GB2078468A - Broadcast information transmission - Google Patents

Abstract

A gale warning message is transmitted in binary code by non- audible phase modulation of the carrier of a LF broadcast transmitter. The message data is recovered by a receiver 12, detector 14 and data recovery and CRC check circuit 16. Gale warning messages are identified by a header code in the data and are routed to a control unit 20, store 22 and display device 24. The display device shows the sea area(s) for which the warning was issued and details of the gale such as wind force and direction and expected duration. <IMAGE>

Description

SPECIFICATION Broadcast information transmission The present invention is concerned with the broadcast transmission of information utilizing existing public broadcast transmissions. Many systems based upon this idea have been used or proposed. For example, television broadcasts in the U.K. currently carry teletext information and radio broadcasts can carry information modulated on to the carrier at frequencies well above the audio band. The present invention is concerned with a system which can be used when the desired area of coverage is large.

This requirement arises in particular in relation to the broadcasting of gale warnings or other weather information to shipping. In the U.K. these are currently broadcast, as flashes, inter alia on the B.B.C's LF (200 kHz) Droitwich transmitter which has a very wide coverage. Messages which are broadcast only as flashes interrupting normal programme material can only be picked up by a person listening with at least half an ear to the normal transmission. It is desirable to transmit the information in such a way that a receiver can be set up to respond automatically in one of various possible ways such as are suggested below.

With a view to meeting such requirements the invention provides a low frequency radio receiver as set forth in claim 1 below, to which reference should now be made. Experiments have shown that binary signals can be reliably transmitted utilizing a degree of phase modulation such that it is inaudible to listeners under all but very exceptional circumstances, such as when the programme material is already severely distorted, even without the superimposition of the phase modulation.

The digital signals are preferably organised in blocks or frames, each starting with a start code or frame alignment word. The frame preferably ends with a CRC (cyclic redundancy check) word allowing error detection/correction. In the example given below an 8-bit start code is followed by 36 data bits and a 10 bit CRC word. The data and CRC bits are biphase encoded, doubling the number of transmitted bits, so the total frame length becomes 8 + 2(36 + 10) = 100 bits. At 50 baud each frame will take two seconds to transmit.

The value of 36 data bits has been chosen for the specific example given below in which a frame can carry a complete gale warning, made up of sea area identificatiion, wind force and direction and delay. However the invention is not limited to the transmission of any specific data format.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the format of a frame for carrying gale warning data; Figure 2 is a block diagram of a gale warning receiver; and Figure 3 shows a display device for use with the gale warning receiver.

Fig. 1 shows a frame consisting of an 8-bit frame alignment word, a 12-bit fixed header code, a 12-bit sea area code, force, direction and delay codes each of 4-bits, and a 1 0-bit CRC word. With the exception of the alignment word all bits are biphase encoded so, as already noted the whole frame requires 100 bits at the channel data rate (which is 50 baud).

No detailed description of the transmitter will be given since an existing transmitter merely requires the addition of a phase modulator and a 50 baud binary data source which can incorporate manual or computer controlled facilities for assembly of the data to be trarismitted.

By way of example only, the phase modulation parameters may be as follows. The biphase coding is such that a source code of "O" is coded and transmitted as 01 while "1" is coded as 1 0. Transmitted binary 1 causes a phase advance of the carrier by 11 if from its unmodulated value and a binary 0 causes the carrier phase to be retarded by 114 ; thus the phase deviation has a peak-topeak value of 22 < . The spectrum of the 50 baud biphase modulating signal is shaped using a filter with a response of - 3dB at 25 Hz and extinction at 37.5 Hz. A similar filter is recommended for the receiver, so that the overall channel response is - 6 dB at 25 Hz, with a 50% cosine roll-off.Biphase encoding ensures that there is no d.c. -- component in the phase-modulation and, therefore, no longterm carrier phase-shift.

The frame of Fig. 1 is employed as follows.

A gale warning is identified by a specific header code. Since 1 2 bits allow 4096 header codes, the system allows many different types of distinguishable messages to be transmitted. It is envisaged that, as well as gale warnings there will be regular time code transmissions, e.g. every minute, identified by another specific header code and giving GMT in hours (0 to 23) and minutes.

The 1 2 bit sea area code allows reasonable flexibility in the context of the waters around the British Isles which are divided into 1 4 inner sea areas, 1 4 outer sea areas and 9 Atlantic sea areas, a total of 37 areas. Although B.B.C. gale warnings do not normally cover the Atlantic areas, the invention will make this feasible since it is anticipated that the phase modulated digital data will have a greater useful range than the ordinary AM transmissions on 200kHz. 37 areas can be distinguished by a 6-bit code but a 12-bit code allows groups of sea areas also to be designated so that a single warning can be sent out for a group of areas, rather than having to send individual warnings in succes- sion.

4 bits more than suffice to convey gate force if forces 6 to 1 2 are to be covered plus cancellation of the warning (wind force less than 6). Coverage from 6 to 12 rather than 8 to 1 2 is envisaged for the benifit of small-boat sailors.

4 bits allow wind direction to be specified to a resolution of 22+". If greater resolution is required one of the gale force bits can be taken over into the direction code. It is desirable also to be able to indicate whether the wind direction is steady, veering or backing.

A further two bit code may be used to carry this information or the receiver may work out the information itself from successive transmissions, i.e. by comparing the old wind direction with the new.

Finally, 4 bits allow adequate information to be given as to delay which is currently expressed by the terms: Imminent - up to 6 hours away Soon - 6 to 12 hours away Later - over 12 hours away.

A two-bit code can carry this information (freeing two bits for the steady, veering, backing information if required) or a four bit code can carry the actual delay in hours, up to 1 5 hours.

Fig. 2 shows one form of receiver embodying the invention. An omnidirectional aerial 10 feeds a fix-tuned, permanent standby receiver 1 2 connected to a phase modulation detector 14 which detects all the phase modu lated data. A conventional data recovery and CRC check circuit 1 6 recovers the bits following each recognised alignment code 01110001, corrects any single bit errors, rejects frames with more than a single bit errors, and passes on the data bits to a type sorting circuit 18 which basically decodes the gale warning and time code headers and routes the last 24 message bits on the outputs labelled gale warning messages and time codes respectively.

The gale warning messages pass to a control unit 20 interacting with a store 22. These elements may be very simple because the basic need is merely to latch the 12+4 + 4 + 4 message bits and actuate display devices in accordance with the decodes of the bit groups. However, as explained below, other facilities may be incorporated and the control unit 20 can have built into it more or less any range of desired facilities and may be based upon a microprocessor, for flexibility of design. The control unit is connected also to a display device 24 which displays both the relevant sea area and gale details. The control unit can also actuate an alarm device 25, shown as an audio alarm device, whenever a gale warning message comes in.

The time codes on the other hand are routed to a consistency check circuit 26 which is connected to an internal clock circuit 28 feeding a time display 30. The consistency check circuit provides an indication of satisfactory working by way of indicator 32 so long as time codes are coming in regularly and exhibit consistency with the internal clock.

The circuit can also correct small internal clock discrepancies to compensate for drift in the rate of the internal clock. If the receiver is not working properly, i.e. not decoding mean ingful time codes, it can also be arranged to actuate the alarm device 25.

Fig. 3 shows one form which the display 24 can take. An outline map of the British Isles and adjacent shores also shows the afore mentioned inner and outer sea areas, demarcated by boundary lines 40. Within each area is mounted an LED 42 or other form of indicator. The control unit 20 causes those LEDs which pertain to the sea areas for which a gale warning is issued to light up, in accor-8 dance with the decode of the 1 2 bit word designating sea areas.

Wind direction is indicated by a rose of say 1 6 LEDs 44. That LED which indicates wind direction is illuminated in accordance with the decode of the direction word. The gale force is shown digitally by a display 46 in accordance with the decode of the force word. The delay is also shown digitally by a display 48 in accordance with the decode of the delay word.

A single LED 32' corresponds to the indicator block 32 in Fig. 2. The time display 30 of Fig. 2 is also shown as a digital display in Fig.

3.

Very many alternatives exist in relation to the display devices. Wind direction may be displayed alphanumerically, e.g. by numerical bearing or compass point bearing, gale force may be expressed in knots instead of on the Beaufort scale, and so on. Very many different visual display devices also exist, including LEDs and LCD displays, and can be used as is appropriate.

As so far described the receiver can only display one gale warning and will display that most recently received - which may not be the most relevant warning to the sea area in which a vessel finds itself. Some selection facility is required and may take various forms. The display device can incorporate a preselector for indicating the sea area currently occupied by the vessel. For example switches could be located in sea areas together with the LEDs 42, the appropriate switch being closed. The selection data is then fed back to the control unit (connection 50 in Fig. 2) which selects for storage and display only gale warnings relating to say the selected sea area or a directly contiguous area. Instead of switch selection, area selection may be automatic from a navigational computer or other navigational aid which keeps track of the vessel's position.Such selection can be effected remotely. Another possibility is to provide for storage of all current gale warnings in the store 22 and to provide a selector whereby the user can bring up for display all different warnings in turn. One implementation of this possibility would be based on the display of Fig. 3 with switches in the sea areas as already suggested above. The switches can be push button switches which incorporate the LEDs 42. The control unit lights up all LEDs in areas for which a gale warning has been issued. The gale warning message is displayed for whichever of these areas has been selected by pushing the button. Such selection remains effective after the button is released (until another is pressed) and the area to which the message pertains is indicated by causing its LED to flash.An expensive receiver in a public area such as a harbour area or on a yacht club can have enough display devices to show simultaneously all of the current gale warnings.

It may also be useful to provide two displays for a ship's receiver for the purpose of showing the two most recent gale warnings for the selected area, enabling the user to see the likely course of the gale and also whether the wind is steady, veering or backing. If the delay times for each warning are counted down hour by hour, the user will have a good idea of the weather he may expect. Furthermore the "no gale" warning can be issued with its delay time and be treated like any other gale warning, i.e. be displayed with its delay. The user will then be able to see from the two displayed delay times the anticipated duration of the gale. Similar facilities can be provided with a simpel display device and a push button or other switch for selection between the first and second stages of the gale.

Claims (8)

1. A low frequency ratio receiver for use with data carrying gale warning messages, comprising a phase modulation detector and data recovery circuit arranged to extract binary data from an amplitude-modulated transmission on a carrier which is also phase-modulated by the binary data, a control unit and store arranged to store at least some of the binary data, and a display device arranged to display the stored data, the display device comprising means for displaying at least a plurality of: sea area(s) of the warning, wind force, wind direction, and delay to the gale.

2. A low frequency receiver according to claim 1, comprising a sea area display means in the form of a map showing sea areas, and indicator devices in the individual areas.

3. A low frequency receiver according to claim 2, wherein each area also includes a manually operable device for selecting that area, for display or any gale warning pertaining thereto.

4. A low frequency receiver according to claim 1, 2 or 3, comprising a wind direction display means in the form of a rose of individually operable indicator devices.

5. A low frequency receiver according to any of claims 1 to 4, comprising a digital display means for the delay to the gale, and means for counting down the displayed delay in accordance with the elapse of real time.

6. A low frequency receiver according to any of claims 1 to 5, wherein the store is arranged to display a plurality of gale warnings and means provided for selecting which stored warning is displayed.

7. A low frequency receiver according to claims 1 to 5, wherein the store is arranged to display a plurality of gale warnings and there is a plurality of display devices arranged to display different one of the stored warnings.

8. A low frequency receiver according to claim 7, wherein the display devices display successive stages of the gale for a sea area.