GB1578227A

GB1578227A - Method and apparatus for the transmission and reception of encoded information

Info

Publication number: GB1578227A
Application number: GB31178/77A
Authority: GB
Inventors: Colin Finch
Original assignee: Australasian Training Aids Pty Ltd
Current assignee: Australasian Training Aids Pty Ltd
Priority date: 1976-07-28
Filing date: 1977-07-25
Publication date: 1980-11-05
Also published as: SE7708611L; CH621901A5; SE428513B

Description

(54) IMPROVEMENTS IN OR RELATING TO A METHOD AND APPARATUS FOR THE TRANSMISSION AND RECEPTION OF ENCODED INFORMATION (71) We, AUSTRALASIAN TRAIN ING AIDS PTY., LTD., a Company incorporated under the Laws of New South Wales, Commonwealth of Australia, of 161-169 Fallon Street, Albury, New South Wales, Australia, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to methods and apparatus for encoding and decoding signals such as binary data signals.

Many systems have been proposed before the encoding and decoding signals in which the system is adapted so that no output is provided if the received signal contains spurious noise or codes. The present invention seeks to provide an improved method and apparatus for encoding and decoding signals.

According to one aspect of this invention, there is provided a method of encoding and subsequently decoding a data signal having a predetermined switching rate, said encoding comprising the steps of producing a guard tone having a frequency substantially greater than said switching rate, additively combining said data signal with said guard tone to produce a summed composite signal, producing a radio frequency carrier signal, and modulating said carrier signal with said composite signal, the modulated carrier being transmitted, the decoding comprising the steps of demodulating said signal to be decoded to restore said composite signal, processing a portion of said restored composite signal in a data channel to remove said guard tone therefrom and thereby restore said data signal, processing a further portion of said restored composite signal in a guard tone channel to remove said data signal therefrom and thereby restore said guard tone, producing a guard tone control signal indicating the presence of said guard tone in said guard tone channel, processing a further portion of said signal to be decoded by comparing the level of said signal to be decoded or a signal representative thereof in a carrier level channel with a fixed reference level and producing a carrier level control signal indicating when the level of said signal to be decoded is greater than said reference level, and gating said restored data signal in response to said control signals, whereby said restored data signal is passed to an output only when said guard tone is present in the guard tone channel and the level of said signal to be decoded is greater than said fixed reference level.

Preferably the method further comprises the steps of amplifying said modulated carrier before it is transmitted.

Preferably the frequency of said guard tone is about 200 times the frequency of the switching rate.

Conveniently said step of modulation comprises frequency shift keying modulation of said carrier signal. Preferably the amplitude of said guard tone is a low percentage of the amplitude of said data signal, and most preferably the amplitude of the guard tone is about five percent of the amplitude of the data signal.

Advantageously the frequency of said carrier signal is between 27 MHz and 33 MHz. Conveniently the frequency of said guard tone is in the order of 10 KHz.

Preferably said data switching rate is in the order of 50 Hz and, conveniently, the signal is a binary signal.

Conveniently, the method further comprises the steps of producing a local reference frequency and mixing said signal to be decoded with said local reference frequency to obtain an intermediate frequency signal having a level determined by the carrier level of said signal to be decoded, and wherein, in said step of comparing the level of signal to be decoded with said fixed reference level, use is made of the level of the intermediate frequency signal.

Preferably said intermediate frequency signal has a centre frequency of in the order of 460 KHz.

Conveniently the said signal to be decoded is received at a broadband radio frequency antenna, and preferably the method includes the step of amplifying and limiting said received signal.

Conveniently the carrier signal is frequency modulated with said composite signal and said step of demodulating comprises frequency demodulation.

Preferably said step of gating further comprises checking for errors in said restored binary data signal.

Advantageously said step of checking comprises parity checking of said restored binary data signal.

According to another aspect of this invention there is provided an apparatus for encoding a data signal having a predetermined switching rate, comprising means for producing a guard tone having a frequency substantially greater than said switching rate, means for additively combining said data with said guard tone to produce a summed composite signal, means for producing a radio frequency carrier signal, and means for modulating said carrier signal with said composite signal.

Preferably the apparatus further comprises means for amplifying and transmitting said modulated carrier.

According to yet a further aspect of this invention, there is provided an apparatus for decoding a signal comprisiing a carrier signal modulated with a composite signal, said composite signal comprising a data signal of predetermined switching rate and a guard tone having a frequency substantially greater than said switching rate, the apparatus comprising means for demodulating said signal to be decoded to restore said composite signal, data channel means for processing said composite signal to remove said guard tone therefrom and thereby restore said data signal, guard tone channel means for processing said composite signal to remove said data signal therefrom and thereby restore said guard tone, means for producing a guard tone control signal indicating the presence of said guard tone in said guard tone channel means, carrier level channel means for comparing the level of said signal to be decoded or a signal representative thereof with a fixed reference level and producing a carrier level control signal indicating when the level of said signal to be decoded is greater than said reference level, and means for gating said restored data signal in response to said control signals, whereby said restored data signal is passed to an output of said gating means only when said guard tone is present in the guard tone channel and the level of said signal to be decoded is greater than said fixed reference level.

Preferably the data signal is a binary signal.

Preferably the apparatus further comprises means for producing a local reference frequency and means for mixing said signal to be decoded with said local frequency to obtain an intermediate frequency signal having a level determined by the carrier level of said signal to be decoded and wherein the carrier level channel means is connected for comparing the level of said intermediate signal to be decoded with said fixed reference level.

Conveniently the apparatus further comprises broadband antenna means for receiving said signal to be decoded, and means for amplifying and limiting said received signal.

Preferably the carrier signal is frequency modulated with said composite signal and demodulating means comprises means for frequency demodulation of said signal to be decoded, and said gating means further comprises means for checking for errors in said restored binary data signal.

Conveniently said checking means comprises means for parity checking of said restored binary data signal.

According to another aspect of this invention there is provided apparatus for decoding a signal comprising a radio frequency carrier signal frequency shift-key modulated with a composite signal, said composite signal comprising a binary data signal of predetermined switching rate and a superimposed guard tone having a frequency substantially greater than said switching rate, the apparatus including a decoder comprising antenna means for receiving said modulated carrier signal, means for producing a local reference frequency, means for mixing said received modulated carrier signal with said local reference frequency to obtain an intermediate frequency signal having a level determined by the carrier level of said signal to be decoded, means for demodulating said intermediate frequency signal to restore said composite signal, data channel means for processing said composite signal to remove said guard tone therefrom and thereby restore said binary data signal, guard tone channel means for processing said composite signal to remove said binary data signal therefrom and thereby to restore said guard tone, means for producing a guard tone control signal indicating the presence of said guard tone in said guard tone channel means, carrier level channel means for comparing said intermediate frequency signal with a fixed reference level and producing a carrier level control signal indicating when the level of said intermediate frequency signal is greater than said reference level, and means for gating said restored binary data signal in response to said control signals, whereby said restored binary data signal is passed to an output of said gating means only when said guard tone is present in the guard tone channel and said intermediate frequency signal level is greater than said fixed level.

Preferably said intermediate frequency signal demodulator comprises, in series, an FM quadrature detector, and a broadband audio frequency amplifier, the output of said FM quadrature detector comprising a broadband audio frequency signal containing said binary data signal.

Conveniently the switching rate of said binary data signal is a predetermined audio frequency, and said data channel means comprises an audio frequency filter for passing only said binary data signal, and an audio frequency pulse amplifier and shaper for supplying at an output said binary data signal to restored form.

Advantageously said guard tone is an audio frequency tone, said guard tone channel means comprising, in series, an audio frequency filter for passing only said guard tone, a limiting amplifier, and an active band pass filter for supplying at an output said guard tone in restored form.

Preferably said guard tone control signal producing means comprises a clamp circuit responsive to said restored guard tone and having an output at which is maintained a first logic level when said restored guard tone is present and a second logic level when said restored guard tone is absen:.

Conveniently said carrier level channel means comprises, in series, a detector for integrating said intermediate frequency signal, a low pass filter for providing a DC signal having a level which varies in dependence on the level of said carrier in said received modulated carrier signal, and a comparator for comparing said DC signal with a predetermined DC reference level and having an output at which is maintained a first logic level when said DC signal is greater than said DC reference level and a second logic level when said DC signal is less than said DC reference level.

Advantageously said gating means comprises an AND gate having an output, a first input for receiving said restored binary data signal, and at least one second input responsive to the levels at said clamp circuit and comparator outputs, whereby said restored binary data signal is passed to said AND gate output only when both said clamp circuit and comparator outputs are at said first logic level.

The apparatus may further comprise circuit means for decoding and checking for errors in the restored binary data signal appearing at said AND gate output.

Preferably said decoding and error checking means further checks parity of said restored binary data signal.

The apparatus may further comprise circuit means for producing and sending said modulated carrier signal, comprising means for producing said guard tone, means for producing said radio frequency carrier signal, means for generating said binary data signal, means for modulating said binary data signal with said guard tone to produce said composite signal, means for modulating said carrier signal with said composite signal, and means for amplifying and transmitting said modulated carrier signal.

From the foregoing it will be appreciated that the received signal is preferably analysed for three known parameters: (1) that the carrier signal level is above a predetermined threshold for the duration of each received segment of encoded information. The carrier may be measured directly, or indirectly by detecting an intermediate frequency signal level, (2) that an identification signal is present for the duration of the received binary signal segment, and (3) that each bit of encoded information has transitions in a known format.

In order that the invention may be more readily understood and so that further features thereof may be appreciated, the invention will now be described by way of example with reference to the accompanying drawings, in which: Figures la--le show wave forms of the signals to be transmitted by the apparatus of Figure 2; Figure 2 is a block diagram of a preferred frequency modulation transmitter for generating signals of the type illustrated in Figure 1; Figure 3 is a block diagram of a receiver; and Figures 4, 5 and 6 show a detailed circuit diagram of the preferred receiver of Figure 2, each of Figures 46 showing a respective portion of the preferred receiver circuit.

Referring initially to Figure la, there is shown a typical signal sample of binary information of a two voltage state. Thus the low voltage portions of the signal represent one binary digit and the high voltage portions represent the other binary digit.

The specific information content is not of importance to the present invention, although it is to be understood that suitable check bits may included, and the binary information may be in a known form, for example the information may have a specified number of bits in each segment to be transmitted.

Figure lb shows the same binary information having a relatively high frequency signal superimposed thereon. For convenience, this superimposed signal will be referred to hereafter as a "guard tone" signal. Figure Ic shows a carrier which has been frequency modulated by the same binary coded information and guard tone signal. The guard tone signal component is represented on the modulated carrier by the small amplitude modulations at the upper and lower portions of the wave form of Figure Ic. The FM signal of Figure Ic is that which is transmitted from the apparatus shown in the block diagram in Figure 2 and received by the apparatus of Figures 3 to 6.

The FM carrier frequency is preferably in the band between 27 and 33 MHz. The binary data switching rate is preferably in the region of 50 Hz, while the guard tone frequency preferably has a frequency of about 200 times the frequency of the switching rate such as of the order of 10 KHz. It can thus be appreciated that there are distinct frequency characteristics for the three portions of the transmitted signal.

Although the representations of Figures lb and Ic are not to scale, it will be understood that the amplitude of the guard tone signal is small compared to the amplitude of the binary data signal level, and is preferably about 5 percent thereof. The frequency modulation of the carrier is preferably of the type known in the art as frequency shift keying (FSK) transmission.

In Figure 2 a block diagram of a transmitter for producing a signal of the type given in Figure Ic is shown. The transmitter comprises an oscillator 100 which generates the guard tone signal and a binary information generator 102 of known type for generating binary coded signals representative of information to be transmitted. The outputs of oscillator 100 and generator 102 are fed to the respective inputs of a summing amplifier 104. The wave form output of amplifier 104 comprises the digital information with the guard tone signal impressed thereon, as shown in Figure lb. The output of amplifier 104 is passed to a frequncy modulator 106, where the carrier signal from oscillator 108 is frequency modulated and in turn passed to a radio frequency amplifier 110 where the signal is amplified for subsequent transmission by antenna 112.The signal propagated by antenna 112 is that shown in Figure Ic.

Reference will now be made to the block diagram of Figure 3 which illustrates a preferred embodiment of a receiver according to the present invention. The receiver allows only signals of the type generally illustrated in Figure Ic to be recognised as coded binary information, and is adapted to reject interference signals.

It is to be noted that the "front end" of this circuit is like that of a conventional FM receiver, fixed to receive signals over a narrow frequency range. Following the receiver "front end" are three distinct signal channels, comprising: (I) The data channel (2) the guard tone channel; and (3) the carrier level channel.

For a data output to be provided at the right-hand portion of the circuit of Figure 3, appropriate signals must be detected simultaneously in all three of these channels, as will be apparent from the following description.

The frequency modulated radio frequency (RF) carrier, in the range of 2733 MHz, and, for example, comprising the signal illustrated in Figure Ic, is applied via a broad band receiver antenna 200 and an antenna matching stage 202 to a two-stage RF band pass filter 204. An image trap 206 is used to provide image frequency attenuation in band pass filter 204. The output of filter 204 is supplied to two RF amplifiers 208, 210 connected in cascade.

Each of amplifiers 208, 210 has a respective image trap 212, 214 associated therewith, and the amplifier 210 includes a control 216 for setting the RF gain. A typical effective range for the RF gain control is about 30 dB.

The output from RF amplifier 210 is combined in mixer 218 with the output of a local oscillator 220. Oscillator 220 is tuned at a fixed, preselected frequency. The intermediate frequency (IF) output from mixer 218 is applied to an IF band pass filter 222 and IF limiting amplifier 224 which contains a discriminator. The limiting amplifier serves to limit the IF signal voltage swing to the succeeding stages. In the preferred embodiment, the IF centre frequency is about 460 KHz.

The output of amplifier 224 is applied to the input of a tuned FM quadrature detector 226 as well as to the input of a carrier level detection channel. The audio frequency output from the discriminator contained in amplifier 224 is relatively broad-band in nature, but may be limited somewhat by the band-width of the quadrature detector circuit 226. The broadband output signal of detector.226 is passed through a broad-band audio frequency amplifier 228 and is then separated into two paths, the primary path being a data channel and the secondary path being a guard tone channel.

In the data channel, the output from amplifier 228 is a signal comprising the digital information at a switching frequency of about 50 Hz with the 10 KHz guard tone signal superimposed thereon. This composite signal is fed into the data channel as well as into the guard tone channel. In the data channel the signal is fed to a low pass audio frequency filter 230 which removes components having a frequency above about 100 Hz. The output from filter 230 comprises the binary data, with the guard tone channel and the high frequency components of the data bits removed. The signal resembles somewhat a sine wave signal. This signal is passed to an audio frequency pulse amplifier and shaper 232 which serves to shape the signal as shown.

The restored binary data is provided at one input of an AND gate 234.

The output signal from amplifier 228 is also applied to the guard tone channel. This signal is first passed through a high pass filter 236A where low frequency components below about 7 KHz are removed. The resulting signal comprises substantially the 10 KHz guard tone signal plus high frequency spikes representative of the transitions between data bits. This signal is fed into limiter 236B where the gain and balance are set to amplify the guard tone components to a peak-to-peak value of, for example, 6 volts. Greater magnitude signals, such as the transition signals, are limited to 6 volts as well. Thus, the output of guard tone limiter 23613 comprises substantially only an amplified and restored guard tone.

The output of guard tone limiter 236B is applied to high pass filter 238A, and then to a 12 dB/octave band pass filter 238B, where peaked signals representative only of the guard band signal are passed. The output from filter 238B occurs only if there is a 10 KHz signal present in the input signal. The output signal of filter 238B is fed to a clamp circuit 240. The clamp circuit maintains its output at 0 volts ( a logic 0 signal) when no guard tone is detected in the guard tone channel, and at +9 volts (a logic I signal) when the guard tone is detected in the guard tone channel. The purpose which the clamp circuit 240 serves will be described in greater detail in the following description of the carrier level channel.

The input to the carrier level channel is typically a 460 KHz centre frequency IF signal with a nominal deviation of +5 KHz representative of the 10 KHz guard tone signal. The actual carrier (or IF equivalent) is in effect switched between the limits of 455 and 465 KHz -- the FM modulation range. The modulation rate of the deviation is approximately the 50 Hz binary data switch rate, and superimposed on this modulation is the 10 KHz guard tone signal at a level of approximately 5 percent of that of the binary data. The IF signal is rectified in a detector 242 and then applied to a low pass filter 244, which provides a DC signal of appropriate rise and fall time which is applied to one input of a level detecting circuit 246.Circuit 246 comprises a comparator which continuously compares the signal from filter 244 with a reference level and provides a logic 0 output (0 volts) when the carrier level is at or below the reference level, and a logic 1 (for example, +9 volts) when the signal from filter 244 is greater than the reference level. Provision is made for setting the reference level at a suitable value.

The output of comparator 246 is provided to the second input of AND gate 234, and serves to enable gate 234 to pass the binary data from amplifier and shaper 232. However, if the clamp circuit 240 has its output connected to ground (0 volts), the output of comparator 246 will be shunted to ground and gate 234 will be disabled. Thus, it can be seen that the guard tone must be detected in the guard tone channel and the IF level detected in the carrier level channel must be above the reference threshold in order to pass binary data to the output of gate 234.

The output of gate 234 is preferably supplied to a standard decoder, which may include means for parity checking or error detection.

Referring now to Figures 4, 5 and 6, a more detailed description of the preferred embodiment of the receiver of Figure 3 is given. Initially, it will be noted that a suitable power supply for the receiver circuit is shown in the lower portion of Figure 5, providing reference voltages at +12 volts, +9 volts, and +4.5 volts with respect to a 0 volt or earth line.

The modulated radio frequency signal is supplied from the antenna matching circuit 202 of Figure 3 to terminals 9, 10 of band pass filter 204. The filter 204 comprises a coupling transformer T2 tuned to the carrier frequency and resonating with capacitor C3.

Capacitor C1 is used to RF ground the antenna input to the 0 volt terminal of the supply source. Transformer T2 is tapped and applied to the second stage of the band pass filter, the second stage comprising transformer T3, which is tuned by capacitor C4. Each stage of band pass filter 204 has associated with it a portion of an image frequency attenuation circuit or image trap 206. The portion of image trap 206 associated with the first stage of filter 204 is made up of transformer Tl and capacitor C2, while the portion of trap 206 associated with the second stage of filter 204 is made up of transformer T4 and capacitor C5. The two sets of transformers Tl, T2 and T3, T4 are mutually dependent in their tuning action.

The output of filter 204 is applied to the first gate gl of a dual gate MOSFET amplifier TRI of radio frequency amplifier 208. Transistor TRI may be, for example, of type FT0601. The output of amplifier 208 is tuned by transformer T6 in combination with capacitor C9, while transformer T5 in combination with capacitor C8 comprises an image trap 212 coupled to transformer T6.

The gain of the first stage of amplifier 208 is set by the bias voltage present in the second gate g2 of the dual gate amplifier.

This bias voltage is determined by a voltage divider comprising resistors R35 and R2.

Capacitor C45 acts as a second gate bypass for transistor TRI.

The output from amplifier 208 is taken from the secondary winding of transformer T6 and applied to the first gate gl of a dual gate MOSFET amplifier TR2, also preferably of type FT0601. The output of transistor TR2 is tuned by the network comprising transformer T8 and capacitor C12. An image trap 214 comprising transformer T7 in combination with capacitor C14 is used in conjunction with transformer T8 of amplifier 210. It is to be noted that the image and signal tuning is interdependent. Transistor TR2 serves as a variable gain amplifier with the gain adjustment being determined by the voltage divider comprising resistors R7 and R8.

Resistor R7 may be adjusted to set the RF gain between input terminals 9, 10 of filter 204 and the junction of the RF trap comprising capacitor C13 and coil L5 in the mixer 218 of Figure 5.

An RF limiter stage is included in the output of amplifier 210 and comprises diodes D7 and D8 connected back-to-back across the primary winding of transformer T8. The effect of diodes D7, D8 is to limit the primary voltage of transformer T8 to approximately 0.7 volts peak-to-peak maximum. Direct current feed for the two stages of the RF amplifiers 208, 210 is supplied via low pass filters comprising choke LI and capacitor C10, and choke L2 and capacitor C11.

The gain adjustment is preferably set to obtain 100 millivolts peak-to-peak at the junction of capacitor C13 and choke L5 for an input of -58 dBm CW at the antenna input terminals 9, 10 of filter 204.

Referring now to Figure 5, the input from amplifier 210 to the mixer stage 218 is supplied via terminals 6, 4 of integrated circuit ICI. Integrated circuit ICI is preferably of type LM371 and comprises a mixer, local oscillator and IF amplifier all within one envelope. Resistor R10 serves to match the output of amplifier 210 to the input impedance of the mixer.

The local oscillator tuned circuit comprises transformer T9 and capacitor C26 tuned to the local oscillator frequency, which is approximately 30 Mllz. The tuned circuit is connected to pin 10 of integrated circuit ICI. The local oscillator crystal XLI is a third overtone type and operates in the series-resonant condition. The crystal is preferably resonant at 29.265 MHz, while the carrier frequency is preferably 29.725 MHz, providing an intermediate frequency (IF) signal of 460 KHz. The crystal feed is tapped approximately one quarter along the inductance winding T9 of the tuned circuit and the crystal output is taken from a phase shift network comprising choke L6 and capacitor C18 to the oscillator feedback point, terminal 1 of integrated circuit ICI.

Choke L6 is adjustable and is used to set the local oscillator frequency.

Integrated circuit ICI also includes an internal mixer and product amplifier function. The product output of the integrated circuit is tuned to 460 KHz by the network comprising transformer T10 and capacitor C16. An RF trap to remove remnant radio frequencies from the IF output is included and comprises capacitor C13 and choke L5 operating in series resonance and tuned to approximately 30 MHz.

The output of transformer T10 is coupled via capacitor C20 to the input of integrated circuit IC2. This integrated circuit (IC2) is a complex integrated circuit comprising two limiting IF amplifiers, an FM quadrature detector circuit and an audio frequency amplifier within a single envelope. Circuit IC2 is preferably of type LM374. The output tuning of the first limiting amplifier of IC2 is provided by transformer Tl l in combination with capacitor C17, and amplifier IC2 is tuned to 460 KHz.

Capacitor C62 serves to remove excess radio frequencies during high RF input conditions.

The output from the first limiting amplifier of IC2 is taken from terminal 5 of transformer Tl l and split in two directions.

The first direction is to the IF level detector 242 and the second is to the quadrature detector circuit 226 of IC2 via capacitor C23. Terminal 4 of IC2 is the input to the second limiting amplifier, used to ensure a constant input voltage to the quadrature detector. The signal applied to the quadrature detector is split into two paths.

The first is to a linear amplifier of IC2 into a summing point, and the second is through a small coupling capacitor, internal to IC2, to terminal 6 of IC2. Capacitor C32 serves as a DC isolating capacitor for the tuned circuit comprising transformer T12 and capacitor C25. This network acts as a shunt phase shifting circuit at terminal 6 of IC2 and feeds the remaining input to the summing point of the integrated circuit.

The output of the summing point internal to IC2 is the sum of the original and phaseshifted inputs to the quadrature detector and is seen as a signal with amplitude and phase proportional to the phase difference between the original and the phase shifted inputs. Since such quadrature detectors are well known in the art, detailed discussion of the internal circuitry of IC2 is not believed necessary.

The network comprising transformer T12 and capacitor C25 is tuned to the IF centre frequency (460 KHz) and the IF input frequency varies about 460 KHz by the deviation (+5 KHz) at the modulation rate.

The output is seen as an audio frequency signal having a frequency at the modulation rate and amplitude proportional to the instantaneous frequency deviation about 460 KHz. Capacitor C28 is a filter used to by pass residual RF components in the detector output signal. The FM quadrature detector output is applied to a peak detector circuit whose basic function herein is as a broad-band audio-frequency pre-amplifier with output at terminal 8 of IC2.

The detector audio frequency output at terminal 8 of IC2 represents the modulation from the IF signal, without the presence of the IF frequency itself. This audio frequency signal is substantially broad-band in nature and is limited only by the IF amplifier band shape characteristics. The magnitude of the output from the terminal 8 of IC2 is determined by the Q of transformer T12 in combination with capacitor C25. The nominal band width of the quadrature detector is +10 KHz about a centre frequency of 460 KHz. The audio frequency output from terminal 8 of IC2 is used in both the guard tone channel and the data channel.

The carrier level detecting circuit comprises detector 242, low pass filter 244 and IF level comparator 246. The function of this circuit is to detect the presence of a suitable level of the IF signal, which in turn represents the presence of the correct carrier frequency and amplitude.

Operation of the carrier level detection channel is as follows. A DC bias circuit (shown in Figure 5) comprising resistors R22 and R36 in combination with bypass capacitor C37 is used to set the 4.5 volt bias at the positive terminal of an operational amplifier IC4, the amplifier IC4 forming a part of comparator 246 shown in Figure 6, and preferably comprising an integrated circuit of type 776. The bias voltage is applied via the secondary winding of transformer Tl l in the limiting IF amplifier through a detector diode D3, limiting resistor R12 and shunt terminating resistor R34. The resulting voltage at terminal 3 of IC4 is approximately +4.3 volts.

The magnitude of the voltage at terminal No. 3 of IC4 will be varied by an amount proportional to any signal which appears in the secondary winding of transformer TI 1 and is rectified by diode D3 and capacitor C36 of detector 242. It will be understood that the voltage at the input of low pass filter 244 is therefore the combination of the fixed DC voltage originating in the DC bias circuit plus the voltage proportional to the detected IF signal level.

The IF signal level voltage will vary as a function of time and therefore a low pass filter 244 comprising resistors R12 and R34 and capacitor C35 is provided effectively to present the average value of these two voltages to terminal 3 of IC4.

The remaining input to operational amplifier IC4 is taken from a voltage divider comprising resistors R26, R33 and R23. The values of these components are such that by proper adjustment of variable resistor R33, the voltage at terminal 2 of amplifier IC4 may be made higher or lower than the voltage at terminal 3 thereof. By this means it is possible for the output of IC4 at terminal 6 to be switched between the limits of the +9 volt source and 0 volts.

Accordingly, if the level adjustment resistor R33 is set so that the voltage at terminal 2 is higher than that at terminal 3 under no IF input conditions, then the voltage at output terminal 6 of IC4 will be approximately 0 volts.

Provided the difference between inputs 2 and 3 of IC4 in the quiescent state is less than the difference in voltage between inputs 2 and 3 with a carrier present, then the output at terminal 6 of IC4 will switch to +9 volts. By this means it is possible positively to detect the presence or absence of a carrier signal of pre-determined amplitude by suitable adjustment of resistor R33. The gain of amplifier IC4 is set by the ratio of resistors R27/R25 and is preferably about 180. Capacitor C38 serves to bypass any transient effects at terminal 2 of amplifier IC4. The logic level output of IC4 drives one input of the AND gate 234 of Figure 6. It is to be noted, however, that the output of amplifier IC4 is further limited by the clamp circuit of the guard tone channel, as will be described in greater detail below.

Referring now to Figure 6, the broadband audio frequency output from terminal 8 of IC2 is applied to the input of a low pass filter network 230, comprising resistor Rl l in combination with capacitor C33.

Capacitor C29 in the filter serves to eliminate any residual IF frequencies present. The output at the junction of Rl l and C33 is applied via transistor TR7 and capacitor C34 to the negative input of a pulse amplifier IC3. Amplifier IC3 preferably is an integrated circuit of type 776. The output from filter 230 effectively contains no frequencies above 100 Hz. The network comprising resistors R14, R16 and capacitor C27 is used to set the quiescent operating point for circuit IC3 by control of the bias voltage applied to the input terminals 2, 3 of the amplifier. The external gain of IC3 is set by the ratio of resistors R4/R13 and is preferably about 212.

Because of the presence of capacitor C27, the only AC input to the amplifier is via capacitor C34. The quiescent operating point of the output of EC3 is midway between the +9 volts supply and 0 volts, and under normal signal conditions the output will switch between the limits of +9 volts and 0 volts (neglecting saturating losses in the amplifier). The output of the audio frequency amplifier is applied via diode D5 and resistor R20 to an output switch circuit comprising AND gate 234 to be described later. Diode D5 functions as a threshold device, so that signals below 6 volts peak will not affect the AND gate 234.

The guard tone channel involves detection, amplification, and processing of the 10 KHz signal superimposed on the normal data channel information. The broad-band audio-frequency output from terminal 8 of IC2 is applied via a high pass filter network comprising resistor R38 and capacitor C48, to one input of operational amplifier IC7. This amplifier is preferably of type 725HC. The class and operating point of IC7 is set by the +4.5 volt bias circuit.

Circuit IC7 acts as a limiting amplifier having an adjustable limit point (gain), the gain setting being provided by suitable adjustment of resistor R43, and is determined by the level of the 10 KHz signal necessary to operate the active band pass filter 238 following limiter 236.

The normal signal content of the 10 KHz modulation is approximately 20 millivolts peak-to-peak, with the gain control range set to have limits of 10--30 millivolts peakto-peak. This guard tone signal level is set high enough to operate the filter, with an overlap of 3 dB at the point where IC7 limits. All other signals normally present are of much greater amplitude regardless of their origin. Therefore, IC7 is driven well into the limiting condition and the output thereof is a well "squared" signal containing the 10 KHz fundamental input frequency, as well as many harmonics.

The harmonic signals at the output of limiter 236 are of a substantially lesser amplitude than the fundamental by a sufficient amount that the active filter 238 will not be operated thereby. Thus, only a signal with sufficient amplitude to operate the active filter 238 will be tuned at the filter frequency of 10 KHz.

The output at terminal 6 of IC7 is attenuated by a factor of 330 by resistor R54 in combination with resistor R55 and is applied to a high pass filter 238a comprising resistor R46 and capacitor C53. This high pass filter serves further to attenuate lower frequencies and forms a part of the effective band pass characteristic of filter 238.

The active band pass filter 238 comprises in part an integrated circuit IC6, which is an operational amplifier of type 776HC. The amplifier has a DC gain of about 0.5, which is determined by the combination of resistors R47, R48 and R45. The AC gain is frequency selective, and is determined by the network of resistors R47, R48 and capacitor C55, C56. The negative feedback is greatly reduced at a particular frequency, determined by the phase characteristics of the network, the network being tuned to a particular frequency by adjustment of variable capacitor C56. Resistor R49 is used to set an internal characteristic of the operational amplifier.

The guard band clamp circuit 240 is provided with the output of amplifier IC6 via capacitor C57 and resistor R52 to the base of a transistor TR5. A diode D10 is present to clip any excess reverse bias signals and to provide output loading for operational amplifier IC6. Resistor R53 provides a DC clamp source for transistor TR5 and to serve as a primary load for amplifier IC6.

Transistor TR6 is driven from the collector of transistor TR5 and acts as an invertor.

When TR6 is in its conductive state, a short circuit to 0 volts is placed on the carrier level detection signal line to output AND gate 234.

The provision of data output is conditional on the presence of the suitable binary data signal from the audio frequency amplifier of IC3 and a signal from the IF level comparator circuit 246 indicating the presence of both the guard tone and a suitable IF level. A logic signal (0 volts or +9 volts) indicating the IF signal level appears at the output of terminal 6 of IC4 and is applied via resistors R28 and R19 to the base of transistor TR3. When the output at terminal 6 of IC4 is at +9 volts TR3 will operate, provided the guard tone clamp TR6 is not conductive. If the guard tone clamp TR6 is operated due to non-detection of the guard tone, the output of IC4 will be clamped at 0 volts, closing AND gate 234.

A time constant provided by resistor R28 and C39 is included in IF level comparator 246 to eliminate transient effects and to increase the rise time of the signal to transistor TR3. Diode D9 shunts resistor R28 to decrease the discharge time of capacitor C39 and therefore more rapidly to re-establish the inhibit condition should the output of IC4 fall to 0 volts from +9 volts.

It can therefore be seen that the transistor TR3 will be switched to a conductive state when the guard tone is detected and, simultaneously, the IF signal level is above a predetermined threshold value. Transistor TR3 will remain in a non-conductive state if the guard tone is not detected or the IF signal is below the threshold value.

It will be appreciated that the embodiment described above is in relation to a transmitter wherein frequency modulation is used. However, it will be understood that similarly coded signals may be transmitted in amplitude modulationtype transmission or any other type of transmission including transmission with a DC level over a transmission wire, and that with appropiate modification to the receiving apparatus any interference signals can be similarly blocked from passing to a decoder.

These and other modifications may be made without departing from the spirit and scope of the present invention, which is defined by the following claims. For example, the invention may be applied to signals other than binary signals.

WHAT WE CLAIM IS: 1. A method of encoding and subsequently decoding a data signal having a predetermined switching rate, said encoding comprising the steps of producing a guard tone having a frequency substantially greater than said switching rate, additively combining said data signal with said guard tone to produce a summed composite signal, producing a radio frequency carrier signal, and modulating said carrier signal with said composite signal, the modulated carrier being transmitted, the decoding comprising the steps of demodulating said signal to be decoded to restore said composite signal, processing a portion of said restored composite signal in a data channel to remove said guard tone therefrom and thereby restore said data signal, processing a further portion of said restored composite signal in a guard tone channel to remove said data signal therefrom and thereby restore said guard tone, producing a guard tone control signal indicating the presence of said guard tone in said guard tone channel, processing a further portion of said signal to be decoded by comparing the level of said signal to be decoded or a signal representative thereof in a carrier level channel with a fixed reference level and producing a carrier level control signal indicating when the level of said signal to be decoded is greater than said reference level, and gating said restored data signal in response to said control signals, whereby said restored data signal is passed to an output only when said guard tone is present in the guard tone channel and the level of said signal to be decoded is greater than said fixed reference level.

2. A method according to claim I wherein said modulated carrier is amplified before it is transmitted.

3. A method according to claim I or 2 wherein the guard tone has a frequency of about 200 times the frequency of the switching rate.

4. A method according to any one of the preceding claims wherein said step of modulating comprises frequency shift keying modulation of said carrier signal.

5. A method according to any one of the preceding claims wherein the amplitude of said guard tone is a low percentage of the amplitude of said data signal.

6. A method according to claim 5, wherein the amplitude of the guard tone is about five percent of the amplitude of the data signal.

7. A method according to any one of the preceding claims wherein the frequency of said carrier signal is between 27 MHz and 33 MHz.

8. A method according to any one of the preceding claims wherein the frequency of said guard tone is in the order of 10 KHz.

9. A method according to any one of the preceding claims wherein said data switching rate is in the order of 50 Hz.

10. A method according to any one of the preceding claims in which the data signal is a binary signal.

I 1. A method according to any one of the preceding claims wherein said decoding further comprises producing a local reference frequency and mixing said signal to be decoded with said local reference frequency to obtain an intermediate frequency signal having a level determined by the carrier level of said signal to be decoded, and wherein, in said step of comparing the level of signal to be decoded with said fixed reference level, use is made of the level of the intermediate frequency signal.

12. A method according to claim 11 wherein said intermediate frequency signal has a centre frequency of in the order of 460 KHz.

13. A method according to any one of the preceding claims wherein said signal to be decoded is received at a broadband radio frequency antenna.

14. A method according to claim 13 further comprising the step of amplifying and limiting said received signal.

15. A method according to any one of the preceding claims wherein said carrier signal is frequency modulated with said composite signal and said step of demodulating comprises frequency demodulation.

16. A method according to any one of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TR3 will be switched to a conductive state when the guard tone is detected and, simultaneously, the IF signal level is above a predetermined threshold value. Transistor TR3 will remain in a non-conductive state if the guard tone is not detected or the IF signal is below the threshold value. It will be appreciated that the embodiment described above is in relation to a transmitter wherein frequency modulation is used. However, it will be understood that similarly coded signals may be transmitted in amplitude modulationtype transmission or any other type of transmission including transmission with a DC level over a transmission wire, and that with appropiate modification to the receiving apparatus any interference signals can be similarly blocked from passing to a decoder. These and other modifications may be made without departing from the spirit and scope of the present invention, which is defined by the following claims. For example, the invention may be applied to signals other than binary signals. WHAT WE CLAIM IS:

1. A method of encoding and subsequently decoding a data signal having a predetermined switching rate, said encoding comprising the steps of producing a guard tone having a frequency substantially greater than said switching rate, additively combining said data signal with said guard tone to produce a summed composite signal, producing a radio frequency carrier signal, and modulating said carrier signal with said composite signal, the modulated carrier being transmitted, the decoding comprising the steps of demodulating said signal to be decoded to restore said composite signal, processing a portion of said restored composite signal in a data channel to remove said guard tone therefrom and thereby restore said data signal, processing a further portion of said restored composite signal in a guard tone channel to remove said data signal therefrom and thereby restore said guard tone, producing a guard tone control signal indicating the presence of said guard tone in said guard tone channel, processing a further portion of said signal to be decoded by comparing the level of said signal to be decoded or a signal representative thereof in a carrier level channel with a fixed reference level and producing a carrier level control signal indicating when the level of said signal to be decoded is greater than said reference level, and gating said restored data signal in response to said control signals, whereby said restored data signal is passed to an output only when said guard tone is present in the guard tone channel and the level of said signal to be decoded is greater than said fixed reference level.

16. A method according to any one of the

preceding claims wherein said step of gating further comprises checking for errors in said restored binary data signal.

17. A method according to claim 16 wherein said step of checking comprises parity checking of said restored binary data signal.

18. An apparatus for encoding a data signal having a predetermined switching rate, comprising means for producing a guard tone having a frequency substantially greater than said switching rate, means for additively combining said data with said guard tone to produce a summed composite signal, means for producing a radio frequency carrier signal, and means for modulating said carrier signal with said composite signal.

19. An apparatus according to claim 18 further comprising means for amplifying and transmitting said modulated carrier.

20. An apparatus according to claim 18 or 19 wherein the frequency of the guard tone is about 200 times higher than the frequency of said switching rate.

21. An apparatus according to any one of claims 18 to 20, wherein said modulating means comprises means for frequency shift keying modulation of said carrier signal.

22. An apparatus according to any one of claims 18 to 21 wherein the amplitude of said guard tone is a low percentage of the amplitude of said data signal.

23. An apparatus according to claim 22, wherein the amplitude of the guard tone is about five percent of the amplitude of said data signal.

24. An apparatus according to any one of claims 18 to 23, wherein the frequency of said carrier signal is between about 27 MHz and 33 MHz.

25. An apparatus according to any one of claims 18 to 24, wherein the frequency of said guard tone is in the order of 10 KHz.

26. An apparatus according to any one of claims 18 to 25 wherein said data switching rate is in the order of 50 KHz.

27. An apparatus according to any one of claims 18 to 26 wherein said data signal is a binary signal.

28. An apparatus for decoding a signal comprising a carrier signal modulated with a composite signal, said composite signal comprising a data signal of predetermined switching rate and a guard tone having a frequency substantially greater than said switching rate, the apparatus comprising means for demodulating said signal to be decoded to restore said composite signal, data channel means for processing said composite signal to remove said guard tone therefrom and thereby restore said data signal, guard tone channel means for processing said composite signal to remove said data signal therefrom and thereby restore said guard tone, means for producing a guard tone control signal indicating the presence of said guard tone in said guard tone channel means, carrier level channel means for comparing the level of said signal to be decoded or a signal representative thereof with a fixed reference level and producing a carrier level control signal indicating when the level of said signal to be decoded is greater than said reference level, and means for gating said restored data signal in response to said control signals, whereby said restored data signal is passed to an output of said gating means only when said guard tone is present in the guard tone channel and the level of said signal to be decoded is greater than said fixed reference level.

29. An apparatus according to claim 28, wherein said data signal is a binary signal.

30. An apparatus according to claim 28 further comprising means for producing a local reference frequency and means for mixing said signal to be decoded with said local frequency to obtain an intermediate frequency signal having a level determined by the carrier level of said signal to be decoded and wherein the carrier level channel means is connected for comparing the level of said intermediate signal to be decoded with said fixed reference level.

31. An apparatus according to any one of claims 28 to 30 wherein the frequency of the guard tone is 200 times the frequency of the switching rate.

32. An apparatus according to any one of claims 28 to 31 wherein the frequency of said guard tone is in the order of 10 KHz.

33. An apparatus according to any one of claims 28 to 32 wherein said binary data switching rate is in the order of 50 Hz.

34. An apparatus according to any one of claims 28 to 33 wherein the frequency of said carrier signal is between 27 MHz and 33 MHz.

35. An apparatus according to claim 30 or any claim dependent thereon, wherein said intermediate frequency signal has a centre frequency in the order of 460 KHz.

36. An apparatus according to any one of claims 28 to 35 further comprising broadland antenna means for receiving said signal to be decoded.

37. An apparatus according to any one of claims 28 to 36 further comprising means for amplifying and limiting said received signal.

38. An apparatus according t5 any one of claims 28 to 37, wherein said carrier signal is frequency modulated with said composite signal and demodulating means comprises means for frequency demodulation of said signal to be decoded.

39. An apparatus according to any one of claims 28 to 38, wherein said gating means further comprises means for checking for errors in said restored binary data signal.

40. An apparatus according to any one of claims 28 to 39, wherein said checking means comprises means for parity checking of said restored binary data signal.

41. An apparatus for encoding and subsequently decoding a signal comprising the combination of an apparatus as claimed in any one of claims 18 to 27 with an apparatus as claimed in any one of claims 28 to 40.

42. Apparatus for decoding a signal comprising a radio frequency carrier signal frequency shift-key modulated with a composite signal, said composite signal comprising a binary data signal of predetermined switching rate and superimposed guard tone having a frequency substantially greater than said switching rate, the apparatus including a decoder comprising antenna means for receiving said modulated carrier signal, means for producing a local reference frequency, means for mixing said received modulated carrier signal with said local reference frequency to obtain an intermediate frequency signal having a level determined by the carrier level of said signal to be decoded, means for demodulating said intermediate frequency signal to restore said composite signal, data channel means for processing said composite signal to remove said guard tone therefrom and thereby restore said binary data signal, guard tone channel means for processing said composite signal to remove said binary data signal therefrom and thereby to restore said guard tone, means for producing a guard tone control signal indicating the presence of said guard tone in said guard tone channel means, carrier level channel means for comparing said intermediate frequency signal with a fixed reference level and producing a carrier level control signal indicating when the level of said intermediate frequency signal is greater than said reference level, and means for gating said restored binary data signal in response to said control signals, whereby said restored binary data signal is passed to an output of said gating means only when said guard tone is present in the guard tone channel and said intermediate frequency signal level is greater than said fixed reference level.

43. An apparatus according to claim 42 wherein said intermediate frequency signal demodulator comprises, in series, an FM quadrature detector, and a broadband audio frequency amplifier, the output of said FM quadrature detector comprising a broadband audio frequency signal containing said binary data signal.

44. An apparatus according to claim 42 or 43 wherein the switching rate of said binary data signal is a predetermined audio frequency, and said data channel means comprises an audio frequency filter for passing only said binary data signal, and an audio frequency pulse amplifier and shaper for supplying at an output said binary data signal to restored form.

45. An apparatus according to any one of claims 42 to 44 wherein said guard tone is an audio frequency tone, said guard tone channel means comprising, in series, an audio frequency filter for passing only said guard tone, a limiting amplifier, and an active band pass filter for supplying at an output said guard tone in restored form.

46. An apparatus according to any one of the preceding claims wherein said guard tone control signal producing means comprises a clamp circuit responsive to said restored guard tone and having an output at which is maintained a first logic level when said restored guard tone is present and a second logic level when said restored guard tone is absent.

47. An apparatus according to any one of the preceding claims wherein said carrier level channel means comprises, in series, a detector for integrating said intermediate frequency signal a low pass filter for providing a DC signal having a level which varies in dependence on the level of said carrier in said received modulated carrier signal, and a comparator for comparing said DC signal with a predetermined DC reference level and having an output at which is maintained a first logic level when said DC signal is greater than said DC reference level and a second logic level when said DC signal is less than said DC reference level.

48. An apparatus according to claims 42 to 47 wherein said gating means comprises an AND gate having an output, a first input for receiving said restored binary data signal, and at least one second input responsive to the levels at said clamp circuit and comparator outputs, whereby said restored binary data signal is passed to said AND gate output only when both said clamp circuit and comparator outputs are at said first logic level.

49. An apparatus according to any one of claims 42 to 48 further comprising circuit means for decoding and checking for errors in the restored binary data signal appearing at said AND gate output.

50. The apparatus of claim 49 wherein said decoding and error checking means further checks parity of said restored binary data signal.

51. An apparatus according to any one of claims 42 to 50 further including circuit means for producing and sending said modulated carrier signal, comprising means for producing said guard tone, means for producing said radio frequency carrier signal, means for generating said binary data signal, means for modulating said binary data signal with said guard tone to produce said composite signal, means for modulating said carrier signal with said composite signal, and means for amplifying and transmitting said modulated carrier signal.

52. An apparatus according to any one of claims 42 to 51 wherein said antenna means comprises a broadband antenna.

53. An apparatus according to any one of claims 42 to 52 further comprising circuit means for amplifying and limiting said received signal.

54. An apparatus according to any one of claims 42 to 53 wherein the frequency of said guard tone is about 200 times the frequency of said binary data signal switching rate.

55. An apparatus according to any one of claims 42 to 52 wherein the frequency of said guard tone is about 10 KHz, and the frequency of said binary data switching rate is about 50 Hz.

56. An apparatus according to any one of claims 42 to 55 wherein the frequency of said carrier signal is between about 27 MHz and 33 MHz.

57. An apparatus according to any one of claims 42 to 56 wherein said intermediate frequency signal has a center frequency of about 460 KHz.

58. An encoding and subsequent decoding method substantially as herein described with reference to the accompanying drawings.

59. Apparatus for encoding a binary data signal substantially as herein described with reference to, and as illustrated in Figures 1 and 2 of the accompanying drawings.

60. Apparatus for decoding a signal substantially as herein described with reference to, and as illustrated in, Figures 3 to 6 of the accompanying drawings.

61. An apparatus for encoding and subsequently decoding a signal substantially as herein described with reference to and as illustrated in the accompanying drawings.