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Covert listening device with SC audio masking

SRT-105 was a covert listening device (bug), developed in 1974 by an unknown party 1 for the US Central Intelligence Agency (CIA). The device uses a sophisticated audio masking scheme, known as subcarrier modulation (SC), with random noise injected into the signal's baseband (SC/Noise).

The device measures 19.7 x 13.3 x 2.7 mm – about the size of a penny – and should therefore be considered subminiature for the era. It is cast in epoxy resin, and weights just 1.4 grams. The image on the right was taken from the manual.

It operates by frequency modulating (FM) the intercepted audio onto a 40 kHz subcarrier (SC), which is then frequency modulated onto the 350 MHz carrier. As an extra secrecy measure, a strong (random) noise signal, is injected into the device's baseband audio channel, which defeats the squelch of an ordinary surveillance receiver.

Compared to other bugs of the era, such as the SRT-91 — which was developed by the NRP in the same year — the SRT-105 is extremely small and much easier to conceal. Nevertheless, it is a continuous wave (CW) transmitter, which was easily detected with the available TSCM equipment. Although its noise-injected subcarrier modulation masking scheme was very sophisticated, it could be detected and demodulated within seconds by Audiotel's Scanlock Mark VB bug tracer.

  1. From surviving documents it is known that – unlike many other bugging devices – this particular model was not developed and manufactured by the Dutch Radar Laboratory (NRP). However, the NRP did developed a suitable surveillance receiver for it – the SRR-90 – under the Easy Chair research contract.

The diagram below is taken from the original instruction manual, and shows the layout of the device [A]. The body of the transmitter, which is cast in epoxy resin, measures just 19.7 x 13.3 mm, with the top of the Knowles electret microphone extending from the upper side. At the right is a cut-out with two terminals, to which the 1.2 to 3V battery voltage should be supplied.

At the top-left is a recessed socket to which the antenna (wire) should be connected. The wire should be approx. 20 cm long. The frequency of the continuous wave (CW) FM-carrier can be adjusted between 350 and 385 MHz with a small screw at the bottom left. An adhesive aluminium label, fitted at the flat side of the device's body, shows the model and serial number. This label was usually removed from the device, prior to installation in the target area.

Compatible receivers
Surveilance receiver SRR-90-A Surveilance receiver SRR-90-B SRR-153 surveillance receiver
Subcarrier demodulator (for audio frequencies)
Audio masking
To hide the RF carrier and its modulation from regular surveillance receivers, professional bugs often use a special technique that is known as audio masking. The SRT-105 uses a sophisticated masking scheme, in which a 22 kHz frequency modulated subcarrier, is frequency modulated onto the 290 MHz RF carrier, whilst at the same time injecting noise in the channel's baseband.

FM subcarrier, frequency modulated onto an RF carrier

This technique is known as subcarrier audio masking and defeats any non-compatible receiver. It is safe enough to hide the signal from an unexperienced eavesdropper and to prevent accidental demodulation in a standard receiver. A similar technique is used in the CIA's SRT-153 and in the OPEC bug, although the latter injects a strong 50 Hz hum into the baseband rather than noise.

 More about subcarrier audio masking

The SRT-105 was supplied with a tiny instruction manual that is shown in the image on the right. It measures approx. 105 x 85 mm (roughly DIN A7 format) and the pages are held together with a plastic ring. It was made in October 1974.

The manual contains operating and installation instructions, various measurement charts, the technical specifications and the circuit diagram.

 Download manual


Despite the fact that the SRT-105 features a professional-grade audio masking technique, bugs of this type can be discovered relatively easy with a bug detector, such as the Audiotel Delta V, or with a professional bug tracer, like the Audiotel Scanlock Mark VB shown in the image on the right.

 More information

Scanlock Mark VB in operation

Because of the wide variety of subcarrier-modulated bugs used by the CIA, produced by different contractors, there were sometimes difficulties when trying to receive their signals on an SRR-153 or the earlier SRR-90 receivers. In many cases it was unclear whether this was caused by the bug or by the receiver.

Especially for this purpose, the NRP developed the UVK-153 transmitter tester, which is able to check every aspect of a bug.

 More information

UVK-153 transmitter tester

Circuit diagram
Below is the circuit diagram of the SRT-105 as it is presented in figure 4-1 of the instruction manual [A]. At the top right is a single-stage oscillator, which forms the actual transmitter. It is coupled directly to the antenna. The circuit at the top left is the audio masking noise generator. The signal from the noise generator is amplified and fed directly to the oscillator/transmitter.

The bottom half of the circuit diagram comprises the Knowles BL-1681 electret microphone (at the center), a two-stage audio amplifier (Q1 and Q2) and the 40 kHz subcarrier oscillator (Q3 and Q4 at the right), the output of which is also fed to the oscillator transmitter at the top right. The circuit around Q7, Q8 and Q9 at the bottom left, is a voltage stabilizer for the subcarrier stage.

Wanted item
Unfortunately, we have only found the instruction manual of the SRT-105, but not the actual device itself. Crypto Museum are currently looking for an original SRT-105 transmitter for its collection. It you have such a transmitter, or if you can provide other information about this device, please contact us.

  • Frequency
    350 to 385 MHz (tunable)
  • Subcarrier
    40 kHz ± 3kHz (0 - 55°C)
  • Modulation
  • Masking
    Random noise
  • Voltage
    1.2 to 3V DC
  • Current
    2.5 mA to 5.8 mA (1.35V to 2.7V)
  • Power
    0.4 mW to 2.9 mW (at 25°C)
  • Dimensions
    13.3 x 19.7 x 2.7 mm (0.525 x 0.775 x 0.105 inches)
  • Weight
    1.4 grams
  • Temperature
    0 to 55°C (-20 to +71°C with reduced performance)
  1. Instruction Manual SRT-105
    October 1974.
  1. Anonymous source, SRT-105 instruction manual
    Retrieved March 2017.
Further information
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Crypto Museum. Created: Thursday 09 March 2017. Last changed: Tuesday, 16 January 2018 - 17:47 CET.
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