Abstract: An apparatus and method reducing interference between multiple frequency modulated, FM, voice transmissions present in the same frequency bandwidth by receiving a radio signal which includes a first FM voice signal to be demodulated along with one or more additional FM voice signals which are present within a modulation bandwidth of the first FM voice signal, detecting the presence of a carrier frequency signal of the first FM signal and determining a lack of modulation of the detected carrier frequency signal by a voice signal, and squelching the received signal when the carrier signal is unmodulated.

Abstract: The mobile telephone detects bad data packets by decoding speech parameters represented by the packets and comparing those speech parameters against acceptable ranges of speech parameters. If the speech parameters do not lie within the acceptable ranges, the packets are discarded. This speech parameter-based detection of bad data packets is particularly useful in mobile telephones receiving variable rate data packets wherein an error in determining the rate of a packet results in the entire packet being decoded at an incorrect rate, thereby likely causing an annoying audible artifact in the decode speech signal. Such rate detection errors can occur even though a CRC or other conventional error detection check fails to detect the bad packet. In an exemplary implementation, the mobile telephone receives signals encoded with TIA/EIA/IS-95-A standards.

Abstract: Carrier squelch decisions are made by performing the steps of receiving an input signal (D), free of DC offset, having desired and undesired signal components, extracting the desired signal components (M) from the input signal, weighting a function of the input signal (D) by a function of the desired signal (M) to produce a weighted signal (W), determining a characteristic of the weighted signal (W), and performing carrier squelch when the characteristic exceeds a threshold.

Abstract: A portable telephone communications system has a portable telephone terminal (300) and a repeater unit (100), and a remote receiver unit (200), wherein the repeater unit (100) includes a main power supply control device which includes detection means triggered by a signal from the telephone terminal arising on receipt of an incoming call, and including infra-red transmission means for transmitting the signal as a frequency-modulated signal to the remote receiver unit; the remote receiver unit (200) includes an infra-red receptor, demodulation means (205) and means for outputting the demodulated signals as audible voice signals, and further comprises a noise level comparator (209d) which compares the voltage of noise generated when no infra-red signals are received with a predetermined level and delivers an output signal when said voltage exceeds the predetermined level, and a time counter (210) which outputs a reset signal when the output from the noise comparator has been generated for a predetermined time i

Abstract: A SONAD (110) control system (100) detects a received signal strength (RSSI) for a radio frequency (RF) signal (102), selects a threshold transfer function (400-404) in response thereto, generates a threshold control signal in response to the transfer function, and utilizes the threshold control signal to select the SONAD threshold value. During operation, the control system (100) decreases the attenuation of background noise levels for weak RF signals.

Abstract: A receive squelch circuit detects a pulse width of an input data signal to prevent errors from occurring during detection of received data. The squelch circuit adds a function of detecting a pulse width to an input side to detect normal and abnormal pulse signals of received data and adjust the squelch circuit operations. The squelch circuit includes a plurality of current sources for each supplying a predetermined amount of current in accordance with a supplied power and a plurality of MOS transistors for detecting a pulse width of an input differential data signal. A plurality of voltage comparators compares the detected pulse width of the input differential data signal with a standard value and judges whether the detected pulse width is normal.

Abstract: A communication system having a radio master and a radio slave for communicating a communication signal and a control signal via a common channel is constructed by a receiver to receive a communication signal, a transmitter to transmit a control signal, and a limiter to limit an output of the signal received by the receiver for a period of time corresponding to a transmission of the control signal and a reception of a predetermined signal by the receiver. The limiter has a key to request the transmission of the control signal and limits the output of the reception signal in accordance with the key operation. The generation of an uncomfortable sound which is caused by a communication of the control signal can be prevented.

Abstract: A radio receiver operates primarily in a first operating mode to process received signals (410). When a carrier signal detect is triggered while in the first operating mode (415, 420), the receiver temporarily operates in a second operating mode which has receiver parameters selected to reduce carrier detect falsing when compared to the first operating mode (430). Preferably, in the second operating mode, the receiver's frequency and time response characteristics are modified to facilitate carrier squelch detection. Carrier squelch is performed if a carrier signal detect is triggered while the receiver operates in the second operating mode (440, 450, 460, 470).

Abstract: An improved squelch circuit which utilizes a phase-locked-loop (PLL) arrangement to track the carrier signal and identify when audio communication should be curtailed. The lock detect circuit of the PLL is used for a signal-to-noise monitor. As the loop locks and unlocks, the lock detect line pulses. These pulses are integrated to give a direct current (DC) level. This DC level is proportional to the signal-to-noise ratio of the carrier signal. The signal-to-noise level is monitored by the squelch gate until a threshold is passed. Falling below the threshold results in a squelching of the audio, when the ratio exceeds the threshold, then audio is commenced. The circuit is enhanced by the ability to automatically adjust the threshold level once the PLL has locked onto the signal.

Abstract: An apparatus and method reducing interference between multiple frequency modulated, FM, voice transmissions present in the same frequency bandwidth by receiving a radio signal which includes a first FM voice signal to be demodulated along with one or more additional FM voice signals which are present within a modulation bandwidth of the first FM voice signal, detecting the presence of a carrier frequency signal of the first FM signal and determining a lack of modulation of the detected carrier frequency signal by a voice signal, and squelching the received signal when the carrier signal is unmodulated.

Abstract: A facility is provided in wireless communications system which causes a base station to substitute predetermined signals for speech and/or noise signals that are received from a wireless station that has entered a muting request, in which the predetermined signals, may be, for example, silence.

Abstract: Detection of in-band noise in a radio communications voice channel is provided by mixing a signal having a known frequency into a radio signal containing a voice channel signal in an in-band range, thereby translating the voice channel signal out of the in-band range. The in-band range is thereafter examined for the presence of noise signal energy whose presence can be indicative of in-band noise. Squelching of the voice channel is provided in the event that in-band noise energy exceeds a predetermined threshold.

Abstract: A circuit arrangement for brief mute switching of the reproduction of an LF signal in a radio receiver for performing additional control operations is described, in which the mute switching is effected by varying the current flow through the control path of an interrupter by means of a reversing switch, as a consequence of the change in charge of a capacitor acted upon by a blanking signal.

Abstract: A device and method for controlling HF single side band radio squelch operation in which a serial tone burst indicates the start and end of a transmission and identifies an on-going transmission. Additionally, receivers in a power saving sleep mode can be activated by receipt of a set of serial tone bursts that have been transmitted with sufficient duration for the receiver to be assured the opportunity to detect them. During the subsequent transmission of information, single "keep-alive" serial tone bursts are transmitted at random time intervals. The end of a transmission is signalled by another set of serial tone bursts. The bursts are provided with the transmitted signal on its frequency and preserve signal bandwidth.

Abstract: A scanning apparatus for scanning a video recording for objectionable content is disclosed that includes an audio processor for analyzing the recording and recognizing the objectionable audio, and audio control gate for manipulating the objectionable audio. The scanning apparatus also includes an amplifier that amplifies the signal and converters to convert the signal from analog to digital format prior to analysis and digital to analog format after analysis. The method of scanning the video recording for objectionable content includes the steps of analyzing the recording and manipulating the recording. The method first includes the steps of amplifying the recording signal, separating the audio portion of the recording from the composite recording, digitizing the portions of the recording, and storing the composite portion of the recording in a loop while analysis is performed on the audio portion of the recording.

Abstract: A method for providing improved carrier based data operated squelch (CDOS) includes routines for recovering the carrier signal and for analyzing the received signal. The received signal is sampled at each transition point of the carrier signal as well as the halfway points between the transition points. In addition, extra samples are taken between these samples. Using the samples, a carrier locked loop routine recovers the carrier signal. A CDOS routine analyzes the extra samples to determine the presence of data. If two successive samples are different, this means that a transition has occurred. The CDOS routine analyzes these transitions to determine whether the input signal is encoded with data. If the CDOS routine determines that the input signal is encoded with data, a CDOS counter is incremented. As the input continues to look like data, the value by which the CDOS counter is incremented is increased. When the CDOS counter reaches a certain threshold, the CDOS mutes the output circuit.

Abstract: A receiver with an automatic receiving-station switching function for automatically switching a current receiving frequency from a reception frequency of a broadcast wave in reception to a reception frequency of a broadcast wave of another station, which is in a good reception condition, when a reception condition of the former broadcast wave becomes poor. After detecting deterioration of the reception condition of a received signal, when the level of a detected signal obtained by detecting the received signal is kept equal to or lower than a threshold value for a predetermined time or longer, the receiver sets muting means to a signal cutoff state to stop the audio output before the activation of the automatic receiving-station switching function. Even if the reception condition becomes poor, the muting means does not cut off an audio signal until the low level of the audio output in the currently receiving broadcasted program continues.

Abstract: An FM receiver capable of instantaneous muting while receiving a broadcast includes a switch circuit provided in series in an audio signal line in the front end of a deemphasizing capacitor, a variable impedance element connected to the capacitor in parallel, and a waveform shaping circuit for smoothing the rise and fall of a muting signal, wherein the muting signal is fed to the switch circuit as a control signal to turn the switch circuit off during the muting signal period, and wherein an output signal of the waveform shaping circuit is fed to the variable impedance element to change the impedance of the variable impedance element correspondingly to the level of output signal of the waveform shaping circuit.

Abstract: In a wireless sound monitoring system, ambient sounds are monitored by a microphone and transmitted over a phase or frequency modulated transmitter to a receiver. A sound actuated switch responsive to ambient sounds louder than a predetermined level turns the transmitter on and causes a hidden tone to be generated. A delay circuit caused the hidden tone to be terminated before the transmitter is turned off when the sound level decreases. In the receiver, a mute circuit responds to the hidden tone to control the output of a loudspeaker, and thus, the hidden tone prevents a squelch tail from being heard. The monitoring unit also includes a pulse generator to cause the hidden tones to be generated at regular intervals, but with short pulse widths.

Abstract: An audio editing device for use with a television including a memory device for storing data representative of predetermined audio signals. Also included is a comparator for comparing the digital form of an input audio signal with the data of the memory device and, upon making a match, generating a first flag at a beginning of the data. A second flag is also generated at an end of the data upon locating a match. Finally, a squelch circuit comprises a flag sensor adapted to, upon the detection of the first flag, squelch the data until the sensor detects the second flag whereat the squelch circuit resumes the unmanipulated passage of audio signals to a television set.

Abstract: A digital squelch system (100) includes a timing signal generator (102) for generating a reference window. An edge detector (106) detects edges in the input signal during this reference window. An accumulator (110) accumulates the number of coherencies. An overflow detector (112) coupled to the accumulator (110) detects overflow conditions therein. A latch coupled to the overflow detector (112) squelches and unsquelches the radio (200) when the overflow or underflow conditions have been detected, respectively.

Abstract: A squelch circuit (111) that is contained in an communications receiver (100) estimates (403, 405) the energy of an input signal in an audio band and in a first portion of the audio band. The squelch circuit (111) then determines (407) whether the energy in the first portion of the audio band is at least a predetermined proportion of the total energy in the audio band. When the energy in the first portion of the audio band is at least a predetermined proportion of the total energy in the audio band, the squelch circuit increases (409) the audio band energy estimate to produce an updated audio energy estimate. The squelch circuit (111) can then use the updated audio energy estimate to adjust (413) a so-called squelch threshold that is subsequently used to determine (419) the presence of the radio frequency signal at an input (101) to the communications receiver (100).

Abstract: When a portable radio (110) receives a transmission from a mobile radio (105), the portable radio (110) mutes a speaker volume of the portable radio (110) for a duration of the transmission from the mobile radio (105). In an arrangement of the mobile radio (105) and the portable radio (110) where both radios transmit and receive on a same communication resource channel and both radios are in close proximity, acoustic feedback may be avoided.

Abstract: A receiver has a random counter, which outputs a timer signal to a control circuit. A zero-cross point detection circuit generates a detection signal which is input to the control circuit. When the random counter outputs a timer signal and the zero-cross detection circuit detects a zero-cross point, a muting function is activated. Simultaneously, different frequencies are scanned for broadcasts containing the same program and having higher field strengths. If on re-checking zero-cross point detection, a zero-cross point is found or a predetermined interval has expired, then the muting function is deactivated. The frequency with the greater field strength is selected.

Abstract: A receiver 700 having an audio output device 326 is disclosed. The receiver 700 receives a carrier signal modulated with data, voice, or a combination of both. The receiver 700 includes a demodulator 708 for demodulating the received carrier signal. The demodulated audio signal 302 is coupled to a delay circuit 300 where it is delayed and coupled to a detector 318. The detector 318 detects the presence of data or voice in the demodulated audio signal 302 and mutes or unmutes the audio output device 326 respectively. The delay circuit 300 stops any portion of the data signal to reach the audio output device 326.

Abstract: A wireless fully submersible transmitter includes the substantially water-resistant casing within which there is located an audio frequency circuit, a radio frequency circuit, antenna, and a rechargeable battery. A microphone is connected through the casing between the inside of the casing and a position external the casing. A receiver includes a squelch circuit to eliminate the volume when the transmitter is submersed within water such that the RF signal from the transmitter is lost. The transmitter is operable from outside of the casing by a switch which interacts with an operable reed switch on the inside of the casing. An antenna is located inside the casing using the circuit board as the ground.

Abstract: A receiver circuit for receiving both digital and analog modulated signals has a high frequency amplifier for amplifying digital and analog modulated high frequency signals, a down conversion block for converting an output signal from the high frequency amplifier into an intermediate frequency signal, a root Nyquist bandpass filter for limiting the intermediate frequency signal from the down conversion block to a frequency band based on root Nyquist characteristics, an intermediate frequency signal linear amplifier for linearly amplifying the intermediate frequency signal from the root Nyquist bandpass filter, a limiter amplifier for limiting to a constant amplitude the intermediate frequency signal from the intermediate frequency signal linear amplifier, a received signal intensity detector for detecting received signal intensity information from the intermediate frequency signal from the intermediate frequency signal linear amplifier, a synchronizing clock signal generator for generating a synchronizing clo

Abstract: An FM noise reduction circuit including a level detection circuit receiving an intermediate frequency signal. The level detection circuit rectifies the intermediate frequency signal, and outputs a rectified intermediate frequency signal. A first frequency filter passes the rectified intermediate frequency signal in a first frequency range, and outputs a first voltage responsive to the portion of the rectified intermediate frequency signal passed in the first frequency range. A first interference signal detection circuit includes a second frequency filter. The second frequency filter passes the rectified intermediate frequency signal in a second frequency range and outputs a second voltage responsive to the portion of the rectified intermediate frequency signal passed in the second frequency range. The second interference signal detection circuit includes a third frequency filter.

Abstract: A radio communication device (10) comprises a radio transceiver (12), having a received signal strength indicator (RSSI) (16), an audio section (22), and a controller (16) for deactivating the audio section whenever the RSSI indicates the the received signal strength is below a predetermined threshold. The controller also operates as a counter (26) for indicating the length of time that the received signal strength has been below the predetermined threshold.

Abstract: A receiver squelch circuit has a programmable DC threshold level. The squelch circuit includes input circuitry that responds to an incoming signal by generating a decision signal when the DC voltage level of the incoming signal crosses a predetermined threshold level. A programmable control network selectively varies the predetermined threshold level.

Abstract: For realizing a correct measurement of a receiving condition even if the bandwidth of intermediate frequency amplification is narrow, the gain of a variable gain amplitude detector circuit is controlled by an output signal of a function generator circuit so as to cancel the bandwidth characteristic of an intermediate frequency amplifier circuit, to flatten the total bandwidth characteristic of a circuit for detecting an amplitude value of a received signal, thereby eliminating the influence of angle modulation related components in the received signal or desired wave.

Abstract: A receiver includes a plurality of antennas, a selection switching unit for selectively switching between the plurality of antennas to output one of the respective output signals of the plurality of antennas having the largest induced voltage, and a unit for converting the selected one antenna output signal to an audio signal to be outputted as an output signal of the receiver. The receiver further includes a muting control unit for detecting the level of the induced voltage of the selected antenna output signal so as to output a muting control signal when the detected level of the induced voltage becomes smaller than a predetermined value, and a muting unit for muting the audio signal for a predetermined time on the basis of the muting control signal. The selection switching unit performs its antenna switching operation on the basis of the muting control signal.

Abstract: A noise suppressing circuit has a gate for controlling a composite signal applied to a stereo demodulator of an FM tuner, and a detector for detecting a noise included in the composite signal and for producing a noise dependent signal for controlling the gate to close it. A pilot signal detector is provided for detecting a pilot signal included in the composite signal for producing a pilot dependent pulses. In response to the noise dependent signal and to the pilot dependent pulses, gate control pulses are applied to the gate, thereby closing the gate in synchronism with the pilot signal.

Abstract: A muting circuit for eliminating transient signals generated due to power supply "turn-on" and "turn-off" is presented. In one embodiment, a junction in a signal chain is AC shunted to ground independent of the DC status of the junction. This shunting is in response to a signal generated by a power supply controlling microprocessor when the microprocessor energizes or de-energizes the power supply for a portion of the signal chain coupling signal to the acted upon junction. In another embodiment, the junction is AC coupled to ground during turn-on through a diode in series with a capacitor with the diode first being forward biased and then reverse biased to remove the AC shunt. The biassing of the diode is accomplished by application of two different sources of voltage to respective electrodes of the diode.

Abstract: Differential signals are employed in a distributed audio system to improve noise immunity of the low level differential signals transmitted between a main unit and a power amplifier. The power amplifier may be in an auxiliary output unit which is locally muted by employing a field-effect transistor connected between the differential signals. When the field-effect transistor is turned on by a mute signal, the common mode rejection of the differential amplifier is employed to mute the amplifier output.

Abstract: A FM receiver quickly suppressing defective effect of multipath reflection interference is disclosed. A field strength detecting circuit (8) is responsive to output signals from preceding stage amplifiers (51) in an IF amplifying circuit (5) for detecting the field strength of the received signal. In response to the field strength detecting circuit (8), a soft muting control circuit (10) is operated, whereby a muting circuit (7) gradually mutes the output signal of a detector circuit (6). Furthermore, an alternate current signal detecting circuit (12) detects the alternate current signal component included in the output signal from the field strength detecting circuit (8), whereby the occurrence of multipath reflection interference is detected.

Abstract: A communication control console having both data and voice capability. Data signalling as transmitted on a voice channel is converted into an internally compatible format, regardless of the original signalling type, for use by the console. Conversely, data messages originating from the console are converted from the internal format to the appropriate external signalling format.

Abstract: A receiver 100 used for recovering modulation signals modulated on a carrier signal is disclosed. The receiver 100 includes a detector 204 and a decoder 206 for detecting the presence of a non-valid coded squelch signal and decoding such signal in the recovered modulation signal. The receiver 100 further includes a synchronizer 208 for synchronizing the detected non-valid coded signal. Receiver 100 is placed in a battery saver mode when a non-valid coded squelch signal is detected. The battery saver mode includes monitoring the recovered modulation signal for a change in the non-valid coded squelch signal. The battery saver mode is departed once a change in the non-valid coded squelch signal is detected.

Abstract: A method and apparatus for extending the range of portable RF transceivers and for providing a select-call system includes a hybrid remote control system to extend the range of a portable transceiver and to permit the operator of a vessel or vehicle to communicate hands-free through the utilization of his own full power base station and antenna, in which a modified full duplex system is used to control transmit and receive cycles from the portable unit so that the system can operate regardless of interference on the channel to which the base station is timed. Also provided is a selective calling system in which a recipient's transceiver is turned on by the transmitting transceiver through punching in the recipient's call letters or at least its address. As an added feature, the transmitting station automatically transmits its own call letters which are displayed at recipient's receiver. This system thereby quiets all receivers until the particular transceiver is digitally addressed.

Abstract: In a television receiver apparatus is provided for muting an audio power amplifier having continuous operating supply voltage applied thereto from a standby power supply even when the receiver is in an "off" condition so that spurious noise and hum coupled to the power amplifier will not be amplified and heard by the viewer. The muting is applied via a microprocessor controller which controls and senses the power supply status of the receiver as well as mutes the power amplifier in both the "off" and the "on" conditions.

Abstract: An improved method of statistically determining the presence of noise on an FM communications channel permits digital signal processing techniques to more reliably squelch unnecesssary segments of an FM receiver reducing noise and power consumption. Digital samples of an FM signal having real (in-phase) and imaginary (phase quadrature) components representing the real and imaginary components of a phasor rotating about the origin of a Cartesian coordinate system at a rate indicative of a signal on the channel are mathematically operated on. Successive digital samples are multiplied by the complex conjugate of temporally adjacent samples producing a new stream of phasors representative of the phase change between samples. The phase changes between samples are processed to produce a number that indicates the probability of the channel having noise present.

Abstract: Self-programming scanning receiver having superheterodyne circuitry, microprocessor controlled synthesized frequency tuning circuitry, frequency validation means, a learn mode to invoke the frequency validation means for all frequencies tunable by the tuning circuitry, listen mode for scanning of frequencies learned in the learn mode, and an ignore mode for erasing frequencies from permanent memory.

Abstract: A system and method to eliminate the squelch-tail in radio receivers used for data communications. The transmitter adds a brief period of unmodulated carrier signal to the end of each data transmission. A sensor in the receiver detects this period and mutes the output of the receiver before the receiver's normal noise sensing circuitry senses that the transmission has ended. A circuit may be added to the transmitter to correct the last data bit sent in the data transmission.

Abstract: A significantly improved scanning method and apparatus is disclosed which allows tone channel guard decode and scan to optimally coexist on the priority channel. In an exemplary embodiment of the present invention, tone channel guard decoding processing is divided into three processing windows. The first window (hereinafter referred to as SHORTLOOK) serves to indicate whether channel guard might be present. If the SHORTLOOK window processing indicates that the correct channel guard might be present, then a further processing window, hereinafter referred to as SECONDLOOK, is initiated. If at anytime during the SECONDLOOK time period, a second predetermined threshold is exceeded, the system is informed that the correct channel guard tone pattern is most probably present. The audio is then unmuted, even though the SECONDLOOK time period expires before the channel guard tone pattern is capable of being reliably detected in accordance with conventional standards.

Abstract: Unwanted audible noise on a mixed voice data channel is reduced by disabling an audio output device in a radio receiver or transceiver when the receiver determines that the RF signal received contains binary digital bit data. Alternatively, the receiver enables the audio output device when it determines that the RF signal received does not contain binary digital bit data.

Abstract: A channel discriminator circuit is specifically adaptable for use in association with a radio receiver in which DC voltage signals are generated which are proportional to the FM signals received by the radio receiver and an inhibit circuit will generate an inhibit signal denoting the reception of FM signals above a predetermined detection threshold corresponding to an assigned carrier frequency. The channel discriminator circuit will determine whether the FM signals received are centered within a predetermined band width corresponding to the assigned carrier frequency by comparing the DC voltage signal generated with upper and lower reference voltage limits generated by a potentiometer network and will cause the inhibit circuit to generate an inhibit signal only if the DC voltage representing the FM signal received is within the predetermined limits established by the potentiometer network.

Abstract: An addressable PCM music broadcast receiver which decodes a contraction identification code such as an address code allocated to the receiver and transmitted by a broadcasting station which performs a pay PCM music broadcast. The receiver is capable of receiving a contracted broadcast or channel on the basis of the decoded result. The receiver further has a demonstration function of receiving a non-contracted broadcast or channel for only a given time.

Abstract: An FM receiving circuit is provided with a front end section operative to select an FM signal from FM broadcasting signals and producing an intermediate frequency signal on the basis of the FM signal, and an audio signal producing section supplied with the intermediate frequency signal for producing audio signals and responsive to a first control signal for eliminating noise components due to a rapid variation in the electric field strength of the FM signal, and the audio signal producing section is associated with a level detecting circuit supplied with an intermediate frequency signal transferred from the audio signal producing section and producing a voltage signal representative of an envelope of the intermediate frequency signal, a low-pass filter circuit providing a time constant for producing the first control signal on the basis of the voltage signal and responsive to a second control signal for variation in the time constant, a high-pass filter circuit operative to differentiate the voltage signal fo

Abstract: An improved FM communication system is disclosed in which noise bursts and pops in the audio output of the receiver, due to weak and Rayleigh-faded signals, are minimized by selectively attenuating the audio signal in the receiver. The amount and rate of attenuation for the receiver noise reduction circuits is determined by a control signal derived form either the average discriminator noise at frequencies above 3 kHz, or from a received RF signal strength indicator, or a combination of both. The audio output is attenuated during weak signals or Rayleigh fades only when the attenuation control signal increases above a predetermined signal-to-noise ratio threshold. Also described are further improvements to noise attenuation devices used in combination with the receiver noise reduction circuits to enhance audio quality in particular applications, e.g., FM repeater systems which may incorporate multiple transmitters in a simulcast configuration.

Abstract: An improved FM companding communication system is disclosed in which noise bursts and pops in the audio output of the receiver, due to weak and Rayleigh-faded signals, are minimized by selectively attenuating the audio signal in the receiver prior to expansion. The amount and rate of attenuation for the noise reduction circuits is determined by a control signal derived from either the average discriminator noise at frequencies above 3 kHz, or from a received RF signal strength indicator, or a combination of both. The audio output is attenuated during weak signals or Rayleigh fades only when the attenuation control signal increases above a predetermined signal-to-noise ratio threshold. Preferably, attenuation increases at a rate greater than the decrease in the received signal strength. The invention includes a circuit for modifying the expansion characteristics of the expandor to minimize signal clipping in the output audio stage of the receiver due to limited dynamic range.