GB2286753A - Transfer of an analog control signal through a medium - Google Patents

Abstract

An analog control signal is transferred through a medium having band pass characteristics, for example a telephone line 117. A first signal of a fixed amplitude is generated having a first frequency f1 within the passband of the medium. A second signal of a second frequency f2 is generated within the passband of the medium. The amplitude of the second signal is controlled with the analog control signal RSSI to provide a modified second signal. The first signal and the modified second signal are sent through the medium; their amplitudes are compared 170 at the receiving end of the medium; and an analog control signal is provided dependent on the relative amplitudes of the first signal and the modified second signal. A method to control a remotely controlled unit is also provided as well as a method to control a radio communication system and a method to control apparatus. Remotely controlled apparatus is also provided. <IMAGE>

Description

METHOD AND APPARATUS RELATING TO TRANSFER OF AN ANALOG CONTROL SIGNAL THROUGH A MEDIUM Field of the Invention The present invention relates in general to a method and to an apparatus to transfer an analog control signal through a medium having band pass characteristics, for example a telephone line. It also, separately, relates to a method to control a remotely controlled unit and to a method to control a radio communication system as well as to control apparatus and remotely controlled apparatus.

Background to the Invention Telephone lines or similar transmission paths are frequently unsuitable for transfer of low frequency or direct current control signals because normally band limiting elements exist preventing the transmission of signals below a certain frequency (e.g. 300 Hz) and because unknown attenuation of an analog signal occurs depending on the length of the line and other factors.

It would be useful to use such a line for the transfer of RF receiver signal strength indicator (RSSI) output voltage to a distant comparator facility in a multi-site RF communication system.

In prior art arrangements, a single signal is sent, the frequency of which is modulated in accordance with the low frequency signal.

(For example it is swept from 300 to about 350 Hz, where 300 Hz may represent a low RSSI value and 350 Hz a high RSSI value.) To filter these signals out of an overlying audio signal, a filter with a large bandwidth (300-400Hz) is necessary. This cuts out a large part of the wanted audio range.

Summarv of the Invention According to a first aspect of the invention, a method is provided to transfer an analog control signal through a medium having a given frequency passband, from a transmitting end to a receiving end thereof, comprising the steps of: generating a first signal of a fixed amplitude and of a first frequency within the passband of the medium; generating a second signal of a second frequency within the passband of the medium, controlling the amplitude of the second signal with the analog control signal to provide a modified second signal; sending the first signal and the modified second signal through the medium; comparing the amplitudes of the first signal and the second modified signal at the receiving end of the medium; and providing an analog control signal dependent on the relative amplitudes of the first signal and the modified second signal.

In accordance with a second aspect, the invention provides a method to control a remotely controlled unit by a controlling unit, connected to each other by the aid of a medium having a band pass characteristics, comprising the steps of: in the controlling unit, generating a first signal of a fixed amplitude and of a first frequency within the passband of the medium; generating a second signal of a second frequency within the passband of the medium; controlling the amplitude of the second signal with the analog control signal to provide a modified second signal; and sending the first signal and the modified second signal through the medium; and- in the remotely controlled unit: comparing the amplitudes of the first signal and the second modified signal at the receiving end of the medium; providing an analog control signal dependent on the relative amplitudes of the first signal and the modified second signal; and using the adjusted signal to control the remotely controlled unit.

The method may be used to control a radio communication system having at least two base stations.

Thus, there is also provided a method to control a radio communication system, having a plurality of base stations each connected to a control unit by means of a medium having a given frequency passband, comprising the steps of: in each base station: generating a first signal of a fixed amplitude and of a first frequency within the passband of the medium; generating a second signal of a second frequency within the passband of the medium; controlling the amplitude of the second signal with the analog control signal to provide a modified second signal; and sending the first signal and the modified second signal through the medium; and in the control unit: comparing, for each base station, the amplitudes of the first signal and the second modified signal at the receiving end of the medium; providing, for each base station, an analog control signal dependent on the relative amplitudes of the first signal and the modified second signal; and comparing, in the control unit, the analog control signals for the different base stations and selectively attenuating further signals from the different base stations dependent on a comparison of the analog control signals.

In accordance with further aspects of the invention, control apparatus and remotely controlled apparatus are provided in accordance with the claims.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawing Fig. 1 shows a remote control system in accordance with a first embodiment of the invention.

Fig. 2 shows an example of a low frequency signal represented as being transmitted through a medium having band pass characteristics, in accordance with the preferred embodiments of the invention.

Fig. 3 shows a radio communication system in accordance with a second embodiment of the invention.

Fig. 4 shows a radio communication system in accordance with a third embodiment of the invention.

Fig. 5 shows details of the mobile and base station of the system of Fig. 4.

Detailed Description of the Preferred Embodiment Fig. 1 shows a first preferred embodiment of the invention. A radio communication system 100 comprises a controlling unit, in this case a base station 101, a controlled unit 102, and a console 103. The base station contains a receiver 105, having a receiver (Rx) circuitry 104 connected to an antenna 106. The receiver produces an audio signal and a RSSI signal (Received Signal Strength Indicator signal). The audio is filtered in notch filter 107 (or in a high pass filter). The RSSI signal is connected to a frequency generator 110. The frequency generator generates two frequencies fl and 2 by means of the circuits 111 and 112 respectively. The circuit 112 is connected to an amplitude modulator 113 which is controlled by the RSSI signal from the receiver.The signal from the first circuit 111 and the signal from the amplitude modulator 113 are connected, together with the filtered audio signal from the receiver, to an adding circuit 115.

The output of the circuit 115 is fed to a transmission medium, in this case a transmission line 117.

The transmission line 117 is connected to a pre-amplifier 120 in the controlled unit 102. The amplified signal is connected to a notch filter 125 and to an A/D converter 140. The digitised signal is connected to a digital signal processor 145. In Fig. 1 functional blocks of the digital signal processor are shown. Two decoders 150 and 155 are connected to the digitised signal to detect signals with tlie frequencies fl and 2 respectively.

The decoded signals are fed to two RMS detectors 160 and 165. The ratio between the two signals from the RMS detectors is established in the divider 170. The output of the digital signal processor controls the amplifier 130 which receives a signal from the notch filter 125. The output of the amplifier 130 is sent to the console 103 which contains a loud speaker 135.

The operation of the radio communication system will now be described. When a radio signal appears on the antenna 106, the receiver circuitry 104 establishes the signal strength of the received radio signal and demodulates the radio signal to an audio signal. The signal strength of the received radio signal, represented as RSSI (Receive Signal Strength Indicator) in Fig. 1, is a low frequency signal which can be used, for example, to adjust the volume at the other end of the transmission line 117.

The transmission line 117, which may be a normal telephone line, has normally band limiting elements, and therefore has a band pass characteristics. The lower cut off frequency for a telephone line is in the order of 300 Hz.

The circuits 111 and 112 of the frequency generator 110 generate a first signal of a first frequency fl and a second signal of a second frequency n. The frequencies fl and f2 are higher than the lower cut off frequency of the band pass characteristics of the transmission line 117 and lower than the higher cut off frequency of the band pass characteristics of the transmission line. They are at the lower end of the audible range. Suitable frequencies are 300Hz and 310Hz, or 300Hz and 305Hz. The amplitude of the second signal is set in proportion to the RSSI signal in the modulator 113. The first signal and the second signal are added together with the audio signal from the receiver 105. The added signal is sent through the transmission line to the controlled unit 102.

In Fig. 2 the pass band characteristics of the transmission line 117 is shown. The first signal of the first frequency fl and the second modulated signal of the second frequency f2 are indicated at F1 and F2 respectively. It is also indicated schematically how a RSSI signal (shown to the left in Fig.

2) modulates the amplitude of the second signal. Preferably, the difference between the first and the second frequencies is small comparatively with the frequency range of the band pass characteristics of the transmission line, to minimise the band width used by the first and second signals on the transmission line. The first and second frequencies are preferably placed close to either the lower cut off frequency of the band pass characteristics of the transmission line or the higher cut off frequency of the band pass characteristics of the transmission line to simplify the removal of the first and second signals in a later stage, as will be described below.

In the controlled unit 102, the signal on the transmission line 117 is amplified by a pre-amplifier 120 and passed on to a notch filter 125 and an A/D converter 140. Notch filters 107 and 125 are tuned to attenuate signals of the first and second frequencies fl and n. Notch filters are efficient filters and can have only about 20Hz or less of bandwidth. After the filtering, the audio signal from the base station is retrieved. It should be understood that the notch filter could be exchanged for a high-pass filter in the case when the first and the second frequencies, fl and n, of the first and second signals are placed close to the lower cut off frequencies of the band pass characteristics of the transmission line.Accordingly, a lowpass filter may be used when the first and second frequencies, fl and n, of the first and second signals are placed close to the higher cut off frequencies of the band pass characteristics of the transmission line.

The digitised signal from the A/D converter 140 is fed to the digital signal processor (DSP) 145. The decoders 150 and 155 filter the digitised signal and signals of the first frequency fl and the second frequency f2 are extracted respectively. These extracted signals are fed to RMS detectors 160 and 165 to establish the amplitudes of a signal of the first frequency fl and a signal of the second frequency f2, respectively. The amplitudes of the signals will depend on the attenuation along the transmission line 117.

The relative amplitude of the signal having the second frequency 2 (from the RMS detector 165) relative to the amplitude of the signal having the first frequency fl (from the RMS detector 160) will be unaffected by the attenuation along the transmission line and so will indicate the modulation by the RSSI signal carried out in the amplitude modulator 113.

The RSSI signal will be retrieved by adjusting the amplitude of the signal of the second frequency f2, from the RMS detector 165, using the amplitude of the signal of the first frequency fl, from the RMS detector 160, as a reference. This is carried out by the dividing circuit 170 where a value of the amplitude of the signal of the second frequency 2 is used in the numerator and a value of the amplitude of the signal of the first frequency fl is used in the denominator. The RSSI signal is retrieved as the quotient produced by the dividing circuit.

The retrieved RSSI signal is (by way of example) used to adjust the amplification of the amplifier 130. The input of the amplifier receives the retrieved audio signal from the notch filter 125. The adjusted retrieved audio signal produced by the amplifier 130 is forwarded to a loud speaker 135 in a console 103.

The decoders 150 and 151, the RMS detectors 160 and 165 and the divider 170 are all realised by software routines run in the DSP 145. Of course, these functions can also be implemented in hardware.

A radio communication system 300 is shown in Fig. 3, this being a second embodiment of the invention. Base stations 305, 310, 315 and 320 are connected to a control unit 102 by the aid of telephone lines 355, 360, 365 and 370. The telephone lines have normal band limiting elements with a lower cut off frequency and a higher cut off frequency. In the control unit 102, the wires from the base stations are connected to input connectors and therefrom to a selector switch 325 and to a decode and control unit 330. An output from the decode and control unit controls the selector switch. The common contact of the selector switch is fed to a notch filter 327. The notch filter is connected to a console 103 which comprises a console control unit 335 and a loud speaker 340.

In accordance with the first embodiment, each base station 305, 310, 315 and 320 establishes the signal strength of the received radio signal, labelled RSSI signal, and generates a first and a second signal of a first frequency fl and a second frequency f2, respectively, both being higher than the lower cut off frequency of the band pass characteristics of the telephone line and lower than the higher cut off frequency of the band pass characteristics of the telephone line, where the first signal has a fixed amplitude and the amplitude of the second signal is modulated with the RSSI signal. The first signal and the second modulated signal are sent through the telephone line together with an audio signal demodulated by the receiver circuit of the base station from the radio signal.

At the control unit 102, the telephone lines from the base stations are connected to a selector switch 325 and to a decode and control unit 330. For each signal received from each corresponding base station the control unit 330 establishes, in accordance with the first embodiment, the amplitude of a first signal of the first frequency fl and the amplitude of a second signal of the second frequency 2. The amplitudes of the signals will depend on the attenuation along the telephone line to the corresponding base station.

The amplitude of the second signal will also depend on the amplitude control with the RSSI signal carried out in the corresponding base station.

To compensate for the attenuation along the telephone line, the second signal is adjusted by the use of the first signal as a reference, and the RSSI signal is retrieved by dividing a value of the amplitude of the second signal with a value of the amplitude of the first signal. In this way, the decode and control unit 330 establishes the RSSI signals corresponding to each base station.

In the decode and control unit 330 the RSSI signals corresponding to each base station are compared, and the signal from the base station having the strongest RSSI signal, is selected by the aid of the selection switch 325, which is controlled by the decode and control unit 330. Before the audio signal from the selected base station is fed to the console 103, signals of the first and second frequencies are attenuated by the aid of a notch filter 327. In the console, the selected audio signal reaches the control unit of the console 335 and eventually is fed to the loud speaker 340.

In the radio communication system described in the second embodiment, the base station, which receives the strongest signal from a mobile radio 345, will be selected. In most cases, it will be the base station which is closest to the mobile radio, but in some cases, due to radio shadows, another base station may receive a stronger signal. Of course, the selection of base station may be changed at any time.

The decode and control unit 330 may be implemented in hardware or by one or several digital signal processors, in accordance with the first embodiment.

In Fig. 4 a third embodiment of the invention is shown. This embodiment is closely related to the second embodiment shown in Fig. 3, and the same reference numbers have been used for identical parts. The third embodiment differs from the second embodiment in that the selection switch 325 has been exchanged by a signal assembly unit 425, and in the function of the decode and control unit 430. The signal assembly unit 425 is connected to the base stations by the aid of the telephone lines 355, 360, 365 and 370. The decode and control unit 430 establishes a first signal of the first frequency fl, a second signal of a second frequency f2 and RSSI signals corresponding to each base station. These functions at the decode and control unit 430 are identical to the corresponding functions in the decode and control unit 330 of the second embodiment.

In a multi-site radio communication system, comprising more than one base station, a radio signal transmitted by a mobile radio may be received by more than one base station. In those cases, it would be advantageous to combine the received signals from several base stations to improve the signal to noise ratio. A problem with combining the signals is that they arrive at the combining point (325 or 425) out of phase. It would be advantageous to eliminate the phase differences to enable combining of the signals.

Phase differences between the signals arise as a result of (a) different distances between the mobile radio 345 and the base stations 305 to 320 and (b) different lengths of lines 355-370. In order to measure the relative phase differences along the different paths, the following arrangement is used.

Referring to Fig. 5, the mobile radio 345 is shown as having a private line (PL) tone generator 501 which generates a tone of about 67 to 250Hz which is fed to the mobile radio's transmitter 502 for transmission in addition to any voice or other signal. This tone is a sub-audible tone which is normally filtered out at the receive end. In the base station 305, this tone is received in receiver 104 and detected in PL detector 504. The tone is multiplied by a factor of 3 (or factor of 2 or 4, depending on the tone frequency) in multiplier 505. This multiplication factor brings the tone into the passband range of the transmission line 355. The multiplied PL tone is now used in place of the signal with the frequency fl. Replacing the higher amplitude signal with the multiplied PL tone minimises phase jitter.The amplitude of the signal of frequency f2 can be controlled by the RSSI signal from the receiver 104 The two signals are combined in circuit 115.

Referring again to Fig. 4, the various multiplied PL tones are received at decode and control unit 430 which performs phase comparisons between the signals. Taking, for example, the signal from base station 305 as a reference, the phase delays (positive or negative) of the signals from base stations 310-320 relative to this signal are measured. Decode and control unit 430 provides phase correction values to amplifiers 482-486 which adjust the phases of their output signals accordingly. The phase corrected signals are added in combiner 490.

Simple adding of the signals in combiner 490 gives simple additive diversity and a resulting improvement in signal-to-noise ratio (SNR) in reception. Improved SNR can be achieved by maximal ratio diversity combining, in which the amplitudes of the signals received are adjusted by decode and control unit 430 using weighting factors which depend on the relative amplitudes of the received signals. Selection of appropriate factors for maximal ratio diversity is well documented in the technical literature.

Claims (7)

1. A method to transfer an analog control signal through a medium (117) having a given frequency passband, from a transmitting end to a receiving end thereof, comprising the steps of generating a first signal of a fixed amplitude and of a first frequency (fl) within the passband of the medium; generating a second signal of a second frequency (f2) within the passband of the medium, controlling (113) the amplitude of the second signal with the analog control signal to provide a modified second signal; sending the first signal and the modified second signal through the medium; comparing (170) the amplitudes of the first signal and the second modified signal at the receiving end of the medium; and providing an analog control signal dependent on the relative amplitudes of the first signal and the modified second signaL

2.A method according to claim 1 where the frequencies of the first and second signals are relatively close to a lower end of the passband of the medium.

3. A method according to either one of claims 1 and 2 where the difference between the frequencies of the first and second signals is small relative to the frequency range of the passband of the medium.

4. A method to control a remotely controlled unit (102) by a controlling unit (100), connected to each other by the aid of a medium (117) having a band pass characteristics, comprising the steps of: in the controlling unit generating a first signal of a fixed amplitude and of a first frequency (fl) within the passband of the medium; generating a second signal of a second frequency (f2) within the passband of the medium; controlling (113) the amplitude of the second signal with the analog control signal to provide a modified second signal; and sending the first signal and the modified second signal through the medium; and in the remotely controlled unit: comparing (170) the amplitudes of the first signal and the second modified signal at the receiving end of the medium; providing an analog control signal dependent on the relative amplitudes of the first signal and the modified second signal; and using the adjusted signal to control the remotely controlled unit.

5. A method to control a radio communication system, having a plurality of base stations (305-320) each connected to a control unit (102) by means of a medium (355-370) having a given frequency passband, comprising the steps of: in each base station: generating a first signal of a fixed amplitude and of a first frequency (fl) within the passband of the medium; generating a second signal of a second frequency (f2) within the passband of the medium; controlling the amplitude of the second signal with the analog control signal to provide a modified second signal; and sending the first signal and the modified second signal through the medium; and in the control unit (102): comparing, for each base station, the amplitudes of the first signal and the second modified signal at the receiving end of the medium; providing, for each base station, an analog control signal dependent on the relative amplitudes of the first signal and the modified second signal; and comparing (330), in the control unit, the analog control signals for the different base stations and selectively attenuating (325) further signals from the different base stations dependent on a comparison of the analog control signals.

6. Control apparatus (102) having an output for outputting a signal to a medium having a band pass characteristics, comprising: first generating means (111) for generating a first signal of a fixed amplitude and of a first frequency (fl)within the passband of the medium; and second generating means (112) for generating a second signal of a second frequency (f2), within the passband of the medium; and modulating means (113), connected to the second means, for modulating the amplitude of the second signal in accordance with a control signal; and transmitting means (115), connected to the output, for transmitting the first signal and the modulated second signal.

7. A remotely controlled apparatus (102) having an input, for receiving a signal from a medium having a band pass characteristics, comprising: first detecting means (150), connected to the input, for detecting an amplitude of a first signal of a first frequency (fl); and second detecting means (155), connected to the input, for detecting an amplitude of a second signal of a second frequency (f2); and adjustment means (170), connected to the first and second means, for producing an adjusted signal by adjusting the amplitude of the second signal using the amplitude of the first signal as a reference, and operational means (130), connected to the adjustment means for control by the adjusted signal.