Binary Detecting and Threshold Circuit

My invention relates to an improved binary detecting and threshold circuit, and particularly to such a circuit for detecting a repeated binary word without the necessity for bit synchronization.

In some radio communication systems, it may be desirable or necessary that some transmissions on a particular radio carrier frequency be heard only by a selected receiver or receivers, and that some other transmission on the same particular radio carrier frequency be heard by a different selected receiver or receivers. Such a system has been implemented by means of a continuous tone controlled squelch system, sometimes referred to as CTCSS. However, such systems have required audio tones of frequencies outside and usually below the range of the information or voice frequencies, and such frequencies are difficult to filter. For this reason, some of those systems use binary signals in which a predetermined binary word comprised of a predetermined number and arrangement of binary bits are continuously and repetitively transmitted. In such systems, only the radio receiver or receivers which are programmed to detect the predetermined binary word will be activated so that the transmission is heard.

Accordingly, a primary and general object of my invention is to provide a new and improved binary detecting circuit.

Another and more specific object of my invention is to provide a new and improved binary detecting and threshold circuit for use in radio communication systems.

Another object of my invention is to provide a new and improved binary detecting circuit that can detect binary bits relatively accurately without the necessity of providing synchronization with the desired binary bit time interval.

In such a radio communication system, the signals representing the predetermined binary word are subject to fading and distortion in the radio transmission path. When these signals are demodulated, the binary bits which are reproduced may be inaccurate, so that a decision must be made whether to activate the receiver.

Accordingly, another object of my invention is to provide a new and improved binary detecting circuit for determining the binary significance of bits which may be distorted or deformed during a radio transmission.

Another object of my invention is to provide a new and improved binary signal threshold circuit that permits the threshold to accurately reflect operating conditions in a binary signal radio communication system.

Briefly, these and other objects are achieved in accordance with my invention by an improved detecting circuit having an input for the series of binary bits forming a repeating binary word. A plurality of logic sampling means are connected to the input for producing phase shifted logic samples of each input bit at respective outputs. A shift register is respectively coupled to each of the sampling means outputs. The sampling means and shift registers are respectively arranged so that the logic samples stored in each shift register are circulated, or so that each new sample is placed in the shift register and the oldest sample is removed from the shift register. A plurality of generating means are provided for producing the predetermined binary word. Comparing means are respectively coupled to the generating means and the shift registers for comparing the binary word in the generating means with the samples in the shift registers. The shift registers are operated at a rate greater than the rate of the input binary bits, so that the binary word in the generating means can be compared with every sequence of samples in the shift register between input bits. And finally, threshold means are connected to the comparing means for producing an output signal in response to a predetermined number of proper comparisons.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the claims. The structure and operation of my invention, together with further objects and advantages, may be better understood from the following description given in connection with the accompanying drawing, in which:

FIGS. 1A and 1B show a diagram of an improved detecting circuit in accordance with my invention;

FIGS. 2A through 2G show wave forms for illustrating the operation of my detecting circuit of FIGS. 1A and 1B;

FIG. 3 shows a diagram of an improved threshold circuit in accordance with my invention for use with my detecting circuit of FIGS. 1A and 1B, and

FIGS. 4A through 4F show wave forms for illustrating the operation of my threshold circuit of FIG. 3.

FIGS. 1A and 1B are to be considered together, with the right hand leads of FIG. 1A being connected to the respectively positioned left hand leads of FIG. 1B. As mentioned earlier, my invention is intended for use in a radio communication system in which a predetermined binary word having a plurality of binary bits is repetitively transmitted to activate or energize the output of a selected radio receiver or receivers. Since the binary bits can be used to modulate radio frequency carriers in various ways, such as amplitude or frequency modulation, I show and contemplate a radio receiver 10 which may demodulate any desired type of signals and produce these signals in appropriate form. In the receiver 10, the binary bits are produced at one output and the audio information or intelligence is produced at a second output. The two outputs are preferably filtered so as not to interfere with each other. The audio output is applied to an audio output circuit 13 which is enabled to produce an output in response to a decode signal. To provide a sufficient number of distinct binary words, I have assumed that each repeated binary word comprises 23 binary bits having logic 1's and 0's in a predetermined sequence to form a predetermined binary word. However, a word could include more or less bits. These binary bits are assumed to have a bit rate BR, which typically may be 135 bits per second. Common to my circuit is a sample clock 11 which produces pulses at a rate that is an integral multiple greater than the bit rate BR. I include a plurality of similar circuits, for example three, each of which is supplied with the received binary bits and samples them at a time that is shifted with respect to the other circuits. In the embodiment shown, I use three such circuits so that the sample clock 11 operates at a rate three times the bit rate BR. While the rate of the sample clock 11 is preferably very accurate, the actual time that each clock pulse is produced relative to the time a binary bit begins does not have to be controlled. That is, the clock pulses do not have to be synchronized with the binary bits. This sample clock 11 is connected to a phase divider 12 having three outputs, each of which produces output pulses at the bit rate BR, these pulses being time or phase shifted relative to the other pulses. Thus, the phase divider 12 produces respective short duration signals .phi.1, .phi.2 and .phi.3, each having a rate BR and phase relations that are preferably 120 degrees apart. It is to be understood that more or less circuits can be used, for example 2 or 4 or more such circuits. For a given plurality N, the phase divider 12 would produce N different outputs, each having a rate BR and a phase relation of 360 degrees/N with respect to the other outputs. Each output from the divider 12 is applied to a respective circuit. For the phase 1 (.phi.1) output, this circuit is shown horizontally across the top portion of FIGS. 1A and 1B. The phase 2 (.phi.2) circuit is shown horizontally across the center portion of FIGS. 1A and 1B, and the phase 3 (.phi.3) circuit is shown horizontally across the lower portion of FIGS. 1A and 1B. Each of these circuits is substantially similar. Corresponding circuit elements are given the same reference numeral, this numeral being followed by a suffix numeral 1, 2 or 3 which respectively indicates .phi.1, .phi.2 or .phi.3. For the .phi.1 circuit, binary data bits from the radio receiver 10 are applied to a sampler circuit 14-1. This sampler circuit 14-1 takes a sample of the binary logic of the binary data bits under the control and at the time of the .phi.1 signal or pulse from the phase divider 12. The samples taken are applied to a multiplexer 15-1. The multiplexer 15-1 is essentially a single pole, double throw switch having one input terminal connected to the sampler 14-1, and the other input terminal connected to the output of a shift register 16-1. The output terminal of the multiplexer 15-1 is connected to one of its input terminals under the control of the .phi.1 pulse from the phase divider 12. The output signals from the multiplexer 15-1 are applied to the input of the shift register 16-1. If, as assumed above, the predetermined binary word comprises 23 bits having a predetermined sequence of 1's and 0's, then the shift register 16-1 comprises 23 stages. In addition to being applied to the multiplexer 15-1, the output of the shift register 16-1 is also applied to one input of a comparator circuit 17-1. The other input for the comparator circuit 17-1 is derived from a code generator 18-1 which sequentially produces the binary bits forming the predetermined binary word. The comparator 17-1 compares the logic of the two bits applied to its inputs, and produces one output if the bit logic is the same and a different output if the bit logic is different. The comparator 17-1 may include a counter or similar circuit which, in response to a predetermined number of outputs indicating proper comparisons between bits from the shift register 16-1 and bits from the code generator 18-1, produces an output. This output is applied to a latch circuit 19-1 which produces an output L1 for a selected time period of at least the remaining time of the bit in which the comparator 17-1 produced an output, plus the time of one more bit.

Phase 1 pulses or signals from the phase divider 12 are also applied to a clock and counter circuit 20-1. This circuit 20-1 produces a stream of clock pulses at a rate higher than the bit rate BR of the .phi.1 pulses from the phase divider 12. For a 23 bit word, this rate is preferably at least 23.times.24.times.BR, where 23 is the predetermined number of bits forming the binary word, and 24 is 23+1 to give added pulses for reasons that will be explained. For a bit rate BR of 135 pulses per second, the circuit 20-1 produces pulses at a rate of 23.times.24.times.135 or 74,520 pulses per second. On receipt of a .phi.1 signal, the circuit 20-1 produces 23.times.24 or 552 pulses at its output, after which no more pulses are produced until another .phi.1 signal is applied. The circuit 20-1 produces these 552 pulses at the 74,520 pulses per second rate in response to each .phi. 1 signal from the phase divider 12. These counted pulses are applied to a pulse eliminator circuit 21-1 and to a pulse producer circuit 22-1. The pulse eliminator circuit 21-1 eliminates one pulse out of every 24 supplied to it, and supplies the remaining pulses to the clock input of the shift register 16-1. The pulse producer circuit 22-1 produces a single pulse in response to every 24 pulses applied to it, and supplies each of these single pulses to the reset input of the comparator 17-1. The .phi.1 signals are also applied for the reset input of the latch circuit 19-1.

The other portions of my circuit for phase 2 (.phi.2) and phase 3 (.phi.3) pulses or signals are similar to that described for phase 1 (.phi.1) signals. If more phases are used, each additional circuit would be similar. All of the circuits operate in a similar manner, but would be started at displaced time relations or phases, the exact time or phase depending on the number of phases or samples desired. For three phases, the time relation would be 120 degrees. For two phases, the time relation would be 180 degrees. These multiple phase samplings eliminate the need for any synchronization with the incoming binary data bits, and provide sufficient samples so that an accurate representation of each bit is obtained, despite any distortion or fading to which the bit may have been subjected.

The operation of the phase 1 portion of my circuit is explained in connection with the wave forms shown in FIGS. 2A through 2G which are plotted along a common time axis. FIG. 2A shows a portion of a received binary data bit; FIG. 2B shows pulses produced by the phase divider 12; FIG. 2C shows pulses produced by the clock and counter circuit 20-1; FIG. 2D shows pulses produced by the pulse eliminator 21-1 and the bit sample number produced by the shift register 16-1; FIG. 2E shows the predetermined bit number produced by the code generator 18-1; FIG. 2F shows the pulses produced by the pulse producer 22-1; and FIG. 2G shows the time occurrence of comparisons taking place in the comparator 17-1. The locations of these waveforms are shown by corresponding letters in FIGS. 1A and 1B.

Prior to the time T0, I have assumed that the circuit has been operating long enough to have filled the shift register 16-1 with bit samples. At the time T0, I assume that the phase divider 12 produces a .phi.1 pulse shown in FIG. 2B. This .phi.1 pulse does several things. It causes the sampler 14-1 to take a binary sample of the binary bit being received, and also causes the multiplexer 15-1 to switch its input from the shift register output to the sampler output so that the binary sample is applied to the shift register 16-1 at this time. If, as assumed, the shift register 16-1 is full, this latest sample replaces the oldest sample because the timing in the shift register 16-1 is under control of the .phi.1 pulses. The .phi.1 pulse causes the counter 20-1 to begin producing the counted stream of 23.times.24 or 552 pulses shown in FIG. 2C. These counted pulses cause the code generator 18-1 to begin producing the sequence of predetermined binary bits forming the binary word. The sequence output of these bits is shown in FIG. 2E. These same counted pulses are also applied to the pulse eliminator 21-1 and to the pulse producer circuit 22-1. The pulse eliminator 21-1 eliminates every 24th pulse as shown in FIG. 2D, and the pulse producer circuit 22-1 produces a single pulse every 24 pulses, as shown in FIG. 2F. In comparison 1 indicated between the times T0 and T1 in FIG. 2G, bit samples 1 through 23 produced by the shift register 16-1 (as shown by the numbers in FIG. 2D) are respectively compared with the predetermined code bits 1 through 23 produced by the generator 18-1 (as shown by the numbers in FIG. 2E) in the comparator 17-1. Samples 3 through 21 and code bits 3 through 21 are omitted to save space. If a predetermined number of comparisons are proper, that is if binary 1's or 0's from the shift register 16-1 occur at the same time as binary 1's or 0's from the code generator 18-1, the comparator 17-1 produces an output that causes the latch circuit 19-1 to latch and produce an output L1. This predetermined number of proper comparisons may have any value, although I prefer 21. After comparison 1 is completed, and at the time T2, counter pulse 24 is eliminated from the shift register 16-1. However, this pulse 24 is supplied to the code generator 18-1, so that predetermined code bit 1 is still produced even though no new bit sample is produced by the shift register 16-1. This pulse 24 is also supplied to the pulse producer 22-1 which produces a pulse 1 (FIG. 2F) that resets the count in the comparator 17-1 to zero. This count reset eliminates any ambiguity or error which might result from bit sample 23 (produced by the shift register 16-1) being compared with code bit 1 (produced by the code generator 18-1) at the time T2. At the time T3, the counter 20-1 produces pulse 25 to start comparison 2. This pulse 25 is produced as pulse 1 by the pulse eliminator 21-1, and causes the shift register 16-1 to produce bit sample 1. This pulse 1 also causes the code generator 18-1 to produce code bit 2, since code bit 1 was produced in response to pulse 24. During comparison 2, bit sample 1 is compared with code bit 2, bit sample 2 is compared with code bit 3, and so on until bit sample 22 is compared with code bit 23, and bit sample 23 is compared with code bit 1. Thus in comparison 2, the bit number forming the predetermined code and the bit numbers of the samples are shifted with respect to each other after comparison 1. Comparison 2 ends at the time T4. At the time T5, counter pulse 48 is eliminated so that shift register pulse 23 remains. Also, pulse producer pulse 2 is produced to reset the comparator 17-1. At the time T6, counter pulse 49 starts the third comparison of bit sample 1 and code bit 3, and so on. This operation continues on for additional comparisons through comparison 22 which ends at the time T7. Between the times T6 and T7, similar operations begin with phase divider pulses .phi.2 and .phi.3 and the remainder of the circuits of FIGS. 1A and 1B. At the time T8, counter pulse 528 is eliminated, and pulse producer pulse 22 is produced to reset the comparator 17-1. At the time T9, counter pulse 529 starts the 23rd comparison of bit sample 1 and code bit 23, and so on ending with bit sample 23 and code bit 22 at the time T10. At this time T10, 551 counter pulses have been used, and every possible predetermined code bit sequence, each beginning with a different code bit but in the same predetermined order, has been compared with the same sequence of bit samples in the shift register 16-1. Since 23 bits form a binary word, there are 23 different bits that the sequence can start with. Consequently, all 23 comparisons must be made in the time of one binary bit. While each of the comparisons described above started with a different code bit but the same bit sample, it will be appreciated that each comparison could have started with a different bit sample but the same code bit. As shown in FIG. 2D, comparison 1 begins with bit sample 1 and code bit 1; comparison 2 begins with bit sample 1 and code bit 2; comparison 3 begins with bit sample 1 and code bit 3; and so on to comparison 22 which begins with bit sample 1 and code bit 22; and finally comparison 23 which begins with bit sample 1 and code bit 23. At the time T11, counter pulse 552 (the last of a clock sequence) is produced. This pulse 552 causes the pulse producer 22-1 to produce pulse 23 which resets the comparator 19-1. After this time bit sample 1 is at the output of the shift register 16-1, and code bit 1 is at the output of the code generator 18-1.

During comparisons 1 through 23 each of the pulse producer pulses 1 through 23 resets the comparator 17-1, so that the count in the comparator 17-1 can start over for each comparison sequence. Actually, only one sequence out of the 23 sequences should or will be correct. For that correct sequence, 23 correct comparisons should be made by the comparator 17-1. Hence, any reasonable count or threshold, such as 20 or 21, in the comparator 17-1 may be required for the comparator 17-1 to produce an output. This output can be used to set the latch 19-1 to provide a high output L1 indicating proper comparison. If, after a reset, 20 or 21 L correct bit comparisons are made in a sequence, the chances are great that all bit comparisons in that same sequence will also be correct. Persons skilled in the art will appreciate the relative merits of having a high threshold in the comparator for accuracy, as opposed to a low threshold in the comparator in order to produce a rapid output so that the receiver or other device is activated or energized.

At the time T12, another phase divider .phi.1 pulse is produced. This causes the sampler 14-1 to sample the logic of the binary bit being received, and causes the multiplexer 15-1 to connect the shift register input to the sampler 14-1 and receive the new bit sample 1' in the place of the old bit sample 1. The logic of the other bit samples 2 through 23 remain the same. This .phi.1 pulse resets the latch 19-1, and starts the clock and counter 20-1 to produce another 552 pulses that produce 23 comparisons as described above. .phi.2 and .phi.3 pulses are also produced to cause their respective circuits to function in the same way. However, as mentioned earlier, the respective phases 2 and 3 are operated at 120 degree relationships with each other and with respect to phase 1. Thus, with multiple sampling of each binary bit at multiple phase relations, the logic samples of the binary bits will be averaged or corrected. If any one or more latch outputs are produced, an indication can be provided to a receiver or other device that the proper binary word is present, and this indication can be used to enable the output circuit 13 of the receiver 10, and provide a listener with the received intelligence or other information. However, in applications where a more accurate way is needed to determine when a receiver should be activated, I prefer to use a threshold circuit such as described below.

FIG. 3 shows a circuit diagram of a preferred threshold circuit in accordance with my invention for use with my detecting circuit of FIGS. 1A and 1B. My threshold circuit utilizes a plurality of two input AND gates 30 and 35. The inputs of gate 30 are coupled to latch circuits 19-1, 19-2; the inputs of gate 31 are coupled to latch circuits 19-2, 19-3; and the inputs to gate 32 are coupled to latch circuits 19-3, 19-1. The outputs of these AND gates 30, 31, 32 are coupled to a three input OR gate 36. The output of the OR gate 36 is applied to the enable input of a counter 38 which is clocked by the .phi.1 pulse. The output of the counter 38 is applied to a threshold circuit 41 which produces an output in response to a predetermined count, for example 8. This part of the circuit forms a fast detect circuit. The output of the threshold circuit 41 is applied to one input of an OR gate 43. The output of the OR gate 43 is applied to a turn-off delay circuit 44 which produces a decode signal in response to an output from the OR gate 43, and continues to produce this decode signal for a predetermined time period following removal of the output from the OR gate 43.

My threshold detect circuit also includes a slow detection circuit. In this part of the circuit, the AND gates are connected to the phase divider 12 and the latches 19-1, 19-2, 19-3, so that AND gate 33 receives the .phi.1 and L1 signals; AND gate 34 receives the .phi.2 and L2 signals; and AND gate 35 receives the .phi.3 and L3 signals. The outputs of these AND gates 33, 34, 35 are applied to an OR gate 37 which provides a clocking signal to a counter 40. This counter 40 is enabled as long as an enabling signal is supplied from a divide-by-128 circuit 39. The circuit 39 counts the .phi.1 signals, and produces an enabling signal until a count of 128 is reached, after which it produces a reset signal. As soon as another .phi.1 signal is received, the divider 39 produces an enabling signal again for another 128 .phi.1 signals. The output of the counter 40 is applied to a threshold circuit having two levels of 20 and 4 as will be explained. If, as the case may be, an appropriate count is received, the threshold circuit 42 produces an output which is supplied to the OR gate 43. The threshold circuit 42 requires a count of 20 if a decode signal is not produced. As soon as a decode signal is produced, the required threshold count is reduced to 4. Thus, if a decode signal is lost, the slow detect threshold circuit 42 requires a count of 20 before providing an enabling signal to the OR gate 43. Once a decode signal is produced, the slow detect threshold circuit 42 requires a count of 4, so that a decode signal will continue to be produced if the counter 40 produces 4 outputs before the counter 40 is reset. As should now be appreciated, this type of feedback connection provides hysteresis in the thresholded output which prevents loss thereof (once acquired) in relatively lower signal-to-noise environments than otherwise possible.

The operation of my threshold circuit of FIG. 3 will be explained in connection with the wave form shown in FIGS. 4A through 4F, plotted along a common time axis. FIG. 4A shows the .phi.1, .phi.2, .phi.3 signals produced by the divider 12 of FIG. 1A. FIGS. 4B, 4C and 4D show assumed output latch signals L1, L2, L3 for the latch circuits 19-1, 19-2 and 19-3 of FIG. 1B. FIG. 4E shows the count outputs produced by the fast counter 38, and FIG. 4F shows the outputs produced by the slow counter 40.

Just prior to the time T1, it is assumed that the latch signals L1, L2 are high, (indicating that the .phi.1 and .phi.2 bit samples agree with the predetermined bit code), and that the latch signal L3 is low (indicating that the .phi.3 bit samples disagree with the predetermined bit code). When the .phi.1 signal appears at the time T1, the AND gate 30 is producing a logic 1 which is passed by the OR gate 36 to enable the fast counter 38 to produce a count of the .phi.1 clock pulse as shown in FIG. 4E. Also, a logic 1 at L1 allows AND gate 33 to pass a .phi.1 clock pulse whch is, in turn passed by OR gate 37 to cause the slow counter 40 to produce a count as shown in FIG. 4F. At the time T2, the fast counter 38 does not produce an output even though enabled, since no .phi.1 clock signal is applied to the counter 38. However, the slow counter 40 does produce an output since the .phi.2 signal and the latch signal L2 are high to cause the gate 34 to produce an output. At the time T3, the fast counter 38 produces no output since no .phi.1 clock signal is applied to the fast counter 38. The slow counter 40 produces no output, since the latch signal L3 is assumed to be low at the time of the .phi.3 signal. At the time T4, both the fast and slow counters 38, 40 produce a count signal for the same conditions which existed at the time T1. A slow count is produced at the time T5 as at the time T2.

At time T6, a slow count signal is produced since the latch signal L3 is assumed to have become high before the .phi.3 signal is produced. No fast count is produced because no .phi.1 signal is present. With respect to the time T7, I have assumed that the comparator 17-1 does not produce an output after the time T3 and before the time T4, so that the latch 19-1 produces a low L1 output at the time T7. Hence, the slow counter 40 does not produce a count at the time T7. The fast counter 38 does produce a count at the time T7 because of the high latch signals L2 and L3.

At the time T8, the slow counter 40 produces a count because the .phi.2 signal and high latch signal L2 are present. No fast count is produced, since the .phi.1 signal is not present. At the time T9, the slow counter 40 produces an output because of the .phi.3 signal and the high latch signal L3. No fast count is produced, since the .phi.1 signal is not present. At the time T10, the fast counter 38 produces a count in response to the .phi.1 clock signal and the high latch signals L2, L3. No slow count is produced since the latch signal L1 is low. At the time T11, the slow counter 40 produces a count in response to the .phi.2 signal and the high latch signal L2. No fast count is produced, since the .phi.1 signal is not present. At the time T12, the slow counter 40 produces a count in response to the .phi.3 and high L3 signals. And, at the time T13, the fast counter 38 produces a count in response to the .phi.1 signal and the high latch signals L2, L3. Other conditions are also possible. But if 8 fast counts or 20 slow counts are produced, a decode signal is produced to enable the audio output circuit 13 of the receiver 10.

This threshold circuit is ideal for radio receiving conditions, and provides the best balance and selection between fast operation for rapid receiver activation on the one hand, and for reliability under fading or adverse conditions on the other.

It will thus be seen that I have provided a new and improved detecting circuit and threshold circuit. While I have shown only one embodiment of my circuit, persons skilled in the art will appreciate the modifications that may be made. In particular, my detecting circuit may use any number of phase signals, depending upon design preference. However, I feel that three equally spaced phase signals provide an optimum choice. Similarly, in my threshold circuit, either the fast threshold circuit or the slow threshold circuit could be omitted. Therefore, while my invention has been described with reference to a particular embodiment, it is to be understood that modifications may be made without departing from the spirit of the invention or from the scope of the claims.