Signal demodulation method and apparatus

A method and apparatus for separating multiple frequency information signals. This invention provides a method and apparatus for separating a communications signal, which is comprised of multiple component information waves of distinct frequencies, into its component waves for demodulation. The extraction of the component information waves is accomplished by determining the mid-cycle zero crossing points of the component information waves by reference to a reference wave, determining the amplitude of the combined wave at these zero crossing points and generating amplitude equations for each of these zero crossing points. These amplitude equations are solved to determine the amplitude coefficients of the component information waves, thereby extracting the transmitted information.

FIELD OF THE INVENTION
 This invention is the field of filters for electromagnetic signals and in
 particular the field of filter systems for separating the components of
 multiple frequency signals.
 BACKGROUND OF THE INVENTION
 The conventional method for separating out an information signal of a
 specific frequency from signals of other frequencies, which together with
 the specific frequency comprise a combined signal, and from noise, is a
 filter comprised of inductor and capacitor circuits. Such filters
 interfere with and alter the wave and, therefore, fail to reproduce the
 wave in the pure form in which it was transmitted. The bandwidth of the
 retrieved wave is substantially larger than the bandwidth of the wave as
 transmitted. It should be noted, however, that the increase in the
 bandwidth is not a function of the propagation of the wave. It is a
 function of the filtering process.
 Another problem with conventional filters is that they require multiple
 cycles of the input signal in order to ramp up. Until the ramping up is
 complete, the filtering will be ineffective. This substantially limits the
 amount of data that can be transmitted by multiple frequency signals and
 extracted by conventional filters. A filtering method is needed which will
 allow a single cycle to be filtered and data to be accurately extracted
 from each cycle.
 The inventor herein has had three prior patents issued which disclose
 methods and apparatuses for digital information transfer. They are U.S.
 Pat. Nos. 5,517,528, 5,640,422, and 5,689,529 to Johnson, and are
 hereinafter referred to as the "prior related patents". Also, a prior
 application, U.S. application Ser. No. 08/879,755, has been filed by the
 inventor herein which discloses certain methods and apparatuses for
 filtering electromagnetic signals. The present application discloses an
 additional method and an additional apparatus for filtering
 electromagnetic signals.
 Continuous combined information waves which are simultaneously generated at
 multiple frequencies, combined and transmitted as a single information
 signal are disclosed in the prior related applications and patents.
 Advanced filter systems which allow the utilization of the capabilities of
 the inventions disclosed in the prior related applications and patents are
 also disclosed in the prior related applications and patents. However, a
 filter method and apparatus is needed which will provide for minimizing
 the complexity of the signal separation process, minimizing the bandwidth
 of the extracted multiple frequency components, and enhancing the accuracy
 of the extracted data.
 The limitations of conventional signal extraction methods also impose
 substantial limitations on digital information storage technology. Current
 digital technology allows only one bit per cycle to be transferred from a
 read/write head to a magnetic storage device such as a hard drive, a
 magnetic disc, or an external magnetic drive. These current storage
 systems operate using analog functions. In addition, as data is
 transferred, the spacing between the storage medium and the read/write
 head changes, distorting or degrading the signal amplitude. When the
 distance between the head and the storage medium changes, the energy
 transfer rate is inversely proportional to the square of the distance.
 Therefore, any deviation of the read/write head changes the amplitude of
 the waves. For this reason digital recording procedures can measure only
 amplitude change. No information is derived from actual amplitudes.
 One objective of the present invention is to provide an improved and
 simplified method and apparatus for filtering and separating combined
 multiple frequency signals.
 Another objective of the present invention is to provide for separation of
 individual frequency components of multiple frequency signals while
 minimizing interference and minimizing the bandwidth of each of the
 extracted components, thus further enhancing the accuracy of the data
 transmitted by each of the extracted wave components.
 A still further objective of the present invention is to provide a method
 and apparatus for separating multiple frequency combined signals while
 minimizing bandwidth distortion.
 A still further objective of the present invention is to provide a method
 and apparatus for filtering simultaneous multiple frequency signals of
 distinct frequencies which is simpler, more reliable, and less costly than
 conventional methods.
 A still further objective of the present invention is to provide a filter
 method and apparatus which will increase the storage capacity of magnetic
 digital storage devices and will enhance the accuracy of data transfer
 between a read/write head and a magnetic storage device.
 A still further objective of the present invention is to provide an
 effective filter method and apparatus which is adapted for use with the
 information transfer receiver apparatuses disclosed in the prior related
 applications.
 SUMMARY OF INVENTION
 The present invention provides an improved method and apparatus for
 filtering and separating a received multiple frequency combined signal
 into its various frequency components. It may be used in conjunction with
 the digital information transfer inventions disclosed in the prior related
 applications and patents.
 The method of the present invention comprises a method of extracting
 information from an electromagnetic signal which is comprised of two or
 more waves of distinct frequencies. Under certain preferred embodiments
 the combined information wave is comprised of a reference wave with a
 known wave form, amplitude, frequency and phase and one or more component
 information waves with known wave forms and frequencies and a known phase
 relationship to the reference wave. The frequencies of each of the
 component information waves is distinct from the frequency of each of the
 other component information waves and the reference wave. For certain
 preferred embodiments the frequency of the reference wave is sufficiently
 less than the frequencies of the component information waves so that it
 can be separated from the rest of the combined information wave by a
 typical narrow bandpass filter. The ratio of the amplitude of each cycle
 of each respective component information wave to the amplitude of the
 reference wave is a function of the digital data being transmitted by that
 component information wave.
 Rather than consisting of an arrangement of narrow band pass filters and
 other necessary components as in a conventional filter system or
 consisting of the special filters disclosed in the prior related
 applications and patents, certain preferred embodiments of the method of
 the present invention simply requires one or more computer circuits which
 perform a mathematical analysis on the received combined signal.
 The first step in extracting the information from the combined information
 wave as received by the receiving apparatus is to store the combined
 information wave for processing. The combined information wave is then
 passed through a narrow bandpass filter with frequency selected to match
 the frequency of the reference wave, thereby extracting the reference wave
 from the combined information wave. Next, the zero crossing points of the
 reference wave are determined by locating points on the wave which are
 half way between the positive apex and the negative apex of the wave. Once
 the zero crossing points of the reference wave have been determined, since
 the frequencies and the phase relationship of the component information
 waves with respect to the reference wave are known the zero crossing
 points of each of the component information waves can be determined.
 Amplitude equations are then generated for each of the component
 information wave zero crossing points. Each of these equations is in the
 form of the sum of the unknown amplitude of each non-zero component
 information wave as multiplied by its known phase factor at that point
 which together equal the observed amplitude at the component information
 wave zero crossing point in question. This process results in the
 generation of linear amplitude equations with the number of equations and
 number of unknowns being equal to the number of component information
 waves. Next, these equations are solved for the unknown amplitudes thereby
 extracting the transmitted digital information corresponding to each
 component information wave.
 The receiving apparatus of certain preferred embodiments of the present
 invention includes a combined wave analog to digital converter to digitize
 the combined information wave as it is received and a digital storage
 medium for storing the digitized wave. A narrow bandpass filter extracts
 the reference wave from the combined information wave and then the
 reference wave is also digitized by a reference wave digitizer and passed
 to a reference zero crossing computer where the zero crossing points are
 located on the wave at points where the wave is half way between the
 positive wave apex and the negative wave apex. The residual combined
 information wave which is the combined information wave with the reference
 wave extracted passes to the residual wave digitizer and then is stored in
 the residual wave storage device. The digitized residual wave is then
 passed along with the reference wave zero crossing point information to
 the amplitude equation computer which generates and solves the amplitude
 equations thereby determining the amplitudes of the component information
 waves.
 An embodiment of a transmission apparatus which can be used to generate and
 transmit the combined information wave for which the present invention was
 designed to extract information, would include a reference wave generator,
 a digital to analog generator for converting to analog the digital signals
 which are to be transmitted by each component information wave, a gain
 control device for adjusting the amplitude of the combined information
 wave and a transmitter for transmitting the amplitude adjusted combined
 information wave. Embodiments of transmission methods and apparatuses for
 generating and transmitting the combined information waves from which the
 present invention is designed to extract information are disclosed in the
 prior related inventions and applications.
 A reference wave can be a discontinuous and periodically repeated wave, a
 discontinuous wave which is repeated with the same frequency as the
 component information waves, or a continuous wave. In any event, for most
 preferred embodiments it would have a pre-set amplitude.
 For certain preferred embodiments of the present invention, the ratio of
 the amplitude of a component information wave to the amplitude of the
 reference wave is determined by the magnitude of the digital information
 carried by that component information wave. However, the amplitude of the
 component information wave may merely be a function of the digital data
 transmitted. The amplitude of the component information waves can be
 adjusted at the receiver by comparing the amplitude of the reference wave
 as received to the known amplitude of the reference wave as transmitted.
 The filtration method of the present invention does not interfere with the
 combined wave or the component waves and the information component can be
 removed with little distortion and with a much narrower bandwidth as
 compared with conventional filters.
 Preferred embodiments of the present invention involve a mathematical
 analysis and the use of one or more computer circuits. For preferred
 embodiments, each component information wave has a known wave form and
 frequency. Also, under preferred embodiments, the relative phase or
 positioning of each component information wave with regard the reference
 wave is also known.
 Certain preferred embodiments utilize additional steps to enhance the
 extraction of the component information waves for certain applications.
 For these embodiments, the amplitude of the combined information wave is
 adjusted prior to transmission to a selected amplitude so that all cycles
 of the combined wave have the same pre-set amplitude. Since the amplitudes
 of the reference and information waves are adjusted proportionally when
 the combined wave is adjusted, the amplitude adjustment does not affect
 the accuracy of the information transmitted.
 In the same manner the amplitudes of all of the combined waves in a wave
 stream, can be adjusted so that they are all the same amplitude. This step
 allows the waves to be filtered at the receiver through conventional
 filters or the special filters disclosed in the prior related applications
 and patents. Because all the waves in a particular wave stream are of the
 same amplitude they can pass through a conventional filter. Also, because
 each half cycle is the same amplitude the zero crossing of the reference
 wave can be found by halving the amplitude. Also, a single cycle of the
 resultant wave can be stored and can be repeatedly sent through a filter.
 To the filter this will appear as a continuous wave and will pass through,
 while noise will be filtered out. The waves can then be separated using
 the methods disclosed.
 The step of making all of the combined waves the same amplitude is very
 useful in certain applications, especially those applications where
 passing the signal through conventional filters is required. This is the
 case for telephone line communications. It also has utility in wireless
 communication where communication signals such as the time slotted
 multiple frequency signals disclosed in the prior related applications can
 first be separated from other communication signals channels using regular
 filter techniques. Then the component information waves can be extracted
 using the methods disclosed for the present invention. This process will
 provide a cleaner wave for component wave extraction by the methods
 disclosed.
 A benefit to this type of modulation and these embodiments of the present
 invention is that whatever affects the combined wave affects the component
 information waves and reference wave proportionally. Therefore, the
 effects of noise and interference are minimized.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Referring first to FIG. 1, this figure illustrates a combined information
 wave 1 which is comprised of a reference wave 2 and three component
 information waves, a first component information wave 3, a second
 component information wave 4, and a third component information wave 5.
 The combined information wave can be comprised of a reference wave and any
 number of component information waves, subject only to the processing
 capability of the receiving apparatus. The amplitude of each cycle of each
 component information wave is a function of the digital information being
 transmitted by that component information wave. Under preferred
 embodiments the ratio of the amplitudes of each of the component
 information waves, the first component amplitude 6, the second component
 amplitude 7, and the third component amplitude 8, to the amplitude of the
 reference wave 9 is used to transmit the information. This allows
 amplitude adjustment of the combined information wave to facilitate
 transmission or to facilitate filtering or further processing of the
 signal at receiver. This also preserves the integrity of the data
 transmitted regardless of noise, attenuation or other interference with
 the signal, whether the signal was transmitted by wireless signal or other
 common media such as telephone lines or coaxial cable.
 The process of extracting the digital information carried by each component
 information wave begins, under certain preferred embodiments by splitting
 the combined information wave, and passing the split combined information
 wave 27 through a combined wave analog to digital converter 10, as shown
 in FIG. 2, thereby producing a digitized combined wave 11 and storing the
 wave for further processing in the combined wave storage 12.
 The split combined information wave is also passed through a narrow band
 pass filter, the reference wave filter 14, thereby extracting the
 reference wave from the combined information wave. The extracted reference
 wave 15 is then directed through a reference wave analog to digital
 converter 16, thereby producing a digitized reference wave 17. A digitized
 residual combined wave 18 is generated by a wave subtractor 13 which
 subtracts the digitized reference wave from the digitized combined wave.
 The next step process is the determination of the mid-cycle zero crossing
 point 28 of the reference wave as shown in FIG. 3. This is performed by
 determining the point on the wave which lies on the half way line 29 which
 is equidistant from the positive apex 30 and the negative apex 31 of the
 reference wave. This is accomplished by the reference wave zero crossing
 determinator 19 shown in FIG. 2. Once the reference wave zero crossing
 point has been determined, the zero crossing points of each of the
 component information waves as shown in FIG. 4 can be determined by the
 component wave zero crossing determinator 20 as shown in FIG. 2 through
 the use of the reference wave zero crossing data 21 and the digitized
 residual combined wave. Referring to FIG. 4 again, the first component
 zero crossing point 33 of the first component information wave 3, the
 second component zero crossing point 34 of the second component
 information wave 4, and the third component zero crossing point 35 of the
 third component information wave 5, can be determined by the component
 wave zero crossing determinator because the phase relationship between
 each of the component information waves and the reference wave is known.
 The first component zero crossing amplitude 36, the second component zero
 crossing amplitude 37, and the third component zero crossing amplitude 38
 are determined by the component zero crossing amplitude determinator 23
 shown in FIG. 2 from the residual combined information wave 32 shown in
 FIG. 4 through the use of the component wave zero crossing data 22. The
 component zero crossing amplitude data 24 is used to generate an amplitude
 equation for each of the zero crossing points.
 For the example illustrated in FIG. 1, which is a combined information wave
 comprised of a sinusoidal reference wave and sinusoidal first combined
 component information wave, second component information wave and third
 component information wave, the waves each have the general form of
 a*sin(.omega.t+.phi.) with a being the amplitude coefficient (maximum
 amplitude) of the wave, .omega. being the frequency, and .phi. being the
 phase displacement. Since the phase relationship between the reference
 wave and each of the component information waves is known, amplitude
 equations are generated and solved by the amplitude equation solver 25
 from the component zero crossing amplitude data 24 as shown in FIG. 2. The
 only unknown for each component information wave is the amplitude
 coefficient for a given cycle. The amplitude equations which are generated
 for the example illustrated in FIG. 1 and FIG. 4 are as follows:
EQU a.sub.1 c.sub.1 +a.sub.2 c.sub.2 =A.sub.1.
EQU a.sub.2 c.sub.3 +a.sub.3 c.sub.4 =A.sub.2
EQU a.sub.1 c.sub.5 +a.sub.3 c.sub.6 =A.sub.3
 For the amplitude equations illustrated above, the only unknowns are the
 amplitude coefficients of the component information waves, namely a.sub.1,
 a.sub.2 and a.sub.3. Those three unknown component information amplitude
 coefficients 26 can be determined from the solution of these three linear
 equations by the amplitude equation solver 25 as shown in FIG. 2.
 The present invention may be utilized with component information waves
 which are distributed in the cycle or keyed to the cycle of the reference
 wave in any selected manner. The component information waves for the
 embodiment shown in FIG. 1 and 4 are sinusoidal waves. However, the
 present invention can be utilized with component information waves of any
 selected wave form.
 For embodiments of the present invention used in conjunction with the
 inventions disclosed in the prior related applications, the process is
 repeated for combined information waves in successive cycles of the
 reference wave. In this way, multiple channels of combined information
 waves can each be successfully and accurately separated into its component
 information waves for demodulation.
 The speed at which the extraction process can be accomplished for the
 combined information wave received in each reference wave cycle will be
 dependent on the embodiment of hardware and software utilized.
 Other embodiments of the invention and other variations and modifications
 of the embodiments described above will be obvious to a person skilled in
 the art. Therefore, the foregoing is intended to be merely illustrative of
 the invention and the invention is limited only by the following claims.