GB2030825A - Message broadcast system receiver arrangement - Google Patents

Abstract

An arrangement for use at each receiving station in a message broadcast system, e.g. shore to ship communication, is capable of determining which messages from the broadcast are to be received by the particular station. The arrangement includes means for detecting the level of garble in the message and allows all messages to be received by the station if the level of garble exceeds a predetermined amount.

Description

SPECIFICATION Communication filter arrangements The present invention relates to filter arrangements of the type in which messages are despatched from a central transmitting station to a plurality of receiving stations, each station being equipped with a filter arrangement which allows only those messages addressed to the particular station to be accepted by that station.

A typical environment for such filter equipment could be for communication of a message to a fleet of ships by radio. Currently all messages broadcast from a shore station are received on board all ships of the fleet and the receiving operator determines from identifying designators within the message whether or not the message is addressed to his ship or not. Accordingly the operator receiving the message has to apply a wide variety of skills to determine the identity of the addressed ship. Such skills include complex aspects of perception, judgment and personal knowledge applied to the message content and designator.

Furthermore, the operators function is made increasingly difficult due to garble of the message. Garble can be caused by faults in the transmitting equipment on the shore station or the receiving equipment on board ship, or interference to the radio bearer causing faulty characters of the message to be received. The transmission of a digital signal to a ship is usually in the form of a two-tone audio signal and in transmission from shore to ship it may be reflected several times from the ionisphere and ground or sea surface. Several reflection paths are possible and can cause a reduction in the signal strength of one or both of the frequency components of the two-tone signal and is therefore, another cause of garble.

Accordingly it is an aim of the present invention to provide a filter arrangement for use at a receiving station which allows a secure acceptance of messages addressed to the particular station.

According to the present invention there is provided a filter arrangement for use at each receiving station in a message broadcast system, including means for conditioning a data processing unit in accordance with message identity information for enabling the unit to determine which messages from those broadcast by the message broadcast system are to be received by the receiving station, and the filter arrangement includes means for detecting the level of garble of the messages and for allowing all messages to be received by the station if the level of garble exceeds a predetermined level.

The present invention will be readily understood from the following description of one embodiment and should be read in conjunction with the following drawings in which: Figure 1 shows a block diagram of a filter arrangement according to the present invention.

Figures 2-7 show tables showing a level of tolerance for each readability state, in terms of a percentage match for each designator length.

Figure 8 shows the software input-output scheduler software structure.

Figure 9 shows the man-machine interface software structure.

Figure 10 shows the message processing software structure.

Figure 11 shows the filtering criteria of the filter arrangement.

Referring to Fig. 1 the microcomputer MC is shown and consists of microprocessor MP of the Intel 8080A type constructed from MOS LSI components. The board also includes 2k bytes of programmable read-only memory PROM, -41k bytes of random access memory RAM, a data input port DIP, by which the filter is enabled or disabled and a universal asynchronous receiver and transmitter UART1, all of which are interconnected by a bi-directional bus system which handles all relevant signals for device addressing, control and data transfers.

An Interface IB is also shown and includes a priming interface PI consisting of a 20mA current loop circuit and is connected to UART1. This interface is equipped to handle standard ITA5 (International Telegraph Alphabet code No. 5) type teletype and through this interface the operator can up-date the information stored in the microcomputer MC.

A message interface Ml is provided and includes two electrical line interfaces LIA and LIB, either of which may be selected by a line input selection device LIS. Line interface LIA is arranged to handle standard ITA2 teletype by way of a 20mA current loop circuit for both input and output purposes and line interface LIB is arranged to handle for input purposes TTL (transistor transistor logic) signal levels and for output purposes CCITT V24 (Comite Consultatit Internationale de Telegraphie et Telephonie) type coded signals.

Telegraph characters are transmitted to and from the message interface by way of UART2 which is connected to the bi-directional bus, and similarly telegraph characters are transmitted to and from the primary interface by way of UART1. Both UART 1 and 2 contain all the necessary logic for character detection and transmission including start and stop bit generation. UART1 operates at 110 bits/sec representing 10 characters/sec. Each character is 11 bits in length consisting of 1 start bit, 7 data bits, 1 parity bit and 2 stop bits, UART2 operates at 75 bits/sec. representing 10 characters/sec. Each character is 7jbits in length consisting of 1 start bit, 5 data bits and 12 stop bits.

The software consists of an input-output scheduler which enables the message and priming interfaces to operate simultaneously and on demand transfers control to the message processing software, man-machine interface software which handles the priming functions and the message processing applications software which provide the filtering function.

Analysis of a message designator is performed by the filter and include both general message format checks as well as detailed validation.

The Garble detection function determines the number of errors found in the message.

Knowing the size of the message the average error rate can be calculated for the particular message in terms of a percentage. This figure is used to select one of four readability states 5, 4, 3 or below 3. The following values represent such states: Readability state 5 0-1 % error Readability state 4 1-5% error Readability state 3 5-1 0% error Readability state below 3 over 10% error The readability state of a message determines the tolerance that may be applied to a particular length of designator in terms of a percentage match obtained from a character string matching algorithm. In this way the addressee correlation is related to level of garble.The filter performs address correlation using the character string matching algorithm and it is advantageous to represent the tolerance applied to a certain length of designator by the number of characters found matched.

In order to proceed with a method for determining the tolerance that should be applied to designators subject to garbling it is necessary to know in numeric terms the distribution characteristics of the garble. The nearest approximation is to assume a purely random garbling of characters and using the P.oisson formula the probability of exceeding a specified number of errors in a designator can be calculated knowing the average error rate.

The tables shown in Fig. 2 to Fig. 7 show for each readability state the tolerance in terms percentage match applied to each designator length to give a minimum confidence of 99%.

The tables shown in Figs. 2, 3 and 4 show an overall average confidence of 99.59% at the maximum error rate for each readability state, and the tables shown in Figs. 5, 6 and 7 show a correlation tolerance averaging at 99.92% confidence.

The software will now be described in detail with reference to Figs. 8, 9 and 10 and the block diagram Fig. 1.

Referring to Fig. 8 the input-output scheduler program is shown. The microprocessor MP (Fig. 1) executes the the scheduler program when there is no message traffic to be handled and during idle periods when message processing is complete. The scheduler is responsible for organising all input-output operations from the message interface Ml (Fig.

1) and the priming interface PI (Fig. 1). The scheduler provides two facilities to the manmachine interface software (Fig. 9); priming input function (Step 4) and a priming output function (Step 5). When the man-machine interface software (Fig. 9) requests a character to be input from the priming interface (Step 4) the scheduler responds by suspending execution of the man-machine interface software until a character is input and the scheduler then returns control to the man-machine interface software with the input character (Step 1). Similarly when the man-machine interface software requests an output character to the priming interface it uses the output function of the scheduler to action the request (Step 5).The scheduler suspends the man-machine interface software if the priming interface output is busy and returns control when the output character is accepted (Step 2). When a character has been received by the message interface UART2 (Fig. 1) the scheduler starts execution message processing software (Step 3), and if a character has not been received the program returns to Step 1.

The man-machine interface software stucture will now be described with reference to Fig. 9. It provides the functions necessary for an operator to manipulate a list of designators to be recognised by the filter arrangement. A command set is provided by the software to enable an operator to instruct the filter arrangement to insert a designator, delete a designator or examine the list of designators stored in the filter arrangement.

The software uses the priming input and output facilities of the input-output scheduler (Fig. 8) to receive and send characters to and from the priming interface (Step 1). When receiving characters from the priming interface via the scheduler priming input function the man-machine interface software analyses the command for either insert, delete or examine (Steps 2, 3 and 4). If none of the commands are executed, an error message is output and a further command input is awaited (Step 5). If the command is an insert, further input is taken to be the new designator (Step 6). The list is examined for sufficient space to hold the new designator (Step 7). If there is, it is inserted at the end of the list (Step 8) and a count of the number of designators is updated (Step 9) and the next command is awaited. If there is insufficient space in the list to accommodate a new designator an error message is output to the priming interface (Step 10) and the next command Is awaited. If the command is a delete, further characters identify the selected designator which is removed from the list (Step 11). The number of designators is decreased by one (Step 9) and the next command is awaited. If the command is an examine, the contents of the designator list is output to the priming interface character by character (Step 12).

The message processing software structure will now be described with reference to Fig.

10.

The message processing software is activated from the input-output scheduler (Fig. 8) every time a character is received from the message interface. The first action is to accept that character (Step 1). For every character received a decision is made whether to filter that character or not (Step 2). The decision is made on a number of factors which includes: - the state of the filter enable/disable switch - the state of the priming (primed/un primed) - the number of printing lines received (Step 5) - the current position in the message (Step 6) - the readability state of the message (Step 8) - whether a designator was detected or not (Step 9) The exact criteria are shown in the decision table Fig. 11.

The message is received in ITA2 code and the designators are stored in ITA5 code; therefore to achieve a comparison the message character is converted to ITA5 code (Step 3). Any non-printing characters in the ITA2 set such as null, letter shift and figure shift are not processed further and control is returned to the input-output scheduler (Step 10).

Printing characters are stored in a serially organised store which performs the function of a shift register and in which all designator comparisons and prosign detection is performed (Step 4). A check is made on the line structure of the received message (Step 5). It should have a maximum of 69 printing characters followed by an end of line function (EOLF) comprised of the character sequence carriage return, carriage return, line feed. The end of line function (EOLF) and overlength line check function records the number of deviations from these rules and updates a message error count accordingly. It also counts and records the number of printing lines in the received message.

The message heading is divided into a number of format lines each of which may comprise a number of printing lines. Each format line has one or a number of character sequences, known as prosigns, which indicate the start of the format line. The format lines should be received in a given sequence. Step 6 analyses for the prosigns and records deviations from the correct sequence. For each format line there is an analysis function which determines the deviation from the rules that apply to the construction of that format line.

The current format line is known after Step 6 and Step 7 selects the appropriate function for verification of it and executes it. When Step 7 is executed the number of errors is known and by dividing by the number of characters received the readability state of the message is calculated (Step 8). The final task is to determine if a primed designator has been received (addressee correlation, Step 9).

The readability state of the message is used to select a tolerance table which indicates the permitted tolerance for any designator from 4 to 20 characters long. The tolerance is represented by the minimum number of characters in that designator that need to be matched for that designator to be recognised as wanted.

This matching processes is applied to all designators in the list.

The message processing software then returns control to the input-output scheduler (Step 10) and awaits the next message character.

Claims (8)

1. A filter arrangement for use at a receiving station in a message broadcast system, including means for conditioning a data processing unit in accordance with message identity information for enabling the unit to determine which messages from those broadcast by the message broadcast system are to be received by the receiving station and the filter arrangement includes means for detecting the level of garble of the messages and for allowing all messages to be received by the station if the level of garble exceeds a predetermined level.

2. A filter arrangement according to claim 1 wherein the data processing unit includes storage means for storing information related to message identities, and the arrangement comprises at least one input/output data path connected to a receiver/transmitter device by way of which the stored information can be amended.

3. A filter arrangement according to claim 2 wherein at least one other input/output data path is connected to a receiver/transmitter device through which messages are received and transmitted.

4. A filter arrangement according to claim 2 wherein the input/output data path handles ITA5 type coded signals.

5. A filter arrangement according to claim 3 wherein the input/output data path handles ITA2 type coded signals.

6. A filter arrangement according to claim 3 wherein the input/output data path handles transistor transistor logic voltage levels for input purposes and CCITT V24 type coded signals for output purposes.

7. A filter arrangement according to claim 1 wherein the data processing unit establishes the level of garble of a received message by executing the step of (i) checking the line structure of the received message, (ii) checking and verifying the character sequence of the message heading, (iii) calculating the readability state of the message, (iv) checking whether a message designator has been received, and (v) checking the addressee correlation and determining the level of garble in accordance with the permitted tolerance for the designator length.

8. A filter arrangement as claimed in any preceding claim substantially as described with reference to the accompanying drawings.