405 results about "Wave modulation" patented technology

The present invention provides a spread spectrum clock generator that is capable of preventing phase jumps and jitters and suppressing the occurrence of Electro Magnetic Interference components and that can easily be applied to large scale integrated circuits. The spread spectrum clock generator can be configured with a filter, quantizer, fractional divider, and other elements. Also, this clock generator circuitry can be configured by combination of a delta-sigma ΔΣ quantizer and factional divider so that sine wave modulation and random number modulation can be realized. Thereby, control with digital values can be performed. This clock generator prevents precipitous phase variations in the output high frequency clock and makes fine phase control possible. Consequently, EMI reduction by 20-30 dB can be expected.

The invention relates to a generation and capture system of an encrypted frame hopping spread spectrum signal of an air fleet link, belonging to the technical field of aeronautical data link and radio navigation. The invention discloses a system framework of an encrypted frame hopping spread spectrum signal generation and capture method which can be implemented on a digital signal processor (DSP)and a FPGA (field programmable gate array) of a circuit board, and provides a baseband signal generation algorithm of an encrypted frame hopping spread spectrum system to replace a traditional long periodic sequence spread spectrum code and a compound pseudo code by absorbing the advantages of the easy capture of the compound pseudo code and the difficult breaking of the long periodic sequence spread spectrum code; and meanwhile, the invention provides two advanced and high-performance encrypted frame hopping spread spectrum signal capture algorithms, i.e. a frequency domain parallel capture method of the encrypted frame hopping spread spectrum signal of based on a FFT (fast fourier transform algorithm) and a direct capture method of the encrypted frame hopping spread spectrum signal based on a matching filter and the FFT. The invention solves the problem of difficult capture of the traditional long periodic sequence spread spectrum code. The method can be widely applied to satellite navigation receivers, range measurement systems and communication systems based on a quiescent carrier modulation direct sequence spread spectrum system.

First and second carrier modulators each modulate a carrier having a different frequency from each other with a baseband input signal. First and second variable wavelength optical modulators each convert the modulated signal into an optical signal having a first or second wavelength. An optical multiplexer multiplexes the optical signals, and sends a multiplexed signal to an optical transmission line. A wavelength separator individually outputs wavelength components of the multiplexed signal. First and second optical receivers each convert these wavelength components into an electrical signal. First and second filters each pass only the signal components of each different frequency. First and second burst demodulators each demodulate the modulated signal. With such a structure, a large-capacity optical communication apparatus which is capable of simultaneously using the same wavelength without requiring wavelength management in optical transmitting circuits can be achieved at a low cost.

The invention discloses an SF6 decomposition gas infrared spectrum multi-component detection method and a device. According to the method, a chopped wave modulation wheel and a spectroscopical filtration wheel in a traditional non-dispersion infrared spectrum technology are combined into a whole, a modulation filtration wheel is utilized to realize simultaneous measurement of multi-component gas,and a long path multiple reflection absorption technology is combined simultaneously to increase detection sensitivity. The device comprises an optics unit, a gas circuit unit and an electronics unitarranged in a main machine box. The SF6 decomposition gas infrared spectrum multi-component detection method and the device are suitable for electrical equipment fault monitoring and diagnosing in anelectrical system with SF6 gas being an insulation and arc extinction medium. On-line gas concentration measurement with high speed, high sensitivity and non-interference is realized.

The invention provides a non-sinusoidal time domain orthogonal modulation method which is non-sinusoidal wave modulation based on orthogonal pulse transmission information. Signals to be transmitted are modulated onto a time domain orthogonal impulse group and impulse parameters are regulated to realize the spectrum shifting and the spectrum control of the modulated signals. The modulated signals are band-limited signals, the spectrum characteristic of which can be controlled. The modulated signals can be applied to any communication frequency band. Communication with relative large bandwidth or broadband communication can be realized, and communication with relative small bandwidth or narrowband communication can be realized. In spectrum management, the method can be compatible with an existing frequency band-division and channel-division communication mode of a relative small bandwidth communication system. And the frequency band utilization rate of the system can infinitely approach Nyquist rate with the increase of orthogonal impulses.

The invention discloses a three-dimensional structure metamaterial which comprises at least one layer of forming base material and at least one flexible function layer arranged on the surface of the forming base material. Each flexible function layer comprises a flexible substrate formed by at least one flexible sub-substrate and a plurality of manmade micro-structures corresponding to electromagnetic waves and arranged on the surface of each flexible sub-substrate. The three-dimensional structure metamaterial has an electromagnetic wave modulation function. The three-dimensional structure metamaterial is simple in manufacturing process and low in processing cost, process precision is controlled simply, and the three-dimensional structure metamaterial can replace various structure parts provided with complex curved surfaces and needing to have a certain electromagnetic modulation functions and can also be attached to various structure parts provided with the complex curved surfaces to achieve the required electromagnetic modulation functions. Besides, the three-dimensional structure metamaterial has good electromagnetic response and wide application range in a curved surface expanding and electromagnetic partition mode.

Methods, systems, and devices are described for sensing a phase-cut dimming signal and outputting a control signal compatible with a switching power circuit. Embodiments of the invention generate at least one of a low-frequency pulse-wave-modulated control signal, an analog output control signal, or a digital (e.g., higher-frequency pulse-wave-modulated) output control signal. Some embodiments further provide preloading and / or startup control functionality to allow proper functioning of the circuitry under small-conduction-angle (i.e., highly dimmed) conditions.

The invention relates to a high power LED drive circuit with the emergency function, comprising a rectifying and filtering unit, a reverse excitation unit, a constant current driving unit and an emergency unit, wherein the rectifying and filtering unit is connected with the input end of the LED drive circuit and is used for rectifying and filtering the input alternating current signals; the reverse excitation unit is connected with the rectifying and filtering unit and is used for PWM wave modulation according to load change; the constant current driving unit is connected with the reverse excitation unit and is used for driving the LED to work at constant current; and the emergency unit is connected with the reverse excitation unit and provides power to the constant current driving unit when no alternating current input is detected by detecting the current of the reverse excitation unit. The LED drive circuit is simple and can automatically switch to the emergency power supply so as not to go out even though the mains supply has failure. The leakage current of the cells in the circuit is almost zero, thus ensuring the standby working time of the cells.

The invention provides a programmable optical component based on a scattering medium, an optical field regulation system, and an optical field regulation method, and solves technical problems that a conventional optical component has single function, is high in optical wave modulation cost, has complex apparatus and low integration degree. The system in the invention is successively provided with a light source beam expanding module, a modulation module, a scattering medium microscopic module and a detection module, a data processing module arranged between the modulation module and the detection module. A computer circularly controls a space light modulator to modulate an input optical field, and a detector detects an output optical field; then data shaping is carried out and an optical transmission matrix of the scattering medium is detected; and then a programmable optical component is constructed so as to achieve optical field regulation with methods such as optical phase conjugation, phase retrieval, speckle reconstruction and the like, wherein the optical field regulation includes imaging, chromatic dispersion, abnormal chromatic dispersion, focusing, shaping, etc. The programmable optical component and the system are intelligent, multi-functional, low-cost, easy to operate, highly integrated and strong in operability, and can be applied to the scientific research fields such as military science, bio-medicines, integrated optic, nano photonics, etc.

The invention belongs to the reflection system technical field and relate to a broadband tunable gradient meta-surface-based reflection electromagnetic wave modulator and a design method thereof. A meta unit periodically extends in a two-dimensional plane so as to form the reflection electromagnetic wave modulator (broadband TGMS) of the invention; six TGMS (tunable gradient meta-surface) units with different structural parameters and phases are arranged according to a size sequence so as to form the meta unit which has broadband phase gradient; and the TGMS unit is mainly composed of an upper layer main resonator and auxiliary resonator as well as a middle medium plate and a lower layer metal grounding plate. According to the broadband tunable gradient meta-surface-based reflection electromagnetic wave modulator and the design method thereof of the invention, varactors and double-resonance structures are introduced into the gradient meta-surface units, and therefore, compensation of phase dispersion and continuous regulation of phases can be realized; the switching of surface wave conversion and beam deflection is realized, and beam deflection has very high conversion efficiency in a wide frequency range.

The invention relates to a method and a system for improving phase stability of a photon microwave signal. The method comprises the following steps that: light output by a laser device is divided into two beams of vertical polarized light by a polarized light beam splitter; a reference light is combined with transmission light via a polarized light beam combiner after frequency-shift, and passes through the polarized light beam splitter after passing through a light micro-wave modulation and a light delay link, finally the reference light is interfered by the transmission light after being rotated and polarized by a faraday rotation mirror, and then a beat frequency signal is detected through a low-speed photodetector; the beat frequency signal is subjected to phase comparison with a 25MHz reference signal to obtain a phase disturbance situation of a transmission system, and the signal output by a phase discriminator is used as a control signal of a microwave photonic phase shifter after being processed. The system comprises a DFB (distributed feedback) laser device, the polarized light beam splitter, an acoustooptic frequency shifter, the polarized light beam combiner, an RF (radio frequency) signal generator, a Mach-Zehnder modulator, the photon microwave signal phase shifter, the polarized light beam splitter, an optical interferometer, a low-frequency optical receiver phase discriminator and a proportion integrator.

A digital-to-analog converter (“DAC”) system utilizes chopping modulation technology to remove 1 / f and other baseband noise from a baseband of a signal of interest. Chopping modulation and demodulation circuitry of the DAC operate at a chopping frequency equal to approximately one-half of a digital input signal sampling frequency. Chopping at one-half the sampling frequency allows fold back into the baseband of the input signal's frequency components and reduces fold back of noise, such as quantization noise, residing outside the baseband. In a further embodiment, a notch filter attenuates signals having frequencies around the chopping frequency prior to chopping to reduce fold back of noise into the baseband due to parasitic modulation. Coordination of chopping timing also reduces noises in the output of the DAC system.

For transmitting data, a receiving / backscattering arrangement receives, modulates and reflects or backscatters electromagnetic waves emitted by a base station. The modulation corresponds to the data to be transmitted and is carried out selectively using first and / or second different modulation methods depending on the received field strength of the received electromagnetic waves. Preferably, phase shift keying is used especially or at least at low field strengths at far range, while amplitude shift keying is used additionally or alternatively for high field strengths at close range. The two modulation methods can be superimposed. A circuit arrangement includes two different modulator arrangements to perform the two modulation methods depending on the received field strength. The second modulator arrangement preferably comprises a multi-stage voltage multiplier circuit with a modulated switching device intervening in one of the stages to achieve the modulation.

The invention provides an artificial super-surface electromagnetic wave amplitude modulator based on graphene, and mainly solves the problems of small modulation scope and narrow modulation bandwidth of a conventional electromagnetic wave amplitude modulator. The artificial super-surface electromagnetic wave amplitude modulator comprises a DC power supply and a frequency selection surface, three layers of dielectric substrates and a reflecting plate laminated from top to bottom sequentially, wherein a dielectric layer with relative dielectric constant less than three is arranged between the second dielectric layer and a reflecting plate to expand wave absorption bandwidth, the frequency selection surface is formed by m*n cross period units, a rectangular-ring metal paster embedded in a graphene film is arranged in the center of each cross period unit, metallic branches are connected to the upper and lower sides of the rectangular-ring metal paster, and the horizontal metallic fine leads connect the single cross units in an entire row into one body. The structure is simple, and metal pasters embedded in the graphene films are used to widen the modulation bandwidth and increase the reflection wave modulation amplitude. The reflecting plate with high conductivity is used to reduce the transmission of electromagnetic waves by a wave absorption device. The artificial super-surface electromagnetic wave amplitude modulator is applicable to active electromagnetic hiding and electro-optical switch equipment.

The invention relates to a driving system of a stepping motor with the current negative feedback, which is characterized in that: external signals pass through an optoelectronic isolating circuit and then are inputted into a control circuit. According to the detected signals, the control circuit generates four channels of input signals of A, / A, B and / B; the four channels of the signals, together with two channels of complementary and unipolar PWM waves which are generated by a PWM wave modulation circuit, enter a logic composite circuit, and then the logic composite circuit generates eight channels of outputs and the eight channels of the outputs are sent to a power driving circuit; the power driving circuit inputs control signals into the stepping motor; meanwhile, return signals on a sampling resistance respectively pass through a current negative feedback circuit to adjust the PWM wave modulation circuit, and pass through a current protection circuit to adjust the control circuit. The driving system of the invention has the advantages that a feedback element is added into the previous driving system of the stepping motor to ensure the steady phase current within a certain frequency ranges so as to realize the steady load capacity and more stable operation.

The invention discloses a simulation method of a bistatic synthetic aperture radar sea wave direction spectrum, which comprises the following steps of calculating a JONSWAP sea wave spectrum and a sea wave vertical displacement according to sea wave parameters; obtaining BiSAR echo signals comprising sea wave orbital velocity and accelerated velocity according to a radar wave modulation principle and BiSAR system parameters; carrying the matched filtering on the echo signals according to a BiSAR distance and Doppler imaging algorithm to obtain an analysis formula of a BiSAR sea wave image; working out the BiSAR sea wave image by adopting a numerical integration method; and at last, carrying out two-dimensional FFT operation on the BiSAR sea wave image to obtain the BiSAR sea wave direction spectrum. The BiSAR sea wave image can be worked out by adopting the numerical integration method without producing the echo signals and processing the image in a time domain, therefore, the invention has higher operation efficiency.

The invention relates to the field of strong directivity parametric array loudspeaker. In the loudspeaker, audio signal and ultrasonic carrier signal are modulated, and by utilizing the strong directivity of the ultrasonic carrier, the audio signal beam with strong directivity is obtained. In particular, the invention relates to a parametric array loudspeaker and a method for treating signal thereof; the method uses conversion transducer units with different frequencies to form a transmitting array to transmit the ultrasonic carrier modulation signal after power amplification, thereby improving ultrasonic radiant intensity, outputting stable enough signal, making audio signal select a proper carrier frequency, further expanding frequency response of audio frequency and improving loudspeaker performance; and the method can be widely applied to the filed of controlling noise jamming.

The invention discloses a high-voltage high-capacity impulse voltage generator. The impulse voltage generator comprises a charging capacitor, a wave head resistance, a wave tail resistance, a voltage divider, a load and a sphere gap; the impulse voltage generator is provided with a plurality of impulse voltage generating units consisting of the charging capacitor, the wave head resistor and the wave tail resistor; and the wave head resistor is arranged in a discharging loop. The impulse voltage generator is characterized in that: in partial or all impulse voltage generating units, wave modulation capacitors are arranged in parallel at two ends of the wave head resistor. The generator overcomes the defect that the wave modulation resistor of the traditional method cannot generate standard thunder impulse, can effectively inhibit adverse effect generated by stray inductance, and generates standard thunder impulse voltage; and the generator can generate the standard thunder impulse by increasing a few elements based on the conventional equipment, and has the advantages of small volume, low cost and the like.

The invention provides a modularization multi-level converter submodule grouping stair wave modulation method. The method comprises the steps of dividing submodules in a commutation chain into N submodule groups, rounding reference voltages of the submodule groups to obtain rounded reference stair wave voltages Vstair_wave, distributing trigger pulses to submodules inside the submodule groups, and carrying out voltage-sharing among the submodule groups. According to the modularization multi-level converter submodule grouping step wave modulation method, the submodules are divided into the N submodule groups, each submodule group is taken as a controllable voltage source, stair wave modulation is adopted in the submodule groups, the N submodule groups adopt rounding correction, and therefore a purpose similar to carrier phase-shifting is achieved; and due to the fact that voltage stability control is carried out among the submodule groups by means of a proper voltage-sharing control method, difficulty of the stair wave modulation is reduced, and requirements of the modulation algorithm and the voltage-sharing control algorithm for software and hardware are greatly reduced.

The invention discloses a microstrip resonance structure Terahertz wave modulation apparatus and the method thereof. The apparatus comprises a Terahertz wave source, a microstrip resonance structure array, a high-resistance silicon substrate, a Terahertz wave detector and a modulatable semiconductor laser; the microstrip resonance structure array is sputtered on the high-resistance silicon substrate; the Terahertz wav from the Terahertz wave source is propagated to the microstrip resonance structure array, and is propagated out through the Terahertz wave detector by the Terahertz wave modulated by the high resistance silicon substrate; the modulatable semiconductor laser is disposed on the same direction of the Terahertz wave source; the included angle between the laser shot direction of the modulatable semiconductor laser and the incidence direction of the Terahertz wave from the Terahertz wave source is 0-30 degrees; and the microstrip resonance structure array is composed by a plurality of microstrip resonance structure units. The invention achieves the advantages of high modulation speed, large modulation bandwidth, small size, compact, easily machined, and simple and convenient operation; is not sensitive to Terahertz wave, can meet the high speed communication demand of Terahertz wave.

The invention discloses a modulation method of a three-phase Z-source neutral point clamped multi-level photovoltaic inverter. The method comprises the following steps of: obtaining line voltage signals Vac*, Vba* and Vcb* through initial modulating waves Va*, Vb* and Vc* and multiplying by a coefficient K respectively; obtaining a zero sequence signal v0 through a zero sequence signal calculating unit; adding zero-sequence signal with KVac*, KVba* and KVcb* to obtain final modulating waves Va**, Vb**, and Vc**, and comparing the modulating waves Va**, Vb** and Vc** with a carrier wave to obtain logical signals Sa, Sb and Sc; comparing the carrier wave with VH and VL to obtain a through connect signal ST; and making the logical signals Sa, Sb and Sc and the through connect signal ST pass through a logic circuit to obtain a switching signal. The invention has the beneficial effects that a single-carrier-wave modulation method is provided, so that the problem of excessive complexity in realizing the conventional multi-carrier-wave modulation and space vector modulation is solved; and meanwhile, the common-mode voltage of a system is kept constant, so that the drain current of the system is effectively restrained. A switching signal generating circuit is simple in structure, and can be realized by adopting an analog circuit as well as a digital circuit.

The invention relates to an amplifier with ultralow direct current (DC) offset at an input end and an analog / digital (A / D) converter. The amplifier with the ultralow DC offset at the input end comprises a chopped wave modulator, a sampling instrument, a correlated double sampling (CDS) instrument and an amplifying / integration unit which are connected in sequence. A chopped wave demodulator is arranged in a circuit behind the sampling instrument. The chopped wave modulator and the chopped wave demodulator are used for eliminating residual DC offset generated due to imperfection of the CDS instrument and the other circuit components. On the other hand, the CDS instrument can also eliminate residual DC offset of the chopped wave modulator and the chopped wave demodulator. Due to the fact that the chopped wave modulator and the chopped wave demodulator modulate the residual DC offset of the CDS instrument and other circuit components at the clock frequency of 2*ck_chop to generate high-frequency modulating signals, input signals and the modulating signals do not overlap in a frequency domain. Consequently, the modulating signals can be eliminated at a low-pass filter which is connected with an output end of the amplifier with the ultralow DC offset at the input end. Therefore, the purpose of eliminating the residual DC offset of sampling signals is achieved.

The invention belongs to the technical field of amplifiers, and particularly relates to a current feedback chopper modulation instrument amplifier working under a micro quiescent current. The amplifier consists of a blocking condenser, a current feedback chopper amplifier, an N-bit mismatch compensation capacitor array, a ripple canceling circuit, a biasing circuit and a clock frequency dividing circuit. The microcurrent and current feedback chopper modulation instrument amplifier has the characteristics of alternating current coupling, high input impedance, ultra-low offset voltage, low noise, high common mode rejection ratio, high power supply rejection ratio, micro-power consumption and the like; the circuit is particularly suitable for a wearable health monitoring system biopotential acquisition circuit adopting dry electrodes, and can eliminate semi-potential imbalance between electrodes in a rail-to-rail mode. The simulation result of one embodiment of the invention shows that the common-mode rejection ratio of the instrument amplifier is greater than 120 dB, the equivalent input impedance is greater than 500 M Ohm, and the noise energy efficiency factor NEF is equal to 4.5.

The invention discloses a double-shaft multiplexing optical fiber gyro and a signal modulation / demodulation method thereof; two optical fiber rings which have 1:3 of optical fiber length and have mutually-vertical sensitive shafts are used on hardware configuration for carrying out parallel multiplexing by an optical fiber coupler; in the signal modulation / demodulation method, a square wave modulating signal is adopted to demodulate, the cycle of the square wave modulating signal is equal to the accumulated value of light transmission time in the two optical fiber rings, the duty ratio s 50 percent, the peak-to-peak value is not equal to the voltage value of multiple half-wave voltages, and the mean value is zero; in the cycle of the square wave modulating signal, the output signal of the optical fiber gyro is divided into four parts with equal length according to time sequence, and the signal in each part is sampled, according to the demodulation formula, the sampling values are calculated, so as to obtain angular velocity information with two different directions. The method can effectively reduce the equivalent cost of the double-shaft multiplexing optical fiber gyro and has high application value and popularization value.

The invention discloses a compact closed gas insulation impulse voltage generation device. The compact closed gas insulation impulse voltage generation device comprises a charging device and a wave modulation device and further comprises a sealed insulation shell, a frame type insulation support, a first shed insulator, a second shed insulator, a fifth shed insulator and a gas insulation multilevel impulse voltage generator, wherein the gas insulation multilevel impulse voltage generator is fixed in the sealed insulation shell through the frame type insulation support, the sealed insulation shell is provided with an inflation and deflation port, and the sealed insulation shell is grounded. The size and weight of the device can be reduced effectively, and the compact closed gas insulation impulse voltage generation device can adapt to a field test of ultrahigh / extra-high voltage substation equipment.

The invention relates to a method for enlarging a range of a high-precision closed loop fiber-optic gyroscope assisted by MEMS (Micro-electromechanical Systems), which is particularly suitable for the high-precision closed loop fiber-optic gyroscope working in a large angular speed range and existing under a large-angle acceleration input environment. The method comprises the following steps: coaxially mounting an MEMS gyroscope and the high-precision closed loop fiber-optic gyroscope; sensitively inputting an angular speed; utilizing a closed loop digital phase step wave modulation-demodulation detection method to obtain an output angular speed of the high-precision closed loop fiber-optic gyroscope; collecting the output angular speed of the MEMS gyroscope; revising the high-precision closed loop fiber-optic gyroscope output of the fiber-optic gyroscope. According to the method provided by the invention, an output result of the high-precision closed loop fiber-optic gyroscope is revised according to a difference value so that the high-precision fiber-optic gyroscope can work normally under the condition of high angular speed, the aim of enlarging the range of the closed loop fiber-optic gyroscope is realized and the application of the high-precision fiber-optic gyroscope is expanded.

The invention discloses a method for measuring the eigen frequency of a fiber-optic gyroscope based on sawtooth-wave modulation of even-multiple eigen requency, and belongs to the field of fiber-optic gyroscopes. The measurement method comprises the steps that a sawtooth-wave phase modulation signal with the frequency close to the eigen frequency of the even-multiple fiber-optic gyroscope is modulated and generated, and then the signal is superimposed on a square-wave bias modulation signal to generate a superposed modulation signal; two beams of light emitted and transmitted by an optical-fiber sensitive ring are subjected to the phase modulation of the superposed modulation signal, and interference occurs; an interference signal is detected to obtain a phase error generated by the two beams of light during the phase modulation; the frequency of the sawtooth-wave phase modulation signal is changed, the phase error is detected, when the phase error is zero, the frequency of the sawtooth-wave phase modulation signal is exactly even multiples of the eigen frequency, and therefore the eigen frequency of the fiber-optic gyroscope is obtained. Without changing the basic structure of the original digital closed-loop fiber-optic gyroscope, the method for measuring the eigen frequency of the fiber-optic gyroscope can achieve the high-precision measurement of the eigen frequency and provide a means for relative researches.

The present invention discloses a real-time normalization apparatus and method of the PGC demodulation in a sinusoidal phase modulation interferometer. An optical setup containing a measuring interferometer and a monitoring interferometer is constructed. An electro-optic phase modulator is placed in the common reference arm of the two interferometers. High-frequency sinusoidal wave modulation and low-frequency triangular wave modulation are applied to the electro-optic phase modulator at the same time. Sinusoidal modulation is used for generating phase carrier, and PGC demodulation is performed to obtain quadrature signals containing the phase information to be measured. Triangular wave modulation makes the quadrature signals change periodically. Ellipse fitting is performed on the Lissajous figure corresponding to the quadrature signals, and real-time normalization of the PGC demodulated quadrature signals is achieved. By calculating the variation of the phase difference between the two interference signals, the measured displacement is obtained, and nanometer scale displacement measurement is achieved.

A system and method serves to generate a composite audio signal capable of modulating a radio frequency carrier to a standard television channel for cable transmission. The system includes inputs for a first audio signal, a second audio signal, and a video signal. A third audio signal is generated which is the sum of the audio signals. A fourth audio signal is generated from the difference of the audio signals. The fourth audio signal is pre-emphasized twice and a timing signal is extracted. A first frequency pilot signal is generated from the timing signal. A second frequency subcarrier signal is also generated from the timing signal. The pre-emphasized fourth audio signal is compressed and modulated. The composite audio signal is generated from the third audio signal, the compressed and modulated fourth audio signal, and the pilot signal.