Abstract: A digital radio receiver generates coherent signals for synchronous detection without use of a phase-locked loop by employing an adaptive notch filter to cancel the desired signal. The cancelling signal is employed as the coherent signal for synchronous detection. The invention obtains faster locking to the desired signal, a wider capture range, and more efficient software coding in a digital signal processing receiver.

Abstract: An AM stereo receiver in which a synchronous in-phase signal is determined comprises a low pass filter circuit for determining an average DC value of the synchronous in-phase signal, a reference voltage supply for providing a reference voltage indicative of a predetermined portion of the DC value of the synchronous in-phase signal occurring during a 100% IF signal level condition, a comparison circuit for comparing the average DC value of the synchronous in-phase signal to the predetermined reference, and an output circuit outputting a signal indicative of an out-of-lock condition when the average DC value of the synchronous in-phase signal falls below the predetermined reference.

Abstract: A digital radio receiver generates coherent signals for synchronous detection without use of a phase-locked loop by employing an adaptive notch filter to cancel the desired signal. The cancelling signal is employed as the coherent signal for synchronous detection. The invention obtains faster locking to the desired signal, a wider capture range, and more efficient software coding in a digital signal processing receiver.

Abstract: A synchronous AM detector and processor requiring a reduced number of external components and fewer integrated circuit pins comprises an audio processor having a first filter operation controlled by a control voltage and an AM stereo decoder including a lock detector and a phase locked loop having a second filter operation controlled by the control voltage. A single control node is coupled to the audio processor and the phase locked loop, the control node providing the control voltage for the audio processor and the phase locked loop. The voltage at the control node is biased normally high, capable of being pulled low by the audio processing circuit and capable of being pulled low by the lock detector. An RC circuit decays the rise time of the control voltage at the control node after the control voltage has been pulled low. Circuitry is added to control the first filter operation of the audio processing circuitry responsive to the control voltage at the control node.

Abstract: An AM stereo input signal is processed by an envelope detector, an in-phase detector and a quadrature phase detector to yield outputs which may carry short noise impulses. Each output is low pass filtered to derive an average signal level and each output is ratiometrically compared to the corresponding average to detect a tic or modulation substantially above the average. In response to a tic a hold circuit generates a flag for a period which depends on the quality of the input signal. A dual mode deemphasis and sample/hold circuit normally filters the audio signal in each channel and is effective for the duration of the flag to hold the existing signal in the circuit to blank out the noise impulse.

Abstract: A receiver for receiving a stereophonic signal with upper and lower sidebands carrying a modulating signal includes independent sideband circuitry for providing upper and lower sideband signals. Selector circuitry responds to the level of audible noise in each of the upper and lower sideband signals for selecting that one of the sideband signals having a lower level of audible noise relative to the other. An ISB highpass filter filters the latter sideband signal to provide a highpass filtered sideband signal. Stereo detector circuitry provides left and right stereophonic audio signals. At least one audio lowpass filter filters the left and right stereophonic signals to provide corresponding lowpass filtered left and right stereophonic audio signals. At least one signal combiner combines the highpass filtered sideband signal with each of the lowpass filtered left and right stereophonic audio signals to provide corresponding composite left and right audio signals.

Abstract: An improved AM-FM combined stereo receiver is provided having a single processor for removing, in the FM mode, arctangent distortion, and in the AM mode, to process the phase term from arctangent ##EQU1## to determine ##EQU2## Further, the radio includes a single discriminator circuit wherein, in the FM mode, the discriminator tank circuit is used to convert frequency deviation to baseband audio and, in the AM mode, the discriminator tank circuit is used as the frequency-determining element for the voltage controlled oscillator (VCO), the VCO being used to translate the intermediate frequency (IF) signal to baseband.

Abstract: A digital signal processing (DSP) radio receiver employs a conventional analog RF tuner to produce an analog intermediate frequency. The receiver performs DSP functions of digitally sampling the analog intermediate frequency at a sampling rate f.sub.s, concurrently mixing, filtering, and sample-rate reducing the sampled intermediate frequency using a digital filter for inherently generating a near-zero complex intermediate frequency signal, and synchronously detecting the complex IF signal. In a preferred embodiment, the invention is employed to reproduce AM stereo signals.

Abstract: An adaptive audio processor for stereo AM broadcast signals includes independent signal processing paths. Each path has a variable lowpass filter and a matrix and variable Q 10 kHz notch filter to form LEFT and RIGHT channel signals with a 10 kHz notched pass band determined by adjacent channel noise. A 10 kHz pass band signal representative of adjacent channel noise from one of the signal processing paths is compared to a reference to generate a correction signal that is fedback to control the lowpass and notch filters. The correction signal adjusts the pass band and Q of the lowpass and notch filters so that the effects of adjacent channel noise are minimized. Other receiver signals which are reflective of the quality of the received broadcast signal, e.g., the AGC signal, an excess modulation signal, and a receiver microprocessor signal are similarly employed to control the lowpass and notch filters.

Abstract: In this radio receiver, the charge on a capacitor (14) controls the bandwidth of filters in the receiver's audio circuits. When tuning the radio, a bandwidth control circuit (13) forces this capacitor (14) to discharge, thereby forcing the audio path to be bandwidth restricted, thereby minimizing interstation noise reproduction.

Abstract: Stereo decoders for use in AM stereo receivers are disclosed which have two different modes of operation, one of which suffers adverse effects from the presence of co-channel interference and employs envelope detection to develope an (L+R) representative signal. When such interference is detected the decoder switches to a second mode of operation, which is less affected by the interference, and also switches to synchronous detection to develop the (L+R) representative signal.

Abstract: An audio detector circuit forms L+R and L-R audio signals from an intermediate frequency compatible quadrature amplitude modulated signal in the form (1+L+R)cos(fct+.phi.) where .phi. contains phase modulated L+R and L-R signals. An envelope detector generates an L+R audio signal and in-phase and quadrature phase detectors produce L+R and L-R audio signals, respectively. The difference between L+R outputs of the envelope and in-phase detectors are amplified to generate a cosine correction signal. Each detector includes a differential operational amplifier having an field effect feedback transistor coupled between each amplifier output and the corresponding input and an field effect transistor coupling the compatible quadrature amplitude modulated signal to the operational amplifier inputs.

Abstract: A receiver for receiving an amplitude modulated signal with upper and lower sidebands, comprising: an independent sideband decoder for providing demodulated upper and lower sideband audio signals, a combiner for differentially combining the upper and lower sideband audio signals to provide a noise signal, a quality detector apparatus for providing upper and lower quality signals representative of undesired noise in the upper and lower sideband audio signals respectively, and a selector responsive to the quality signals for processing the upper and lower sideband audio signals to provide an output audio signal with reduced noise. The quality detector includes a correlator for correlating the noise and audio signal in the associated sideband to provide the quality signals.

Abstract: In the case of a method for the generation of an amplitude-modulated ISB transmission signal from different AF signal (NF1,2), the signal vectors are split into signal components (x1,y1 and x2,y2, respectively), components sums (X,Y) are formed from the signal components and an amplitude signal (A(t)) and a harmonic phase signal are derived from the component sums. Amplitude signal and phase signal are then further processed separately and combined at the single transmitting tube (43) by combined amplitude modulation and phase modulation into the final transmission signal.

Abstract: Disclosed is a circuit for controlling the activation of a stereo indicator and a tune indicator in an AM stereo receiver. In the preferred embodiment, a pilot tone detector is employed to determine if the input signal is a stereo signal and an AGC is used to detect a preselected improvement in the amplitude of the received signal. A capacitor is charged in response to the stereo or the amplitude condition. When the charge level of the capacitor reaches a first predetermined level, a tune indicator driver is enable to allow a tune detect circuit to drive the indicator. This prevents the tune indication from activating before the receiver is stabilized. When the charge level of the capacitor reaches a second predetermined level, a stereo indicator driver gradually activates the stereo indicator. This prevents sudden power drains from adversely affecting the circuit.

Abstract: An AM Stereo receiver is described which includes an AGC having an output which indicates whether the received signal has significantly decreased in level and having a clamping circuit for controlling the voltage at the input of a VCO. The clamping circuit is further defined as having a window detector for sensing a change in the voltage at the input of the VCO according to a predetermined threshold; a selectable controller, responsive to the window detector, for adjusting the voltage at the input of the VCO; and a window reduction circuit, responsive to the output signal of the AGC, for altering the predetermined threshold to provide the controller with a more sensitive indication of a drift in the voltage at the input of the VCO. The receiver additionally includes a conventional locked loop driver for steering the VCO according to the phase of the received input signal.

Abstract: 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.

Abstract: An improved radio receiver that maintains a stable predetermined IF frequency upon receipt of input carrier signals of differing frequencies. The IF signal (14) is processed through a signal demodulation unit (16) that includes a tangent processing unit (22) to yield a control signal that varies as a tangent function of both the demodulated IF signal and a reference signal from a reference oscillator (17). This control signal is used to control the frequency and phase of a reference signal (26) used by the IF stage (12) to center the IF signal with respect to the IF stage characteristics.

Abstract: An audio processor for adaptively rejecting undesired noise and interference in an audio input which includes a variable Q, 10 kHz switched capacitor notch filter. The Q of the notch filter is varied by a control loop responsive to the 10 kHz content of the audio signal. The notch filter includes a summer section that is clocked at a predetermined frequency, and an integrator section which is clocked at a multiple of the predetermined frequency in order to prevent center frequency variation with changes in Q of the notch filter.

Abstract: In an AM stereophonic receiver a voltage is developed in response to negative overmodulation in the received signal, to detection of a pilot tone, to detection of a minimum or no signal strength condition, and to external control. Predetermined levels of this voltage are utilized to enable/disenable various function circuits within the receiver to optimize the operation.

Abstract: A tuner includes circuitry for conditioning an audio signal including (i) a pilot signal canceller, (ii) a noise limiter, (iii) a notch filter, and (iv) a de-emphasizer; as well as an attenuator for RF signals. In an exemplary embodiment, an AM stereo pilot signal canceller utilizes a 25 Hz signal which is added or subtracted from the left and right channels to eliminate the pilot signal. The noise limiter includes a capacitor which is switched into the feedback circuit of an operational amplifier when the input is at a selected level. The diverting of the feedback signal to the capacitor causes the amplifier to saturate thereby removing the noise from the signal. The notch filter includes first and second band pass filters which are centered at a selected frequency, e.g. 10 kHz, for removing a 10 kHz beat note. The passed band is phase shifted so that it is 180 degrees out of phase with the original signal and then added back with the original signal to remove the selected band.

Abstract: An improved radio receiver that maintains a stable predetermined IF frequency upon receipt of input carrier signals of differing frequencies. The IF signal (14) is processed through a signal demodulation unit (16) that includes a tangent processing unit (22) to yield a control signal that varies as a tangent function of both the demodulated IF signal and a reference signal from a reference oscillator (17). This control signal is used to control the frequency and phase of a reference signal (26) used by the IF stage (12) to center the IF signal with respect to the IF stage characteristics.

Abstract: In an AM stereo receiving apparatus for decoding an AM stereo broadcasting signal of an ISB system, an envelope detecting circuit 6 detects an output of an intermediate frequency amplifying circuit 5 and a DC removing circuit 7 removes a DC component therefrom, so that a stereo sum signal (L+R) is obtained. A quadrature detecting circuit 9 detects quadrature of the output of the intermediate frequency amplifying circuit 5. A multiplying circuit 10 multiplies the stereo sum signal (L+R), which is amplified a times as much by a first amplifying circuit 8, and the output of the quadrature detecting circuit 9. An addition circuit 12 adds the output of the quadrature detecting circuit 9, which is amplified b times as much by a second amplifying circuit 11, and the output of the multiplying circuit 10.

Abstract: A simplified stereo signal decoder is disclosed for use in an AM stereo receiver which receives composite AM stereo broadcast signals comprising a radio frequency carrier wave having amplitude modulation representing stereo sum (L+R information and phase modulation representing stereo difference (L-R) information. The decoder makes novel use of a common, commercially available integrated circuit (IC) that normally is used as a tone detector or a frequency-modulation (FM) detector. The decoder provides synchronous detection of the (L-R) information, combined two-mode phase-locked loop (PLL) recovery of the carrier component and enabling of the (L-R) signal output, and delayed enabling of the (L-R) signal output for a "stereo bloom" effect. The decoder is particularly useful for decoding independent sideband (ISB) AM stereo broadcast signals.

Abstract: This invention discloses an AM stereo transmission method characterized in that when left and right audio signals L and R are transmitted as an AM stereo signal which is represented by S=A cos(.omega.t+.phi.) wherein A denotes an amplitude, .omega. a carrier angular frequency and .phi. a phase angle given by tan.sup.-1 (L-R)/1+L+R, the audio signal of low frequency is transmitted at an amplitude A=(1+L+R), and the audio signal of high frequency is transmitted at an amplitude A=.sqroot.(1+L+R).sup.2 +(L-R).sup.2. The modulator according to the present invention comprises a matrix circuit for providing signals (L+R) and (L-R), an orthogonal modulator to produce an AM stereo signal .sqroot.(1+L+R).sup.2 +(L-R).sup.2 cos(.omega.t+.phi., an amplitude controller, an amplitude control signal generator circuit and a low-pass filter. In the modulator the said AM stereo signal S is changed to the signal (1+L+R)cos(.omega.t+.phi.) for audio signal of low frequency by controlling the amplitude controller.

Abstract: In a radio having an in phase multiplier (A) for providing an in phase detected signal, a detection unit for receiving this signal and for determining whether this signal includes any negative polarity components which would indicate that a viable broadcast signal does not exist on the incoming frequency of interest. This detection unit can include a comparator (11) for comparing the incoming signal against a selected reference value. The invention can also include a signal quality evaluation unit (58) for providing either a digital or analog quality signal that can also effect the output and be used by appropriate processing circuitry to further refine the channel selection process.

Abstract: An AM stereo receiver applicable to receive AM signals containing ID signals which represent different AM stereo systems. The AM stereo receiver includes an IF circuit for generating an IF signal based on a received signal, a PLL circuit for locking the frequency of the IF signal, a clock circuit for generating a clock signal based on a signal obtained from the PLL circuit, and ID signal detector for detecting any one of the ID signals and for producing a detected ID signal. A circuit for distinguishing which one of the different AM stereo systems does the detected ID signal represent includes a pulse generator for generating a first pulse signal having a pulse width as a function of the frequency of the detected ID signal, a counter for counting the number of the clock pulses occurring during said first pulse signal, and distinguish circuit for distinguishing each detected ID signal from different ID signals based on the counted result of the counter.

Abstract: A circuit serves to extract one of two or more pilot tones which are modulated on an audio-frequency signal to select an appropriate decoder, and which cause a changeover to the appropriate decoder or of a decoding signal processor. To this end, one of the stereo signals is digitized by means of a clocked analog-to-digital converter, and the audio signal is separated from the pilot-signal-containing component by means of a pilot-tone-range filter and then averaged over a number of clock periods of the analog-to-digital converter by means of a sampling averager. The output signal of the averager is applied to the inputs of pilot-tone filters, then rectified to measure the signal energy, and applied to the inputs of low-pass filters. The outputs of the low-pass filter control a logic circuit which causes the two audio-frequency signals to be applied to the input of the appropriate decoder.

Abstract: An AM stereo receiver uses an AM detector (208) and an asynchronous PM detector (206) to decode AM stereo signals. The AM stereo broadcast signal has a carrier signal phase modulated with a modified phase shifted left channel minus right channel (L-R) audio signal, which is then amplitude modulated with a phase shifted left channel plus right channel (L+R) audio signal.The AM detector (208) within the receiver decodes the phase shifted L+R signal, while the PM detector 206 within the receiver decodes the modified phase shifted L-R signal. A phase shifter (214) restores the L+R and L-R signals to their proper phase relationship. A matrix circuit (228) regenerates the left and right channel signals from the L+R and L-R signals.

Abstract: A synchronizing circuit used for both an AM intermediate frequency signal and an FM composite signal. A phase detector, voltage-controlled oscillator, frequency divider and phase shifter are connected in a phase-locked loop. The division factor of the frequency divider is controlled to different values for the AM and FM signals.

Abstract: A control circuit in an AM stereophonic receiver examines a detected signal for occurrences of overmodulation and, when a first given level of overmodulation is reached, begins to turn down the correction signal used in the demodulation and, if overmodulation continues, would eventually eliminate the correction signal. At a second given level of overmodulation, the circuit will begin to attenuate the stereo difference signal from the receiver output and eventually eliminate it. When the overmodulation is reduced, operating conditions return toward normal.

Abstract: A variable Q filter network in an AM stereophonic receiver provides elimination of any adjacent channel carrier interference while providing the function of a tone control, either automatic or manually controlled. Parts count and costs, IC pinouts and the possibility of intermodulation are all reduced.

Abstract: The correction signal of the AM stereo system is filtered through a lowpass filter thus providing compatible signals at all normal modulation levels and frequencies, but allowing the signal to become pure quadrature at high frequency, high modulation levels. The inverse of the process can be utilized in the receivers if desired. The improvement is needed only for narrow channel or restricted sideband broadcasting.

Abstract: A correction control circuit for use in compatible AM stereophonic receivers of the type utilizing a cosine correction signal to remove inherent distortion from the received and detected signals. During periods of weak or noisy signals or when co-channel interference is present, excess cosine correction signal causes a reduction in the amount of stereo difference signal component in the output signals, and in the amount of correction provided. In a worst case signal situation, there will be no difference signal component in the output and the monophonic output not be corrected.

Abstract: To permit transmission of traffic information by amplitude modulation in the medium wave band without disturbing audio signals being broadcast, a stereo pilot tone of frequency f.sub.pt below audible range is generated, as well as first and second sinusoidal signals of f.sub.1 =m.sub.1 /n.multidot.f.sub.pt and f.sub.2 =m.sub.2 /n.multidot.f.sub.pt, in which m.sub.1, m.sub.2, and n are different integers. The carrier is modulated by the stereo pilot tone and the first and second sinusoidal signals in such a manner that the phases thereof are different and further, when added, they do not essentially exceed the amplitude of the stereo pilot tone.

Abstract: Certain, but not all AM stereo radio broadcasting and reception systems suffer from a severe form of reception anomaly wherein the perceived stereo image moves from side to side, back and forth, which effect has come to be known as "Stereo Platform Motion" or simply "Platform Motion". A prior art patent proposed a method for sensing reception conditions conductive to Platform Motion and then switching the receiver from stereo to monophonic reception to avoid Platform Motion. The present invention enables the AM stereo receiver to maintain stereo operation under such reception conditions, and instead of switching the receiver to monophonic operation, it switches the receiver to a different method of stereo reception that does not suffer from Platform Motion.

Abstract: An AM stereo demodulator with which the transition from the stereo receiving mode to the monaural receiving mode upon a reduction in the field strength of the received signal is made gradual so as to eliminate an abrupt change in the sound quality. For stereo demodulation, a pair of reference signals are generated which are .pi./2 different in phase for stereo reception. As the field strength of the received signal approaches the level where a phase-locked loop in the demodulator would become unlocked from the carrier of the received signal, the phase difference between the two reference signals is gradually reduced to .pi./4.

Abstract: Disclosed is an AM stereo receiver capable of receiving signals representing at least two different types of AM stereo systems, but using an integrated circuit (IC) decoder designed to properly decode received signals representing only one of the two types of systems. The receiver includes circuitry coupled to the IC which adapts the IC for also properly decoding received signals representing the second type of AM stereo system as well. Circuitry is also disclosed for automatically detecting which type of AM stereo system is being received and for configuring the receiver's decoding circuits to properly decode the received stereo signals.

Abstract: An analog divider circuit as for use in AM stereophonic receivers provides minimum phase variation with control voltage level change, by means of a feedback circuit. The division process is performed by subtracting logarithms of currents produced by the dividend and divisor voltages, then deriving the antilog of the difference or remainder. At the critical point in the process, where junction capacitances do not accurately track the change in the control level, cross-coupled capacitors stabilize the circuit by feeding back a signal from the circuit output to minimize any undesired phase changes.

Abstract: The tone detector provides accurate detection of a single predetermined tone frequency within a mixture of frequencies, utilizing a carrier signal source at a relatively high frequency. The high frequency signal is divided down to two frequencies, one just above and one just below the tone. The divider output is then maintained at the frequency of the tone by continuously adjusting the divisor number between the two divisor values. Multiplying the received signal or a portion thereof by the divider output provides a detect signal when the tone is present in the received signal with sufficient amplitude.

Abstract: The AFC gain of the local oscillator of an AM stereophonic decoder is linearized by the use of difference channel (L-R) information for controlling the capacitance of two varactor-type devices coupled across and in series with the oscillator inductance coil. The improvement helps to prevent "falsing" of the pilot tone indicator and the mono/stereo mode circuitry, particularly at the ends of the tuning range of a manually tuned receiver.

Abstract: In an AM stereo system wherein a low frequency pilot tone is added to the difference channel signal before modulation of the carrier, "image shifting" and audio distortion in a receiver, due to residual pilot tone signal, are removed by separating out a portion of the pilot tone after it has been filtered and phase shifed for enabling the stereo mode of operation. The separated signal is then inverted and delayed appropriately for each of the PLL and audio circuits.

Abstract: An AM stereo receiver decoder is shown. An AM detector produces the stereo L+R signal and a PM detector produces the L-R signal. The PM detector is created from a conventional FM detector that employs an input limiter driving a balanced multiplier. The limiter also drives a tuned circuit which provides quadrature drive to the multiplier. An integrator connected to the FM detector converts the response to a PM decoder. A large value inductor is simulated to appear across the integrator so as to create a low modulation frequency resonance at a subaudible frequency thereby providing a controlled pilot tone response. The inductor is simulated by the action of a first G.sub.m amplifier driving a capacitor which drives a second G.sub.m amplifier having an output coupled back to the input of the first G.sub.m amplifier. The capacitor is switched by means of a series connected switch that disconnects the capacitor when the AM exceeds a predetermined value.

Abstract: A pilot signal detecting circuit for use in an AM stereo receiver capable of receiving and demodulating AM stereo signals broadcast according to any of several different AM stereo modulation methods, in which the particular method employed in broadcasting the AM stereo signals is determined by detecting the individual pilot signal and providing an indication of the particular method employed in modulating the broadcast AM stereo signal. Replicas of each of the different pilot signal frequencies in each of the different AM stereo modulation systems are generated and a signal containing the pilot signal is extracted from the demodulated received AM stereo signal. The replicas are compared with the extracted pilot signal to determine the identity of the AM stereo modulation system employed in the received AM stereo signal.

Abstract: 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.

Abstract: Decoding or demodulating circuitry can produce stereo right channel and left channel signals from AM stereo signals that have been coded according to any of the well known methods. A stereo sum signal made up of the left and right channel stereo signals added together, and a stereo difference signal made up of the difference between the stereo left and right channel signals are developed and fed to a matrix to interconnect these signals to extract or separate the left and right channel stereo signals. At least one of the known methods of stereo coding requires phase shifting of these two developed signals and suitable phase shift circuitry, along with a switching circuit provides the matrix with the appropriate signals when that method of stereo coding is employed at the broadcast side.

Abstract: A phase-locked loop circuit comprises an oscillator, a phase comparator which compares the phase of an input signal with the phase of an oscillator signal, a detector which detects when the phase-locked loop circuit is locked within a predetermined frequency range and produces a corresponding lock detecting signal, and a filter circuit including a variable current source which produces a variable current in response to a change of state of the lock detecting signal to control the bandwidth of the filter circuit, a filter element which receives the variable current, a differential amplifier which receives the phase-compared signal, a current mirror circuit which receives the variable current from the variable current source, and a buffer circuit connected to the filter element which supplies an output signal to the oscillator to lock the frequency of the oscillator signal to the frequency of the input signal.

Abstract: An AM stereo demodulator having monophonic and stereophonic modes of operation and adapted to demodulate a QAM signal whose quadrature-phase signal has a low frequency slot containing substantially no information signal therein. The demodulator includes a low-pass filter for filtering the quadrature-phase signal so as to provide a filtered signal containing only signals in the low frequency slot of the quadrature-phase signal. A window comparator compares the amplitude of the filtered signal with two threshold levels and provides a first output signal when the filtered signal is outside of the range bounded by the threshold levels. A circuit is included for placing the AM stereo demodulator in the monophonic mode of operation in response to the first output signal.

Abstract: A muting circuit for an AM stereophonic receiver of the type that having a detector circuit operating upon an AM stereophonic signal, the carrier of which is phase-modulated by the difference signal of first and second stereo channel signals, for detecting the difference signal. The muting circuit includes a muting signal generating circuit for detecting the output level of the detector circuit exceeding a predetermined value and for generating a muting signal at least for periods when the predetermined value is exceeded, and a muting switch circuit for attenuating the output of the detector in response to the muting signal.

Abstract: A stereo receiver is described which is capable of receiving any two or more of the five currently proposed AM stereo system broadcast signals. The multisystem receiver includes circuitry which is used in various configuration to demodulate the stereo signal components from broadcast signals of any one of two or more of the proposed systems. Selection of appropriate elements of the receiver's circuitry for demodulating any one of the received signals is performed automatically in response to the detection of the presence of a corresponding pilot signal which is unique for each of the five different AM stereo systems that have been proposed. The receiver includes apparatus which detects the presence of such pilot signals and controls the automatic switching of such receiver circuitry. Also described is the application of such apparatus for reliably detecting the presence of the desired pilot signal in a single system stereo receiver.