NO144863B - RECEIVER DEVICE WITH CONTINUOUS STRUCTURE FOR SEISMIC INVESTIGATIONS. - Google Patents

Description

Selective call system, preferably for wireless connection between load and mobile,

or between mobile stations.

The present invention relates to a selective call system, preferably for a wireless connection between fixed and mobile, or between mobile radiotelephony stations,

where a coding system is used where it

for each digit in a dean's call number, a code signal consisting of two is sent out

simultaneously transmitted speech frequency modulation signals of a group of five frequencies, and where, in addition to the digit code signals, a sixth audio frequency code signal is transmitted.

From German patent document 908 150 it is

known such a system, where it immediately

in front of each code signal, a preparatory signal is sent out which, by means of an alternating current polarized — or a direct current relay

cancels the blocking or connects the code receiver in the short period of time the code signal is received. The preparation signal can have a

frequency of 50 or 150 Hz or it may consist of direct current surges, and is intended for

thread transfer.

In the case of wireless transmission where the quality

of the transmitted signals can vary greatly,

it is common to use a relatively narrow

prescribed frequency range of 300—3000

Hz. The known system is therefore not very suitable

for wireless transmission, and in addition

that by expanding the frequency band so that

frequencies of 50 or 150 Hz should be able to

let through, it would easily form as a result of distortion in the transmission equipment

harmonic overshoots that would degrade the quality. When using such low

frequencies such as 50 or. At 150 Hz, it is difficult to keep the dimensions of the frequency-selective circuits within reasonable limits, given the fact that technical development is moving in the direction of miniaturization.

Selective calling systems for wireless connection are also known, and they can be broadly divided into two main groups: 1. The calling code is transmitted as rapid pulses of two tones (e.g. 600 and 1600 Hz). 2. The call code is transmitted as a combination of two or more tones that are sent out simultaneously in relatively long time intervals.

It is also known to use resonant relays or electric swing circuits for frequency-selective reception of the emitted tones or tone combinations.

In the present case, a resonant relay has been used as a frequency-selective link.

When planning a selective call system for connection to the public telephone network, great emphasis must be placed on the system's operational security. The system must be as immune as possible to false calls caused by faulty decoding, radio noise, voice signals and other disturbances, while at the same time it must provide the greatest possible certainty that the selective call signals lead to calls with the desired mobile subscriber.

The latter can often be difficult with VHF radio connections due to "rapid fading" or "gaps" in the field strength from the fixed transmitter to the mobile receiver when the latter is in motion. American research has shown that these "holes or dead zones 1 the field strength can cause the signal to the receiver to drop out for up to 100 msec. A system with rapid pulsing between 2 tones, as mentioned under point 1, under such conditions can hardly provide the desired security that the call goes through without error, as individual pulses can be lost when passing through such dead zones if the duration of a single pulse is less than approx. . 100 msec.

The purpose of the invention is therefore for wireless transmission to provide a selective call system where the frequency of the preparation signal is in the voice band itself and with relatively long code signals in a decadal digit system, which can easily be expanded for an arbitrary number of digits and which has the greatest possible security against errors -decoding and fake calls, and can easily be adapted to standard automatic telephone equipment.

According to the invention, this is achieved by the fact that the sixth code signal, which lies within the voice frequency band, is sent out in each of the spaces between the digit code signals, so that all these individual signals follow one another in a continuous sequence, that the system's code receiver for recording the call is equipped with a set of cascaded flip-flop circuits which, on receipt of the alternately transmitted digit code signal and the sixth code signal, are triggered in turn to working position to give an output signal, that the flip-flop circuits are connected so that the first circuit in the cascade is triggered by the first digit code signal, while the subsequent circuits are only triggered if they receive the respective code signal and the output signal from the previous circuit at the same time, and that the circuits' output signals are emitted only from the time their code signals cease and have calculated duration, so that the flip-flop - the circuit switches back to rest position and thereby interrupts the decoding of the call number if time the gap between the beginning of two consecutive code signals exceeds a certain limit!

The advantages of sending a sixth audio-frequency rhodulation signal between each digit code is, firstly, to be able to utilize all digit combinations in a multi-digit number, and secondly, to prevent the code receiver from failing to decode signals that are disturbed by, for example, fading. Without an indication of the transition from one digit to the next, you will not be able to repeat the same number as a neighboring digit in a code number, e.g. - - 11 - -- - 22 - - etc. The code receiver would then be able to decode this incorrectly as one digit. If one were to loop through all number combinations --11-- --22- - etc., with a 5-digit code system, for example, one would lose 40% of the total number of combination possibilities. In the case of wireless transmissions as mentioned above, where fading occurs, it is possible that the signal strength for a short period of time decreases to zero or to such low values that they are not registered by the code receiver. If such a short-term minimum occurs at an unfortunate time so that the code receiver receives the first and last part of the digit information, it would, if an additional signal had not been introduced between each code combination, decode this as a repetition of the same digit.

The selective call is composed of six different frequencies within the voice band and is sent out partly individually and partly as combinations of two tones. Five of the tones are used for the digit coding itself according to the system two tones out of five (which gives ten combination possibilities corresponding to the numbers 0 to 9), the sixth tone is used for digit shift, i.e. indication of transition from one digit to the next. A complete call signal e.g. a three-digit number will thus consist of the following single signals:

1. Tone combination for 1st digit

2. Digit shift tone

3. Tone combination for 2nd digit

4. Digit shift tone

5. Tone combination for 3rd digit

All these individual signals follow each other in a continuous sequence, and the duration of each individual signal can be e.g. 250 msec. A call will then only take 1.25 seconds, while the individual signals are so long-lasting that they will go through with a fair degree of certainty even in areas with strong rapid fading.

As can be seen, more than two tones are never sent at the same time. By ensuring that these two tones provide maximum modulation of the VHF transmitter, a good signal/noise ratio is achieved.

A device for such selective calling, where a monostable flip-flop circuit is used for time-limited recording of each code signal, and where this circuit is controlled by an "and" circuit, is characterized according to the invention in that in the "and" circuit a capacitor is arranged to keep the flip-flop circuit in working position for some time after the incoming control signals to the "and" circuit have ceased.

A preferred device of this kind is, according to the invention, characterized by a diode-resistance coupling from two of the "and" circuit's control inputs to the base of the transistor in the flip-flop circuit which is current-carrying (open) in the coupling's rest position, with the aim of preventing that the flip-flop circuit switches over to working position as long as control signals are present on the two mentioned inputs.

An embodiment of the invention will be described in more detail with reference to the drawings. Fig. 1 shows a connection diagram for a selective call system according to the invention. Fig. 2 shows a connection diagram for a recording circuit according to the invention. Fig. 3 shows a time diagram for explaining the system's operation.

From the output of the radio receiver, the signals are fed via input terminals 1 and 2 to the coil of a resonant relay 3. This relay has a number of tongues 4—9 which are tuned to the frequencies fl—fs. When the relay coil is supplied with an alternating voltage with a frequency that corresponds to one of the mentioned resonance frequencies fl-fs, the relevant relay tongue will swing up and make intermittent contact with the fixed contact belonging to the same tongue. The fixed contacts are marked 10—15. All the relay tongues are connected to minus potential on the equipment's power supply.

From each of the relay contacts 10—15, connections go to connection points 16—20 in a cross-connection field which in practice is designed as a "code plug". When a relay tongue oscillates and forms intermittent connections with its associated fixed relay contact, the associated connection point in the cross-connection field will have a negative potential. The capacitors 27-32 serve to smooth out the voltage pulses caused by the intermittent contact.

The aforementioned cross-connection field also has a number of other connection points 21-26. The number of these connection points depends on the number of digits the system is built for. Two such connection points are required for each digit, in the selected example the receiver is built for a three-digit call number, and the number of connection points is therefore equal to six.

Units 27, 28, 29, 30 and 31 are transistorized register circuits. They are basically the same in structure, except that some of them have two control inputs 33, 34 or 34, 35, others have three inputs 33, 34, 35, depending on their place in the function row, as well as an output 36. The register circuit's task is to transmit control pulses in a rather special way from inputs to outputs. The control pulses from the output of a register circuit are fed to the input 33 of the next register circuit in the function series.

Fig. 2 shows the circuit diagram for a register circuit. In principle, it is composed of a monostable flip-flop circuit, and an "and" circuit with two, respectively three inputs, which controls the flip-flop circuit. The coupling also contains a capacitor, two diodes and a resistor which gives the entire register circuit special characteristics.

These desired special properties are the following: 1. The register circuit must give a pulse on the output only after receiving negative control pulses on all inputs at the same time. 2. It should give a pulse on the output only if the control pulses on inputs 34 and 35 cease at the same time. 3. It must not give a pulse on the output as long as there are control voltages on inputs 34 and 35. The pulse on the output must start the moment these two control voltages disappear. 4. The pulse on the output must have a limited duration, after which the entire circuit tilts back to rest.

With support in fig. 2, the operation of the register circuits shall be explained in more detail below.

The connection to the left of the dashed line in the sketch is the aforementioned "and" circuit connection. The connection on the right is a monostable flip-flop circuit, also called "Smith-Trigger".

The "And" circuit works as follows:

The resistors 37, 38 and 39 are individually small compared to the resistor 40. The resistors 41 and 42 are of the same order of magnitude as each of the resistors 37, 38 and 39, and are in series with the emitter line of the transistor 43. In rest position, current flows from minus through resistor 40 and further through 37, 38 and 39 in parallel to plus. As 37, 38 and 39 are small compared to 40, point 44 (base transistor 43) will assume a potential close to the voltage source's plus potential. A rather weak current will therefore set in through the transistor 43 and point 45 also assumes a potential close to the plus potential. If one or two of the resistors 37, 38 and 39 are removed or blocked for current flow, the potentials at points 44 and 45 will rise somewhat, but not sufficiently for the flip-flop circuit to "flip over". Each of the resistors 37, 38 and 39 can be blocked for current coming through the resistor 40 by applying a sufficiently large negative voltage to the respective inputs 33, 34 and 35. The diodes 46, 47 and 48 prevent the negative control voltages from reaching point 44 .

If all three inputs 33, 34, 35 receive a negative control voltage, all three resistors 37, 38 and 39 are blocked for current flow from resistor 40, and this now only carries base current to transistor 43. The voltage drop across resistor 40 therefore drops to a fraction of the voltage drop in the rest position, and point 44 assumes a potential close to the current source's minus potential. This causes transistor 43 to open and have very little internal resistance. The capacitor 49 will therefore very quickly be charged to near full minus potential through the transistor 43 and the resistor 50 which is relatively small and is inserted to limit the maximum current through the transistor.

When one or more of the control inputs 33, 34, 35 lose their control voltage again, point 44 will return to resting potential, transistor 43 is blocked, and capacitor 49 is discharged through resistors 41 and 42 as well as through the base of transistor 50.

Transistors 50 and 51 together with resistors 52, 53, 54, 55 and 56 form a normal monostable flip-flop circuit. In the rest position, transistor 50 is blocked and transistor 51 is open. The resistance 56 is small and the voltage between the output 36 and the plus terminal is therefore small.

With negative control voltage on all three inputs, as mentioned in front of the capacitor 49 will quickly charge up to near full negative potential, and the potential at point 45 will rise accordingly. With an ordinary monostable flip-flop circuit, this would cause the connection to instantly "flip over" when point 45 had reached a certain potential.

To prevent this (cf. the desired, special properties of the register connection, point 3 above), the connection contains two diodes 56 and 57 and a resistor 58. The diodes are connected on the cathode side to respective inputs 34 and 35, on the anode side to a field - read resistance 58. This resistance is, on the other hand, connected to the base of transistor 51. The diodes 56 and 57 have the task of preventing mutual coupling between the inputs 34 and 35.

The effect of this connection is that as long as a negative control voltage is applied to the inputs 34 and/or 35, the same negative control voltage(s) will be passed across the resistor 58 to the base of transistor 51. With a correct dimensioning of the components, this additional negative voltage to the base of transistor 51 prevents the circuit from "flipping" over even with full negative voltage across capacitor 49. But as explained above, capacitor 49 will be charged up and "prepare" for flipping, as long as all three inputs have negative control voltage.

The moment the negative control voltage is removed from inputs 34 and 35, the base of transistor 51 loses its extra negative voltage and the circuit flips over. Thereby, transistor 51 is blocked and the voltage between output 36 and plus rises close to the applied operating voltage. The voltage on output 36 is used to control input 33 on the next register circuit in the function chain.

The duration of the voltage pulse on output 36 is essentially determined by the discharge time for the capacitor 49. This discharge time is determined by the size of the capacitor and by the values of the resistors 41 and 42, and by the base resistance of the transistor

50. To increase the pulse length to some extent

with given capacitor and resistor values, a positive feedback is introduced from the collector of transistor 51 to the base of transistor 50 through resistor 59.

The duration of the output pulses must be at least equal to the duration of each of the received tone signals in a call, that is, in the example chosen here, at least 250 ms.

In fig. 3, a function/time diagram is set up for a correctly received three-digit call, that is, for a call that matches the inserted code plug in the relevant receiver. The order of operation is as follows: 1. The two-tone combination for the first digit is received. Two of the relay tongues fl—f5 swing up and, above the code connection in the code plug, a negative control voltage is applied to the inputs 34 and 35 in the register circuit 27 (fig. 1. This circuit does not have input 33.

The capacitor 49 in this circuit is charged and prepares a pulse for the output 36. 2. As the tone combination for the first digit ceases after 250 ms, circuit 27 flips over and applies a negative control voltage to input 33 in circuit 28. At the same time, shift tone fs is received, and this causes a negative control voltage on input 34 in register circuit 28. (Register circuit 28 does not have input 35). Capacitor 49 in this circuit will therefore be charged and prepare a pulse for output 36 in circuit 28. 3. Since the shift tone after 250 m sec. up- hears, circuit 28 will flip over and provide control voltage to input 33 in circuit 29.

At the same time, a two-tone combination is received

for another digit, and this will effect

negative voltage on the inputs 34 and

35 in circuit 29.

4. When the two-tone combination for the second digit ceases, circuit 29 will flip over

and provide control voltage to circuit 30 etc.

5. Finally, circuit 31 will after receipt

of the two-tone combination for the last digit flip over and supply control voltage to

the indicator circuit 32. This can e.g.

be equipped so that it provides short-term

acoustic signal after which it ignites

a warning light that stays on

until it is switched off manually by the called subscriber.

As will be apparent from the foregoing

will be the conditions for receiving a call

that the correct tone combinations are received i

in the right order and at the right tempo. Each of the register circuits excepted

the first depends both on its having

received its correct signal and of that it

previous circuit in the chain has received

its correct signal, and that the time interval between the two signals is zero or within the tolerances that the system allows.

Claims (3)

1. Selective call system preferably for wireless connection between fixed and mobile, or between mobile radiotelephony stations, where a code system is used where for each digit in a decadal call number a code signal consisting of two simultaneously transmitted speech frequency modulation signals of a group of five frequencies is emitted, and where in in addition to the digit code signals, a sixth audio frequency code signal is emitted, characterized in that the sixth code signal that lies within the speech frequency band is sent out in each of the spaces between the digit code signals, so that all these individual signals follow each other in a continuous sequence, that the system's code receiver for recording the call is equipped with a set of cascaded flip-flop circuits which, upon receiving the alternately transmitted digit code signal and the sixth code signal, are triggered in turn to work position to emit an output signal, that the flip-flop circuits are connected so that the first circuit in the cascade is triggered by the first digit code signal, while the subsequent circuits are only triggered if they receive the respective code signal and the output signal from the preceding circuit at the same time, and that the circuit's output signals are emitted first from the moment their code signals cease and have calculated duration, so that the flip-flop circuits flip back to the rest position and thereby interrupt the decoding of the call number if the time interval between the beginning of two consecutive code signals exceeds a certain limit. 2. Selective call system according to claim 1, where a transistorized monostable flip-flop circuit is used for time-limited recording of each code signal, and where this circuit is controlled by an "and" circuit, characterized in that in "and" the circuit is equipped with a capacitor to keep the flip-flop circuit in working position for some time after the incoming control signals to the "and" circuit have ceased. 3. Selective call system according to claim 2, characterized by a diode-resistance coupling from two of the "and" circuit's control inputs to the base of the transistor in the flip-flop circuit which in the coupling's rest position is current-carrying (open), in the purpose of preventing the flip-flop circuit from tipping over into working position as long as control signals are present on the two mentioned inputs.