WO1997016909A2

WO1997016909A2 - Detection of multitone signals

Info

Publication number: WO1997016909A2
Application number: PCT/GB1996/002637
Authority: WO
Inventors: Peter John Garrard; Peter Hibbittt
Original assignee: Advanced Technology (Uk) Limited
Priority date: 1995-10-31
Filing date: 1996-10-30
Publication date: 1997-05-09
Also published as: GB9522243D0; WO1997016909A3; GB2306723A

Abstract

A means is disclosed for the communication of data over noisy channels, in particular, using multitone signals. The receiver samples the signal for a period substantially longer than 40 ms, and the samples are decoded, if possible, to multitone symbols. A signal is accepted as valid when at least some predetermined numbers of the samples are decoded to the same symbol.

Description

DETECTION OF MULTITONE SIGNALS

The present invention relates to low power communication of data signals within a noisy environment and in particular to the recovery of a required data signal from a noisy signal.

One known method of communicating data is a method where the digital data is converted into analogue dual tone multi frequencies (DTMF) which are then transmitted. Analogue signals are used because the recovery of an analogue signal is feasible at much lower levels than a digital signal. The DTMF signal is then detected by a receiver. To help ensure the tone received is a valid signal an analogue delay means is employed. This monitors the input tone signal for a period of typically up to 40ms until the signal is judged to be effective and only then is the input signal processed. The tone detector is normally a phase-lock loop. We have found that the present communication method is not satisfactory for an electronically noisy environment and for the processing of a noisy signal.

According to one aspect of the present invention there is provided a device for the communication of noisy data comprising transmission means for the encoding of a digital signal into at least two analogue tone frequency signals and the transmission of the analogue tone frequency signal through a transmission medium, receiving means for detecting and decoding the analogue tone frequency into a digital signal, in which the detecting process comprises sampling the analogue tone frequency over a period of time substantially greater than 40ms, the samples of the analogue tone frequency each being decoded when possible into a digital signal, and a valid digital signal being accepted for outputting when at least a predetermined number of the samples taken in said period of time give rise to the same digital signal.

Thus, during the sampling period, due to noise some samples will not be capable of being decoded into a valid digital signal, but other samples will be successfully decoded. It is necessary for a predetermined number of samples to be successfully decoded during the sampling period, for the "average" of the successfully decoded digital signals (they should all be the same) to be taken as the valid digital signal.

The predetermined number is preferably greater than 20% of the samples.

Preferably the analogue tone frequency is in the form of a multi tone multi frequency. Alternatively the analogue tone frequency might be simplified to the form of a dual tone multi frequency.

Also preferably the analogue tone frequency signal is transmitted in the form of radio waves. Alternatively the analogue tone frequency signal is transmitted through electronic circuitry and/or electrical wiring.

Preferably the transmission unit comprises microprocessor means, logic encoder means and transmitter means.

Preferably the microprocessor means comprises a plurality of microprocessors. The logic encoder means preferably comprises a plurality of logic encoders. It is also preferable that the transmitter means comprises a plurality of transmitters. Preferably the receiver means comprises a receiver and a microprocessor. Alternatively the receiver means comprises a plurality of receivers and microprocessors.

Also preferably the receiver device samples the analogue tone frequency signal ten times in at least one half of one second.

By way of example only the invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows a representation of the flow of communication signals in a system in accordance with the invention;

Figure 2 shows a dual tone multi frequency matrix;

Figure 3 shows how the transmission time or number of samples is increased in dependence on the signal to noise ratio;

Figure 4 shows a representation of the transmitted analogue communication signal; and

Figure 5 shows a representation of the received decoded digital signal.

With reference to Figure 1 , a device 1 for the communication of noisy data comprises a dual tone multi frequency (DTMF) encoder 2 comprising a digital input 3 and an analogue output 4, a transmitter 5, a receiver 6 and a dual tone multi frequency decoder 7 comprising an analogue input 8 and a digital output 9. The digital input 3 receives a digital signal. The digital signal is split into separate strings 10 containing four digital bits. Each string 10 comprises a different combination of digital bits.

With reference to Figure 2 the separate strings 10 may be represented by a combination of two of the tone frequencies of columns 1 1 , 12 and 13 and rows 14, 15 and 16. The combination of the two tones forms an analogue dual tone multi frequency signal which is transferred from the analogue output 4 to the transmitter 5. The dual tone multi frequency encoder may be in the form of a telephone tone dialler such as a Holtek™ HT-9101.

The dual tone multi frequency signal is then transmitted by the transmitter 5. The encoder 4 is included within the Holtek™ HT-9101. The transmitter 5 could be in the form of a radio transmitter such as a Scantronic™ 4689-UK-00 module.

The receiver 6 receives the transmitted signal and supplies it to the analogue input 8. A typical receiver and decoder which may be used is a Holtek™ HT-91 70 device. The receiver could be in the form of a radio receiver such as a Scantronic™ 4666-U -00.

Referring to Figure 4 the receiver device 6 may be modified so that the dual tone multi-frequency signal 20 received is sampled over an extended period of data transmission time 26 and the samples 22 are then examined. Those samples of the dual tone multi-frequency signal that are capable of being decoded to give a digital signal are decoded. Other samples, due to noise 24 , may be incapable of being decoded. A minimum requirement, say 20%, is set for the number of samples that must be successfully decoded in the extended period. Provided that this requirement is met, then the digital value of the successfully decoded samples is taken as the valid digital signal. This gives improved data recovery for a noisy signal over a greater distance of up to 1000 metres. This method also provides more reliable data recovery for noisy signals compared with conventional methods.

It will be appreciated that the number of digital bits contained in each separate string may vary and is not limited to 4 bits.

The dual tone multi-frequency signal 20 represents a compression of eight bits of data 30 per sample frame 22. The eight bits of data are represented by a combination of continuous tones for one frame 22. Each consecutive signal 20 can comprise a different dual tone multi-frequency signal representing a different string of eight bit data. Ten samples 28 of the signal 20 are taken per frame 22. Two of the ten samples 28 taken must be successfully decoded for the sample to be taken as a valid signal. The time taken for each sample frame 22 is one half of one second. The valid samples are converted back into digital signals 32 by the decoder 7. As the dual tone multi-frequency signal 20 represents eight bits of data 30 the total sample time 26 can be increased without a significant decrease in processing time compared with known methods of communicating data.

It will also be appreciated that each string may represent a predetermined command.

Figure 3 shows how the transmission time, or number of samples, is increased to deal with a reduction in the signal to noise ratio. It will also be appreciated that the number of analogue tone frequencies transmitted concurrently is not limited to 2. That is, a multi tone multi frequency signal can with advantage be used.

Claims

1. A device for the communication of noisy data characterised in that the said device comprises transmission means arranged to encode a digital signal into at least two analogue tone frequency signals and to transmit the analogue tone frequency signal through a transmission medium, receiving means adapted to receive the analogue tone frequency signal from the transmission means and arranged to detect and decode the analogue tone frequency into a digital signal, the receiving means being arranged to perform a detecting process comprising sampling the analogue tone frequency over a period of time substantially greater than 40ms, the samples of the analogue tone frequency each being decoded when possible into a digital signal, and a valid digital signal being accepted for outputting when at least a predetermined number of the samples taken in said period of time give rise to the same digital signal.

2. A device as claimed in claim 1 in which the predetermined number is greater than 20% of the samples.

3. A device as claimed in claim 1 and claim 2, in which the analogue tone frequency is in the form of a multi tone multi frequency.

4. A device as claimed in claims 1 and claim 2 in which the analogue tone frequency is in the form of a dual tone multi frequency.

5. A device as claimed in any of the preceding claims in which the analogue tone frequency signal is transmitted by the transmission means in the form of radio waves.

6. A device as claimed in any of the preceding claims in which the transmission means is in the form of a unit comprising microprocessor means, logic encoder means and transmitter means.

7. A device as claimed in claim 6 in which the microprocessor means comprises a plurality of microprocessors, the logic encoder means comprises a plurality of logic encoders and the transmitter means comprises a plurality of transmitters.

8. A device as claimed in any of the preceding claims in which the receiving means is arranged to sample the analogue tone frequency signal ten times in one half of one second.

9. A device for the communication of noisy data substantially as described herein with reference to Figures 1 to 5 of the accompanying drawings.