Showing papers on "Demodulation published in 1977"

Simplified MSK Signaling Technique

Abstract: Modulation and demodulation in the minimum (frequency)-shift keying (MSK) format are reduced to the form of coherent biphase keying. Considerable simplification of circuitry is thus effected without sacrifice of performance. When the ratio of the carrier frequency to the bit rate is high, then the technique described here does not require the precise relative phasing of any pair of oscillators in the transmitter. Experimental results demonstrate the feasibility of synthesizing the one filter that is unique to this signaling scheme. In the present instance, the filter is realized as a surface acoustic wave device.

Software Implementation of a PN Spread Spectrum Receiver to Accommodate Dynamics

Abstract: To optimize the threshold of a pseudonoise (PN) spread spectrum modem for use over an aircraft/satellite communications link at SHF, the effects of Doppler must be taken into account. Reconstitution of carrier phase by a Costas loop to coherently demodulate the PSK data and also the delay-lock error voltage has typically been the practice in PN modems intended for ground applications. To accommodate the platform dynamics, the Costas loop must have a relatively wide bandwidth, and this implies a significant threshold degradation. An alternate implementation employs a noncoherent carrier tracking loop which maintains frequency lock rather than phase lock. Now, the delay-lock error voltage is noncoherently demodulated. For the airborne application, analysis and simulations show this implementation will extend the receiver's tracking threshold significantly (up to 6 dB) for the worst case dynamics profile. An experimental project was undertaken to modify an existing ground PN modem (AN/USC-28, ADM version) for flight test. A software implementation of the digital tracking algorithms was selected where a HP-2100A minicomputer controls carrier frequency and PN code phase via digital phase shifters. The Costas demodulator for extracting PSK data resides entirely in software, and is completely segregated from PN tracking. In laboratory testing of the receiver with simulated dynamics and in actual flight tests, the demonstrated performance was found to approach closely the goals established by the analyses and simulations.

Filter and demodulation arrangement

Abstract: A digital filter and demodulation arrangement for passband signals, whose corresponding baseband signal has a bandwidth limited to a given maximum frequency The passband signals are filtered according to two bandpass characteristics which, apart from their asymmetrical distortion relative to their central frequency, are versions from one another shifted 90° in phase The filtered passband signals are demodulated with an in-phase carrier and a quadrature carrier and the demodulated signals are combined to a baseband signal In the digital filtering process the sampling frequency is reduced from a value higher than twice the highest frequency in the passband signals to a value which is not higher than twice said maximum frequency in the baseband signal and in the digital demodulation and combination processes the reduced sampling frequency is also used so that a considerable reduction in the internal processing speed is obtained

Spread spectrum communication system

Abstract: An improved spread spectrum communication system having a transmitter subsystem and a receiver subsystem is disclosed. The transmitter subsystem comprises a data source of 50 bps rate which is modulo-2 added to a C/A (clear/acquisition) code of 1,023K chips/sec in an EXCLUSIVE OR gate, phase modulated on a carrier signal at about 154 f o where f o = 10.23 MHz in a phase shift keying modulator and transmitted. The receiver subsystem comprises an antenna for receiving the phase modulated signal, a plurality of filter/amplifier mixer stages for reducing the phase modulated signal to baseband, an in-phase CCD matched filter channel and a quadrature phase CCD matched filter channel for determining the presence of the C/A code and the phase of the incoming C/A code for synchronizing a replica of the C/A code with the incoming C/A code, a mixer for beating the replica C/A code signal with the C/A code signal, a Costas loop for automatic phase control, a sampler for sampling the baseband signal, an integrator for integrating the sampled signals, and a clocked comparator for comparing the integrated signals with a zero voltage at a 50Hz rate to demodulate the data code. Two embodiments of the CCD matched filter are disclosed. The first comprises a CCD having a length equal to five times the number of stages for each period of the C/A code. The second comprises a CCD transversal filter having a feedback loop for use in its transversal filter mode and in a recursive filter mode to enhance C/A code signals relative to noise.

Microprocessor Implementation of High-Speed Data Modems

Abstract: This paper describes the application of a commercially available microprocessor (Intel 3000 or Signetics 3000) to a flexible data transmitter and data receiver for high-speed data modems. For the transmitter a quadrature modulation scheme is chosen; the receiver is based on phase-shift compensation techniques and coherent demodulation with an externally derived digital carrier. For the realization with the given microprocessor it has been necessary to adapt the way of executing the various operations (especially the multiplications for the digital filtering) to the available computational capabilities. The resulting microprocessor implementations are also suitable for application in the current medium-speed synchronous data transmission systems.

Method and apparatus for signal recognition

Abstract: A method and apparatus are provided for sorting signals from compressive receivers in accordance with the type of modulation on the signal, so as to be able to establish the identity of the signal short of fully demodulating it. It has been found that signals from a compressive receiver which are insufficiently sampled for complete demodulation can be identified by "histograms" in which each designated condition, such as the number of consecutive 1's or 0's is graphed against the number of occurrences of this designated condition over a period of time, designated a "data collection interval." The result is a pattern or diagram which is characteristic of the modulation type, such that FSK, DFSK, PSK, hand morse, machine morse, anomolous morse, AM, SSB, and multitones can be distinguished one from another. Moreover, it is possible to recognize two-way voice communication.

Spread spectrum detector

Abstract: The detector of the present invention includes a first and a second signal channel each of which is adapted to receive a spread spectrum signal, or any other similar type multi-modulated carrier signal In the preferred embodiment of the invention, the first signal channel is provided with a correlator for partially demodulating the received signal, as to one modulation signal component, with a demodulation signal derived from the second signal channel The first signal channel is also provided with an additional correlator for demodulating the partially demodulated signal as to another modulation component The second signal channel is comprised of a means for generating a dithered demodulation signal which signal is driven into synchronization with a corresponding modulation signal component in the received signal so as to enable the accurate generation of the demodulation signal utilized by the first signal channel

Maximum A Posteriori Estimator for Suppression of Interchannel Interference in FM Receivers

Abstract: In this paper an FM detector capable of suppressing the degradation in receiver performance due to the presence of an interfering signal is presented. Optimum receiver structures based on maximum-aposteriori estimation procedures are first derived and then a practical demodulator based on the optimum receivers is examined. The receiver consists of two phase-locked loops (PLL) interconnected in a manner to permit one PLL to lock onto and track the Stronger received FM signal while the other loop tracks the weaker of the two received FM signals. The detector has the capability of demodulating both the desired received FM as well as the interferer even for the case when both signals are co-channel. Experimental results demonstrating such capability even in the presence of strong input Gaussian noise are presented.

Metal detector system with ground effect rejection

Abstract: A metal detector uses a transmitting search coil inductively coupled to a receiving coil for detecting the presence of metal objects near the surface of the ground within the field of the coils. An oscillator generates a signal transmitted by the transmit coil, and the signals detected by the receive coil are coupled to the signal inputs of two synchronous demodulators. The output of the oscillator also is applied at different phases to the reference signal inputs of the two synchronous demodulators. The outputs of these demodulators then are passed through low pass and bandpass filters having a low cutoff frequency which is higher than the highest frequency components generated in the synchronous demodulators due to ground effects. The signals passing through the bandpass filters then are applied respectively to the signal input and reference signal input of a third synchronous demodulator, the output signal of which has an amplitude representative of the presence of metal objects and the polarity of which is an indication of the type of metal being detected. Undesired signals produced by ground effects are reduced by a considerable amount, and the output of the third synchronous demodulator is applied to a suitable indicator circuit.

Demodulation Procedure for Very Wide-Band FM

Abstract: Since conventional FM demodulators are incapable of accurately demodulating very wide-band frequency-modulated signals, a new approach to the problem is needed. Based upon a theorem by Sandberg, a practical procedure is presented for demodulating not only typical FM signals but also those employing modulation bandwidths and/or frequency deviations on the order of the cartier frequency. The proposed demodulator utilizes an iterative recovery procedure. The first iteration is analogous to demodulation by a conventional zero crossing discriminator and results in severe distortion of the modulation under wide-band conditions. This distortion is reduced by successive iterations. Because, in practice, the iterative process is truncated after a finite number of terms, a procedure for computing the coefficients of these terms for minimum mean-squared error is given. Using computer simulation, the demodulation technique is demonstrated to work effectively for several very wide-band FM signals.

Differential encoding and decoding scheme for digital transmission systems

Abstract: Digital transmission systems operating over microwave radio generally employ phase modulation rather than amplitude modulation because phase modulation is less sensitive to non-linearities in the transmitter. Coherent demodulation is typically employed in the receiver to ensure maximum immunity to the thermal noise of the radio receiver. This, in turn, requires that the receiver include circuitry to recover an unmodulated carrier from the incoming digital signal for use as a phase reference in the demodulation process. Unfortunately, the phase of the recovered carrier is subject to ambiguities which may result in the demodulated digital signal being transposed and/or inverted, resulting in gross transmission errors. Differential encoding of the digital signal, prior to modulation, overcomes this problem but may create additional problems, such as the inability of the receiver to monitor transmission errors by means of the parity bits which are included in the digital signal to be transmitted. The instant invention comprises an encoding algorithm which encodes the digital signal prior to modulation so that phase ambiguities in the recovered carrier signal become unimportant, yet at the same time preserves the ability of the receiver to check for transmission errors. The algorithm comprises two separate but complementary encoding rules, one of which is employed if the last three outputs of the encoder are logical equivalents and the other if they are not. A similar decoding algorithm is employed at the receiver.

Charge coupled device filters and modems incorporating such filters

Abstract: A Modem utilizing process technology compatible CCD and IGFET circuits and sub-systems incorporated on a single I.C. chip. CCD transversal filters are employed as bandpass filters, being tuned for different modes of operation by adjustment of the clock frequency. The bandpass filters are operated in conjunction with a charge amplifier to remove d.c. offset from the analog output of the filters. The demodulator section of the Modem also incorporates low-pass filter employing switched resistors. The modulator section includes waveform generators based on a digital-to-analog converter including switched capacitors having values selected such that a sine wave having minimal distortion is generated when switched in proper sequence. Provision is made for digital inputs to control the frequency and amplitude of the sine wave output. The Modem can be operated under control of a microprocessor.

Carrier synchronization system for coherent phase demodulators

Abstract: This invention concerns a carrier synchronization system for a four-phase coherent PSK demodulator. The four-phase PSK signal is demodulated by an in-phase reference carrier and a quadrature reference carrier both of which are locally generated by a reference carrier source. The in-phase and quadrature components of the four-phase PSK signal that result from these demodulation operations are delayed a time interval substantially equal to T/2 seconds, T being the signaling period. The product of the delayed in-phase component and the sign of the quadrature component is compared with the product of the delayed quadrature component and the sign of the in-phase component, to produce a phase error signal. This error signal is used to control the adjustment of the reference carrier source.

Digital SSB-FDM communication system derived from a complex band-pass digital filter bank and by a filter breakdown process

Abstract: An SSB-FDM modulator derived from a complex band-pass digital filter bank and by a filter breakdown process comprises a discrete Fourier transform (DFT) processor capable of giving phase offsets to complex output signals thereof. In a broader sense, DFT includes inverse DFT (IDFT). The processor is called an offset discrete Fourier transform (ODFT) processor and supplied with real baseband sample sequences. The phase offset complex output signals are frequency-selected by a complex band-pass digital filter unit operable at a sampling frequency for the baseband sample sequences into a real SSB-FDM signal. An SSB-FDM demodulator likewise derived comprises a complex band-pass digital filter unit for frequency-selecting a real SSB-FDM signal into complex sample sequences of the respective baseband channels. The complex sample sequences are ODFT'ed into real baseband sample sequences. For simplified processing of signals transmitted between the modulator and demodulator, use is made of at least one dummy baseband channel. The phase offsets may be provided in the modulator by a post-processor following a conventional DFT processor and in the demodulator by a pre-processor prior to conventional DFT. If desired, frequency spectra of the sample sequences of predetermined one or ones of the baseband channels may be reversed in the modulator prior to digital filtering and in the demodulator after the ODFT.

Method of and means for conveying and recovering supplementary message signals superimposed upon a multilevel signal stream in a digital transmission system

Abstract: A multilevel mainstream of digital signals, organized in a recurrent frame, is monitored at a transmitting end by a pulse generator GP1 which emits a marking pulse whenever that mainstream has a modulable waveform, i.e. a symbol represented by either the lowest or the highest amplitude level. A waveform generator GF controlled by pulse generator GP1 produces a bipolar signal 19 whose pulses are to be superimposed upon the modulable waveforms coinciding with the occurrences of the marking pulses, the superimposition being carried out in an analog adder S1 which extends the amplitude excursion of the mainstream signal beyond its limiting level. The mainstream frame is divided into a plurality of subframes during the first of which the bipolar signal 19 is a frame-alignment word with an invariable first part and a variable second part, the latter specifying the number of clock cycles -- corresponding to the number of mainstream symbols -- elapsed between the start of the frame and the appearance of the first modulable waveform. In subsequent subframes, the bipolar signal 19 carries message bits from one or more supplementary signal channels. At a receiving end, a threshold circuit CD emits confirmation pulses 24 upon detecting any modulable waveform in the incoming composite signal stream and further emits demodulation pulses 25 if that waveform is amplitude-modulated beyond the normal highest or lowest level. From these two pulse sequences 24, 25 a comparator CF1 determines, during a first subframe, whether the fixed part of an incoming alignment word has a predetermined configuration whereupon a second comparator CF2 checks, with the aid of a counter CL2 stepped by extracted clock pulses and a shift register R2 controlled by that counter, whether the variable second part of that alignment word corresponds to the number of elapsed clock cycles. Upon verification of alignment, counter CL2 is enabled by a logic network LA to allow the readout of demodulation pulses 25 during the following subframes as a replica of the supplementary message bits. Otherwise, logic network LA initiates an alignment search until the requisite synchronism is achieved.

Encoding and transcoding apparatus for video disc systems

Abstract: Color encoding format for a color picture information record, and recording apparatus therefor, are disclosed for recording of discs for use with both (1) disc players designed to provide PAL-type outputs for driving PAL-type color television receivers, and (2) disc players designed to provide SECAM-type outputs for driving SECAM-type color television receivers. In players, up-conversion of recovered composite signal precedes comb filtering to separate luminance and chrominance components. Decoding of up-converted chrominance component, to obtain baseband color-difference signal information is followed by remodulation of a carrier pursuant to a desired output format. Comb filter arrangements of U.S. Pat. No. 3,969,757 are used when up-conversion shifts color subcarrier to a frequency in vicinity of PAL subcarrier frequency, but bearing odd integral multiple relationship to half the line frequency. Differing comb filter arrangements are employed when up-conversion shifts color subcarrier to PAL subcarrier frequency. Demodulation/remodulation arrangement involving the separate detection and remodulation use of two color-difference signals, as well as system requiring detection and remodulation use of only one color-difference signal, are disclosed.

Clock generator for video signal processing

Abstract: A clock generator is provided which demodulates the burst subcarrier portion of a video signal containing time base errors. The demodulation is with respect to an external subcarrier reference signal so that the demodulator output represents an accurate vector summation of the instantaneous phase and amplitude difference of the incoming video signal relative to the external reference. The demodulator output is utilized to remodulate the external reference signal to provide a rephased subcarrier reference signal which is frequency multiplied and waveshaped to produce a clock signal which is synchronized to the incoming video signal.

Estimation and tracking of parameters of narrow-band signals by iterative processing

Abstract: An iterative procedure for optimally estimating the phase angle of a sinusoid of known frequency is presented. The sinusoid is observed in additive Gaussian noise, and the phase angle is assumed to be a Gaussian random variable. The maximum {\em a posteriori} probability (MAP) criterion is used. It is also shown how other signal parameters, such as amplitude and frequency, can be optimally estimated and tracked. Examples are given in which these techniques are used to perform optimum demodulation of angle-modulated signals. The results should dispel the commonly held belief that the best one can do in approaching optimum demodulation of angle-modulated signals is to use a phase-locked loop.

Detection of Digital FSK and PSK Using a First-Order Phase-Locked Loop

Abstract: A classical problem in digital frequency-shifted keyed (FSK) demodulation is the evaluation of the bit error probability performance when an estimator-correlator that incorporates a phaselocked loop (PLL) is employed. Although some attention has been devoted to this problem in the past, an accurate account of the mechanism which produces decision errors has not yet been advanced. This paper examines a special case, viz., a first-order PLL preceded by a wideband IF filter, of the above problem using a new approach which is based upon the renewal Markov process theory and the Meyr distribution. In particular, the ad hoc approach of invoking the Gaussian assumption on the decision variable and patching it with a correction term based on Rice's click theory is not used. Rather, the effective noise is properly characterized by unfolding the renewal Markov process associated with the loop phase error. As a slight extension of the results, the performance of the above PLL detector operating on low data rate PSK is given and demonstrated to be approximately 3 dB superior to that of FSK reception. The theory and analysis presented herein apply to the special case where the bandwidth of the IF filter preceding the first-order PLL is required to be several times the data rate because of frequency uncertainties due to channel Doppler and oscillator instabilities, but the frequency deviation to data rate ratio may be chosen small (if desired) to optimize system error probability performance. In addition to presenting results for the case where the oscillator instabilities are assumed absent and channel Doppler is prefectly tuned out at the receiver oscillator, the effects of small residual Doppler on bit error probability performance is considered. In all cases tested, excellent agreement was obtained between theory and computer simulation results.

A Worst-Case Crosstatk Comparison Among Several Modulation Schemes

Abstract: The effects of worst-case crosstalk (from a time-domain point of view) on unsynchronized frequency division multiplexed (FDM) constant envelope modulations are considered assuming coherent hard-decision receivers and no filtering for bandwidth constraint. Continuous-phase frequency shift keying (FSK) is found to be superior to various forms of phase shift keying (PSK).

Circuit for demodulating a pulse width modulated signal

Abstract: In a video tape recorder having a tape on which are recorded "0" and "1" binary coded bits of a pulse width modulated signal as for identifying frames of a video signal also recorded on the tape, the tape being advanced past a head detecting the pulse width modulated signal at speeds variable with the fast forward, normal and slow motion mode of operation of the recorder so as to change the bit frequency of the pulse width modulated signal in correspondence with the speed of the tape, a circuit which demodulates the "0" and "1" bits of the detected pulse width modulated signal by comparing the coded pulse width of each succeeding bit (B i +1) with the width of the preceeding bit (B i ) to make the demodulation substantially independent of the bit frequency and the corresponding speed of the tape so that the circuit operates in each mode of recorder operation. In the preferred embodiment of the circuit, each bit is quantized into periodic pulses which correspond in number to the width of the bit. The number of pulses corresponding to the pulse widths of successive bits are then digitally compared in a comparator to obtain a two-level discrimination signal. More particularly, only selected higher value digits of the pulse numbers are compared to simplify the comparison and the comparator changes the level of the discrimination signal only during a preselected portion of each bit relative to the preceeding bit to assure accurate demodulation. The preferred circuit also has a circuit for deriving an edge pulse signal from the coded pulse width of each bit and a logic circuit operating on the edge pulse signal and the discrimination signal from the comparator for providing a signal corresponding only to bits of the one value, for example the "1" bits.

Wide acquisition range MSK demodulator input circuit

Abstract: An input circuit for a wide acquisition range minimum shift keying demodulator includes a pair of phase-locked loops (PLL). Each PLL is designed to receive from a frequency doubler a respective one of two discrete spectral lines in the frequency spectrum separated by a frequency equal to the bit rate. Each PLL operates at one of three frequencies including its own search position frequency and the search position frequency of the other PLL plus or minus the bit rate. A logic decision circuit responsive to the PLLs operates with a frequency synthesizer connected to a mixer at the input of the doubler so that when one PLL locks to one spectral line, the other PLL is operated at its respective frequencies to lock to the other spectral line.

Suboptimal Continuous Shift Keyed (CSK) Demodulation for the Efficient Implementation of Low Crosstalk Data Communication

Abstract: A suboptimal receiver of constant envelope, continuousfrequency MSK-type modulations called Continuous Shift Keying (CSK) is proposed that halves demodulator complexity with no more than a 1.6 dB loss in detectability in AWGN. The technique is especially applicable to a frequency division multiple access system composed of many unsynchronized user terminals.

Differential detection systems with non-redundant error correction

Abstract: The differential detection system comprises a demodulator for a modulated carrier transmission system for binary signals, in which a symbol detected from the phase difference between the received signal delayed by one signaling interval and the signal received in the present signaling interval is converted into data and a symbol detected from the phase difference between the received signal delayed by two signaling intervals and a signal received in the present signaling interval is converted into parity. The data and the parity are applied to a decoder for a rate 1/2 single error correcting self-orthogonal convolutional code effecting demodulation with non-redundant error correction. The correction system is also applicable to demodulate a carrier wave modulated by m (an integer) level digital data.

Digital frequency shift signal demodulator

Abstract: An FS signal demodulator includes a zero-crossing detector which detects zero-crossings of frequency shift signals (FS), a pulse converter which generates a train of "0" and "1", "0" representative of a time slot from the first detected zero-crossing through a given period of time and "1" representative of a time slot from that period through the next succeeding zero-crossing, a memory means receiving the outputs of the pulse converter and thus containing the train of the pulses with the length corresponding to modulation rate, a counter means counting the number of "1's" or "0's" contained within the memory means in response to clock signals, and a determination circuit which effects frequency discrimination by comparing the counts of the counter means with a given limit.

Threshold extension fm demodulator apparatus for wide band width fm signals

Abstract: A tunable oscillating limiter threshold extension demodulator capable of demodulating video and other wide band width FM signals is described. A steering signal is generated to tune the oscillating limiter in response to the video output signals from an FM discriminator to which input signals from the oscillating limiter are applied. The steering signal is generated so as to be in proper phase relationship to track the frequency of the FM signal for modulating frequencies within certain critical regions and is reduced in amplitude over another region where proper phase relationship is not achievable because of unavoidable delays in the system. The steering signal generating circuit may be provided by a filter having a phase response characteristic such that the total phase lag of the system is small at the low frequency end of the band width and about 360° at the high frequency end of the band width. The filter presents substantial attenuation (in the form of a notch) in the frequency region of the band width where the phase delay may be about 180° and would otherwise provide improper oscillating limiter steering action. In the case of color television signals, for example, significant spectral components of the signal, specifically the components due to the sync and equalizing signals are present in the low frequency end of the signal band width while the significant color or chromatic subcarrier spectral component is at the high frequency end of the signal band width. Accordingly, when applied to tune the band pass filter of the oscillating limiter the frequency to which the oscillating limiter is tuned is caused to substantially match the instantaneous frequency of the color television modulated FM signal at the frequencies of these significant components and the noise threshold in terms of the minimum signal to noise ratio which the demodulator can operate properly is thereby improved.

Method and apparatus for transmitting auxiliary channel over digital communications system

Abstract: An auxiliary channel, for example, an engineering orderwire, is derived onn existing time-division multiplex carrier system by modulating the pulse repetition rate of the digital bitstream. The modulating signal itself may be an analog or a digital signal. The modulator and demodulator both include a first-in, first-out shift register acting as a buffer, and the desired modulation is effected by varying the rate at which the digital information is fed out of the FIFO buffer at the transmitting end of the carrier system.

Optimized Designs for Fiber-Optic Cable Television Systems

Abstract: System design trade-offs for the application of fiber optics to CATV are presented with the system partitioned into trunking and local distribution. Performance and cost comparisons for analog Intensity Modulation (IM), analog Frequency Modulation (FM), and digital (PCM) approaches are presented. The performance and cost effects of multiplexing are presented for all modulation bases. The paper concludes with a summary showing that multiplexed digital fiber optics systems are best for most of the trunking areas, with analog approaches recommended for local distribution.

Phase locked loop signal demodulator and squelch circuit

Abstract: An FM demodulator and detector suitable for integration including a phase locked loop for locking to an FM signal and recovering the modulating signal and squelch circuitry coupled to the phase locked loop for gating out the recovered modulating signal. The phase locked loop includes an emitter coupled multivibrator whose phase shifted output is applied together with the FM signal to the squelch circuitry for quadrature phase detection, followed by filtering to derive an indication signal for controlling the gating of the recovered modulating signal.