Apparatus for preventing leakage of a weak signal from a speaker

An apparatus for preventing leakage of a weak signal from a speaker is provided. The apparatus passes a normal speaker drive signal coming through speaker signal lines and blocks a weak signal generated by the sounds around the speaker, thereby preventing wiretapping the speaker.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0104624, filed Oct. 24, 2008, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to an apparatus for preventing leakage of a weak signal from a speaker, and more particularly, to an apparatus for preventing leakage of a weak signal from a speaker, which passes a normal speaker drive signal coming from an audio system through speaker signal lines and blocks a weak signal generated by the sounds around the speaker.

2. Discussion of Related Art

A speaker, which is a device for generating sound, can be maliciously used for eavesdropping in some situation, since it has a structure similar to a microphone.FIGS. 1A and 1Bare schematic diagrams showing structures of a speaker100aand a microphone100brespectively, andFIG. 1Cis a schematic diagram showing a structure of a broadcast speaker for announcement,100c.

Referring toFIG. 1A, the conventional speaker100ahas a structure including a voice coil120asurrounding a permanent magnet110aand a diaphragm130aattached to the voice coil120a. When an output current of a signal generator D1is transmitted through speaker signal lines140aand flows to the voice coil120a, the diaphragm130amoves according to the electromagnetic force by the Lorentz force law, thereby generating sound.

Referring toFIG. 1B, the microphone100bhas the similar structure as the speaker100a. When the diaphragm130bis moved by microphone-around sounds, a current flows in the voice coil120baccording to electromagnetic induction. The current is transmitted to a signal receiver D2through the microphone signal lines140b.

As described above, the speaker100ahave basically the similar structure as the microphone100b, so a current may be generated by speaker-around sounds and flows in the voice coil120aof the speaker100a, as in the microphone10b. In this case, when the current flowing in the voice coil120ais leaked through the speaker signal lines140a, the conversation around the speaker may be eavesdropped.

Specifically, when broadcast speakers for announcement are installed at multiple positions, conversations around all speakers connected to the common signal lines may be eavesdropped simultaneously if a person branches off the common signal lines.

Referring toFIG. 1C, the broadcast speaker100cgenerally includes a transformer160which is used to minimize the transmission power loss but may make eavesdropping easy because it amplifies a weak signal of the voice coil120a.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for preventing a weak signal generated by the sounds around the speaker from being leaked through speaker signal lines.

According to an aspect of the present invention, there is provided an apparatus for preventing leakage of a weak signal from a speaker, including: a bidirectional threshold element configured to conduct only when a voltage difference between both sides is larger than a predetermined value, wherein the bidirectional threshold element passes a speaker drive signal coming through speaker signal lines and blocks a weak signal coming from a voice coil of the speaker.

The bidirectional threshold element may include two parallel diodes connected in an opposite direction or two serial Zener diodes connected in an opposite direction.

The bidirectional threshold element may include at least one varistor.

The apparatus may further include: a low pass filter for blocking a noise or an abnormal signal whose frequency is above the audible frequency. The low pass filter may include inductors and capacitors.

The apparatus may further include: a high pass filter for bypassing a noise or an abnormal signal whose frequency is above the audible frequency to another path. The high pass filter may include capacitors.

The apparatus may further include: a limiter for limiting a voltage difference between the speaker signal lines to be less than a predetermined value. The limiter may include a varistor.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

An apparatus for preventing leakage of a weak signal from a speaker according to exemplary embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings.

FIGS. 2A and 2Bare schematic diagrams of a speaker200which includes a device240for preventing leakage of a weak signal according to an exemplary embodiment of the present invention.

Referring toFIGS. 2A and 2B, the speaker200includes a permanent magnet210, a voice coil220surrounding the permanent magnet210, a diaphragm230connected to one side of the voice coil220, and the device240for preventing leakage of a weak signal from a speaker according to exemplary embodiments of the present invention. The device240is connected to one line of the voice coil220and becomes conductive only when a voltage difference between both sides is larger than a predetermined value.

According to an exemplary embodiment of the present invention, the speaker200has the same elements as the conventional speaker100a, except the device240.

The device240includes a bidirectional threshold element240aconfigured to conduct only when the voltage difference between both sides is larger than the predetermined value.

The bidirectional threshold element240ais composed of two parallel diodes241connected in an opposite direction, or two serial Zener diodes243connected in an opposite direction. In another exemplary embodiment, the bidirectional threshold element240ais composed of at least one varistor.

FIGS. 3A and 3Bare schematic diagrams showing the speaker200which further includes a transformer260according to an exemplary embodiment of the present invention.

Referring toFIGS. 3A and 3B, when the speaker200further includes the transformer260, the bidirectional threshold element240ais directly connected to one of the speaker signal lines250or placed between the voice coil220and the transformer260.

The position of the bidirectional threshold element240ais determined by the magnitudes of leakage signals before and after the amplification by the transformer260. This will be described below in detail.

First, if the threshold voltage of the bidirectional threshold element240ais much higher than the magnitude of the leakage signal in the voice coil220and slightly higher than that of the leakage signal amplified by the transformer260, the bidirectional threshold element240ashould be directly connected to one of the speaker signal lines250as shown inFIG. 3A.

Second, if the threshold voltage of the bidirectional threshold element240ais slightly higher than the magnitude of the leakage signal in the voice coil220and lower than that of the leakage signal amplified by the transformer260, the bidirectional threshold element240ashould be placed between the voice coil220and the transformer260as shown inFIG. 3B.

The reason why the position of the bidirectional threshold element240adiffers is to minimize the distortion of the speaker drive signal.

By the way, the bidirectional threshold element240abecomes conductive unexpectedly due to noise or abnormal signals, then the weak signal in the voice coil220may be leaked from the speaker.

Accordingly, the present invention prevents the bidirectional threshold element240afrom being abnormally conductive by adding a low pass filter (LPF) and a limiter to block a signal, whose frequency is above the audible frequency, or a high pass filter (HPF) and a limiter to bypass an above-the-audible-frequency signal to another path. This will be described below in detail.

FIG. 4Ashows an example in which an LPF270aand a limiter280are connected to the bidirectional threshold element240aaccording to an exemplary embodiment of the present invention, andFIG. 4Bshows an example in which the LPF270aofFIG. 4Ais implemented with two inductors and one capacitor and the limiter280ofFIG. 4Ais implemented with a varistor.

Referring toFIGS. 4A and 4B, the LPF270apasses an audio signal but blocks an above-the-audible-frequency signal and the limiter280restricts a voltage difference between the speaker signal lines250to a permitted value, thereby preventing the bidirectional threshold element240afrom being abnormally conductive.

FIG. 5Ashows an example in which an HPF270band a limiter280are connected to the bidirectional threshold element240aaccording to an exemplary embodiment of the present invention andFIG. 5Bshows an example in which the HPF270bofFIG. 5Ais implemented with a capacitor and the limiter280ofFIG. 5Ais implemented with a varistor.

Referring toFIGS. 5A and 5B, the HPF270bbypasses an above-the-audible-frequency signal but blocks an audio signal and the limiter280restricts a voltage difference between the speaker signal lines250to a permitted value, thereby preventing the bidirectional threshold element240afrom being abnormally conductive.

Operation of the bidirectional threshold element240aaccording to an exemplary embodiment of the present invention will be described below in detail through test results.

FIG. 6is a measured graph of voltage-current characteristics of the bidirectional threshold element240aaccording to an exemplary embodiment of the present invention.

In the measurement, as changing the voltage of the power supply, we measured the current flowing through a resistor of 31Ω connected to the bidirectional threshold element240aas shown in the left-top portion ofFIG. 6. We measured two types of the bidirectional threshold element240a, one of which was composed of general purpose diodes and the other was composed of Schottky diodes.

As shown in the graph ofFIG. 6, there is a dead zone in which no substantial current flows. The dead zone is in a range of −0.3 V˜+0.3 V in the case of general purpose diodes and in a range of −0.1 V˜+0.1 V in the case of Schottky diodes.

That is, in the bidirectional threshold element240aaccording to an exemplary embodiment of the present invention, the threshold voltage are 0.3 V and 0.1 V in case of general purpose diodes and Schottky diodes respectively.

FIG. 7Ashows a test setup for measuring a voltage amplitude of a weak signal generated in the conventional speaker as changing the input sound intensity to the speaker;FIG. 7Bis a graph showing a maximum voltage amplitude of a weak signal measured at the front (channel1) and rear (channel2) of a transformer160according to the input sound intensity to the conventional speaker inFIG. 7A.

By using a signal generator D1, an audio amplifier (AMP), and the conventional speaker100aas shown inFIG. 7A, we radiated 1 kHz acoustic waves into the broadcast speaker for announcement100c, placed 50 cm apart from the speaker100a, and measured the sound intensity of the acoustic waves incident to the speaker100cby using a sound level meter D4.

As changing the sound intensity, peak-to-peak voltage amplitudes of signals observed at the front (channel1) and rear (channel2) of the transformer160were measured by an oscilloscope. At this time, Inter-M CS-03 was used as the broadcast speaker for announcement,100c.

At a conversation sound intensity of 70˜75 dB as seen fromFIG. 7B, the peak-to-peak voltage amplitudes of channel1and channel2were less than 2 mV and 25 mV respectively. At a high sound level of 95.6 dB such as noise in a factory, etc., the peak-to-peak voltage amplitudes of channel1and channel2were less than 7 mV and 200 mV respectively.

When the bidirectional threshold element240awith the threshold voltage of 0.3 V, which is implemented by general purpose diodes, is connected to a point of channel2(seeFIG. 3A) or the bidirectional threshold element240awith the threshold voltage of 0.1 V, which is implemented by Schottky diodes, is connected to a point of channel1(seeFIG. 3B), the leakage of a weak signal in voice coil can be prevented.

FIG. 8Ashows a test setup for measuring a voltage amplitude of a normal speaker drive signal coming from an audio amplifier (AMP) as changing the output sound intensity measured by a sound level meter (D4) 50 cm apart from the broadcast speaker for announcement,100C;FIG. 8Bis a graph showing a maximum voltage amplitude of a speaker drive signal measured at the front (channel1) and rear (channel2) of a transformer according to the output sound intensity from the speaker inFIG. 8A.

By using a signal generator D1, an audio amplifier (AMP), and the broadcast speaker100cas shown inFIG. 8A, we radiated 1 kHz acoustic waves into a sound level meter D4, placed 50 cm apart from the speaker100c, and measured the sound intensity of the acoustic waves incident to the speaker100cby using a sound level meter D4. As changing the sound intensity, peak-to-peak voltage amplitudes of signals observed at the front (channel1) and rear (channel2) of the transformer160were measured by an oscilloscope. At this time, Inter-M CS-03 was used as the broadcast speaker for announcement100c

Even at a sound intensity of 62.5 dB as seen fromFIG. 8B, the peak-to-peak voltage amplitudes of channel1and channel2were at least 1 V and 15 V respectively.

In conclusion, we can install the bidirectional threshold element240aimplemented by Schottky diodes at a point of channel1or the bidirectional threshold element240aimplemented by general purpose diodes at a point of channel2.

FIG. 9shows a test setup for checking the effect of the signal leakage prevention device240according to an exemplary embodiment of the present invention.

As shown inFIG. 9, we radiated the conversation sounds recorded in an MP3 player D5by using the AMP and the conventional speaker100a, and measured the sound intensity incident to the speaker200, placed 50 cm apart from the speaker100a, by using a sound level meter D4. Then, we tried to listen to that sound by wiretapping the speaker200and using a Low Noise Amplifier (LNA).

According to the results, even when the sound intensity was 90 dB, we could not listen to the conversation sounds due to the device240. However, when the device240was removed from the speaker200, we could clearly listen to the conversation sound, even when the intensity of incident sound was only 70 dB.

FIG. 10shows a test setup for checking whether the speaker200, which includes the signal leakage prevention device240according to an exemplary embodiment of the present invention, normally operates.

As shown inFIG. 10, we radiated the conversation sounds recorded in an MP3 player D5by using the AMP and the speaker200including the device240, and measured the sound intensity by using a sound level meter D4paced 50 cm apart from the speaker200.

In normal operation, the sound intensity was about 76 dB. However, in case that the device240was removed, the sound intensity increased into about 82 dB.

Even though the device240for preventing leakage of a weak signal reduces the sound intensity by about 6 dB, the present invention make a broadcast speaker for announcement operate normally without serious sound quality degradation while blocking a weak signal generated by the sounds around the speaker, thereby preventing tapping through the speaker.

While the present invention has been shown and described in connection with exemplary embodiments thereof, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention as defined by the appended claims.