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Bodil B
Telephone line carrier bug - under construction

BODIL B (Bulgarian: Бодил) 1 was a Cold War covert listening device (bug) for room surveillance, developed around 1978 in Bulgaria, at a time when the country was part of the Warsaw Pact. 2 The device was also used by other WP countries, like the DDR (East-Germany), and by the USSR.

The device is housed in a cylindrical aluminium enclosure, that is 47 mm long, has a diameter of 11 mm and weights just 12 grams. At the front is a removable cap behind which a subminiature BT 1751 microphone with built-in buffer – made by Knowles 3 in the USA – is located. At the rear are two teflon wires, by which the device can be connected to an analogue 'host' telephone line.

The device does not contain a radio transmitter, but instead acts as a parasitic device on the host telephone line, using the line as a power source and and a transport medium for its intelligence.
The BODIL telephone line carrier bug

Carrier modulation is used to conceal the presence of the bug on the line, which means that the (analogue) telephone line can be used as normal, without the bug being overheared during the conversation. BODIL is remotely activated by injecting a 30 kHz carrier wave into the phone line.

The injection is done at the tapping point, which is commonly somewhere outside the target area or even outside the building. As its frequency is above the audible range (supersonic), it can not be overheard accidentally by any calling parties.

Inside the device, the 30 kHz carrier frequency is multiplied to 60 kHz and phase-modulated (PM) with room audio, before it is returned to the line. A Bodil B2 receiver – installed at the tapping point – can be used to demodulate the 60 kHz signal and retrieve the audio, so that it can be recorded on tape for analysis and transcription.
Protective cap removed

The protective metal cap at the front end of the device is normally glued in place, but has been removed here so that we can check the interior. According to a test report of June 1981, the East German security service, the Stasi, had approved the device for operational use and was going to order 700 bugs, 500 receivers and ~200 rented-line interfaces over the course of five years [4].

We should like to thank Detlev Vreisleben in Germany for giving us access to BODIL and its technical and historical documentation, obtained from the archives of the former Stasi [1][2]. Without his help, this page would not have existed.

  1. Бодил (Bodil) is the Bulgarian word for Thorn.
  2. In its capacity as a highly-developed technological country, Bulgaria was often referred to as the Silicon Valley of the Eastern Bloc at the time [3].
  3. Ironically, many of the Knowles subminiature microphones were developed during the 1970s with funding from the US Central Intelligence Agency (CIA).

The BODIL telephone line carrier bug The sound port of the Knowles microphone protruding the protective cap Bodil with wiring BODIL transmitter Protective cap removed Close-up of the Knowles 1751 microphone
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The BODIL telephone line carrier bug
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The sound port of the Knowles microphone protruding the protective cap
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Bodil with wiring
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BODIL transmitter
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Protective cap removed
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Close-up of the Knowles 1751 microphone

A complete BODIL B bugging system consists of the following items:

  • BODIL B1
    This is the actual bug as shown above. It is powered by a strong 30 kHz AC signal, sent to it via an analogue telephone line. The device produces a modulated 60 kHz signal that is sent as a supersonic (subcarrier) PM signal via the telephone line to a BODIL-B2 receiver.

  • BODIL B2
    This is the base station that is connected to the telephone line at the tapping point. It contains a 30 kHz signal generator (used for activating the BODIL-B1 bug) and an FM demodulator for supersonic (subcarrier) phase-modulated signals, sent by BODIL-B1.
The diagram below shows how and where the two devices are connected to the telephone line. At the left is the POTS 1 telephone exchange from which multiple two-wire cables (here shown in red/blue) are connected to homes and offices. Somewhere along the way to the target area, the wires are tapped and the BODIL-B2 receiver is covertly connected to the line. This can be done in a patch case, at an underground cable junction or amplifier, or at the telephone exchange itself.

The BODIL B1 bug is installed in the target area and is connected to the telephone line, in parallel to the telephone set. By default, the bug is inactive. In order to activate it, the B2 receiver has to be switched on and sends out a 30 kHz / 850 mV activation signal. This signal is rectified into a DC voltage that is just high enough to power the bug. Inside the bug, it is also doubled to 60 kHz, which is then phase-modulated (PM) with the ambient sound and returned to the line.

This process is known as carrier modulation, and is frequently used in radio bugs as an audio-masking technique. As the 30 kHz and 60 kHz signals are relatively far apart, they can easily be discriminated at the receiving end by using band filters. Instead of the loudspeaker, it was also possible to use an additional telephone line to guide the signal to a state security listening post.

The security service of the former DDR (East Germany) – the Stasi – also used the central antenna installation (CAI) of an appartment building as the transport medium for BODIL. Especially for this application, a transformer-based modification was suggested, that allowed the B1 bug and the B2 receiver to be connected directly to the coaxial cable of the central antenna system [C]. In such cases the bug was usually placed inside or near the antenna wall socket of the TV/radio set.

  1. POTS = Plain Old Telephone System. Common expression for the old analogue telephone system.

The transmitter, also known as BODIL-B1, is shown in the image on the right. It is extremely small – especially considering its age – and can easily be hidden in the target area.

It was connected to the telephone line, which means it had to be installed inside or near the telephone set, or a cable junction box. In case the TV antenna cable was used as a transport medium, the transmitter was often hidden inside the antenna wall socket or inside the TV set.
BODIL transmitter

Receiver   wanted item
A special receiver, known as BODIL-B2, was used for remote activation of the transmitter and for demodulating the returned masked audio signal.

The image on the right shows a typical BODIL-B2 receiver [1], which is currently missing from our collection.
BODIL-B2 receiver. Image kindly supplied by Detlev Vreisleben [1].

Block diagram
The block diagram below shows how the transmitter works. At the far right is the telephone line from which the 30 kHz activation signal is received (sent by the receiver). This signal is filtered and then rectified, creating just enough DC voltage to power the circuit and the microphone.

Block diagram of the BODIL-B1 transmitter

The 30 kHz is also doubled to 60 kHz and fed to the modulator, where the amplified sound from the microphone is added, resulting in an PM modulated 60 kHz subcarrier signal that is injected into the telephone line. The 30 kHz and the 60 kHz signals are way above the audible range, so that they can pass the cable unnoticed. Yet they are sufficiently low to pass the existing cables and junction boxes, especially if the distance between transmitter and receiver is not too large.

Block diagram of the BODIL-B2 receiver)

The receiver is connected between the telephone exchange and the subscriber line, as shown in the above block diagram. At the left is the 30 kHz generator that delivers the activation signal for the bug. The returned modulated 60 kHz signal is filtered, demodulated, amplified, filtered again and compressed, and is then fed to the headphones and/or a recorder. Alternatively, the signal can also be delivered directly to an external state security listening post via a rented line. The receiver can be powered from the mains, by internal batteries, or from the (optional) rented line.

BODIL-B1 interior seen from both sides. Click for a closer look.

The interior of the BODUL-B1 transmitter is easily accessible. All you have to do is remove the protective cap from the front end of the enclosure and carefully pull the Knowles microphone that sticks out below the cap. If the teflon wires are not blocked, the interior should come out easily.

The unit is built on a silver-plated frame and consists of two printed circuit boards (PCBs), each constructed from a ceramic substrate. One board holds the power circuit, whilst the other one holds the frequency doubler and modulator.

At one end of the frame is a 'large' transformer that is needed to obtain a galvanic separation between the bug and the telephone line. On top of the transformer is a Knowles BT 1751 or 1752 microphone with a built-in buffer. The three wires of the microphone are guided through a hole at the center of the cylindrical transformer.
Bodil-B1 interior

Microphones like the Knowles BT 1751 are extremely sensitive and offer a very good audio quality in the smallest possible package. For that reason they were commonly used in the hearing aids of the 70s and 80s. Ironically, they were developed with help from the Central Intelligence Agency.

As part of their miniaturization strategy, the CIA funded much of the research and development at Knowles at the time. They did that, of course, with the intention to use them in their own bugs, but despite Cold War export restrictions, they could not avoid that many of them found their way into Eastern Bloc countries, commonly under the guise of local hearing aid manufacturing.

Another point of interest is the sophisticated manufacturing skills of the Bulgarians at the time. The image on the right shows a close-up of a series of embedded laser-trimmed resistors.
Miniature components bonded to the ceramic PCB substrate

Each resistor is consists of a small rectangular ceramic chip on which resistive material has been evaporated. Gold-plated contact strips are present at either end. Seven such resistors are directly bonded to the ceramic PCB substrate and are fixed in place with a drop of transparent epoxy.

Protective cap removed Bodile circuit protected by a transparent plastic sleeve Modulator board Another view of the modulator Power board Another view of the power circuit Bodil-B1 interior Miniature components bonded to the ceramic PCB substrate
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Protective cap removed
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Bodile circuit protected by a transparent plastic sleeve
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Modulator board
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Another view of the modulator
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Power board
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Another view of the power circuit
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Bodil-B1 interior
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Miniature components bonded to the ceramic PCB substrate

Circuit diagram
Below is the circuit diagram of the transmitter. At the bottom right is the telephone line (a/b) from which the 30 kHz activation signal is received. This signal passes transformer Tr1 and is then rectified in D5-8 into a 60 kHz pulsating voltage. Behind diode D4 this signal is smoothed by C5 into a stable DC voltage that powers a 3-stage amplifier consisting of transistors T1-T3. The voltage for the FET (T4) is delivered by a stabilizing circuit around D1, D2, D3 and C2.

The signal from the electret microphone (Knowles BT-1751), is amplified by the 3-stage amplifier (T1-T3) and is then fed to the gate of T4, which acts as a voltage controlled resistor (much like a potentiometer). The output of T4 is mixed (modulated) with the pulsating 60 kHz signal from the bridge rectifier (D5-8) and fed to transformer Tr2, which injects it into the telephone line (a/b).

  1. БОДИЛ Б (BODIL B), Technical manual and operating instructions
    Bulgaria, 1979. Original manual in Russian language.
    BStU, 21 pages marked BSTU 0178—0199. 1

  2. Bodil, Beschreibung
    DDR, date unknown. Hand-written description (German).
    BStU, 10 pages, marked BSTU 0223—0232. 1

  3. Bodil Anpassung, Beschreibung des Vorslages
    Hauptmann Bräunig, Suggested modification (German).
    Date unknown. BStU, 2 pages marked BSTU 0221-0222. 1

  4. Erprobungsbericht System 'Bodil B'
    Oberstleutnant Sillmann, Test report System 'Bodil B' (German).
    Berlin (DDR), 17 June 1981, BStU, Abt. 26 Nr. 1551. 1
  1. Document from BStU archives [2], kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, BODIL, technical and historical documentation
    Personal correspondence, April - May 2018.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. Wikipedia, Bulgaria
    Retrieved, May 2018.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated to BStU.

Further documentation
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Crypto Museum. Created: Wednesday 02 May 2018. Last changed: Sunday, 06 May 2018 - 06:14 CET.
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