Hazeltine Corporation

Bernard D Loughlin

Multi-system or 'universal' AM stereo receivers are provided which require no switching of the receiver's stereo decoder circuitry regardless of which AM stereo system signal is being received. In accordance with one aspect of the invention, acceptable stereo sound is achieved for all AM stereo systems by introducing a selected amount of distortion correction in the L-R channel of the universal receiver and a selected phase difference between the L+R and L-R channels. In addition, perceived stereo separation in AM stereo and other stereo receivers is enhanced by also properly selecting the gain in the L-R channel in relation to the L+R channel.

Apparatus for processing stereo signals and universal AM stereo

In a superheterodyne receiver which employs a frequency synthesizer to generate local oscillator (LO) frequencies, there is provided apparatus which determines when an undesirable frequency relation exists, which may produce a condition known as a 'tweet', and which adjusts the frequency of the LO signal so as to prevent, reduce or eliminate the tweet.

Tweet elimination, or reduction, in superheterodyne receivers

An improved transmitter for a compatible independent sideband (ISB) AM stereo system develops a phase modulated carrier where the modulation represents the stereo difference signal inverse modulated by the stereo sum signal in accordance with a selected modulation function. The phase modulated carrier then is amplitude modulated by the stereo sum signal. This simplified modulation scheme provides the second-harmonic sideband correction required to develop a true single sideband type signal wherein left and right stereo information are transmitted in separate lower and upper sidebands, which can be demodulated without distortion in ISB AM stereo receivers. Simplified receivers which use inverse modulation are also disclosed.

Independent sideband AM multiphonic system

In a receiver for composite amplitude and angle modulated signals and having a non-flat automatic gain control characteristic, there is provided apparatus for providing distortion correction to the quadrature component produced by the angular modulation in accordance with the amplitude modulation component. In accordance with the invention, the output of an amplitude demodulator is modified to remove the carrier signal level dependent magnitude multiplier and develop a modification signal which is used to inversely amplitude modulate the quadrature signal component to provide distortion correction over a range of carrier signal levels. Such inverse modulation can be provided before or after quadrature demodulation of the received signal.

Distortion correcting AM stereo receiver with non-flat AGC

Multiple low frequency tones are employed for stereo indication in an AM stereo radio system. The tones are modulated on a carrier wave which is also multiplex modulated by stereo related audio signals. Detection of the presence of one of the tones in a received signal provides an indication of the possibility of stereo related modulation components in the received signal. Detection of the presence of both tones in the received signal provides confirmation that stereo related modulation components are in the received signal. In a preferred embodiment, 15 Hz and 30 Hz phase-related tones are used. A stereo receiver incorporates 15 Hz and 30 Hz phase detectors which form interrelated phase-locked loops with a 30 Hz reference oscillator. The presence of both tones in the proper phase relationship indicates reception from an AM radio station which is broadcasting in stereo.

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Centerport, NY, United States of America

Bernard D Loughlin