DE2021941A1 - Transmission system for the encrypted transmission of audio frequency information signals - Google Patents

Description

Dr. Herbert Scholz ZPHN.

^Mlt Va / WG

Applicant: NV PHILIPS'GLOEILAMPENFABRIEKEN
File: PM-4351

Registration from! May 4, 1970

Transmission system for the encrypted transmission of audio frequency information signals, "

The invention relates to a transmission system for encrypted transmission of audio frequency information signals, e.g. speech signals in which the encryption ("scrambling") is obtained by the The transmitter and the receiver are provided with oscillators which are fed back under the control of two synchronously acting Shift registers, each from a single shift pulse generator are controlled, can be changed in frequency according to a prescribed mustfi, which is in the ■ Frequency changing output signal of the oscillators with the Sound signal to be sent out or the distorted one received Signal combined to generate the distortion or demodulation will.

109883/0740 ^8AD

-2- ZPHN. 4331

In such a system, the shift registers acting as control elements continuously run through a specific one "Duty cycle", the duration of which is determined by the number of shift register elements in the shift register will. At the output of the last stage of the shift register, a periodic square-wave signal of irregular shape occurs, the duration of which is equal to the duration of one duty cycle of the shift register. At any point in time, the size is the Distortion determined by the binary information present in each shift register. The shift register elements become for this purpose, e.g. via an assembly device made up of resistors with the frequency-determining circuit of the oscillator, the output signal of this oscillator being combined with the sound signal to be transmitted will. The receiver is equipped with the same oscillator that is controlled by a shift register as with the transmitter and whose output voltage is fed to a demodulator. To get the correct demodulation of the received signal should be the frequency of the signal that is combined with the received signal by the receiving oscillator is, at every point in time, equal to the frequency of the signal to be transmitted by the transmitter-side oscillator with the signal combined signal, i.e. that exact synchronization must take place between the shift registers.

A system of the type mentioned in the opening paragraph is known from Swedish patent specification II907 / 67.

The synchronization of the

109883/0740

-3- ZPHN. 433 "!

Shift register in the transmitter and in the receiver by the fact that in the transmitter the from the shift register he - '* ugtf; ^T T, pirlstntister is combined with the distorted speech signal and these signals are transmitted together. In the Frapfanger, the received signal, which contains the pulse pattern generated by the shift register in the transmitter, is correlated with the pulse pattern taken from the receiving-side shift register. Only the component of the received signal which corresponds to the pulse pattern generated by the receiving-side shift register contains a (cross) correlation or a (cross) correlation product which is or is not equal to zero. With exact synchronization of the shift registers in the transmitter and in the receiver, the mentioned (cross) correlation or the mentioned (cross) correlation ™ product is maximal, which property can be used to control the receiving-side shift register in order to obtain the desired synchronization.

This synchronization method comprises a continuous one Control of the effect of the shift register, but has the disadvantage that the aforementioned pulse pattern to be transmitted contains a comparatively large amount of information which, when correctly transmitted, covers a broad frequency band requires. In practice it has been found that the. Transmission of this information via common communication channels Difficulties are caused by which application in this synchronization method is less desirable.

The aim of the invention is to create a system of the type mentioned in the introduction, in which the synchronizing

10988 3/07 40

• AD ORiGlNAL

-h- ZPHN. '+331

information required is relatively small, while the one needed for the transmission of this information Bandwidth is also so small that this information can be transmitted safely via common communication channels, can.

According to the invention, the transmitter contains a device to merge the tone signal distorted by frequency changes with one taken from the Schieheim pulse generator Control signal whose frequency lies in the frequency band of the audio signals to be transmitted and whose phase information via the shift register at the transmitter end Includes pushing pulses while the receiver is using a device is provided for extracting the control signal mentioned before the demodulation, which control signal to the The shift pulse generator on the receiving side is emitted so that a fixed phase relationship *? between the mentioned Control signal and the signal generated by the shift pulse generator is achieved.

In this embodiment s form synchronization is obtained by the fact that only a discrete frequency, namely the fundamental sinusoidal frequency of the control voltage is exceeded, of which the disc pulses are derived in the transmitter, whereby the continued pushing in the shift registers of the transmitter and the receiver is synchronous. The phases of the shift registers are made equal to one another within the relevant operating cycle at special intervals, for example in an interval preceding the information transfer, in the

109 883/0740

-5- ZPHN. 4331 '

Transmit impulses with a certain repetition frequency which define a predetermined starting position of the shift register on the transmitter side. ■

In the case of carrier frequency transmission, as a result of frequency changes in the demodulating signal, it may be desirable to transmit two discrete synchronization frequencies, with the information about the times of occurrence of the shift pulses in the transmitter from the difference of the two transmitted Frequencies is derived, which difference signal of any changes in the absolute value of the frequencies is independent.

Some embodiments of the invention are shown in FIG Drawing and are described in more detail below, Show it:

1 shows a block diagram of a transmission system for voice-encrypted transmission, in which the Synchronization obtained by the method according to the invention will;

FIG. 2 shows a pulse signal taken from a shift register in the system of FIG. 1, which is used to distort the Speech signal is used, and

3 shows some voltage diagrams to explain the mode of operation of the receiver according to FIG. 1.

As shown in Fig. 1, this is from the microphone 10 resulting speech signal is fed to a modulator 11, which is designed, for example, as a symmetrical mixer stage and to which the signal generated by the oscillator 12 is also fed. The frequency of the oscillator 12 varies between different levels. A continuously acting back-coupled sliding

109803/0740

021941

PHN. 4331

register 13 determines the current frequency of the oscillator 12, the individual shift register elements of the Shift register 13 via an assembly device 14 made up of resistors with the frequency-determining Circle of the oscillator 12 are connected.

The shift register 13 can, for example, be based on the ones described in Swedish patents 11907/67 and 12524/67

described manner and is controlled by a shift pulse generator which is composed of a control oscillator 15 and a divider stage 16 arranged between the control oscillator 15 and the input circuit of the shift register 13. The control oscillator 15 emits, for example, a frequency of 2γ kHz, which is divided by the divider stage 16 by a factor k , so that a pulse series with a frequency of 625 Hz is fed to the shift register.

The oscillator 12 is tuned to an intermediate frequency which is the upper limit of the spectrum of the speech signal, e.g. 3 kHz, while the frequency jumps brought about by the shift register 13 are at most equal _ + _ 400 Hz, which means that a signal emitted by the oscillator whose frequency varies irregularly between 2600 and 3 ^ -00 Hz. An example of such a thing The frequency-changing signal generated by the oscillator 12 is shown as a function of time in FIG.

In the case of the signal generated by the mixer 11, the lower sideband of the mixed product is used, its frequency

10 9 8 83/07 4 0

-7- ZPHN. ^? + 33i

is equal to the instantaneous frequency output by the oscillator 12 minus the frequency of the speech signal. That The spectrum of the speech signal is also inverted by the action of the mixer 11, which results in the interpretation such a disturbed signal is made more difficult without the use of the correct receiving equipment «

The disturbed speech signal can be fed to the receiver directly or modulated on a carrier wave which receiver is shown in FIG.

In the receiver, the distorted speech signal is passed through a filter 17 to a demodulator or a mixer 18 supplied, which (which) is constructed in the same way as the transmitter-side modulator 11.

The filter 17, the effect of which will be described in detail below, is permeable to the entire frequency band of the speech signal. The mixer stage 18 is also supplied with the output voltage of an oscillator 19, which oscillator 19 corresponds to the transmitter-side oscillator 12 and is controlled by a shift register 20 via an assembly device 21 made up of resistors. The shift pulses for the shift register 20 are taken from a shift pulse generator which contains a control oscillator 22 which is connected to the input of the shift register 20 via a divider stage 25. The shift register 20 corresponds completely to the shift register 13 on the transmitter side and emits an identical series of pulses. If the shift registers 13 and 20 act synchronously, a signal is fed to the demodulator 18, its current frequency

109883/074 0

-8- ZPHN. ^

exactly equal to the frequency of the oscillator 12 to the Mixer 11 output signal, "-, ■ i ,. A in output of the demodulator 18 a mixed product occurs which corresponds completely to the original speech signal.

To achieve synchronization of the shift registers The following measures are taken in the sender and in the receiver.

In the transmitter, the control voltage is taken from the divider stage 16, which is connected to the shift register 13, the frequency of which is, for example, 62 5 Hz, which control voltage is fed to a device 32 via a line 2h and is combined with the distorted speech signal in this device. The aforementioned control voltage can optionally be filtered before it is combined with the distorted speech signal, so that only the sinusoidal fundamental frequency is transmitted. The transmission of the control voltage of the shift register can take place continuously during the entire call or, if necessary, in an intermittent manner if the pulse generator on the receiving side has sufficient frequency stability.

The signal that has arrived at the receiver, consisting of a distorted speech signal and the control voltage of the Shift register exists, is, as already mentioned, fed to the filter 17. In filter 1? becomes the control voltage of the transmitter-side shift register 13 is selected from the received signal and fed to a phase detector 25,

109883/0740

SAD ORSOiHAL

-9- ZPHN. 4331

the filter 17 at the same time, prevents the control signal the demodulator 18 trains leads act. To the selection the control voltage, the filter 17 has a very narrow Passband with a bandwidth that is only a few Hz is greater than the frequency of the control voltage, so that the suppression of the speech signal in this frequency band the quality of the speech signal is hardly affected.

To achieve the effect mentioned, the filter 17 is provided with an amplifier 26 fed back via a DoppeIt-T network 27 and with an input resistor 28, a resistor 29 in the transmission path of the speech signal and a resistor 30 which has the output of the amplifier 2, 6 the output of the resistor 29 is fed back.

In the phase detector 25, the selected control voltage with a frequency of 625 Hz with that of the divider stage taken signal compared, which is used to control the shift register 20. The output of the phase detector 25 »which contains information about the phase difference of the signals fed to the phase detector 25 is via an amplifier 31 the frequency-determining circuit of the oscillator 22 supplied. In the closed control loop formed in this way, the negative feedback maintaining an output voltage of the phase detector 25 counteracted. Is in the steady state that is, the signal emitted by the divider stage 23 is in phase with the output signal of the transmitter-side divider stage 16 and becomes the information in the shift register elements

109883/0740

-10- ZPHN. 4331

pushed away synchronously.

The shift registers ij and 20 can, however, have a different phase within the working cycle in the case of synchronous advancement. In order to maintain phase equality, pulses are transmitted via the connecting line in a speech-free time interval, for example a time interval preceding the transmission of speech signals. The frequency of these synchronizing pulses, which are taken from the control oscillator 15 in the illustrated embodiment, is equal to 2-g-kHz. The duration and the shape of these pulses are determined by the AND gate 35 which connects the output of the oscillator 15 to the assembly device 32. The AND gate is opened and closed by means of the bistable flip-flop 36, which is controlled by two decoding elements 37 and 38. The first decoding element 37 is connected to the trigger input of the bistable flip-flop 36 via an AND gate 39, while the second decoding element 38 is coupled to the reset input of the flip-flop 36. The AND gate 39 is opened by the voltage taken from a monostable flip-flop kO and remains open during the switching interval of the flip-flop. The flip-flop 4o is controlled by a switch 41 which emits trigger pulses via a differentiation circuit 42 to the flip-flop 4o. The switch 41 is activated, for example, when the start button is pressed to transmit a call.

109883/0740

in the

The decoding elements 37 and 'j8 are connected to all shift register elements of the Schi ^r - ■ - * ■ ^! --f.:· γ-γιϊγρ 1 3 connected and thus scan the current position of the shift register. Each decoder element is matched to the generation of an output pulse at a specific position of the shift register. For example, the decoder 38 delivers an output pulse at the point in time at which all the shift register elements of the shift register 13 are switched and, for example, assume a position that is indicated in binary with "1", while: - end the decoder 37 a number Shift pulse periods generated before the mentioned time pulses. Under the condition that the AND gate 39 is open, the flip-flop 36 is toggled when the shift register reaches the position in the duty cycle to which the decoder 37 is sensitive. A short time interval thereafter, the position is reached to which the decoder 38 responds, after which the flip-flop 36 is reset. During the switching interval of the flip-flop 36, the AND gate 35 remains open and a signal, the frequency of which is 2500 Hz in the example shown, is fed via the AND gate 35 to the joining point P, at which a joining with the signal of 625 Hz he follows. The trailing edge of the synchronization pulse, the repetition frequency of which is 2500 Hz, defines the point in time at which the shift register 13 assumes the aforementioned position, in which all shift register elements indicate the state "1".

The reset time of the flip-flop ko is selected in such a way that a number of synchronizing

109883 / 07AO

• AD ORIGINAL

pulses, e.g. 3 to 5> are fed to the line. Through the Resetting the flip-flop '+ υ ν- - ■ ... ■ <! > i «i / nd-'Jattor 39 closed, whereby the flip-flop 36 never V. *. nohr switched and the synchronization pulses no longer occur.

In the receiver, the synchronization pulses are selected using a filter ^ 5, which is tuned to the pulse repetition frequency of 25ΟΟ Hz. The filter h5 is provided with an amplifier k6 which is fed back via a double-T network k7. The selected synchronization pulses are fed to an integrator ^ 8 which is provided with an amplifier 50 which is fed back via a capacitor k9 and in whose input circuit two diodes 5I and 52 are included. The output voltage of such an integrator increases gradually with each period of the AC voltage supplied. A limiter is included in the output circuit of the integrator, which limits the voltage in the output circuit to a certain value. When the synchronization pulses cease to exist, the capacitor ho discharges through a resistor

53, where the discharge time is given by the time constant of the RC circuit formed by the capacitor 49 and the resistor 53 is determined.

The integrator '48 controls a Schmitt toggle switch

54, which during the period in which the output voltage of the Integrator ^ 8 exceeds a certain value, a Gives off tension. The one generated by the Schmitt flip-flop 5'4 Voltage pulse controls after amplification in an amplifier 55 via an AND gate 56 the input of the shift

109883/0740

-13- ZPHN. 4331

register 20. When a voltage pulse of the The shift "" r * - "g; iste: releme: rvte of the shift register are assigned to the TJnd gate 5i> 20 compulsorily in a certain position, e.g. the. Position indicated above with "1", guided. If the Pulse, the shift register is triggered and responds again to the shift pulses originating from the divider stage 23 at.

The output voltage of the integrator 48 is shown in FIG. 3a, while FIG. 3b shows the output voltage of the Schmitt flip-flop 54. The level of the output voltage of the integrator 48, at which the Schmitt flip-flop is switched on, is denoted by S in FIG. 3a. The time t .. coincides with the beginning of a synchronization pulse when the voltage at the output of the integrator 48 increases. At the _{instant t o} , the Schmitt flip-flop 54 is switched on. At the point in time t "no more synchronization pulses occur and immediately thereafter - at the point in time tu - the voltage at the output of the Schmitt flip-flop circuit also disappears.

On condition that the AND gate 56 is during the duration of the synchronization pulse remains open, increases the shift register takes a predetermined position at the end of this pulse.

The AND gate 56 is controlled by means of a counter 57 to which the synchronization pulses occurring at the output circuit of the amplifier 55 are fed via an AND gate 58. The counter 57 is in such a way on an input

109883/0740

- \ k- ZPHN.

of the AND gate 58 fed back that this AND gate is automatically closed and according to the payer 57, one certain number of pulses, e.g. 3 to 5 »no synchronization pulses more can be fed. During the interval in which the counter received synchronization pulses is advanced, the AND gate 56 is held open. When the predetermined number of pulses is reached, the AND gate 56 is closed and there are still none

More pulses fed into the shift register 20.

The counter 57 is reset to zero with the aid of an integrator 59 which works in the same way as the integrator 58 is built up, but that of the selected control voltage is controlled by 625 Hz, which is used to synchronize the Advancing the shift register is used. At the beginning of the conversation, the voltage at the output of the integrator 59 constructed in the manner already described for the integrator ^ 8. This voltage remains during the entire conversation φ at the mentioned limiting level, the counter 57 then can perform the work cycle described. When there is no voltage at the output of the integrator 59 after termination of the conversation, the counter 57 is then reset to zero again.

The mode of operation of the arrangement described is as follows:

From the time the arrangement is put into operation, the control voltage for advancing the shift register 13 in the transmitter to the receiver. Of the

109883/0740

-15- ZPHN. 4331

controlled oscillator 22 and the dividing stage 23 in the receiver are synchronized with the received bteu-.rsj ^ nung (locked), after some time the divider stage 23 is on The pushing pulse signal is generated, which exactly «ixt the sliding- Pulse signal at the output of the transmitter-side divider stage 16 is in phase.

When the transmitter switch designated by Ί1 in FIG. 1 is closed, the flip-flop kO switches over and the AND gate 39 is thus opened. The bistable flip-flop 36 switches over when the content of the selector register 13 reaches a value such that the decoder 37 responds, after which the flip-flop 36 is reset to the initial position at the point in time at which the content of the Shift register reaches a value to which the decoder 38 responds.

During the interval in which the flip-flop 36 switches, the AND gate 35 remains open and a synchronizing pulse signal of 2500 Hz is delivered to the line. After a certain number of synchronizing pulses has been delivered, the AND gate 35 is closed so that none Synchronization pulses are transmitted more.

In the receiver, each synchronization pulse generates a voltage of the form shown, for example, in FIG. 3 at the output of the integrator 48. During the interval in which the Schmitt flip-flop 5k emits an output voltage which corresponds to the signal shown in FIG. 3t, the shift register receives a voltage at the control input via the

109883/0740

-16- ZPHN. 4331

Amplifier 55 and the AND gate 56, which voltage the shift register elements in the shift ^^ repis ^ er 2 ^r > forcibly leads to the predetermined position. At the end of each synchronization pulse, the shift register 20 is triggered and then responds again to the shift pulses originating from the divider stage 23. The content which the shift register 20 forcibly reaches through the occurrence of a synchronizing pulse is selected to be the same as the content to which the decoder element 38 is sensitive. Since the decoder 38 determines the point in time at which the synchronization pulse is ended, the content of the shift register 20 will be exactly the same as the content of the shift register 13 at the point in time when the synchronization pulse is ended. After receiving a predetermined number of synchronization pulses, the AND gate ^> 6 is closed, after which the shift register 20 is controlled by the shift pulses of the divider stage 23. If it is assumed that the synchronization of the shift pulses is maintained and that the last synchronization pulse -di <? has caused the correct setting of the shift register 20, the synchronous effect of the shift registers in the transmitter and in the receiver is ensured.

Instead of the shift register in the receiver only once

to be set at the beginning of a conversation, assuming that the synchronization will last for the entire duration of the conversation is maintained, the shift register can also be reset intermittently during the call then the call signal briefly is suppressed. Such a method can be desirable, for example, in transmission systems in which synchronization through the occurrence of fading ("fadine")

BAD ORIGIN «-

-17- ZPHN. 4331

can get lost.

This is the case with carrier frequency transmission System in which the information about the control signal of the transmitter-side shift register in a simple harmonic Signal is included, not reliable. After joining with the carrier wave, it cannot be avoided that the frequency of the control signal after demodulation deviates slightly from the desired frequency. As the for synchronization If the required information is in the phase of the control signal, such a frequency deviation is not permitted. If, however, instead of a simple harmonic signal, two simple harmonic signals are transmitted, which e.g. Have frequencies of 600 or 1200 Hz, the problem mentioned is avoided, because in this case the information about the phase of the control signal is included in the difference between the mentioned simple harmonic signals. Even if the amplitude of these transmitted signals changes somewhat over time, the difference remains constant and the correct information is transmitted to the recipient.

An increase in secrecy can be achieved simply by removing the An assembly device built up with resistors, which connects the shift register with the oscillator in the transmitter and in the receiver connects, is replaced by a differently constructed assembly device, in which case the synchronization is adjusted. A second method of increasing confidentiality is to have the shift register elements

109883/0740

-18- * ZPHN.

in the shift register can be connected in a different way, whereby a different frequency recovery cycle is obtained.

Can "sationsmassnahmen in connection with the above-described Synchroni also still an increase of secrecy are obtained in that in the connecting line between the microphone 10 and the modulator 11 of FIG. K a filter 33 is received, that acts as a low-pass and mainly the higher Frequencies in the spoken signal attenuates and its cutoff frequency lies within the frequency band to be transmitted In Fig.- h , this cutoff frequency is, for example, 1 kHz.

In order to prevent the synchronization pulse pattern from being discovered, the two decoding elements on the transmitter side can can be made adjustable, the synchronizing pulses then occur at different times of the duty cycle of the transmitter-side shift register. This can be compensated in the receiver by adding a time delay between the one that detects the synchronization pulses The device and the device which sets the shift register in a predetermined manner are attached will.

The described arrangement can be used not only for the transmission of distorted speech signals, but also for encrypted ones Transmission of information signals in the form of Tone signals or frequency shift signals are used, for example, the 3-out-of-5 code or the like. is used at which each information element by a frequency within of the tape.

109883/0740

Claims (1)

PATENT CLAIMS -19- ZPHNV 43.31 Transmission system for the encrypted transmission of audio frequency information signals, e.g. speech signals, in which the encryption is obtained as a result, that the transmitter and the receiver are provided with oscillators under the control of two synchronously acting feedback shift register, each for see by one Shift pulse generator can be controlled according to a prescribed pattern in which the frequency can be changed the output signal of the changing frequency Oscillators with the sound signal to be transmitted in the transmitter and the received distorted signal in the receiver is, characterized in that to achieve synchronization of the shift registers (13 and 20) of the transmitter and the receiver of the transmitter with a device (32) for joining the audio signal distorted by frequency changes is provided with a control signal taken from the shift register (15, 16), the frequency of which is im Frequency band of the audio signals to be transmitted lies and its Phase information about the shift register on the transmitter side contains supplied shifting pulses, while the receiver with a device (17) for taking the mentioned control signal is provided before the demodulation, which control signal is sent to the receiving-side shift pulse generator (22, 23) is output to establish a fixed phase relationship between the aforementioned control signal and that from the shift pulse generator generated signal. 109881/0740 2. System according to claim 1, characterized in that the receiving-side sliding device (22, 23) into a closed control loop is added, which has a phase detector (25) contains, on the one hand, the aforementioned control signal and, on the other hand, the output signal of the shift pulse generator is supplied and with the help of which the shift pulse generator (22, 23) is controlled to have a fixed phase relationship between the signals compared in the phase detector. 3. System according to claim 1 or 2, characterized in that the control signal is formed by the difference signal obtained from two signals whose different frequencies are in the frequency band of the sound signals to be transmitted lie. k . System according to one of claims 1, 2 or 3 »characterized in that for the synchronization of the initial positions of the shift registers (13 and 20) the transmitter is provided with a device (35 - ^ 2) which is controlled by the shift register 13 and which operates at least during an interval that precedes the information transmission, generates pulses with a certain repetition frequency and transmits them to the receiver, which pulses define a predetermined position of the shift register 13, and that the receiver is provided with a device (^ 5 - ^ 6) for receiving the said pulses is, the shift register 20 of the receiver being moved to the same starting position with the aid of these pulses. 3/0740 BAD ORlGiNAL -21- ZPHN.Λ331 5. System according to one of the preceding claims, characterized in that a filter is included in the transmitter-side signal modulation circuit, which acts as a low-pass filter and mainly the higher frequencies d _; it attenuates the information signal and its cutoff frequency lies within the frequency band to be transmitted. 1 098 83/07