US3432754A - Matched signal transmitter-receiver arrangement - Google Patents

Description

March l1, 1969 E. SCHWARTZ MATCHED SIGNAL TRANSMITTER-'RECEIVER ARRANGEMENT Filed NOV. 13. 1964 Sheet of 1E 3,432,754 MATCHED SIGNAL TRANSMITTER-RECEIVER ARRANGEMENT Filed Nov. 13. 1964 March l1, 1969 5.1. SCHWARTZ 2 of l5 Sheet :fangen/ef" INVENTOR.- awa/va 5CH/MUZ EPEE/ :77,44 BETWEEN SAMPS ATTORNEYS March l'l, 1969 E. l. SCHWARTZ Sheet 3 of 13 ATTORNEYS March 1l, 1969 Filed Nov. 13, 1964 l. SCHWARTZ MATCHED SIGNAL TRANSMITTER-RECEIVER ARRANGEMENT y INVENTOR:

A rra/m5 ys March 11, 1969 E. 1. scHwAR'rz 3,432,754

MATCHED SIGNAL TRANSMITTER-RECEIVER ARRANGEMENT Sheet 5 of 13 Filed NOV. 13, 1964 5. F 4l; M IM w3 31; 4 f M2M PULSE N0- xNvENToR: w1/Mm I, SC//M/APTZ ORNEYS March 11, 1969' scHwAR'rz MATCHED SIGNAL TRANSMITTER-RECEIVER AHRANGEMENT Filed Nov. 13, 1964 Sheet 6 of 13 F/G. a

lNVENTOR-' /MU/VD I, SC//Wd/@TZ BY g A l j ATTORNEYS March 11, 1969 E, |v, SCHWARTZ 3,432,754

MATCHED SIGNAL TRANSMITTER-RECEIVER ARRANGEMENT Filed uw. 1s. 1964 sheet 7 0111s PESCE/VER W/7'H l5 WORD GEA/E/QATOPS 6 24j 110010001111010110010001111010 /f 24214Y 011001000111101011001000111101 F/G. .9 M

March 11, 1969 E, SCHWARTZ 3,432,754

MATCHED SIGNAL TRANSMITTERRECEIVER ARRANGEMENT Filed Nov. 13, 1964 Sheet 8 of 13 FB. S. E.

March 11, 1969 E. SCHWARTZ MATCHED SIGNAL TRANSMITTER-RECEIVER ARRANGEMENT Filed Nov. 13, 1964 Sheet March 11, 1969 E. l. SCHWARTZ MATCHED SIGNAL TRANSMITTER-RECEIVER ARRANGEMENT Sheet /0 of 13 Filed Nov. 13. 1964 INVENTOR EMI/Va f. SCA/114427Z BY TTORNEYS March ll, 1969 E. l. SCHWARTZ 3,432,754

MATCHED SIGNAL TRANSMITTER-RECEIVER ARRANGEMENT Filed Nev. 1s. 1964 sheet er 13 *Mess/04o final 77 l Mn 84 5' @n.7 90 83 L 27 79 0e wrfm-nlscyMlrr E D/,q l

` T12/66m Nu `Aal/D 88 il E "X c b Q x 9i 5@ f 2 a 4 5 6 (12a /2/ 122 123 /24 125 rz 2 V Q O L l j 7 Cf afa/(wens) Q n g INVENTOR: Y [mf/ww J, swf/Mrz BY fw TTORNEYS United States Patent Office 3,432,754 Patented Mar. 11, 1969 ABSTRACT or THE DISCLOSURE word yields signal at quanducing words, correlators for determining identity of transmitted word, comparator for producing a signal corresponding to output of sampling A/DC, and digitalto-analog converter for producing signal whose amplitude matches that of sampled signal. v

This invention relates to la transmission system for electromagnetic signals having arbitrary amplitude variations with respect to time', -and more particularly to such a system employing a correlation type receiver and transmitter.

Correlation type receivers, otherwise known as matched-filter receivers, are well known.- They are usually employed in transmission systems in which the signal transmitted and to be received has a predetermined pattern of amplitude variation with respect to time. An ex ample of such a system is a radar system, wherein a sig'- nal of known characteristics is transmitted, land upon reection, received by the receiver of the system. For systo be transmitted into a perdetermined number of states or levels, yand providing means for generating a diiferent discrete `signal having a special property, corresponding to each level. The same predetermined number of receivers are employed, each one matched to one of the discrete signals.`The particular discrete signals employed are characterized by the fact that each produces a relatively large output when applied to the receiver which is matched to it, and an insignificant output when applied to all the other receivers. Thus, the chances of misinterpreting the discrete signals, as :by mistaking one for another, arev greatly diminished. Since any arbitrary signal can be broken into the various proposed amplitude levels, any signal can be transmitted by means of the predetermined, and hence known, discrete signals. Thus, it is possible to build a receiver, or more correctly an assembly of receivers, matched to the signals which will be transmitted, regardless'of the nature of the information to be trans- Vmitted.

A feature of the invention is its ability to yield any desired quality of `reproduction of the original information signal at the receiver. The quality of reproduction lis a function of the number of levels into which the original signal is quantized. Thus, the greater the numfber of levels, the higher the quality of reproduction.

' p Another feature of the invention is its ability to quickly transform the amplitude of the original information signal at any instant of time to a particular one of a predetermined number of code signals, and transmit the code signal to a receiver adapted to receive the code signal and reconvert it to a signal having an amplitude level corresponding tothe level of the original signal at the instant it was sampled. This ability to transform any amplitude level to a particular code signal is achieved by utilizing the technique of generating a predetermined number of pseudo-random binary words, and controlling the operation of the ybinary word generator by a means which samtems such as these, a matched filter receiver may readily be built because the manner in which the amplitude of the signal varies is known in advance. A matched filter receiver is one so designed that its output, in response to the transmitted signal to which it is'matched,'will be least disturbed by-the inherent noise in the receiver sys'` i" tem, i.e., Vthe output signal-to-noise ratio of the receiver will be a maximum when it receives the signal to which it is matched. As a result, a correlation type or matched` filter receiver offers a maximum probability of detection of the transmitted signal for any particular power input',

distance betweentransmitter and receiver, and false alarm rate (number of instances per -unit time thatnoise isvin-l terpreted by the receiver as #a transmitted signal).

In the case of communications systems, and in particular, systems for transmitting speech, the lamplitudes v'of the modulating signals vary inno predetermined fashion.' As a result, it has not been possible to'build a receiver matched to the transmitted signal', and consequently,all the advantages of matched transmitter-receiver systems have not fbeen obtainable.

It is therefore the primary object of the present invention to provide a matched transmitter-receiversystem'for use in transmitting signals having arbitrary amplitude variations with respect to time. .l

It is another object of the invention to provide a technique for transmitting and receiving speech, and like ind formation, over greater distances than heretofore'possible' for a given power input and quality of reception.

To accomplish these objectives, I have conceived of: the ideaof fragmentizing or quantizing the information ples the information signal and thereby determines the particular binary word to be generated. The predetermined number of different Words which the word generator is capable of producing equals the number of levels into which the original signal is quantized, whereby one particular pseudo-random binary word can correspond to each quantized level.

As used herein, 'the term pseudo-random binary word is intended to mean a word comprising n bits, wherein any sequence of a particular number of successive bits is non-repetitive until after the nth bit has been generated, the particular number being equal to the number of bits. in the preliminary word initially inserted into the word generator.

In its preferred form, the invention employs an analogto-digital converter for sampling the original information-bearing signal and producing a digital ouptut, or preliminary word, corresponding to the amplitude level of the signal at the instant of sampling. The preliminary word is inserted las an initial setting into a word generator, which may be a shift register employing the standard technique of feed-back with half-adding, thereby determining the particular pseudo-random binary word which Awill be produced by the generator. This particular word corresponds to the amplitude level of the original signal at the time of sampling by the an-alog-to-digital converter, and this same word will be generated each time the amplitude of the original signal is at the particular level to which the word corresponds. The binary or code word m-ay be transmitted to the receiver :by any standard technique, such as by modulating a radio wave.

In the receiver are a number of vvord generators equal to the number of dierent words which can be produced by the` word generator at the transmitter end of the system. Each of the receiver word generators continuously and repeatedly produces a reference word identical to one of the possible words which can be produced by the transmitter word generator. Correlator means in the receiver compare the code word transmitted with all the reference words, and upon sensing correlation between the code word and one of the reference words causes a signal to be engendered having an amplitude corresponding to the amplitude level of the original signal sample which led to the generation of the particular transmitted code word.

The present invention employs a very special type of code and reference words exhibiting a property which enables the invention to perform an autocorrelation function. When a code word of the type employed is correlated with the reference word identical to it, the correlator means produces a relatively large positive output. However, all other correlators comparing that code word with their respective reference words will produce negative outputs. Thus, the output of the correlator corresponding to the transmitted code word can easily be distinguished from the outputs of the other correlators, even under extremely adverse conditions, e.g. noise in the system.

It will be appreciated that the desired behavior of the present invention is dependent upon the code words at the transmitter end of the system being generated at the same rate as each reference word at the receiver end of the system, so that there is no changing time delay between generation of each code word and generation of a series of reference words with which it is to be compared. It is therefore a feature of the invention to provide means for synchronizing, and maintaining synchronization of, all the word generators in the system by means ofthe signal carrying the code words from the transmitter to the receiver, i.e., without the need for a separate synchronization signal. Obviously, if desired, a separate synchronization signal could be employed.

Another feature of the invention is the provision of two word generators at the transmission end of the system operating in leap-frog fashion, i.e., while a preliminary word is inserted into one of the word generators, the other is in operation producing a code word. In this way, a continuous stream of encoded information is produced, without interruption, at the transmitter end of the system.

Other features and advantages of the invention will be apparent from the following description in which reference is made to the accompanying drawings.

In the drawings:

FIG. 1 is a simplide bloc'k diagram of an encoder and decoder according to this invention;

FIG. 2 is a diagram showing a arbitrary informationbearing signal being sampled;

FIG. 3 is a diagram showing the signal of FIG. 2 as reproduced by an encoder and decoder according to this invention;

lFIG. 4 is a simplified diagram of an analog-to-digital converter;

FIG. 5 is a table indicating the conditions of the Hip-flop circuits of the analog-to-digital converter as a result of sampling the signal of PIG. 2;

FIG. 6 is a simplified diagram of a feed-back shift register;

FIG. 7 is a table indicating the pseudo-random words which may be produced by the shift register of HG. 6;

FIG. 8 is a table indicating which particular pseudorandom words are produced by the flrst four samplings of the signal of lFIG. 2;

FIG. 9 is a diagram showing the pseudo-random words entering the decoder, and the reference words produced in the decoder;

PIG. 10 is a more detailed block diagram of an encoder according to this invention;

IFIG. 11 is a block diagram of one of the shift registers of FIG. 10;

FIG. 12 is a more detailed block diagram of a decoder according to this invention;

FIG. 13 is a block diagram of one of the correlators of FIG. 12;

FIG. 14 is a diagram indicating the autocorrelation function of a pseudo-random binary word;

FIG. 15 is a block diagram of means for maintaining synchronization between the encoder and decoder',

FIG. 16 is a schematic diagram of the greatest of circuit of FIG. 15; and

FIG. 17 is a block diagram of another means for maintaining synchronization between the encoder and decoder.

The general mode of operation of an illustrative embodiment of the invention will be described with reference to PIG. l. An encoder for transforming successive samples of an information-bearing signal into code words for transmission to a distant receiver is defined by the upper dot-dash rectangle 15, and a decoder for retransforming the transmitted code words into signals having amplitudes corresponding to the amplitudes of the successive signal samples is defined by the lower dot-dash rectangle 16. The signal samples entering the encoder 15 are labeled analog signal in, and the reconstituted signals leaving the decoder 16 are labeled analog signal out.

The encoder 15 includes a clock 17 running continuously at a predtermined rate. The clock causes actuation of a counter 18 which in turn controls the operation of an analog-to-digital converter 19 (sometimes referred to hereinafter as A/DC). The clock also actuates a word generator 20. At a predetermined count of the counter 18, the A/DC 19 samples the information-bearing signal, and produces a digital output corresponding to the amplitude level of the information signal at the time of sampling. The A/DC 19 is capable of producing a predetermined number of different digital outputs, and consequently is capable of quantizing the information signal, or dividing it, into the same predetermined number of quantized amplitude levels, each of the possible digital outputs corresponding to a diternt quantized level of the amplitude of the information signal. The digital output of the A/-DC 19 is inserted into the word generator 20, thereby conditioning the word generator to produce a particular code word. The word generator is capable of producing a predetermined number of ditfernet code words equal to the number of different digital outputs capable of being produced by the A/DC. The particular code word produced by the word generator 20 is determined by the particular digital signal it receives from the A/DC 19, and any particular code word produced, therefore, corresponds to the quantized level of the information signal amplitude at the time of sampling by the A/DC. Once the digital output of the A/DC has been inserted into the word generator, the clock 17 activates the latter causing it to produce a code word.

The code word may be transmitted to a receiver by means of any standard technique, and the manner in which this is done forms no part of the present invention. For example, the code word may be applied to the modulator 21 of a radio transmitter. The output of the transmitter is applied through an appropriate channel 28 to the demodulator 22 of a receiver, and the code word is thus delivered to the decoder 16.

The decoder includes a clock 23 running continuously at a rate equal to the rate of the clock 17 in the encoder. The clock pulses are applied to a series of word generators 24a, 24b, 24C, and 24u activating them and causing them to produce code words simultaneously. The number of word generators present in the decoder equals the number of different code words capable of being produced by the word generator 20 in the encoder. Furthermore, each of the word generators 24a-n is arranged to continuously and repeatedly produce a different one of the possible code words which may be produced by the word generator 20. Consequently, each time a code word is transmitted from the encoder 15l to the decoder 16, the transmitted code word will be identical to one of the code words produced by the word generators 24a-n lof the decoder, and will differ from the code word produced by every other word generator in the decoder. Associated with each of the word generators 24a-n is a correlator which receives the code word produced by the word generator. Thus, the word produced by the word geneator 24a is applied to correlator 25a, the word produced by the word generator 24b is applied to correlator ZSb, and so forth. The code word received from the encoder is also applied to all the correlators 25a-n, simultaneously.

The correlators compare the code word transmitted from the encoder with the words received from their respective word generators, and all but one correlator produce a small negative output, because the transmitted word does not match the words received by these correlators from their respective word generators. However, since the transmitted word will always be identical to one of the words produced by the word generators 24a-n, one of the correlators will sense agreement between the transmitted word and the word received from its respective word generator. As a result, the output of this correlator will be relatively large and positive. For example, if the transmitted code Word is identical to the word produced by the word generator 24C, the correlator 25e will have a relatively large positive output and each of the other correlators will have a small negative output. The outputs of all the correlators 25a-n are applied to a comparator 26. The comparator responds to the relatively large output of one of the correlators by causing a signal, corresponding to the digital output of the analog-to-digital converter 19 which determined the code word to be produced by the Word generator 20, to be applied to a digitalto-analog converter (D/AC) 27. In response, the D/AC produces an output signal having an amplitude which matches the quantized level of the information-bearing signal at the time is was sampled. Thus, sampling of the information/bearing signal at successive time intervals causes a signal to be produced by the D/AC which approximates the amplitude of the information signal at the time of the sample, and combining the successive outputs of the D/AC produces a signal approximating the information-bearing signal.

The relationship between an original information-bearing signal and the approximate signal produced by means in accordance with this invention can be seen by comparing FIGS. 2 and 3. The curve 29 of FIG. l2 represents a portion of the original signal, such as speech, the amplitude of which varies arbitrarily with respect to time. Sampling of the signal 29 by the A/DC 19 takes place at intervals t1, t2, etc., shown along the horizontal axis, the time T between samples being determined by the counter 18. Since the sampling time At extends for some finite period, each sampling time is defined by two ver-l tical lines, such as the lines 30 and 31 of sampling time t1, spaced apart along the horizontal or time axis. In the specific example to lbe described below, the A/DC is capable of producing 15 different digital outputs. Hence, FIG. 2 illustrates, along the vertical axis, 15 possible quantization levels of amplitude.

At sampling time t1, the amplitude of signal 29 is closest to the twelfth quantization level, i.e, the portion of the signal between the lines 30 and 31 is closest to the twelfth quantization level. As a result of this sampling, the decoder 16 produces a signal 32, beginning at time t1 in FIG. 3, having an amplitude at the twelfth quantization level. The signal 32 maintains this amplitude level until the next succeeding sampling period. By sampling time t2 (FIG. 2), the amplitude of signal 29 has dropped to a point closest to the sixth quantization level. Consequently, the decoder 16 produces a new output signal 33 (FIG. 3), beginning at time t2, having an amplitude at the sixth quantization level. Further comparison of FIGS. 2 and 3 indicates that lthe amplitude of each decoder output signal approximates the amplitude of the original signal 29 at the time of taking the sample to which the decoder signal corresponds. Suitable means of any well known variety can be employed to smooth out the reconstructed signal created by the decoder. The smoothed out reconstructed signal, which approximates the original signal, is indicated by the curve 34. It is apparent that the greater the number of quantization levels and the shorter the intervals T between samplings, the more faithfully will the original signal be reproduced, i.e., the closer will the reconstructed signal 34 conform to the original signal 29.

As mentioned above, the successive samples of the original signal are translated into code words by encoder 15 for transmission to the receiver. It has been found advantageous to` use a pseudo-random sequence of -binary words as the code words because such binary words offer the autocorrelation feature mentioned above and to be discussed in more detail below. However, the invention is not limited to the use of pseudo-random binary words, and any series of words offering the advantages presented by these binary words may be em` ployed. Pseudo-random binary words may be generated by a shift register employing feedback with half addition. According to this technique, a shift register having n stages or ip-op circuits is capable of producing a maximum of 211-1 different pseudo-random words, each word containing 2-1 bits. Thus, a shift register having three stages will produce seven different pseudo-random words each containing seven bits; a shift register having four stages will produce fifteen different pseudo-random words, each containing fifteen bits, and so forth. The particular binary word produced by the shift register, during any cycle, depends upon the initial setting of the shift register stages at the beginning of the cycle. Setting the shift register stages is accomplished, according to this invention by the analog-to-digital converter 19.

The A/DC 19 shown schematically in FIG. 4 has a four stage staircase generator including ip- flop circuits 37, 38, 39, and 40. This A/DC is capable of setting the stages of a four stage shift register which, when actuated, will produce a fifteen bit binary word. Each tiip-iiop of the A/DC has, of course, two possible states: on or oth Ithese states being sometimes referred to hereinafter as 1 or 0. By means of any suitable well known arrangement, not shown, the A/DC is able to sample the original information-bearing signal 29 (FIG. 2), and the flip-flops 37-40 will thereupon flip until the output of the staircase generator approximates the amplitude of the signal 29 at the moment of sampling. In well-known fashion, the flip-flop 37 is arranged when on to produce an output equal to one unit of the maximum fifteen unit output of the staircase generator, the flip-Hop 38 is arranged to produce an output of two units, the flip-flop 39 four units, and the flip-flop 40 eight units. The value of the output produced by each flip-flop is shown above each in FIG. 4. Thus, by appropriate switching of the four flip-flops between their on and off states, any output from one unit to fifteen units can 'be achieved. Each different output level of the A/DC corresponds to one of the quantization levels, shown in FIG. 2, into which the original signal can be divided.

FIG. 5 shows in Table A the quantization level of the signal 29 at each sampling time shown in FIG. 2, and in Table B the state each of the tiip-liops 37-40 assumes as a consequence of each of the samplings. During the first sampling interval t1, the signal 29 is near the twelfth quantization level. Thus, the staircase generator runs until its output equals twelve units. At this point, the liipflops 39 and 40 will be on, and the liip- ilops 37 and 38 will be off. This setting of the flip-Hops will be inserted into the shift register (to be discussed next) and thereby cause the latter to generate a unique fifteen bit pseudorandom binary word. At sampling time t2, the staircase generator will run until the flip- flops 38 and 39 are on and ip- flops 37 and 40 are off, thereby yielding an output of six units which corresponds to the sixth quantization level nea-r which the signal 29 exists at time t2. This new setting of the flip-Hops will be inserted into the shift register in order to cause another unique fifteen bit word to be generated. Further lreference to FIG. 4 will indicate the setting of the four fiip-flops 37-40 at each successive sampling time shown in FIG. 2.

In the present example, the word generator 20 is a feed-back shift register (sometimes referred to hereinafter as FBSR) shown diagrammatically in FIG. 6. The FBSR includes four stages or flip- flop circuits 41, 42, 43, and 44, and the feed-back loop includes a halfadder 4S. Each of the Hip-flops 41-44 bas two possible states: on or off (1 or 0). When pulses from the clock 17 are applied to the FBSR, the latter steps and produces a fifteen bit pseudo-random binary word. The particular word produced depends upon the initial setting of each of the p-fiops 41-44, i.e., on or off, at the beginning of the fifteen bit cycle, and the particular feedback arrangement employed. Just as there are fifteen different possible combinations of states of the four fiipflops 37-40 of the A/DC 19, there are fifteen different possible combinations of states of the four fiip-fiops 41-44 of the shift register. Since each initial setting of the Hip-ops 41-44 causes a unique fifteen bit pseudorandom binary word to be produced when pulses from clock 17 are applied to the shift register, the shift register is capable of producing fifteen different pseudo-random binary words with any particular feed-back arrangement. It follows, therefore, that each fifteen bit binary word produced by the shift register of FIG. 6 corresponds to one of the lfifteen levels into which the original signal can 'be quantized. This is because the particular binary word produced depends upon the initial setting of the flipops 41-44, and this setting is the sa-me as the setting of the flip-flops 37-40 of the A/DC 19 after the latter samples the original signal and its staircase generator steps to produce an output approximating the amplitude level of the original signal at the time of sampling. When the appropriate output is reached, the states of the flipfiops 41-44 are caused, by suitable well-known means, to assume the states of the flip-flops 37-40, respectively.

The discussion above mentions that the four flip-hops 37-40 and the fiip-ops `41--44 yield fifteen different possible combinations of states, and fifteen different binary words. Actually, of course, there are sixteen possible combinations of states, the combination ignored above being the one in which all fiip-ops are off The reason that this combination is not considered is that when the initial setting of each of the flip-ops 41-44 is oli, the shift register when stepped will produce nothing more than a series of bits. Such a series is not actually a pseudo-random binary word, and therefore is not treated as one within the context of the present invention. It should be noted, however, that when apparatus according to this invention is in operation during a period in which the original signal 29 discontinues, i.e., falls to the zero quantization level in FIG. 2, the settings of all the ipflops of the A/DC will be off and therefore the flipops 41-44 of the shift register will be similarly set, and hence a series of 0 bits will be generated and applied to the modulator 21. None of the correlators 25a-n will sense correlation between this transmitted wor and its respective reference word, and so there will be no analog output signal from the decoder. Consequently the series of 0 bits is capable of transmitting the information that the original signal has been discontinued.

FIG. 7 illustrates the lmanner in which the states of the iiip-iiops 41-44 of the shift register of FIG. 6 change as the shift register steps. For the sake of illustration, assume that all the iiip-ops are initially set on. Thus, a l appears under each of the flip-flop reference numerals. As each pulse from clock 17 is applied to the shift register, the state of each p-op shifts one hip-flop to the right, i.e., each iiip-op assumes the on or off state previously existing in the flip-fiop to its left, and the state of the leftward most fiip-op, 44, is determined by the feed-back loop including the half-adder 45. The operation of the feed-back loop is as follows: if both dip- flops 41 and 42 are on (1) or off (0), the flip-flop 44 will be turned off (0); however, if one of the flip- flops 41 and 42 is on and the other is off, the ip-iiop 44 will be turned on. When the first pulse is applied to the shift register (the successive pulses are indicated at the left in FIG. 7) the feedJback loop turns ip-tiop 44 off, because fiip- fiops 41 and 42 are both on, and the on state of each flipflop shifts one flip-flop to the right. Thus, all flip-liops remain on except iiip-op 44. After the second pulse, flipfiop 44 remains off and the flip-flop 43 to its right turns off. With the third pulse, the liip-liop 44 remains off, and the off state shifts to fiip-op 42 as well as flip-hop 43. Now the states of flip- fiops 41 and 42 differ. Hence, when the fourth pulse is applied to the shift register, the feedback loop causes flip-fiop 44 to be turned on. The states of the four flip-flops continue to change in this way throughout the fifteen pulse cycle, and it should be noted that in the course of the fifteen pulse cycle, every possible combination of states of the ip-ops appears (with the exception of all hip-flops off). Upon application of the sixteenth pulse, which is the first pulse of the succeeding cycle, the initial state, in which all flip-ops were on, returns.

The fifteen successive states of the fiip-op 44, outlined in FIG. 7, represent a pseudo-random binary word. The fifteen bits of this word are applied to the modulator 21 for transmission to the receiver and decoder. 'Ihe particular word outlined in FIG. 7 corresponds to the fifteenth quantization level of FIG. 2, because it was generated from an initial setting in which all flip-flops 41-44 were on. It can 'be seen with reference to FIG. 4 that all flip-flops of the A/DC 19 are on when the A/DC samples a signal near the fifteenth quantization level. A quantization level is indicated to the right of each combination of states of the fiip-fiops 41-44, in FIG. 7. Upon sampling a signal at any particular quantization level, the A/DC would initially set the flip-flops 41-44 to the states sho-wn corresponding to that quantization level. From that starting point, the successive states of fiip-flop 44 would represent a unique fifteen bit binary word.

FIG. 8 shows the fifteen successive combinations of states of the fiip-fiops 41-44 as they would actually appear as a result of the first four samples indicated in FIG. 2. The combinations resulting from every other sampling are offset for the sake of clarity. At the sample time t1, the signal 29 is closest to quantization level l2. FIG. 7 indicates that the initial setting of the liip-ops 44-41 corresponding to the twelfth quantization level is l, l, 0, 0, respectively. Hence, this combination is shown opposite time t1 in FIG. 8. The series of times shown at the left in FIG. 8 indicate the times, after the sampling step, that each successive clock pulse is applied to the shift register. rlhe pseudo-random binary word appearing beneath the reference numeral 44 is the code word corresponding to the twelfth quantization level. At the time of the second sampe, t2, the signal 29 is closest to the sixth quantization level. Hence, the initial setting of the flip-fiops 41-44 is as shown opposite time t2 in FIG. 8. The pseudo-random binary word shown beneath reference numeral 44' (the use of the prime appendage to this reference numeral will be explained hereinafter) is the code word corresponding to the sixth quantization level. The remaining combinations shown for sampling times t3 and t4 will, it is believed, be readily understood in view of the above discussion.

FIG. 9 shows, at the top, the code words transmitted to the receiver as a result of the samplings of signal 29 at times t1 and t2. These words should be read from right to left and are identical to the two code words just discussed with reference to FIG. 8. Below these incoming words are illustrated fifteen word generators or feed-back shift registers 24a-o, and fteen correlators 25a-o, within the decoder 16. Each of the FBSRs 24a-o continuously and repeatedly generates a different one of the fifteen possible binary words which can be produced by the FBSR in the encoder 15, and applies it to its corresponding correlator. The specific binary words are shown extending between the FBSRs and the correlators in FIG. 9. The quantization level corresponding to each of the code words is indicated to the left of the FBSRs. Although each of the FBSRs 24a-0 continuously reproduces its assigned word, without interruption between repetitions, a space is shown between the repetitions in FIG. 9 for the sake of clarity. The words transmitted to the decoder are applied to each of the correlators 25a-o. It will be seen that the rst word, resulting from the sampling at time t1, is identical to only one of the reference words produced by the FBSRS 24a-0, namely the word produced by FBSR 241' l( all words in FIG. 9 are to be read from right to left), and the code word produced by this FBSR corresponds to quantization level 12. Thus, correlator 251 will sense correlation between the incoming word and the reference word applied to it. All the other correlators will sense that the incoming word differs from the reference words applied to them. The outputs of the correlators are used as described above with reference to FIG. 1 and to be described below in more detail.

FIG. 10 illustrates in more detail the operation of the encoder 15, particularly, the lmanner in which the various operations of the encoder are controlled. In practice, two word generators or shift registers and 20v are employed, rather than one as shown in FIG. 1. If only one shift register is used, a delay must necessarily exist between the generation of the last bit of one word and the generation of the first bit of the next succeeding word. The delay is a result of the time required for the A/DC',` 19 to reset the iiip-ops of the shift register, after completion of one code Word, in order to determine the next code IWord which will be generated by the shift register. On the other hand, if two shift registers are employed, the flip-Hops of one can be set to condition the shift register to produce an appropriate word, during the time that the other shift register is actually generating a word. In this way, it is possible to obtain a continuous stream of bits from the encoder without any delay whatsoever between the last bit of one word and the first bit of the next succeeding word. In this situation, the time T between samplings (see FIG. 2) will equal the time required to generate one code word. Furthermore, the stream of bits from the encoder remains continuous even during pauses between words or sentences, when speech is being transmitted, since as mentioned above when the amplitude of the original signal drops to zero, the shift register produces a series of "0 bits.

The A/DC 19 of FIG. 10 samples the original signal and sets the flip-flops of one of the shift registers, say the register 20. Pulses from the clock 17 are applied to the shift registers 20 and 20 through AND circuits 48 and 49, respectively. The AND circuits operate alternatively to transmit the clock pulses under control of a Hipop circuit 50 which is in turn controlled by the counter 18. In the present example, the AND circuit 48 transmits pulses to the FBSR 20, whereby the latter generates a code word. The outputs of the FBSRs are passed to the modulator 21 of the transmitter by AND circuits 52 and 53, respectively, and OR circuit 47. The AND circuits operate alternatively under control of the iiip-op circuit 50. In the present example, the AND circuit 52 passes the output of FBSR 20 to Kmodulator 21. While the FBSR is generating a word, the counter 18, which is continuously actuated by clock 17, sends a pulse to the A/DC 19 commanding it to sample the original informationbearing signal. Conversion of the analog value of the original signal to a digital response ordinarily takes about 20-40 microseconds, and after conversion is complete, i.e., after the flip-hops of the A/DC have reached a setting which produces a digital output approximating the amplitude value of the original signal at the time of sampling, a pulse from the counter 18 is applied to a flipflop circuit 51. This flip-flop, as a result, produces a pulse Which causes the inactive FBSR, in this case register 20', to accept an initial setting from the A/DC 19. When the fteenth pulse is transmitted from clock 17 through AND circuit 48 to FBSR 20, assuming that the code words employed are each fifteen bits long, the counter 18 sends a pulse to iiip-op 50, which in turn causes the AND circuit 49 to start transmitting clock pulses to FBSR 20 and the AND circuit 48 to cease transmitting clock pulses to the FBSR 20. At the same time, the liipop 50 causes the AND circuit 52 to discontinue passing the output from the FBSR 20 to the modulator 21 of the transmitter, and causes the AND circuit 53 to begin passing the output from the FBSR 20" to the modulator. Operation of the encoder 15 as just described is continuous. The two FBSRs operate in leap frog fashion to produce a continuous stream of encoded information without gaps or breaks of any kind. It will now be appreciated that the combinations of Hip-flop states appearing on the left in FIG. 8 illustrate the states of flip-ops 41-44 of FBSR 20, and the combinations appearing on the right illustrate the states of flip-flops 41-44 of FBSR 20.

In a practical system, according to this invention, for transmitting, say, a five kilocycle band of speech, the A/DC should convert each analog sample to a digital output at a 20 kilocycle per second rate, which is greater than twice the Nyquist rate to insure high fidelity. Furthermore, it is thought that a practical system requires an A/DC and a shift register each having a minimum of seven stages, whereby each code word will comprise 127 bits and the original information-bearing signal can be quantized into 127 levels. In such a system, the cycling rate required to produce a continuous stream of bits from the encoder is 127 X20,000 cycles, or 2.54 megacycles per second. Thus, the clock 17 would generate pulses at the rate of 2.54 megacycles per second.

FIG. ll illustrates a seven stage shift register of the type which may be used in the arrangement of FIG. 10, tWo such shift registers being shown in FIG. 10. Each of the flip-flops 54-60 receives signals from its respective flip-op of the A/DC, via a pair of conductors 63 and 64, for the purpose of initially setting the state of the tiipflops 54-60. Another conductor 65 carries the pulses from the flip-op 51 (FIG. l0) lto each of the ip-ops 54-60 commanding the latter to accept the settings from the A/DC via the conductors 63 and 64. Thereafter, pulses from clock 17 are applied to each of the flip-flops 54-60 via conductor 66. In response to each clock pulse, the state of each ip-op is shifted one stage to the right by means of conductors 67 and 68 between each two adjacent ilip-flops. The outputs of flip-ops 57 and 60 are added in a half-adder 45, and the sum is fed-back to the first stage ip-flop 54 via conductors 69 and 70. This is a standard technique referred to as half-adding or exclusive OR combining. The output of lthe last stage or flip-flop 60 represents the code word output of `the shift register. The particular stage used to generate the code word is immaterial, and it will be noted that in FIGS. 6 and 7 the output of the first stage was treated as the code word.

The decoder 16 is shown in more detail in FIG. 12. The signal from the transmitter is applied -to the demodulator 22 of the receiver. The receiver sequence of bits, or binary code word, is applied to each of 127 correlators 25. Also applied to each correlator is a reference word, as described above. These reference words could be supplied by 127 individual seven stage feed-back shift registers. However, the same result can be achieved in a simpler fashion. A seven stage FBSR 74 identical to the FBSR of FIG. 1l is provided, and receives pulses from the clock 23. The output of each stage of this FBSR is used as a reference word. Thus, the FBSR 74 produces seven reference words, making it necessary to provide additional means for producing the remaining 120 reference words. The additional means may be a delay register 75 consisting of 120 stages or ip-iiops identical to those in the shift register 74, but containing no feed-back loops. The output of each stage of the register 75 is the same as the sequence generated by the FBSR 74, but delayed by a time dependent upon the number of stages between it and the FBSR. Thus, the 127 reference words are produced by using only 127 individual Hip-flops.

The correlators and comparator will be described in more detail with reference to FIGS. 12 and 13. Each of the 127 correlators is designed to produce a positive pulse each time a bit of the transmitted code word matches a bit of the reference word applied to the correlator, and no output when the transmitted bit and reference bit do not match. This is achieved by introducing the reference word and the transmitted word into a pair of parallel connected AND and NAND circuits. Thus, the reference word is introduced, via conductor 76 into AND circuit 77 and NAND circuit 78, and the transmitted code word is introduced into circuits 77 and 78 via conductor 79. The AND circuit 77 produces a positive pulse each time a l of the reference word matches a l of the transmitted word, but no output when any other combination of bits is applied to it. The NAND circuit 78 produces a positive pulse each time a of the reference word matches a 0 of the transmitted word, but no output when any other combination of bits is applied to it. The outputs of both circuits 77 and 78 are applied to a single OR circuit 80. Thus, it will lbe seen that the OR circuit receives a positive pulse each time two 0 bits or two 1 bits are simultaneously applied to the circuits 77 and 78. The output of the OR circuit 80 is applied to a capacitor 84 through an amplifier 83. In the correlator receiving the reference word identical to the transmitted word, each pair of bits applied to the circuits 77 and 78 will produce a positive pulse, since they will always match. These pulses, after inversion by amplifier S3 will serve to charge the capacitor 84 highly negative. In all the other correlators, the capacitor will contain approximately zero charge, for a reason to be discussed below with reference to FIG. 14.

The negative charge on capacitor 84 is applied to a Schmitt trigger 85, which also has a threshold voltage applied to it. The Schmitt triggers serve as comparators 26. When the negative charge becomes large enough to counteract the threshold voltage, the Schmitt trigger switches. It should be noted that for each transmitted code word, the Schmitt trigger is connected to only one correlator, i.e., the correlator corresponding to the transmitted word, will switch. The Schmitt triggers connected to all the other correlators will remain unaffected. This is because the threshold voltage applied to each Schmitt trigger is adjusted so that the capacitors in the correlators not corresponding to the transmitted word never receive enough charge to counteract the threshold voltage. On the other hand, the threshold voltage need not necessarily be so high that the capacitor must receive 127 pulses before it is charged sutliciently to overcome the threshold voltage. In general, the more intense the noise in the system, the higher the threshold voltage will have to be in order to avoid an inordinate number of false alarms.

When the 127th bit of the transmitted word and each of the reference words have been applied to the correlators, the FBSR 74 sends a pulse, through an AND circuit 86 and conductor 87, to a series of gates 88, one of which follows each Schmitt trigger 85. The gate following the Schmitt trigger which has switched will thereupon produce an output pulse, but the other gates will have no output. The amplitude of the output pulse produced by each gate, when its corresponding Schmitt trigger has switched, is always the same, and equal to the output pulse produced by every other gate. The output of the gate corresponding to the Schmitt trigger which has switched is applied to a decoding matrix (within the box identiiied by reference numeral 27 in FIG. l2), and the latter sets the flip-flops of the D/AC, the particular setting depending upon which gate transmits the pulse to the matrix. As a result of this setting, the D/AC produces a signal whose amplitude approximately equals the amplitude of the original signal at the time of the sample which led to generation of the code word which caused the particular output pulse. The output of the D/ AC is smoothed by a low-pass lilter 89.

FIG. 14 is intended to illustrate the autocorrelation function of the pseudo-random binary words employed in the present invention. As mentioned above, if a particular transmitted code word is correlated with an identical reference word, a positive pulse from OR circuit (FIG. 13) will result as each of the 127 bits in the code word is matched with its corresponding bit in the reference word. Consequently, in the absence of inversion by amplitier 83, a positive charge equal to 127 positive pulse units would build up on an integrator 84, which may be a capacitor. This charge is indicated on the vertical axis of FIG. 14. The point of intersection between the two axes indicates not only zero charge, but zero time delay between the code word and its corresponding reference word. The horizontal axis represents time delay, in bits, between the code word and the same reference word just mentioned. Obviously, however, any such time delay is equivalent to a change in the reference word. Thus 126 time delays at one bit intervals actually represent the other 126 reference words. It will 'be seen that correlation of the code word with each of the reference words which do not match it produces, after comparison of al1 127 bits of the code word, a negative charge of one unit. Thus, there is no chance of any Schmitt trigger switching except the one corresponding to the reference word which matches the transmitted word.

From the above it will be apparent that the successful operation of the present invention depends upon the clock 17 in the encoder running at exactly the same rate as the clock 23 in the decoder. Any difference in clock rates will, of course, cause a time delay between generation of the transmitted code word and generation of the reference word to which it should correspond with the result that the transmitted word and reference word will not match as they should. Even if the two clocks are operating at the same rate, the same disadvantageous result will occur if there is relative motion between the transmitter and receiver, since the transmitted signal will ppear to the receiver to have slipped some number of its.

Synchronization of the word generator in the encoder and the word generators in the decoder can be accomplished by sending an independent synchronization signal to the receiver at the time transmission of a new code word commences. However, the present invention offers the advantage of achieving initial synchronization; and thereafter maintaining synchronization without resort to a separate synchronization signal. Initial synchronization between the word generators of the encoder and decoder can be achieved by repeatedly sending a known code word corresponding to a certain quantization level. In the absence of synchronization, the output of the decoder will be at some other quantization level. The decoder is adjusted by prohibiting one pulse at a time produced by clock 23 from reaching the word generators 24a-n. This can be accomplished by means of a gate or a phase shifter between the clock and word generators. This procedure is continued until the output of the decoder is at the same quantization level as the quantization level corresponding to the transmitted word.

FIG. l illustrates means for maintaining the synchronization of the encoder and decoder word generators despite variations which may occur between the rates of clocks 17 and 23 and despite relative movement between the receiver and transmitter. The word generators or shift registers 24a-n and correlators 25a-n are comparable to those illustrated in FIG. 1. The transmitted code word is applied to each correlator via conductor 90. The output of each correlator is applied to both a greatestof circuit 91, via conductors 92, and to a gate, via conductors 93. The gates are indicated by reference numerals 94a-n, and there is one gate associated with each correlator. An output from the greatest-of circuit 91 is also applied to each of the gates 94a-n. Outputs from all the gates are applied to an amplitude comparator 95, .of any Well known kind, and the output of the amplitude comparator is in turn applied to a phase shifter 96, through which pulses from clock 23 are applied to the shift registers 24a-n.

As described above, when a transmitted code word is decoded, only one of the correlators 25a-n will produce an output of any significance. This assumes, however, that the encoder and decoder word generators are synchronized. The greatest-of circuit 91 responds to asignal from the correlator corresponding to the transmitted word by turning on the two gates associated with the correlators on either side of the correlator applying the signal to the greatest-of circuit. Thus, if the correlator 251: corresponds to the transmitted word, the greatest-of circuit will turn on gates 94a and 94e. Consequently, the ouputs of correlaors 25a and 25e will be applied, through these gates, to the amplitude comparator 95. If synchronization exists, the outputs of correlators 25a and 25e will be equal and negligible, and therefore there will be no output from comparator 95. However, if the word generators in the encoder and decoder are not perfectly synchronized, the output of correlator 25b will be reduced, since the AND circuit 77 (FIG. 13) will conduct for only a fraction of the time it would conduct if synchronization were present. Hence, the output of the OR circuit 80 will similarly -be reduced, and the positive charge which builds up on integrator 84 will no longer be equal to 127 positive pulse units, but rather to some lesser value. For the same reason, in the absence of synchronization, the output of one of the correlators adjacent to the correlator 25b, say correlator 25a, will be correspondingly increased, because the charge which builds up on the integrator corresponding to the correlator 25a will increase and no longer be equal to the charge on the integrator corresponding to the correlator 25C. The comparator 95 will sense the difference between the outputs of correlators 25a and 25e, and Will activate the phase shifter 96 to alter the transmission of pulses from clock 23 to the word generators 24a-n in order to resynchronize those word generators with the Word generator of the encoder. The amplitude comparator 95 continues to adjust the phase shifter until it no longer senses any difference in the outputs of the correlators 25a and 25e. The sense or direction of the error in synchronization is revealed by noting which of the outputs of the correlatros 25a and 25C exceeds the other. Thus, if the output of correlator 25a, which corresponds to a lower quantization level than correlator 25e, is greater than the output of correlator 25e, the phase shifter is adjusted to speed up the application of pulses from clock 23 to the word generators.

It should be noted in passing that FIG. 15 illustrate an alternative to the comparators 84, Schmitt triggers 85, and gates 88 of FIG. 13. The outputs of the greatest-of circuit 91 can be applied to a decoding matrix 97; instead of the outputs of gates 88. The matrix 97 in turn feeds a register comprising seven nip-flops, the setting of which depends upon the value of the output of the matrix. The output of the matrix depends upon the correlator providing the pulse to the greatest-of circuit. For each different matrix output, a unique seven bit binary word will be produced by the register. The register is connected to a binary-weighted resistive adder having 127 possible output amplitude levels. Consequently, a signal approximating the amplitude level of the original signal sample can be produced.

The greatest-of circuit 91 is shown in detail in FIG. 16. A series of transistors a-n, equal in num-ber to the number of correlators 25a-rv (FIG. 15) are arranged with their emitters connected to a single junction by conductor 101, and with their collectors connected to another junction by conductor 102. The output of each correlator is applied to the base 103 of its respective transistor. In such an arrangement, when the base of one transistor receives a signal larger than that applied to any other transistor, the collector voltage of that transistor will drop whereas the collector circuits of the other transistors remain at a relatively high potential. Since all the transistors are connected by conductor 101 to a common emitter load resistor 104, the voltage drop across this resistor will equal that of the largest input signal to all the transistors. Consequently, all the transistors will assume a non-conducting condition except the one receiving the largest input signal, and therefore the latter transistor will conduct.

FIG. 17 illustrates an alternative arrangement for synchronizing the encoder and decoder. In this arrangement, two banks of shift registers and correlators, A and B are provided instead of one. In other words, twice as many shift registers and correlators are present as there a-re quantization levels. All the shift registers are actuated by a clock 2-3 through a phase shifter 96. However, a delay circuit 107 is located between the phase shifter 96" and the 4bank of shift registers 24a'-n'. The circuit 107 has the elect of delaying the application of the clock pulses by some predetermined interval. The delay might be onehalf bit delay, i.e., a time delay equal to one half the time required to generate a bit. The transmitted code word is applied to each of the correlators 25a-n and 25a'-n' via conductor 108. The outputs of correlators 25a-n are applied to a greatest-of circuit 104, and the outputs of correlators 25d-n are applied to another greatestof circuit 105. The output of the greatest-of circuits are applied to an amplitude comparator 106, which in turn controls the phase shifter 96.

Assuming that the encoder word generator and the word generators 24a-n are synchronized, one of the correlators 24a-n, say correlator 24b, corresponding to the transmitted code word will produce a maximum output signal, i.e., a signal equal to 127 units. On the other hand, since there is a one-half bit time delay between the encoder word generator and the word generators 24a-n', the correlators 25a' and 25h will each produce an input signal equal to one-half the maximum output signal of correlator 24h. These outputs are transmitted to the amplitude comparator 106 by ygreatest-of circuits 104 and 105, and under these conditions, the comparator 106 will produce no output. Assume now that the output of correlator 2512 becomes larger than the output of correlator 25a', e.g., the output of correlator 25b' rises to 75% of the maximum output and the output of correlator 25a' falls to 25% of the maximum output. In response to this circumstance, the amplitude comparator 1016 will adjust the phase shifter 96 in order to delay the transmission of the clock pulses to the shift registers. Adjustment of the phase shifter 96' continues until the synchronized condition described above returns.

In order to initially achieve synchronization between a transmitter and receiver having a very high velocity in relation to each other, means are provided for rapidly varying the rate of the clock, which may be an oscillator, until some output is produced -by the correlators. Fine synchronization can then be achieved by the means described above.

The invention has been shown and described in preferred form only and by way of example, and many changes may be made in the embodiment disclosed without necessarily

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