Remote control system, components and methods

What is claimed is:

1. A receiver for receiving a spread spectrum transmitted signal wherein a predetermined parameter of said signal is varied to carry a sequence of digital data chips comprising:

(a) detector means for providing a detector signal representative of said predetermined parameter in said transmitted signal;

(b) chip interval series means for generating a plurality of separate series of predetermined chip intervals such that each predetermined chip interval in each such series is equal to the duration of one chip in said transmitted signal and so that each said series is offset in time from another one of said series by a predetermined offset interval, and subdividing each chip interval in each of said separate series into a plurality of crumb intervals;

(c) crumb-level comparison means for testing the values of said detector signal for the crumb intervals in each chip interval of each of said separate series of chip intervals against a predetermined emplate corresponding to a predetermined series of clock signal values;

(d) selection means for selecting one of said separate series of chip intervals for which the detector signal values in the various crumb intervals best match the template and outputting the selected sequence of chip intervals as a decoding sequence of chip intervals; and

(e) recovery means for assigning a first binary value or a second binary value to each chip interval in said decoding sequence of chip intervals depending upon the value of said detector signal during the chip interval.

2. A receiver as claimed in claim 1 for receiving a radio frequency transmitted signal wherein said predetermined parameter is a parameter of said radio frequency signal, said detector means including means for providing said detector signal representative of said predetermined parameter of said radio frequency signal.

3. A receiver for receiving a spread spectrum transmitted signal wherein a predetermined parameter of said signal is varied to carry a sequence of digital data chips comprising:

(a) detector means for providing a detector signal representative of said predetermined parameter in said transmitted signal;

(b) chip interval series means for generating four separate series of predetermined chip intervals such that each predetermined chip interval in each such series is equal to the duration of one chip in said transmitted signal and so that each said series is offset in time from another one of said series by a predetermined offset interval equal to one-fourth the duration of each said chip interval, and subdividing each chip interval in each of said separate series into two equal crumb intervals;

(c) crumb-level comparison means for determining whether each of said separate series of chip intervals satisfies the condition that the values of said detector signals for the two crumb intervals in each chip interval differ from one another;

(d) selection means for selecting one of said separate series of chip intervals which satisfies said condition and outputting the selected sequence of chip intervals as a decoding sequence of chip intervals; and

(e) recovery means for assigning a first binary value or a second binary value to each chip interval in said decoding sequence of chip intervals depending upon the value of said detector signal during the chip interval.

4. A receiver as claimed in claim 3 further comprising reference value means for providing a reference value, said recovery means including chip level comparison means for comparing the average value of said detector signal during each chip interval in said decoding sequence of chip intervals with a reference value and assigning said first or second binary value to each said chip interval in said decoding sequence depending on whether the value of said detector signal during the chip interval is greater than or less than said reference value.

5. A receiver as claimed in claim 3 for receiving a radio frequency transmitted signal wherein said predetermined parameter is a parameter of said radio frequency signal, said detector means including means for providing said detector signal representative of said predetermined parameter of said radio frequency signal.

6. A spread spectrum method of communicating digital information comprising the steps of:

(a) timing predetermined transmitter chip intervals;

(b) during a preamble period, generating a preamble chip signal having either a first or second binary value during each of said transmitter chip intervals, generating a transmitter clock signal having a predetermined series of different values for different transmitter crumb intervals, each transmitter crumb interval being a rational fraction of one of said chip intervals, and impressing both said transmitter clock signal and said preamble signal on a predetermined parameter of a transmitted signal so that said predetermined parameter of said transmitted signal carries said preamble signal encoded with said transmitter clock signal;

(c) after said preamble period, transmitting said information as a stream of binary chip values impressed upon said predetermined parameter of said transmitted signal;

(d) detecting said transmitted signal at a receiver remote from said transmitter and producing a detector signal representative of the value of said predetermined parameter in said transmitted signal;

(e) generating at said receiver plural separate sequences of chip intervals such that the duration of each said chip interval in each said sequence is equal to the duration of said transmitter chip intervals and so that said sequences of receiver chip intervals are offset in time from one another, and subdividing each chip interval in each of said separate sequences into receiver crumb intervals equal in duration to said transmitter crumb intervals;

(f) determining whether each of said separate series of receiver chip intervals satisfies the condition that the values of said detector signal for the crumb intervals in each chip interval match a template corresponding to said predetermined series of values in said transmitter clock signal, and selecting as a decoding sequence one of said separate series of receiver chip intervals which best satisfies said condition; and

(g) after said preamble period, assigning first or second binary values to said chip intervals in said decoding sequence within said receiver according to the value of said decoder signal during each said decoding chip interval, to thereby provide said decoding chip intervals with binary values corresponding to the binary values in said digital information.

7. A method as claimed in claim 6 wherein said transmitted signal is a ratio frequency signal.

8. A receiver for receiving a spread spectrum transmitted signal wherein a predetermined parameter is varied to carry a digital message including a plurality of bits each encoded into a plurality of chips so that the chips representing each bit include substantially equal numbers of zero and one values, said receiver comprising:

(a) detector means for producing a detector signal representative of said predetermined parameter;

(b) reference value means for determining the time average value of said detector signal;

(c) chip-level comparison means for comparing said detector signal with said time average value and providing a sequence of chips with first or second binary values depending on whether the detector signal is greater or less than the time average value to thereby provide a string of binary data chips representing said message; and

(d) interpretation means for interpreting said string of data chips to thereby recover said message.

9. A receiver as claimed in claim 8 for receiving a radio frequency transmitted signal wherein said predetermined parameter is a parameter of said radio frequency signal, said detector means including means for providing said detector signal representative of said predetermined parameter of said radio frequency signal.

10. A receiver as claimed in claim 8 wherein said interpretation means includes means for selecting predetermined groups of said chips at predetermined locations within said string of chips and applying different decoding schemes to each said selected group depending on the position of the group in said string.

11. A receiver as claimed in claim 10 wherein said means for selecting includes means for selecting said groups of chips so that each such selected group is representative of one of said bits in said message.

12. A receiver as claimed in claim 11 wherein said interpretation means includes assignment means for assigning each said group of chips either for decoding according to an A-decoding scheme or to a B-decoding scheme different from said A-decoding scheme depending upon the position of the bit represented by the group within the message, so that the order in which said groups are assigned to A or B decoding corresponds to a predetermined overlay code.

13. A receiver as claimed in claim 12 wherein said assignment means includes a register defining a plurality of positions including A-blocks and B-blocks of positions, the order of said A-blocks and B-blocks corresponding to said overlay code, said interpretation means including A-decoder means associated with each said A-block for decoding the chips positioned therein according to said A-decoding scheme and B-decoder means associated with each said B-block for decoding the chips positioned therein according to said B-decoding scheme, whereby each said A-decoder and B-decoder means is associated with a predetermined bit position in the message, said assignment means also including means for advancing said string of chips into said register.

14. A receiver as claimed in claim 13 further comprising reference information means for providing a sequence of reference bits corresponding to a sequence of bits in the message, the value of each said reference bit being equal to the expected value of the corresponding bit in the message, said receiver including means for comparing the value of each said reference bit with the value of the corresponding bit in the message.

15. A receiver as claimed in claim 14 wherein said reference information means includes means for providing each said A-decoder means and each said B-decoder means with a reference bit value equal to the expected value for the associated bit position in the message, each said A-decoder means includes means for encoding the reference bit value into reference chip values according to an A-encoding scheme inverse to said A-decoding scheme, each said B-decoder means includes means for encoding the reference bit value into reference chip values according to a B-decoding scheme, each of said A-decoder and B-decoder means including disparity check means for comparing the reference chip values with the message chip values in the associated block of register positions.

16. A receiver as claimed in claim 15 wherein said disparity check means of each said A-decoder and each said B-decoder includes bit-level disparity count means for providing a count of the disparities between the reference chip values and the message chip values compared in the decoder, said interpretation means also including disparity total means for adding the counts provided by a plurality of said bit-level disparity count means and accepting or rejecting the message depending upon the resulting sum.

17. A receiver as claimed in claim 16 wherein said blocks of positions within said register include address blocks and command blocks, the order of said address blocks and said command blocks in said register corresponding to a predetermined address and command bit position scheme, said reference information means including reference address means for providing reference bit values representing a predetermined address to said decoder means associated with said address blocks and for providing plural sets of command bit values representative of plural alternative commands to said decoder means associated with said command blocks, said decoder means associated with each said command block including means for encoding a bit value from each of said sets of command bit values into a set of alternative chip values, comparing each said set of alternative chip values with the values of the chips in the associated block and providing a disparity count for each said comparison, said interpretation means further comprising command signal selection means for separately summing the disparity counts from comparisons for said bit values of said plural alternative commands and selecting the one of said alternative commands having the lowest sum of disparity counts.

18. A remote control system comprising a receiver as claimed in claim 17 and a transmitter, said transmitter including means for storing a transmitter address, selectively operable means for providing one of plural alternative commands, message assembly means for assembling the address stored in said address storage means and the command provided by said selectively operable command means into a message including a plurality of address bits representative of said stored address and one or more command bits representative of said command provided by said selectively operable means, so that said address bits and said command bits are sequenced within said message according to said predetermined address and command bit position scheme, encoding means for encoding each bit in said message according to an A-encoding scheme inverse to said A-decoding scheme or according to a B-encoding scheme inverse to said B-decoding scheme depending upon the position of the bit within the message so that the order in which the bits are encoded according to said A and B encoding schemes corresponds to said predetermined overlay code, and broadcast means for providing said transmitted signal and varying said predetermined parameter of said transmitted signal in accordance with said encoded message.

19. A message transmission system comprising a receiver as claimed in claim 12, message generation means for providing the message to be transmitted as a sequence of bits, encoding means for encoding each bit in said message according to an A-encoding scheme inverse to said A-decoding scheme or according to a B-encoding scheme inverse to said B-decoding scheme depending upon the position of the bit within the message so that the order of A-encoded and B-encoded bits in said message corresponds to said predetermined overlay code, and broadcast means for providing said transmitted signal and varying said predetermined parameter of said transmitted signal in accordance with said encoded message.

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BACKGROUND OF THE INVENTION

The present invention relates to remote-control systems. The invention is particularly useful in control of electrical devices in a building and in control of appliances.

Conventional switches used in building electrical power systems are connected in the power supply wiring of the building between the power source and the loads controlled by the switches. Accordingly, the power wiring of the building must extend to each switch and from each switch to the load. An ordinary wall switch controlling a ceiling light fed by an electric power wire in the ceiling thus requires a branch extending down from the ceiling through the wall to the switch and back up through the wall to the light fixture. Such branch wiring requires expensive materials, such as high voltage cable, junction boxes and the like, to handle the electrical power. Moreover, expensive, skilled labor is required to install such branches during construction of the building. Even more labor is required to install such a switching branch in a completed building, where the branch wiring must be worked through existing walls.

All of these problems are even more severe where a load must be controlled from more than one switch. The conventional "three way" switch arrangement utilized to control a ceiling light from either end of a stairway requires a switching branch extending from the power supply wiring to one switch, from that switch to the other switch and from the other switch to the light. Where a single load is to be controlled by more than two switches, the required interconnections become even more complex and costly.

Low voltage remote control systems bring only low voltages to the switches and use the switched low voltage to control relays or other high voltage switching devices in the building wiring system. These systems eliminate the need for high voltage components in the switching branches, but do not eliminate the need for a wired connection between the switch and the controlled device. Therefore, substantially the same labor costs are involved in installation of these systems. Moreover, the relays required at the controlled devices add significant costs.

Another remote control system which has been utilized to some extent in building wiring is the "carrier current" system. In the carrier current system, the control switch actuates a small radio frequency transmitter which is connected to the building wiring so that the radio signal propagates through the wiring to a receiver mounted on the controlled device. The receiver actuates a relay or electronic switch controlling the power flow to the device. Each transmitter must be directly connected to the wiring, thus restricting the versatility of the system and adding to its cost. Moreover, carrier current systems typically convey information only at relatively low rates and typically can control only a few devices in a given system. There is a considerable possibility of interference between multiple carrier current systems as, for example, where carrier current systems are used in multiple homes served by a common supply transformer. Conversely, carrier current systems cannot pass information between locations served by different supply transformers, and hence are unsuitable for use in large buildings with multiple supply transformers. Also, carrier current systems typically encounter difficulties with spurious signals caused by random electrical noise on the power line. These and other difficulties have limited application of carrier current systems.

Attempts have been made heretofore to eliminate the difficulties associated with wired and carrier current systems by using free space communication for control purposes, i.e., by directing the control signal from a transmitter through free space within the building to a receiver at the fixture. With free space propagation, the transmitter location is unrestricted and the costs of switch wiring are eliminated. Control signals can in theory be propagated through free space as acoustic signals such as ultrasonic waves or as optical signals, i.e., light beams. These techniques are used in limited applications such as television remote controls and the like where there is only a short gap between the transmitter and receiver and where there is direct line of sight communication between the two. As these favorable conditions are not always present in a building wiring control system, these acoustic and optical systems typically are unsuitable for controlling electrical power within a building.

Attempts have been made to utilize radio control systems for certain limited aspects of building power and/or appliance control as, for example, garage door openers, individual power outlet controls and the like. These systems, however, have been unreliable inasmuch as they are subject to unintended actuation by interfering radio transmitters and, conversely, sometimes fail to actuate the controlled device. To avoid interference with other radio equipment, the transmitters used in these radio control systems are required to be low power devices, thus limiting the range of the system. Additionally, the transmitters and receivers used in these radio control devices have not been suited for mounting within the junction boxes normally used in electrical wiring systems. These junction boxes often are metal enclosures which tend to attenuate radio signals. To receive the weak signals provided by the low powered transmitters, the receiver must either be mounted outside of the junction box or provided with an antenna structure extending out from the box, rendering the entire device cumbersome and unsightly. Moreover, most radio control systems available heretofore have required expensive components. For all of these reasons, radio control systems have not been widely adopted in building wiring systems.

Thus, although there has been an acute need for an inexpensive, reliable and versatile wireless or free space power control system suitable for use in a building power system, no such control system has been available heretofore. The same need for a reliable remote control system exists in the case of thermostats, doorbells and other devices which must actuate another unit at a remote location. There is a similar unmet need with respect to wireless control systems for appliances. Although appliances have been provided heretofore with the short range, line of sight optical and acoustic remote control systems mentioned above, and with rudimentary, unreliable radio control units, there has been no truly satisfactory system for wireless remote control of electrical appliances. The need with respect to remote control of appliances has become more acute with the advent of home automation systems. Modern data processing technology can provide a central automation system capable of controlling and coordinating many appliances within the home, and also coordinating building fixtures such as lights, heaters and alarms. Heretofore, the difficulty and expense of communication between the central system and the various appliances has hindered adoption of such systems.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

One aspect of the present invention incorporates the realization that a communication technique known as "spread-spectrum" radio communication can be employed to provide economical, reliable, and versatile wireless remote control of electrical power supply or appliance operation. The term "spread-spectrum" refers to communication systems and techniques in which a carrier signal such as a radio frequency signal has information impressed upon it so that the carrier signal occupies a bandwidth wider than required for transmission of the information itself. Thus the carrier signal, and hence the information, is spread over a wide range of frequencies. According to well-known communication system theory, a spread signal is less susceptible to interference than an unspread signal. Simple forms of spread spectrum communications techniques, such as a common FM radio broadcasting, completely occupy relatively broad regions of the frequency spectrum. These techniques are unsuitable for use in remote control systems, inasmuch as the power levels of the transmitters would necessarily be limited to avoid interference with other users of the frequency spectrum, and there would be a considerable possibility for interference between neighboring systems.

In more sophisticated forms of spread-spectrum communication, the signal is spread by impressing both the information to be carried and a code on the carrier. In a "frequency hopping" scheme, the code is a sequence of discrete frequencies, and the code is impressed upon the carrier signal by switching the carrier signal among the various frequencies according to the coding scheme. In so-called direct sequence coding, the code is impressed upon the carrier signal to vary the carrier signal along with the transmitted information so that both the code and the information cause a particular parameter of the carrier to vary. For example, in a direct sequence scheme using frequency modulation, both the code and the transmitted information are applied to modulate the frequency of the carrier signal such as a radio signal. Thus, the code and information can be combined to produce an encoded information signal and that encoded signal can be impressed on the carrier. Similar direct sequence schemes can be used with other parameters of the transmitted carrier as, for example, in phase modulation, binary phase shift keying, amplitude modulation, frequency shift keying or even simple on/off keying. As used herein, the term "spread-spectrum" refers to techniques where a specific spreading code is impressed upon the carrier signal, and hence includes both direct sequence coding and frequency hopping. Also, the term "modulation parameter" is used broadly herein to refer to the parameter of the carrier signal which is varied in accordance with information and/or code, regardless of whether the particular scheme of variation involves modulation or keying. For example, in both frequency shift keying and frequency modulation, frequency constitutes the modulation parameter.

The receiver in a spread spectrum system decodes the signal and hence reverses the coding operation applied at the transmitter. Where the receiver is arranged to apply a specific decoding scheme, it will be relatively insensitive to signals encoded according to another scheme, even though those signals are transmitted over the same range of frequencies. Accordingly, many spread spectrum systems can occupy the same region of the electromagnetic spectrum without interfering with one another. Stated another way, a single spread spectrum system may occupy various frequencies within a relatively broad range of frequencies, but will not occupy any one frequency for a sufficient period of time to create a serious interference problem.

Spread spectrum systems heretofore have been regarded as suitable only for relatively sophisticated, high cost applications such as military systems, spacecraft communications and the like. According to the present invention, however, it has been found that spread spectrum techniques can be applied to provide a simple and economical system which meets the needs described above for electric power wiring and appliance controls. A system according to this aspect of the present invention may include a control transmitter unit including transmitter address storage means for storing a predetermined transmitter address or accepting a transmitter address from an external source such as a home automation computer, selectively operable trigger means for generating an action signal, and broadcast means for producing a spread spectrum radio signal carrying digital address information representing the transmitter address and digital command information representing the action signal. The radio signal is propagated through free space within the building. The transmitter unit may take the place of an ordinary wall switch or the like. The system preferably also includes a receiver incorporated in the building power supply wiring or, for appliance control applications, within the power supply wiring of a domestic appliance or the like. The receiver most preferably includes recovery means for receiving the spread spectrum radio signal propagated through free space from the transmitter unit and recovering the address and command from that radio signal. As will be appreciated, the receiver must be capable of decoding the information as encoded by the transmitter unit.

The receiver preferably also includes address storage means for storing a preset receiver address and address comparison means for comparing this preset receiver address with the transmitted address, as recovered by the recovery means. Control signal means are also provided for generating a control signal in response to the transmitted command, but only when the transmitted address matches the preset address stored by the receiver Preferably, the receiver also includes action means for controlling transmission of electricity through the wiring of the building or appliance in response to the control signal.

This aspect of the present invention incorporates the realization that spread spectrum transmission can overcome the serious difficulties associated with reception of radio signals within buildings, and particularly within enclosures such as the junction boxes of building wiring systems or appliance enclosures. These enclosures typically are substantially closed metallic boxes having small openings at random locations on their surfaces as, for example, the small cracks left around conduit entries to junction boxes, mounting holes or the like. Such boxes, therefore, tend to attenuate radio signals and prevent them from reaching the interior of the box. Moreover, whatever radio signals do propagate into the inside of the box ordinarily enter through multiple pathways. These factors, together with the additional attenuation and multipath effects created by walls and other building structural elements intervening between the transmitter unit and the receiver typically make it impractical to receive radio signals within such enclosures.

Because spread spectrum signals typically do not interfere with other signals occupying the same frequencies, governmental authorities will permit the use of greater power in spread spectrum signals. Therefore, the system can overcome the attenuation and provide a reasonable signal level within the enclosure. Moreover, the spread spectrum signal is substantially immune to multipath interference. Thus, the system can provide reliable performance even where the entire receiver is disposed within an enclosure. For example, a receiver for controlling a ceiling lamp may be mounted entirely within the junction box utilized to mount and connect the lamp. The transmitter unit may be mounted anywhere within range. In systems for controlling distribution of electrical power within a building, the transmitter unit is preferably isolated from the building wiring system and incorporates a battery power supply.

As will be appreciated, all of these factors greatly simplify installation of the system. The system according to this aspect of the present invention thus provides the long wanted solution to the remote control problems mentioned above. Plural transmitter units and plural receivers may be provided in a single system. These transmitter units and receivers are associated with one another by means of the addresses which they store or accept. Thus, one or several transmitters may be provided with the address of a single receiver. Any one of these transmitters can actuate the receiver and hence can control the associated electrical device. Conversely, several receivers can be provided with the same address so that all will be actuated by the same transmitter or transmitters.

The reliability of the system is greatly enhanced by providing error detecting features in the receiver. Thus, the receiver preferably includes means for recovering the digital information from the transmitted signal in encoded form and decoding the encoded digital information to provide the transmitted address and command. The receiver preferably also includes error detection means for comparing the encoded digital information with predetermined spreading code information and accepting the encoded digital information only if it matches the spreading code information within predetermined tolerances. The control signal means preferably is arranged to generate the control signal only if the encoded digital information is accepted by the error detection means. Thus, received signals must pass twofold tests within the controlled switch. The probability of an interference signal having both the proper code and also carrying the address of a given receiver is extraordinarily low, and hence the system is essentially immune to unwanted actuations caused by interfering signals.

In a particularly preferred arrangement, the transmitted signal includes a preamble signal and another portion carrying the actual message to be conveyed. Thus the signal may carry a digital message including both "preamble" and "information" bits. The information bits typically include bits representative of the address and command. Different codes may be employed with respect to the preamble bits and the information bits. The term "chip" as used herein refers to a bit which is part of a larger sequence representing a bit of encoded information, such as a sequence of 1 and 0 value chips representing a single 1 or 0 valued bit. Each preamble bit may be encoded into a preamble chip sequence according to a preamble code, and each information bit may be encoded into information chip sequences according to an information code different from the preamble code. A predetermined valuation parameter of a carrier signal such as a radio signal is varied in accordance with the chip values. The information chip sequences typically follow the preamble chip sequences in the message sent by the transmitter unit.

The receiver preferably includes means for recovering from the transmitted signal an output stream including the information chip sequences and also including output representative of the preamble. Decoding means preferably are provided for emitting information bit value signals only in response to the information chip sequences. The decoding means may be coupled to the recovery means so that the decoding means receives the entire output stream. However, because the decoding means is responsive only to the information chip sequences, the decoding means will not emit bit value signals in response to the initial portion of the output stream, representing the preamble. Rather, the decoding means will emit an initial bit value signal only when the first information chip sequence passes from the recovery means. The receiver preferably also includes bit level synchronization means for initializing a bit sequence index in response to this initial information bit value signal and means such as a clock for incrementing the bit sequence index in synchronization with subsequent information bit value signals. Thus, the receiver uses the difference in coding between the preamble and information bits to establish the location of the information bits within the transmitted signal and, having found that location, keeps track of the location of each information bit within the signal. Therefore, the receiver can interpret each information bit according to its intended meaning, i.e., either as part of an address or as part of a command.

Preferably the recovery means of the receiver includes detector means for providing a detector signal representing the modulation parameter in the radio signal, reference value means for providing a reference value, clock means for timing a decoding sequence of predetermined chip intervals and chip level comparison means for comparing the value of the detector signal during each chip interval with the reference value. The comparison means thus forms a sequence of 1 and 0 chips by assigning a 1 or 0 value to each chip interval depending on whether the value of the detector signal during the chip interval is greater than or less than the reference value. For example, in a system using frequency modulation, the detector may provide a voltage representing the received signal frequency, and the reference value may be a voltage corresponding to the central or carrier frequency of the frequency modulated signal. If the detector output voltage and hence the radio signal frequency is above the center frequency during a chip interval, the chip is assigned a value of one whereas if the receive frequency is below the center or carrier frequency, the chip is assigned the value zero.

Inaccuracy or "drift" in the transmitter or the receiver may cause confusion between 1 and 0 values. Thus, drift in the transmitter may cause the central frequency to rise above the intended center frequency, so that all of the signal consists of frequencies greater than the intended center frequency. In this case, the receiver will tend to interpret the signal as an uninterrupted stream of 1's, and the message will be lost.

In one preferred system according to the present invention, this problem is obviated by setting the reference value during transmission of the preamble bits. Thus, the receiver is provided with means for determining the mean value of the detector signal during transmission of the preamble bits and adjusting means for adjusting the reference value means substantially to the mean value of the detector signal. Lock means are provided for disabling the adjusting means after this adjustment has been made. Thus, the reference value of the receiver can be set to correspond with the actual characteristics of the radio signal as transmitted by the transmitter. The preamble bits preferably are encoded so that they are "DC free", i.e., so that they include equal numbers of 1 value and 0 value portions. Therefore, the predetermined modulation parameter of the transmitted signal will be above and below its central or threshold level for equal amounts of time during transmission of the preamble bits. The time-average value of the modulation parameter, and the time-average value of the detector signal will accurately represent the central value in the transmitted signal. As the reference value is set from the preamble bits, there is no need for the information bits to be DC free. Therefore, the information code can be one where the chip sequence representing a given bit value has an unequal number of one and zero chip values.

For the comparison means of the receiver to recover meaningful digital information from the signals, the decoding sequence of chip intervals must be synchronized with the chip intervals used by the transmitter. Preferably, the preamble signal is employed to establish this synchronization. In the most preferred synchronization scheme, a transmitted clock signal having alternating high and low periods each one half of the duration of one of the chip intervals used in the chip sequences. These half chip intervals are referred to herein as "crumb intervals." Preferably, the clock signal is combined at the transmitter with preamble chip sequences by a particular combination scheme referred to as "Manchester encoding." In this combination scheme, the two crumb intervals within each chip interval will always have different binary values, and hence the parameter of the transmitted signal will differ from one crumb interval to another within each chip interval. The receiver includes clock synchronization means which generates several, typically four, separate series of chip intervals so that each series of chip intervals is offset in time from the next series, preferably by an offset interval equal to one fourth of the duration of a chip interval. The clock synchronization means also includes means for subdividing each chip interval in each of these four separate series into equal crumb intervals. Crumb level comparison means are provided for determining whether each of the separate series of chip intervals satisfies the condition that the value of the detector signal, and hence the average value of the parameter in the transmitted signal, differ from one another for the two crumb intervals in each chip interval. A series of chip intervals which meets this condition is in synchronization with the chip intervals of the transmitted signal. That series is selected and used as the sequence of chip intervals for decoding subsequent portions of the signal. Stated another way, the clock synchronization means in the receiver tries several different series of chip intervals, and uses the Manchester-encoded transmitted clock signal and preamble bits to check the synchronization of each series.

This particularly preferred synchronization scheme is a special case of a more general scheme which can be used according to this aspect of the invention. In the general scheme, the transmitter includes means for generating a clock signal having discrete values for different crumb intervals, each crumb interval being a fraction of the chip interval, and means for combining this clock signal with the preamble chips so that a predetermined sequence of discrete clock signal values is impressed on the transmitted signal during each chip interval. The receiver includes means for generating plural separate series of chip intervals each offset from the others, subdividing the chip intervals of each said separate series into crumb intervals and performing a matching test on the detector signal for the crumb intervals of each chip interval in each separate series against a template corresponding to the predetermined sequence of clock signal values used by the transmitter. The receiver selects the particular series for which the detector signal best matches the template and uses the so-selected sequence of chip intervals for decoding the remainder of the signal. Desirably, a small even number of crumb intervals are included in each chip interval.

Yet another aspect of the present invention relates to specific features of the decoding apparatus which can be used, for example, as the decoder means of a receiver in the aforementioned systems. The decoder receives an input stream of digital data chips. As received by the decoder, each chip originally has either a first or second binary value, i.e., 1 or 0. A predetermined sequence of chips, N-chips long denotes a valid first bit value, as, for example, a binary 1 bit. The decoder preferably includes transform means selecting successive N-chip sequences from the input data stream and transforming the original values of each chip in each selected N-chip sequence into either a first analog output or a second analog output such that when a sequence includes the series of first and second binary values indicating a valid first bit, every one of the analog values will be equal to the first analog value. Thus, two different transformation schemes may be applied to the original chip values in each selected sequence According to a first or "non-inversion" transformation scheme, an original chip value equal to the first binary value will yield the first analog output whereas an original chip value equal to the second binary value will yield the second analog output. According to the second or "inversion" transformation scheme, the reverse correlation applies. A chip value equal to the first binary value yields the second analog output, and vice-versa. These two different transformation schemes are applied to the chips in each selected N-chip sequence according to the positions of the chips in the sequence. The first or non-inversion transformation is applied to chips in positions within the selected sequence corresponding to the positions of first-value chips in the predetermined sequence denoting a valid first bit value. The second transformation is applied to chips occupying positions in the selected N-chip sequence corresponding to the positions occupied by second-value chips in the same predetermined sequence denoting a valid first bit value.

Merging means are provided for merging all of the analog outputs from the transform means to form a composite analog output. Where a valid first bit value sequence is selected, all of the analog outputs will be equal to the first analog value and hence the composite analog output will be equal to a standard value corresponding to merger of N analog output signals each equal to the first analog value. If any other sequence of chips is selected, at least some of the analog outputs will be equal to the second analog