Abstract: A method and apparatus for detecting a covert audio transmitter in which a microphone which picks up audio frequencies in the room and a radio receiver tuned to an unidentified audio signal are compared to determine if a "bug" is in the room being searched. An innocuous type of noise is introduced into the area to be searched by having persons converse or by turning on a radio, and these audio frequency signals are picked up directly by a microphone associated with the bug. The signal received by the bug causes the bug's RF carrier to be modulated, and this is picked up by the bug detector's radio receiver. The received signal is demodulated and is compared with the audio signal picked up by the bug detector's microphone to determine if the demodulated radio signal corresponds to the sounds in the room. If the demodulated signal from the detector's radio receiver matches the audio information picked up by the detector's microphone, an indicator light is lit to inform the operator that a bug is present.

Abstract: In a radio frequency detector, a high pass filter is provided to attenuate any undesired low frequencies present in signals applied to the filter. The filter output serves as an input to an AND gate. A low pass filter directs power line voltage to a one-shot multivibrator which is biased to respond to the positive-going zero crossing of the power supply to produce an output of limited duration which is applied to the gate as an enabling input. The output of the gate is joined to a second one-shot multivibrator which responds to a pulse generated when the gate is fully enabled causing energization of an alarm actuator.An additional pulse detector optionally is interposed between the gate and the second one-shot to eliminate any random pulses which may occur at the gate output at a frequency below r.f. level, and an additional filter optionally is included between the high pass filter and the AND gate to eliminate the effects on the circuitry arrangement of high frequencies above a predetermined level.

Abstract: A pulse generator is provided for transmitting individual acoustic pulses which can be sensed by an audio surveillance device and retransmitted as modulated electromagnetic information. Circuitry is associated with the pulse generator for controlling the time when a first acoustic pulse is generated relative to the detection of an unmodulated carrier frequency. This circuitry further provides for the generation of a second acoustic pulse in the event that a modulated carrier frequency is detected during the interval when it can be expected that the first pulse has resulted in modulation of the previously unmodulated carrier frequency. Means are included in the circuitry for comparing the respective periods between the times the first and second acoustic pulses are generated and when modulation on the carrier frequency occurs within the predetermined intervals following such pulse generations.

Abstract: Method and apparatus for detecting a transmitter. The harmonics of an incrementally increasing fundamental frequency are mixed with input signals thus forming intermediate frequency signals. Frequencies occupied by background transmitters are identified by surveying the r-f spectrum at a remote point; these frequencies are then bypassed when scanning is performed in the area to be secured. Another method is also disclosed wherein an operator scans the r-f spectrum in an area, and records indications corresponding to the frequencies of signals not emanating from eavesdropping devices so that those frequencies will be bypassed on subsequent scans.

Abstract: A pulse generator is provided for transmitting an acoustic pulse which is sensed by an audio surveillance device and retransmitted as modulated electromagnetic information. Associated with the pulse generator is a timing arrangement and display. The timer is triggered into operation by the generation of the acoustic pulse. The operating speed of the timer is established to be a function of the speed of sound. Means are provided to stop the timer upon receipt of a demodulated signal representative of the retransmitted pulse whereby the display indicates the distance to the surveillance device.