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Telephone Eavesdropping and Detection


Click here to play video.Infinity Transmitter

The infinity transmitter, frequently called the harmonica bug or mittleman device, is used as a stand alone type of eavesdropping device. However, it was at one time considered a type of hookswitch bypass .

Infinity transmitter is connected in parallel to the telephone line and is electronically isolated from the line in much the same way as a high impedance wiretap. To better understand the operating functions of aninfinity transmitter, let's discuss its 20-year history. When the original tone activated infinity transmitter was invented, most telephone company central office exchanges were the electro-mechanical type. This type of exchange allowed for a series of mechanical relays to close as each digit was dialed by the calling (eavesdropping) party. As the last number was being dialed, the last relay made contact and connected the calling party's telephone line to the target's telephone line. Therefore, a voice path existed before the target party's telephone instrument would ring.

One design function of the infinity transmitter was to answer the call upon sensing a special tone, before ringing voltages activated the instrument's ringer. Therefore, the eavesdropper would blow into a small harmonica while dialing the last number. The infinity transmitter would answer the call and instantly trip the ringing voltage to prevent the activation of the instrument's ringer. As with the "keep live" devices, the drawback to this type of attack was that, whenever the device was active, the line would be busy for other callers.

When the telephone industry began changing the electro-mechanical exchanges to electronic switching types of exchanges, the original infinity transmitter became obsolete. With ESS types of exchanges, the called party's telephone line does not make contact with the calling party's line during the ring cycle. Contact is made only after the calling party answers the telephone.

The new style of infinity transmitter, promoted as a home burglar alarm, is programmable to automatically answer the call after the set number of ringing voltages cycles are sent to the device. Once the device answers the call, it will wait for some type of tone signal before activating its built-in microphone. This type of Infinity Transmitter must still be considered a threat. Consider, an eavesdropper runs another telephone line into the target area and connects this Infinity Transmitters. This line would not be connected to any telephone instrument ringers and would therefore never alert the target.

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