clear from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a manner by which character information blocks are sent serially in a CATV system of the prior art; 
     FIG. 2 is a block diagram showing the construction of an adapter used in a sender of a prior art CATV system; 
     FIG. 3 schematically shows an embodiment of a data transmission system according to the present invention; 
     FIG. 4 shows a manner by which character information blocks are sent serially in the data transmission system according to the present invention; and 
     FIG. 5 is a block diagram showing the construction of an adapter used in a sender of the system shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a block diagram schematically showings an embodiment of a data transmission system for a CATV system in accordance with the present invention. In this figure, video signal sources 1a, 1b, . . . , 1n supplying video signals from VTRs or ITVs or video signals of commercial broadcasting programs are connected to corresponding video modulators 2a, 2b, . . . , 2n. Carrier frequencies f 1 , f 2 , . . . , f n  are assigned to the respective video modulators so as to correspond receiving channels. 
     A computer 3 produces character information blocks, each block including character information to be displayed on one picture. The character information blocks are sent through an adapter 4 to an FSK modulator 5 where the character information blocks are frequency-shift-keying modulated using a carrier frequency f d . The video signals output from the video modulators 2a, 2b, . . . , 2n and the character information blocks output from the FSK modulator 5 are sent to a transmission line 14. In order to compensate for attenuation in these signals, an amplifier 6 is provided the middle of the transmission line to amplify the video signals and the character information blocks. 
     The amplified video signals and character information blocks are input to tuners 7a, 7b, . . . , 7m at terminal sides and demodulated thereat to be displayed on video monitors 8a, 8b, . . . , 8m at channels which operators select. 
     FIG. 4 shows the manner in which character information blocks are serially sent in accordance with the present invention. This figure indicates that a series of character information blocks D1, D2, . . . , Di have been sent in a first cycle and that, when the updating of the character information block Di takes place Just after the character information block D1 has been sent in a second cycle, the updated character information block Di is sent immediately behind the character information block D1. 
     Next, an operation of the adapter 4 shown in FIG. 3 will be explained with reference to FIG. 5 schematically showing the construction of the adapter 4. Character information blocks D1, D2, . . . , Dn sent from the computer 3 are input through an input interface 9 to a buffer memory 10 and stored therein. Character information blocks output from the buffer memory 10 are fed through a switch 11 to an output interface 12. The buffer memory 10 and the switch are controlled by a data transmission control unit 13. 
     When the updating of a character information block does not take place, the stored character information blocks are taken out of the buffer memory 10 in the order in which the information blocks are stored in the buffer memory 10, and sent sequentially through the switch 11 to the output interface 12. On the other hand, when an updated character information block is provided from the computer 3 to the adapter 4, the data transmission control unit 13 detects this fact and interrupts the taking of the character information blocks out of the buffer memory 10. The switch 11 is then changed to connect to the input interface 9 and the updated character information block is sent through the output interface 12. At the same time, the updated character information block is stored in a corresponding area in the buffer memory 10. It is noted that whether a character information block is an updated one or not is simply decided in accordance with information supplied from the computer 3. 
     As can be understood from the description above, when a character picture displayed on a monitor which an operator watches should be updated to another, a character information block to be updated is immediately sent to the terminal side, which results in a shortening of the transmission time. Moreover, the transmission side is constructed such that an updated character information block is sent with priority, which serves to simplify a circuit construction of each terminal. 
     The present invention has been described in detail with particular reference to a certain embodiment thereof, but it should be understood that modifications and variations can be effected within the scope and spirit of the invention. For example, in the embodiment describe above, the explanation has been given to the case where the computer provides an updated character information block for one picture. Instead, it is possible to send an actually updated portion of a character information block from the computer to the adapter which, in response to the updated portion, modifies a corresponding information block stored in the buffer memory and sends the modified block. Such an operation can easily be accomplished by defining a PARTIALLY UPDATE information signal as one of the signals which the computer outputs to the adapter and rewriting the buffer memory when the adapter detects the PARTIALLY UPDATE information signal. 
     Further, a signal provided from the computer to the adapter is not limited to a character information block. The computer can provide a control information signal by which a particular action is designated to be taken at the terminal sides.