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Rocking Chair   RC
Room bugging via telephone line

Rocking Chair, abbreviated as RC, was a secret research program carried out at the Dutch Radar laboratory (NRP) on behalf of the US Central Intelligence Agency (CIA), as part of a long term development contract under the name EASYCHAIR. The research aimed to develop a covert listening device (bug), that worked in a similar way to the EASYCHAIR Mark I device, and that would be powered by and activated via a regular analogue telephone line, using a WEC device.

The system involves the use of a so-called Passive Element, or PE, similar to the one used with the Easy Chair Mark I listening device, that is connected to an existing analogue telephone line inside the target area. The PE is powered by a strong RF signal injected into the telephone line from a tapping point outside the building.

This done by means of the Wired Easy Chair, or WEC, shown in the image on the right. The WEC consists of a transmitter, a receiver and a line matching unit or interface that also acts as a duplexing unit for the transmitter and receiver.
WEC Mk I in green suitcase

WEC was originally developed for using an existing analogue telephone set, such as the Ericsson Model 1951 and the Heemaf Model 1955, to intercept a conversation in the target area. However, it could also be used for the Rocking Chair (RC) project without any modifications whatsoever.

To activate the bug, the WEC injects a strong RF signal with a frequency between 20 and 300 kHz directly into the telephone line. As this signal is well outside the audible range, users of the line will not notice its presence. Like the EC I, the bug absorbes the RF energy and converts it into a DC voltage that is used to power a small audio amplifier. Any sound in the room is picked up by a microphone and amplified in the amplifier, which causes modulated sidebands to be generated. These sidebands are fed back to the diodes and eventually, through a transformer, to the line.

The modulated reflected signal is picked up by the sensitive receiver of the WEC unit, that is installed at a covert tapping point outside the target building. In order to separate the weak reflected signal from the strong activation signal (that operates on the same frequency), the WEC contains an advanced duplexer that acts as a cancelling unit. After demodulating the reflected signal, a high-quality reproduction of the original intercepted audio will become available.

 More information about Wired Easy Chair (WEC)

The application of the RC is limited to those situations were a regular (analogue) telephone line is present in the target area, which is connected to the exchange of a Public Switched Telephone Network (PSTN) somewhere outside of the target area. Furthermore, it is necessary that a tapping point can be made inconspicuously on the line between the target area and the exchange. The Passive Element (PE) is now placed in the target area and is connected in parallel to the telephone line. It will be powered by the RF energy from the WEC unit that is placed at the tapping point.

The diagram above shows how this works. The tapping point can be made at an existing junction box, in a building where the cable passes through, at the exchange, at an underground cable joint, or by digging up the cable and cutting into it. For situations where it was difficult to access and operate the WEC at the tapping point unobtrusively, a remote control unit was developed.

The diagram above shows the additional remote control unit (RCU) at the bottom left. It is connected to an extra telephone line that is connected to a remote listening post (LP). The WEC unit can be controlled via this telephone line which also delivers the intercepted audio at the LP.

Passive Element   PE
Below is the circuit diagram of the Passive Element (PE) that was used in the Rocking Chair (RC) system. It is similar (but not identical) to the PE of an Easy Chair (EC) system, such as the EC Mark V. At the far right is the line interface by which the PE is connected to the existing telephone line.

The interface has been dimensioned in such a way, that it provides minimum loading on the line, making it very difficult to detect. Two HG5004 diodes are used to convert the RF signal into a DC voltage that is high enough to power the rest of the circuit. The actual PE is built around three Philips OC44 transistors in a rather strange arrangement. The first two transistors (T1 and T2) are used as pre-amplifiers, whilst the third one is the final amplifier that feeds back the modulated sidebands to the rectifier diodes, which will in turn pass the signal on to the telephone line.

All transistor stages are coupled via miniature Fortiphone 1 transformers. A sensitive dynamic microphone, such as the Shure MC-30, must be connected to the coloured terminals at the left. A 1:10 transformer is provided to allow the use of both high and low impedance microphones.

  1. At the time, Fortiphone was a British manufacturer of heading aids, and was able to supply the smallest possible components. Fortiphone transformers were also used in the PEs of the the EC I thru EC V.

  1. Report on RC-System (R.C. Final Research Report)
    CM302599. NRP/CIA 31 October 1961.
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Crypto Museum. Created: Thursday 30 March 2017. Last changed: Monday, 21 November 2022 - 16:05 CET.
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