The Infinity Transmitter - An Old Bug That Had Its Time

by Howard

There is always a great hush when the term infinity transmitter is mentioned, as if it were some amazing secret device, but it can be simply explained.

The infinity transmitter or "harmonica bug" is a device installed within a target's phone.  This device allows a person to call the phone and listen in on him while he is quite unaware.  This device has a few problems, the biggest of which is that the target's phone must be connected to either a crossbar or step-by-step switch.  The other drawback is that the bug must be installed in the target's phone.

This means one must enter the house, place the bug in the phone, and rewire it as required.  This bug could also be detected if the target were to attempt to use his phone while you were monitoring his activities.  Since you are on his phone line listening to him, he might think it strange that his phone was being used, especially if he has any technical background.  Let's see how to use the bug once it is installed.

Once installed all the observer has to do is call the target's phone number.  After the observer dials the last digit, he sends a specific tone down the phone line which causes the bug to answer the phone before it rings.  The frequency of the tone is user selectable and set during the construction process.  The exact frequency of the tone is quite unimportant.

This type of bug can be used from anywhere there is a phone.  The potential distance is infinite hence the name "infinity transmitter."  Ending the audio visit with the target is just as easy as starting it.  A different frequency tone is sent down the line telling the bug to hang up.  Overall, a very simple concept.

The reason this bug works on step-by-step and crossbar switches is because in these systems the audio and ring generator are connected to the phone called before it is answered.  So it is possible for the bug to answer the phone before the ring capacitor is fully charged by the ring generator.

ESS and DMS switches do not connect the audio to the called phone line until after the phone is answered, making the infinity transmitter useless.  In the case where the user does not apply the pick-up tone immediately, the phone would ring, then stop suddenly.  Therefore some skill is required to avoid tipping the target off to the fact that he is being watched.

Construction of this device should be relatively easy for someone with a little experience in the electronics world.  The bug would be isolated from the phone by using two non-polarized capacitors of 1 µF or better.  It would mainly consist of two frequency detectors.  One would connect the audio from the mouth piece to the phone line and answer the phone when the pick-up tone is detected.  The other would disconnect the audio from the mouth piece from the phone line and hang up the phone when the hang-up tone appears.