GB2334650A

GB2334650A - Coding a transmitted signal for identification

Info

Publication number: GB2334650A
Application number: GB9800331A
Authority: GB
Inventors: Ronald David Lessels Mackie
Original assignee: Healthcare Technology Ltd
Current assignee: Healthcare Technology Ltd
Priority date: 1998-01-09
Filing date: 1998-01-09
Publication date: 1999-08-25
Anticipated expiration: 2018-01-09
Also published as: GB2334650B; GB9800331D0

Abstract

A transmitted signal is coded by a method which comprises generating, in response to a triggering event (1), (e.g. the R-wave in an electro cardiographic signal), a signal comprising a sequence of a plurality of bursts (2). The coded signal is detected by counting the number of bursts (2) received during a predetermined time (3). This avoids interference between signals from a plurality of patients. Application is to signals using radio, line, infra-red or ultrasound.

Description

METHOD FOR CODING A TRANSMITTED SIGNAL This invention relates to a method for coding a transmitted signal, and more particularly is concerned with a simple method for coding a signal which contains relatively little information, such as an address of a sending device or of a receiving device.

In heart rate monitoring it is common practice for a chestworn transmitter to emit a short simple burst of radio waves for each heart beat. A wrist-worn receiver detects the transmitted radio waves, times the beat-to-beat intervals and calculates and displays a heart rate.

Operating range in this application is about 1 metre, nevertheless problems of interference can arise when users cluster sufficiently close together such that their receivers detect signals from more than one transmitter.

There is therefore a need for a method of coding the transmitted signals in a manner which is capable of differentiating various transmitters in order that users can approach within the operating range of another transmitter without encountering mutual interference.

Because the transmission range (of about 1 metre) in heart rate monitoring is so short, the number of potentially overlapping users is small, probably only two or three.

Thus the number of available unique codes needed is also small and may amount to no more than ten.

It is therefore an object of the present invention to provide a method for coding a transmitted signal in a simple manner which permits a small number of transmitters to operate without mutual interference.

According to the present invention there is provided a method for coding a transmitted signal comprising generating, in response to a triggering event, a signal comprising a sequence of a plurality of bursts and detecting the coded signal by counting the number of bursts received during a predetermined time.

It should be borne in mind, however, that the use of a method for coding of this kind is not limited to heart rate monitors and has application in other consumer products.

For incorporation in such products it is necessary for the coding method (and consequently the means of implementing the method) to be simple, without the need for expensive or high-tolerance components.

Moreover, a method for coding of this kind is not limited to use with radio frequency transmissions and can be employed, for example, where a number of units communicate over a common conducting wire with one or more receivers, also connected to the common conductor, and further where transmission is by infra-red (or other) radiation or ultrasound.

The transmitted signal may be generated in response to each triggering event or in response to a predetermined number of triggering events.

The triggering event may comprise the R-wave in an electrocardiographic signal from a beating heart.

The predetermined time may commence with reception of a first burst.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of the time relationship between a triggering event, a number of bursts of transmitted signals and a reception time window of a receiver in one embodiment of a method for coding according to the present invention; and Figure 2 is a diagrammatic illustration of reduced signal traffic when signals are transmitted only once in every four triggering events.

Figure 1 illustrates a specific application of the method for coding according to the present invention as applied to heart rate monitoring in which the occurrence of heart beats is signalled from a chest-worn radio transmitter to a wrist-worn receiver. Nevertheless, it will be understood by the skilled person that the basic principles illustrated in Figure 1 apply to any situation in which an event triggers the prompt sending of a coded message to a receiving device.

Figure la illustrates a triggering event 1, in this case the R-wave in an electrocardiographic signal from a beating heart.

Figure lb illustrates the transmission of a signal comprising a plurality of bursts 2 of radio waves in response to the triggering event 1. In order to differentiate between transmitters, the emitted signal is necessarily more complex than a simple burst of radio waves. Differentiation is achieved by adapting each transmitter to transmit a number associated with that transmitter, the number being represented by a close spaced sequence of a corresponding number of simple bursts 2 transmitted in response to each triggering event 1. Thus, for the code N, the number of bursts is N for each triggering event.

Figure lc illustrates the opening of a time window 3 in response to the detection in a receiving device of a first burst 2 of radio waves. The receiving device then counts the number of bursts 2 received during the time window 3.

Only if the number of bursts 2 received during the time window 3 corresponds to an expected number of bursts corresponding to the predetermined code for that receiving device does the receiving device recognise the signal.

Provided the time window 3 is of sufficiently long duration, and the bursts of radio waves are sent at a sufficiently low rate for clear reception, there are no stringent timing requirements on the transmission and reception procedures. In contrast to the commonly used asynchronous binary serial data communications methods in which relatively expensive quartz crystal oscillators are generally required to control the transfer of data, the accuracy of clocks required for implementation of the present invention is of the order of 20 percent of the mean and such low accuracy can readily be achieved with timing circuits made from low cost resistors and capacitors.

Signal reception and decoding is simple, merely requiring the counting of bursts 2 within the time window 3 which opens when the first burst is received. Clearly, the time window 3 must be sufficiently long to encompass the longest code that can be transmitted at the slowest expected rate.

Because signal duration increases linearly with code number, this method for coding is not suitable where a very large number of different codes is required. Long signals transmitted by neighbouring transmitters have an increased probability of overlapping in time. Such clashes corrupt the signal when received and give rise to reception dropouts because the signal cannot be recognised by any reception device.

The incidence rate of this problem depends upon the signal length and the average time between all signals. Thus, the limit on the usable code range must be assessed for each application, although this is a straightforward procedure in itself. For heart rate monitoring, and transmission at each heart beat at the commonly used frequency of 5 kHz, a code range of about ten represents an acceptable compromise.

Where there are significant numbers of corrupted signals caused by clashes from neighbouring transmitters using the same radio frequency, the same piece of wire, or using infra-red radiation or ultrasonics, it can be arranged that a signal is transmitted only once every few events (say, once every two, three or four events). This is shown in Figure 2 where a signal is transmitted only at every fourth event. The resulting reduction in signal density reduces the signal clash rate very significantly. In the case of a heart rate monitor, this procedure would mean that the reception device would measure the time taken for four heart beats, rather than every beat-to-beat interval. This reduction in information resolution is not significant because in practice when every beat-to-beat interval is measured, the heart rate is almost always averaged before display.

Thus the coding method according to the present invention is of very general application in situations where the instants of occurrence of events require to be conveyed promptly from a sensing site to a monitoring station. This is irrespective of whether the transmission medium comprises conducting wire, radio waves, infra-red radiation, ultrasonics or other means of transmission.

Claims

CLAIMS 1. A method for coding a transmitted signal comprising generating, in response to a triggering event, a signal comprising a sequence of a plurality of bursts and detecting the coded signal by counting the number of bursts received during a predetermined time.
2. A method according to claim 1, wherein the transmitted signal is generated in response to each triggering event.
3. A method according to claim 1, wherein the transmitted signal is generated in response to a predetermined number of triggering events.
4. A method according to any preceding claim, wherein the triggering event comprises the R-wave in an electrocardiographic signal from a beating heart.
5. A method according to any preceding claim, wherein the predetermined time commences with reception of a first burst.
6. A method for coding a transmitted signal substantially as hereinbefore described with reference to the accompanying drawings.