Intelligible crosstalk in microwave radio systems employing single-sideband modulation is substantially eliminated by phase-modulating the AM carrier-wave prior to its modulation by the baseband signal. The phase-modulation may comprise a periodic signal, such as a single sinusoidal tone or pseudo-random noise, or it may comprise an aperiodic signal such as a truly random noise. In this latter event, the coding signal is transmitted to the receiving location to ensure proper demodulation of the encoded signal.
A single carrier is amplitude-modulated by a signal which is a transform (as the sine function) of the audio signal, and one set of sidebands is removed. In a receiver, the signal can be recovered by first deriving the Hilbert transform of the sine transform, and multiplying it by the signum of the derivative of the original audio signal to obtain the cosine transform. The sine and cosine transforms are then decoded to obtain the original audio signal. A circuit for deriving the signum of the derivative of a signal is disclosed.
A quadrature vestigial-sideband (QVSB) method and system with correlated noise removal are embodied in a QVSB receiver with a correlated-noise estimator-subtractor configured to obtain I/Q channel correlated noise estimates for receive-filtered I/Q signals in the QVSB receiver, and to subtract the I/Q channel correlated noise estimates from the receive-filtered I/Q signals to whiten noises entering a quadrature-crosstalk, maximum-likelihood-sequence-estimator (QC-MLSE) of the QVSB receiver.
A single sideband communications system conveying a message through a transmitter to one or more predetermined receivers which are enabled by a coded squelch signal unique to the predetermined receivers is disclosed. The transmitter comprises means to angle modulate both the pilot signal and the information bearing single sideband signal with the coded signal at a predetermined deviation and to transmit the resulting signal. Each receiver frequency control means tracks the frequency excursions of a selected portion of the transmitted signal up to predetermined limits so that the message can be demodulated without frequency distortion being introduced. Each receiver demodulates the coded squelch without interfering with the message demodulation or tracking functions and activates the receiver squelch of those preselected receivers.
Devices and methods for minimizing multifrequency acoustic or electromagnetic noise that is propagated from its source to a remote region where the noise is to be abated by adaptive means are described herein. The noise minimization is achieved by sampling a noise, synthesizing from this sample a counternoise, the source of which is close to the noise source, and allowing the counternoise to propagate toward said region such that the noise and counternoise cancel each other as they arrive at said region. A closed-loop control is used to adjust the amplitude and time delay, or phase in case of monochromatic noise, of the counternoise during the synthesis, until the residual noise at said region becomes zero. For closed-loop control, the residual noise is transmitted back to the counternoise source by a communication link so that it can be used for said synthesis of the counternoise. The closed-loop control maintains the noise null at said region, even when the distance between the noise and said region changes with time.
In an adjacent channel measurement system, a desired channel signal and an adjacent channel signal are generated using a single signal generator. The output from this generator is modulated with a signal from an offset oscillator to produce a carrier and a sideband signal, one in each channel. The ratio of the carrier amplitude to the sideband amplitude is set by the system's modulation index, which is controlled by an audio attenuator on the output of the offset oscillator.
