Abstract:
To transmit encoded information such as for example voice messages between remote transceivers, a transmitter digitizes and encodes an audio signal and then transmits the resulting digital coded signal to the receiver. The receiver decodes the signal and converts it back to an audio signal. The received encoded digital signal is used to reset the timing clock in the receiver so that the timing is synchronized in the receiver and transmitter by the transmitted encoded information. The encoder transmits digital information to a shift register. Encoding switches select certain stages of the shift register and the transmitter transmits the exclusive-OR of the digitized message and the output of the selected stages. This signal is also the input to the shift register. The information transmitted to the receiver is a scrambled digital signal. In the receiver, the coded information synchronizes a clock for the shift register from the encoded digital signal that it receives and selects the stages of the shift register that unscramble the coded information for conversion to an audio signal suitable for the speaker.

Description:
RELATED CASES 
     This application is a continuation-in-part of U.S. application 60/012,465 filed Feb. 28, 1996, on Synchronization Technique and Method and Apparatus for Transmitting and Receiving Coded Signals. 
    
    
     COMPUTER PROGRAM LISTING APPENDIX 
     A Computer Program Listing Appendix on compact disc was submitted in this application. One compact disc and a duplicate copy were submitted containing the file “Patent.asm”, which was created on Nov. 12, 2001, and is 68,827 bytes in size. 
     BACKGROUND OF THE INVENTION 
     This invention relates to synchronization of timing signals and to the transmission of encoded information such as for example the transmission of voice messages between remote transceivers in code. 
     It is known to transmit signals in digital code from one location to another for secrecy. However, most attempts to transmit digitized radio signals in encoded form in real time from one transceiver to another with reliable inexpensive equipment and good fidelity have been unsuccessful. This has been difficult to achieve with a coded signal because of the difficulty in synchronizing the encoding and decoding circuits in the transmitting and receiving apparatuses in real time with reasonable precision. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a novel technique for synchronizing timing circuits not having a connection between them such as for example not having an electrical connection between them. 
     It is a further object of the invention to provide an electromagnetic linkage in which a signal is encoded and transmitted at one location and received and decoded at the other location without the use of a solid connection therebetween. 
     It is a further object of the invention to provide a novel radio for transmitting and receiving encoded signals. 
     It is a still further object of the invention to provide a novel portable transceiver capable of transmitting and/or receiving signals in encoded form from another source. 
     In accordance with the above and further objects of the invention, remotely located timing circuits such as oscillators operating at the same frequency are synchronized together from the signals that are transmitted fron one unit to the other. The signals, upon being received by the second unit, are used to synchronize the timing circuit in the second unit to that in the first unit to permit decoding of the data. 
     In the preferred embodiment, a transmitter digitizes and encodes an audio signal and then transmits the resulting digital coded signal to the receiver. The receiver decodes the signal and converts it back to an audio signal. The received encoded digital signal is used to reset the timing clock in the receiver so that the timing is synchronized in the receiver and transmitter by the transmitted encoded information. The synchronized signals are used in the encoding and decoding of the signal. 
     In one embodiment, the encoder feeds digital information to a shift register. Encoding switches select certain stages of the shift register and the transmitter transmits the exclusive OR of the digitized message at the output of the selected stages. This signal is also an input to the shift register. Thus, the information transmitted to the receiver is a scrambled digital signal. In the receiver, a shift register has the same stages selected as in the transmitter and the coded information synchronizes a clock for the shift register from the encoded digital signal that it receives. The selected stages of the shift register unscramble the coded information for conversion to an audio signal suitable for the speaker. 
     In the preferred embodiment, the transmitter converts the analog audio signal to a digital binary signal with 16 bit words. It counts the binary ones in each 16 bit word, and if the number of ones is an odd number, it inverts the digital word before transmitting with the entire encoded message. It transmits words having an even number of binary ones without inverting them. At the receiver, the words having an odd number of ones (or of zeros) is reinverted and the words are decoded using the same process as the coding. 
     Of course, other characteristics that can be determined from the transmitted word may be used to alter the digital code in a predictable manner. For example, if the coded words include an odd number of bits such as 17 bits but a binary one is always added at a fixed position before transmitting and always subtracted upon receiving, the bit in the fixed position would be ignored and the process repeated or an even number of bits per word could be used but binary zeros could be counted and the word invested if an even number of binary zeros are in the word as described above. 
     From the above description, it can be understood that the synchronizing technique of this invention and the coded transmission system of this invention have several advantages such as: (1) they provide low noise transmission; (2) there is very little cross talk between several different transmitters even when the encoding selections are only slightly different; (3) the equipment is simple and inexpensive; and (4) the code may be easily changed. 
     SUMMARY OF THE DRAWINGS 
     The above-noted and other features of the invention will be better understood from the following detailed description when considered with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a radio system in accordance with an embodiment of the invention; 
     FIG. 2 is a block diagram of a cipher circuit used in the radio system of FIG. 1; 
     FIG. 3 is a schematic diagram of an encoder used in the cipher circuit of FIG. 2; 
     FIG. 4 is a schematic circuit diagram of a decoder used in the cipher circuit of FIG. 2; 
     FIG. 5 is a schematic circuit diagram of a synchronizing circuit for use in the cipher circuit; 
     FIG. 6 is a schematic circuit diagram of another embodiment of cipher circuit using a microprocessor; 
     FIG. 7 is a simplified schematic diagram showing one embodiment of encoding and decoding keys; 
     FIG. 8 is a block diagram showing a manner in which the circuit of FIG. 6 implements the invention; 
     FIG. 9 is a more detailed block diagram of a portion of the embodiment of FIG. 8; 
     FIG. 10 is a more detailed block diagram of another portion of the embodiment of FIG. 8; 
     FIG. 11 is a more detailed block diagram of another portion of the embodiment of FIG. 8; and 
     FIG. 12 is a more detailed block diagram of still another portion of the embodiment of FIG.  8 . 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, there is shown a duplex radio system  10  having a first transceiver  10 A and a second transceiver  10 B. In the preferred embodiment there are a multiplicity of transceivers and one of the transceivers serves as a base station. Each of the trasceivers has a convenient system for selecting a code in which the signals are encoded for transmission between the transceivers. One purpose of the encoding and decoding is to provide a ciphered message but the claimed system also provides the advantages of high fidelity and low cross talk with a simple inexpensive circuit. 
     In this cryptic radio system, a simple selection device is used to select any of many different codes as long as both the transmitter and the receiver that send and receive the scrambled signals are set to the same code, the transmission occurs with good fidelity and little cross-talk with other transceivers that may be in the same region. For the purposes of this description, the transceiver  10 A and the transceiver  10 B are substantially identical and only the transceiver  10 A will be described hereinafter. 
     The transceiver  10 A includes a first antenna  22 A for the transmitter, a second antenna  23 A for the receiver, a circuit  20  for connecting the antenna  22 A to transmit signals, a circuit  21  for connecting the antenna  23 A to receive signals for the transceiver  10 A, a transmitting circuit  12 , a receiving circuit  14  a cipher circuit  16 , an audio circuit  18  and a timing circuit  58 . The audio circuit  18 , the transmitting and receiving circuits  12  and  14 , the circuits  20 ,  21  and the antennas  22 A and  23 A are all conventional and not part of the invention except to the extent that they cooperate with the cipher circuit  16 . 
     The receiving circuit  14  includes conventional units such as the RF stage, IF stage, mixer and local oscillator indicated at  24  and the FM detector indicated at  26 . The transmitting circuit  12  includes conventional FM modulation circuits such as those indicated at  28  and amplification circuits such as those indicated at  30 . With this circuitry, digital encoded signals are received and applied from the FM detector  26  to the cipher circuit  16  and audio signals are provided to the cipher circuit  16  and provided by the cipher circuit  16  to the transmitter circuit  12  for transmission. The cipher circuit  16  is also connected to the audio circuits  18  which include audio stages  32  and  36 , the speaker  34  and the microphone  38 . The audio stage  32  receives signals from the cipher circuit  16  and applies audio signals to the speaker  34  for providing sound to the operator. The audio stage  36  receives signals from the microphone  38  and provide signals to the cipher circuit  16  for transmission. 
     The timing circuit  58  is connected to the cipher circuit  16  through conductors  58 A to provide timing signals to the cipher circuit  16  and through the conductor  58 B to transmit data from the cipher circuit  16  to the timing circuit  58  for synchronization of the timing circuit  58  when it is serving as a slave transceiver to a master trasceiver as far as synchronization is concerned. Most commonly, at least one of the speakers is in headphones. 
     In FIG. 2 there is shown a block diagram of the cipher circuit  16  including an audio input conductor  40 , an audio output conductor  42 , an encoder  44 , a digitizing circuit  46 , a decoder  48 , a data output conductor  50  from the digitizing circuit  46  to the encoder  44 , a data output conductor  52  from the encoder  44  to the transmitter circuit  12  (FIG. 1) a data input conductor  54  to the decoder  48 , a data input conductor  56  from the decoder  48  to the digitizing circuit  46  and a timer circuit  58 . 
     The timer circuit  58  is connected to each of the synchronized circuits of the cipher circuit  16  to provide timing signals through a conductor  58 A to the encoder  44 , the digitizing circuit  46  and the encoder  48 . The conductor  58 B for transmitting data to the timer circuit  58  has incorporated within it a switch  58 C. If the switch  58 C is open, the transceiver  10 A is the master transceiver and the transmitted data from this master transceiver  10 A is used to synchronize the timing circuit in the other transceiver (slave transceiver  10 B). If the switch is closed, then transceiver  10 A is a slave transceiver and data transmitted from the master transceiver  10 B is used to synchronize the timing circuit  58  in this slave transceiver  10 A. 
     In FIG. 3 there is shown a schematic circuit diagram of the encoder circuit  44  and digitizing circuit  46  connected to each other. The encoder  44  includes a 16-bit shift register  64 , selector switches  60 , and a decoding circuit  62 . The selector switches  60  are connected between the outputs of the shift register  64  and the decoding circuit  62  to permit the selecting of a code. The shift register  64  is connected to one input of an exclusive OR gate  66 , the other input being connected to the output of the digitizing circuit  46  which provides coded signals to this input of the exclusive OR gate  66  through the conductor  50 . The output of the exclusive OR gate  66  is transmitted to the transmitter through the conductor  52 . 
     The selector switch bank  60  and the gating circuit  62  of the transmitter enable the stages of the shift register  64  that are selected for transmission. These stages correspond to the stages enabled in a corresponding shift register, selection switches and gating circuit in the decoder  44  of a receiver that receives the signal. The selector switches connect the signals through exclusive OR gates in the gating circuit  62  so that only the bits of the signal in certain stages of the shift register are transmitted to the output conductor  52  for transmission at each clock pulse. 
     In FIG. 4, there is shown a schematic circuit diagram of the decoder  48  connected to the digitizing circuit  46  and of the timing circuit  58 . The decoder  48  includes a bank of shift registers  70 , selection switches  72  and an exclusive OR gate  76 . The shift registers  70 , selection switches  72  and gating circuit  74  correspond to the shift register  64 , selection switches  60  and gates  62  of the encoder  44  (FIG. 3) respectively. The selection switches  72  are selected to correspond to the selection switches  60  so that if one transceiver is intended to transmit signals to a second transceiver, the same numbered switches are closed such as the first switch, the second switch and the fifth switch in both to provide proper decoding of the encoded digital signal. 
     The timing circuit  58 A includes a differentiator  80  and a ripple counter  82  connected together to provide edge signals at the selected frequency. The differentiator  80  forms the edge signal and the ripple counter  82 , driven by the oscillator in the digitizer  46  divides the frequency to the chosen frequency. 
     With this arrangement, signals received from the FM modulator  28  of the decoder through conductor  54  are transmitted to the shift register  70  and to the timing circuit  54 , then selected in the shift register  70  by the selection switches  72  for application to the gating circuit  74  which transmits them to the exclusive OR gate  76 . Timing signals and data signals are also supplied to the timing circuit  58 A which detects the positive and negative transitions of the data by the two differentiators at  80  and transmits them to the ripple counter  82 . An oscillator within the digitizing circuit  46  in some embodiments performs the synchronization. It drives the ripple counter  82 . A microprocesser  90  receives the signal from CODEC  94  and transmits an audio signal to the speaker. 
     In FIG. 5, there is shown a schematic circuit diagram of the timing circuit  58  which is driven by a crystal oscillator  91  through a ripple counter  92  with a tap taken off of it to apply to the circuits being timed through conductor  58 B. The switch  58 C is used to select the slave transceiver for that operation since the data signal applied through conductor  54  is transmitted through switch  58 C when closed to reset the counter  92  in synchronization with input data. The digitizing circuit  46  is a 609 delta modulation circuit sold by MX-COM, Inc. The shift registers  64  and  70  are 4015 B shift registers manufactured by SGC-Thomson Microelectronics, the ripple binary counter  92  is a SGS-Thomson HCC 4020 ripple-carry binary counter-divider, the exclusive OR gates are  4030  exclusive OR gates and the AND gates are 4073 AND gates. 
     In FIG. 6, there is shown a block diagram of another embodiment of the invention having a microprocesser  90 , an oscillator  92 , a CODEC  94  and an amplifier  96 . The microprocesser  90  is programmed with the software contained in the Computer Program Listing Appendix on compact disc to perform the steps of encoding and decoding. The CODEC  94  receives analog information from the microphone, converts it to digital information and feeds the digital signal to the microprocesser or receives decoded digital signals from the microprocesser, converts it to an analog signal and transmits it to the speaker. The microprocesser performs encoding and decoding functions by receiving the digital signal, and in accordance with its software, feeding it at the selected clock rate through a software shift register, selecting certain stages for a scrambled output for transmitting to the receiver or receiving coded digital signals to process in the shift register and to the CODEC. When receiving the encoded digital signal, the CODEC performs the inverse function to provide an analog signal to the speaker. The CODEC  94  serves its design function as a receiver or transmitter. 
     In FIG. 7, there is shown the eight stages of switches  60 , each of which enables the selection of outputs from the hardware shift register  64  in the embodiment of FIG. 3 or its software equivalent in the embodiment of FIG. 6 in accordance with a number on a particular switch. Thus, the selection of the fifth wheel for turning, connects the fifth stage of the hardware or software shift register to the exlusive OR circuits  62 . By setting the same combination of switches on or off in a receiver and a transmitter of the embodiments of FIGS. 4 and 6, the encoder and decoder match to provide communication with little cross-talk even with transceivers operating at a similar frequency and with similar codes. 
     In summary, the encoder  44  includes: (1) an analog-to-digital converter; (2) an exclusive OR gate; (3) a shift register with a certain number of stages; (4) a set of the same number of switches; and (5) a network which obtains the single bit result of the exclusive or of the inputs. The analog-to-digital converter used in this application is a continuously variable slope delta modulator  46  that converts the incoming analog signal into an asynchronous data stream. 
     One input of the gate receives the data and the other is a control signal. This gate passes the data through unaffected if the control input is a logic low. If the control input is a logic high, the data is inverted. The output of this gate represents the data transmitted to the decoder. 
     The transmitted data also flows in a shift register of a fixed number of stages. The shift register contains the memory of a corresponding number of last bits of data that were transmitted. The outputs of this shift register stages are connected through corresponding switches giving a number of possible combinations of switch settings. An exclusive OR network having the same number of inputs and one output is connected to the switches so that the exclusive OR of the outputs of the selected stages may be found. The single bit result is used as a control input of the previously described exclusive OR gate located in the path of the data stream. 
     The decoder consists of: (1) a shift register of the same number of stages as the shift register in the encoder; (2) a set of code switches of the same number; (3) a network which obtains a single bit result of the exclusive OR of the inputs; (4) an exclusive OR gate; (5) a digital-to-analog converter; and (6) a synchronizer. The data stream entering the decoder flows into a shift register. This shift register then contains the same information as the shift register which is located in the encoder. The outputs of the shift register stages are connected to switches. These switches are then connected to an exclusive OR network identical to the one previously described for the encoder. 
     The single bit input of the above network is exclusive OR gated with the incoming data stream. Assuming the decoder settings are identical to the encoder switch settings the following occurs: the bits which, at the encoder, were not inverted, are not inverted here at the decoder. In a similar fashion, those bits which, at the encoder were inverted, are now inverted again restoring them to their original state. The resulting decoded data is fed to a continuously variable sloped delta digital-to-analog converter  46 . The output of this decoder represents the recovery of the analog signal which was applied to the encoder. 
     Both encoder and decoder clocks are obtained by dividing the output of a crystal oscillator using a ripple counter. As the crystal oscillators are not tied to one another, the encoder and decoder clocks drift past one another in phase, causing a loss of the recovered analog signal with each cycle. To prevent this, a synchronizing circuit is used in the decoder. Each transition of the incoming data stream, regardless of direction, is used to generate a short pulse which is then used to reset the ripple counter. This forces the decoder clock to match in the inverse of the encoder clock at the data transition points and the crystal oscillator maintains the clock between transitions. An inverse clock is used to allow the incoming data to settle before being clocked. 
     This process converts the analog audio signal to a digital binary signal in the CODEC 94  with 16 bit words. It counts the binary ones in each 16 bit word in the microprocesser  90 , and if the number of ones is an odd number, it inverts the digital word before transmitting with the entire encoded message. It transmits words having an even number of binary ones without inverting them. At the receiver, the words having an odd number of ones (or of zeros) is reinverted and the words are decoded using the same process as the coding. 
     Of course, other characteristics that can be determined from the transmitted word may be used to alter the digital code in a predictable manner. For example, if the coded words include an odd number of bits such as 17 bits but a binary one is always added at a fixed position before transmitting and always substracted upon receiving, the bit in the fixed position could be ignored and the process repeated or an even number of bits per word could be used but binary zeros could be counted and the word inverted if an even number of binary zeros are in the word as described above. 
     In FIG. 8, there is shown a block diagram  100  illustrating the steps of beginning  102  to begin transmission with a switch and followed by talking into a microphone for real time encoding and transmission. The mircoprocesser is initialized as shown at step  104  and the program determines if the transceiver being spoken into is to be the master or the slave program indicated at step  106 . If it is to be the master transceiver, then the program proceeds to the master program routines  108  and if it is to be the slave transceiver the program proceeds to the slave program routines  110 . 
     In FIG. 9, there is shown a flow diagram of the program for receiving signals for the master transceiver including the steps of selecting the master program routines shown at step  120 , and decoding the incoming data shown at step  122 . In the decoding process, the last incoming encoded data bit is selected as shown in step  124  and shifted into the decode software shift register shown at step  126 . 
     The selected stages of the shift register are set by the manual switches shown in FIG.  7  and the bits are selected as they move through the software shift register as shown at  128  and exclusively-ORed to determine if the result has odd or even number of binary one bits shown at step  130 . If the decision shown as being made in steps  132  indicates an odd number of bits, then the next incoming encrypted data bit is inverted as shown at step  134 . If it is even, then the next incoming encrypted data bit is selected as shown at step  136  without inverting. The steps  134  and  136  are followed by the output data bit shown at  138 . This code may then be converted to the analog audio output signal. Outgoing data is encoded as shown at step  108 B and more specifically in FIG.  10 . 
     As shown in FIG. 10, when an audio signal in analog form is generated by speaking into the microphone of the transmitter, the signal is digitized and encoded as shown at step  140 . The last outgoing data bit after encryption is selected as shown at step  142  and shifted into the encode shift register as shown at step  144 . The shift register has been set by the switches shown in FIG. 7 to the established code and this code is processed in the soft shift register as shown in step  146  to cause the selected stages to be exclusively-ORed to determine if the result has an odd or an even number of binary one bits as shown at step  148 . If a decision is made as shown at step  150  that the result has an odd number of bits, then the program proceeds to step  152  to invert the bit. If it has an even number as shown at step  154 , then the bits are transmitted on without inverting. The output encoded data bit is shown at  156  ready for transmission to the receiver. The program then returns to its beginning routine as shown at step  158 . 
     In the case the switch has been pushed so that the transceiver is a slave transceiver rather than the master transceiver the program as shown at  110 A starts with the slave program routines being selected as shown by the step  160 . Incoming data after this selection is decoded as shown at step  162  by getting the last incoming encoded data bit as shown in step  164  and shifted into a software register shown at step  166 . The switches have already been manually selected so that the program implements the selection of the bits in the shift register as the shift register is stepped from stage to stage in accordance with the code as shown at step  168 . These bits are exclusively-ORed to determine if the result has an odd or even number of binary one bits as shown at step  170 . 
     A decision is made as shown at step  172  and if there are an odd number of binary one bits, the bits are each inverted as shown at step  174  and transmitted on to the output decoded data bit program shown at step  176 . On the other hand if they are an even number of bits, the data bits are shifted to the output decoded data bit program  176  without inverting as shown in step  178 . The data is checked for speed as shown at step  180  and if it is moving faster than the master transmission the clocks are slowed down as shown at step  182  before being converted to audio in sync with the transmission. On the other hand as shown at  184  if the slave program is running slower than the master, it is speeded up so as to maintain synchronization. Upon receiving the audio signal and beginning to transmit, the program moves to the outgoing data section as shown in FIG. 12 at  110 B. 
     In FIG. 12, the transmission of data by a slave transmitter is shown by the program steps  110 B including the step  190  for encoding the audio signal generated by the microphone and proceeding to step  192  to obtain the last digital bit and shifting it into the encode shift register  194 . The selected switches select stages as shown at step  196  for Exclusive-OReding to determine if the result has an odd or even number of binary one bits as shown in step  198 . A decision is made as to whether its odd or even as shown at step  200  and if it is an odd number of bits the bits are inverted as shown at step  202 . If even, they are transmitted on without inverting as shown at stage  204 . The output is then transmitted as shown at step  208  and the program returns to the beginning of the slave program routine as shown at step  210 . 
     From the above description, it can be understood that the synchronizing circuit and coded transmitter and receiver of this invention have several advantages such as: (1) they transmit the signal with high fidelity; (2) there is very little cross talk between several different transmitters using even slightly different encoding selections; and (3) the equipment is simple and inexpensive. 
     Although a preferred embodiment of the invention has been described with some particularlity, many modifications are possible without deviating from the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced other than as specifically described.