The noise blanker of a conventional FM radio is inhibited for several seconds upon closing of the receiver's squelch gate. Following the inhibit period, the blanker is reactivated and again inhibited for the full period upon a subsequent squelch gate closing. Thus, the blanker is inhibited for those signal conditions in which its operation would degrade the overall intelligibility of the processed signal as determined by the squelch control setting.
In a digital wireless subscriber telephone unit for wireless connection with a base station which includes a modem processor whose signal is converted to an analog signal which is then upconverted and filtered to form an IF signal, a deglitching circuit for removing glitch spikes occurring during the transition periods from digital signals to analog signals comprising a timing system, a detector for detecting the transistion periods in order to control the timing system, and a mixer coupled to the output of a D/A converter to mix a blanking signal from the timing system with the analog signal received from the D/A converter during the transistion periods, whereby the output of the mixer is returned to an intermediate reference level.
To a standard AM radio receiver there is connected an impulse-noise suppression system comprising a preliminary blanking gate adapted for connecting to and for interrupting the AM-modulated signal path at the input of the IF section, and an audio blanking gate adapted for interrupting the audio circuit. Both blanking circuits detect impulse noise at the RF amplifier and with appropriate delays blank both points. Audio blanking masks the audio disturbance caused by the blanking in the AM-modulated-signal path. Audio blanking time is preferably from 2 to 3 times the duration of the blanking of the AM-modulated-signal path and is thus kept very short causing a minimum interruption of the wanted audio signal. Associated with the audio-signal-path blanking circuit is a sample and hold circuit for smoothing the blanked audio signal and virtually eliminating an audio disturbance or noise that is otherwise generated by the audio blanking circuit itself.
An apparatus for snubbing or blanking digital signals representing a band of signals that includes an encoded carrier signal transmitted over a power line. An average signal level is compared with the instantaneous signal level to develop a blanking circuit control signal. Additionally, snubbing occurs in the circuit which determines the average signal level to prevent noise from building-up the average signal level. A hold-off signal is used in this circuit to prevent the average signal level from being latched permanently low. A unique infinite impulse filter subtracts out the D.C. offset thereby improving the dynamic range of the blanking. Additionally, the average signal level is used by AGC logic to control the gain at the front end of the apparatus. The state and switching of the AGC is controlled to minimize errors.
Method and apparatus are provided for impulse noise blanking in a radio receiver wherein blanking attenuators are provided in both the wideband, prefilter stage and narrowband, postfilter stage of the receiver so that the bandwidth and location in the receiver in which blanking is provided is optimized dependent upon signal conditions.
Improved noise reducing circuitry for high frequency signal sideband (SSB) receivers is described that substantially eliminates interference caused by impulse noise from a noise source 160. The SSB receiver includes first and second mixers 102 and 106 intercoupled by a crystal filter 104 having a passband that is approximately the same as the noise bandwidth of the antenna circuitry 100 when the SSB receiver is tuned to the lower frequency in its frequency band. A second intermediate frequency (IF) signal from second mixer 106 is coupled to an IF amplifier 122 and a noise amplifier 126. An envelope decector 128 is coupled to the output of noise amplifier 126 and provides an output signal having a magnitude proportional to the envelope of the noise amplifier output. Automatic gain control circuitry 132 is coupled to the output of the envelope detector for generating a gain control signal that reduces the gain of noise amplifier 126 as the magnitude of the envelope detector output increases. An impulse detector 130 is also coupled to the envelope detector output for detecting the presence of impulse noise and generating mute control signal pulses when impulse noise is detected. The mute control signal pulses are coupled to an attenuator 124 which attenuates the output of IF amplifier 122 so that the impulse noise does not create audible interference in the speaker 154 of the SSB receiver.
