GB2251767A

GB2251767A - Multichannel operation of rds

Info

Publication number: GB2251767A
Application number: GB9122683A
Authority: GB
Inventors: John Lawrence Riley; Andrew Kevin Mcparland; Simon John Parnall
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1990-10-25
Filing date: 1991-10-25
Publication date: 1992-07-15
Also published as: AU8749391A; FI931853A0; GB9023307D0; FI931853A; EP0555304A1; JPH06504413A; AU649740B2; WO1992008303A1; GB9122683D0

Abstract

Data, for example, data over a traffic message channel, is transmitted over a radio data system by multiplexing across several available channels. Data is transmitted at intervals over each channel so that data is transmitted at different times on different channels. As a result, the rate at which data is transmitted over the plurality of channels taken in combination is greater than the transmission rate over any one channel.

Description

MULTICHANNEL OPERATION OF RDS The present invention relates to the Radio Data System (RDS) and, in particular, its use as a Traffic Message Channel ('TMC1). Such a system is discussed in J.L. Riley, March 1989 "Some ideas on a Traffic Message Channel (TMC) for the Radio Data System (RDS)" BBC Research Department Technical Memorandum EL-1758, March 1989 TMC data cannot occupy all the available capacity of an RDS channel: other background data must also be present. Other types of RDS data occupy more than three quarters of the data capacity available and, consequently, the capacity available for TMC is likely to be limited to 10-20% of the total, that is, one or two groups per second out of the eleven provided on the RDS system.

The minimum data rates stipulated for the main RDS features prevent the TMC data capacity in any one channel being greater than this. During the gaps between TMC data there is an opportunity to gather TMC data from other channels and effectively increase the amount of TMC data which can be gathered in a given time.

In accordance with the invention, there is provided a method of transmitting data over a radio data system in which the data is transmitted over a plurality of radio data channels, data being transmitted at intervals over each channel such that data is transmitted over the respective channels at different times and such that the channels carry the data transmission in turn, whereby the rate at which data is transmitted over the plurality of channels taken in combination is greater than the rate at which it is transmitted over any one channel.

We have appreciated that an improvement could be obtained if the data were time-multiplexed on different channels so that a receiver could continually re-tune in between receiving TMC groups of data on one channel to acquire similar data on other channels. If a broadcaster has control of several channels, there is an opportunity to spread the TMC data across these channels and to arrange them in such a way that they are time-multiplexed, each occupying different time-slots with reference to the absolute clock time (CT) which each channel can broadcast via the CT features. An intelligent receiver can, therefore, frequency 'hop' from channel to channel to acquire all the TMC data available in an effectively continuous stream.

The TMC groups would need to be "bunched-upn or compressed on each channel so that there is an opportunity, in between processing them, to re-tune to another channel for more data.

The multiplexing would be at a low cycle rate in the order of a second or so. Although this restricts when the TMC data can be inserted on any one channel, it should be possible for an intelligent RDS encoder to arrange the overall transmission of RDS groups so as to maintain the minimum requirements for the repetition of the standard group types.

This method requires synchronisation of the TMC data on different channels but this can be accomplished by reference to the Clock Time (CT) Groups. The necessary phasing information could be signalled as part of the TMC data, in a system message.

A system in accordance with the invention will now be described in greater detail, by way of example only.

The RDS data transmission rate is defined to be 1187.5 bit/sec + 0.125 bit/sec. This implies a Group transmission rate of between 685.0240386 and 685.168269 Groups/minute.

TMC message cycles are restrained to start and finish at minute-edge boundaries governed, when present, by a CT Group 4A.

The end of this Group is defined to occur within 100 ms of the minute/second edge and it occupies one of the 685+ Group slots each minute. Some minute periods will contain 684 other RDS Group slots between successive CT Groups and others will contain 685. Considering the allocation of RDS Group types to these slots, it would be possible to reserve particular slots for TMC groups without affecting the repetition rates of other RDS data.

CT Groups would not interfere with these reserved slots. If other RDS data needed to occupy the reserved TMC slots, the TMC data would not be sent at that time. For example, Type 14B Groups which signal the start of a traffic message broadcast via EON, might be considered of higher priority than TMC and the next TMC data would be 'pushed on' until the next vacant normal slot in the RDS data system.

The switching cycle envisaged would be between about < second and several seconds. Within this cycle, each channel is allocated a time-slot to which TMC data is confined, although not every group period is necessarily occupied by TMC data. Indeed, the basic requirements of the minimum repetition frequencies of the main features must be met first. The precise phases of the time slots allocated to each channel would be signalled in separate system messages carried within the TMC data. In a multichannel system, the time-slots of the other channels and their RF frequencies would also be signalled both within the TMC data and using the EON feature.

As an option, and on any radio channel, the TMC data (Type BA) Groups can be arranged at the RDS encoder to occupy declared time-slots which are defined by a particular phase relationship with the minute or second edge observed after a Clock Time (CT) Group. This phase relationship can be signalled in a system message. If the TMC data, carried on different channels, occupies different time-slots and with a sufficient time-margin between them, a receiver can increase its TMC data throughput.

The allocated time-slots represent those times outside which TMC data is not allowed for that channel. It is not intended that all slots allocated to 'N', for example, would be occupied by TMC Groups; that would infringe the minimum repetition rates for other background RDS data. Where three adjacent slots are indicated, for example, only one or maybe two of these would actually be used for TMC.

The margin of several RDS Groups between TMC-Group slots is required both for re-tuning/RDS decoding and to allow for any time-slot which results from the non-synchronisation of Group boundaries between one channel and another.

Two alternative possibilities A and B are illustrated in the Table appended to this description. In each of these cases, there are two channels, represented by 'N' for National and 'L' for local.

In alternative A with a multiplex cycle of 12 the data transmission rates might be: Channel N Channel L Total TMC capacitv Gps/bunch TMC% Gtslbunch TMC% Groups/cycle Groups/second 1 8.3 0 0 1 0.95 1 8.3 1 8.3 2 1.90 2 16.7 0 0 2 1.90 2 16.7 1 8.3 3 2.85 2 16.7 2 16.7 4 3.80 In alternative B with a multiplex cycle of 18 the data transmission rates might be: Channel N Channel L Total TMC capacity Gps/bunch TMC% Gps/bunch TMC% Groups/cycle Groups/second 1 5.5 0 0 1 - 0.63 1 5.5 1 5.5 2 1.27 2 11.1 0 0 2 1.27 2 11.1 1 5.5 3 1.90 2 11.1 2 11.1 4 2.54 3 16.7 0 0 3 1.90 3 16.7 1 5.5 4 2.54 3 16.7 2 11.1 5 3.17 3 16.7 3 16.7 6 3.81 The two possible phases for TMC data can be identified by a single address bit. Other examples could be defined in a similar manner and these could spread to multiplex cycles longer than about one second as used in the examples given. Further channels could be multiplexed at these longer multiplex cycle times but this would further restrict the broadcaster in the timing of the bunches of TMC data.

It should be possible to re-tune and gather RDS data from different FM channels at a sub-second rate. In order to allow time for synchronisation margins between different transmitters it would probably be unwise to consider more than about two excursions per second but more channels over a cycle of a few seconds might be possible.

A listener accesses TMC data by a single function selection. The receiver is sufficiently frequency-agile and intelligent to perform all the necessary processing to render its operation transparent to the listener.

The receiver recognises a TMC channel by one of two methods: by simply detecting Type 8A RDS groups or by indirect reference from a variant of a Type 1A group. Once a TMC channel is found its data can immediately be decoded and used to build up a stored repertoire of current messages in the receiver. This is a straightforward case of single channel TMC operation.

In a multichannel TMC scenario, some of the TMC data contains system messages which gives information about the format of messages in the tuned channel and the PI codes of other relates TMC channels. In the tuned channel, this information includes phasing data which defines the time-slots allocated to the channel. The receiver then knows when it can expect TMC data and when it is free to do other things. The PI codes'of related channels allows the receiver to acquire their TMC data as well.

The frequency of the other channels and their time-slots can be signalled by the EON feature. Once the receiver has acquired the system information about all the related TMC channels, it can 'hop' between them at will and operate at the full TMC data capacity intended by the broadcaster.

Two experimental receiver systems were used to determine whether adequate response times could be achieved.

RDS Reference Receiver This receiver incorporates a remotely-tunable front-end, RDS demodulator and decoding facilities. For the test, it was programmed in HP BASIC to operate a cycle of alternate tuning to two off-air signals and displaying the decoded PI code in each case. The speed at which it could do this process could be adjusted until the RDS decoding became corrupted. This was found to occur when about a 1h second was allowed for each re-tuning, settling and RDS decoding process. The front-end receiver used in the Reference receiver is not intended for fast switching; it is basically a monitoring receiver. For this application, the time constants in the AGC feedback loop have been reduced to allow a faster re-tuning response.This cannot, however, be expected to approach the switching speed of a synthesiser controlled directly as part of an integrated receiver design.

This 3 second cycling to access two channels could be considered a worst case".

ESVP and SUN The ESVP receiver is capable of re-tuning faster than the Reference receiver. The demodulated FM multiplex signal was fed into an RDS decoder and the data/clock connected to a parallel interface associated with a PC. The PC acted as an interface with the SUN and converted the data bit-stream into a byte-serial form. The PC was connected via a serial interface to a SUN workstation which was programmed in C to decode the RDS data in real-time. The same re-tuning cycle, described above, was implemented with this system and it was found that a re-tuning, settling and RDS decoding process could be achieved in between one-quarter and one-third of a second.

The number 684 happens to have factors of 9, 12, and 18 so that reserved TMC slots could be allocated in such a way as to allow differently phased slots to be used on different channels and with sufficient free groups in between to allow for re-tuning. Two examples are given below, both are for two channels; a national channel denoted by N and a local channel denoted by L. It should be stressed that not all the allocated time-slots can be filled by TMC Groups, because of the need to maintain normal RDS data repetition rates.

TABLE Alternative A. Multiplex cycle of 12

L 1 CT N N N L L L N N N L L L N N N L L L minute/ second edge Alternative B. Multiplex cycle of 18

L 1 CT N N N N N L L L L L N N N N N L L L L L minute/ second edge

Claims

CLAIMS 1. A method of transmitting data over a radio data system in which the data is transmitted over a plurality of radio data channels, data being transmitted at intervals over each channel such that data is transmitted over the respective channels at different times and such that the channels carry the data transmission in turn, whereby the rate at which data is transmitted over the plurality of channels taken in combination is greater than the rate at which it is transmitted over any one channel.
2. A method according to claim 1 in which each data transmission on each channel occurs at a substantially fixed interval or intervals from a clock signal transmitted over the channel at regular intervals.
3. Apparatus for receiving data transmitted over a radio data system by a method according to any of claims 1 or 2, the apparatus comprising tuning means operable to tune automatically to each of a plurality of channels in turn so as to receive data transmitted at intervals on those channels.