Scrambled voice from Salyut 6

Richard S Flagg and Sven Grahn

Based on an internal Kettering Group technical memo dated September 5, 1978



In 1978 scrambled voice signals were received infrequently on 121.75 MHz from Salyut 6. These voice signals sounded as though the normal voice spectrum had been inverted. Early in July 1978, Sven Grahn and Richard Flagg heard the scrambled voice on two successive days while tracking from Sollentuna. Circuitry was constructed and successfully used to unscramble the tape recorded voice transmissions.


A typical speech frequency spectrum is shown in Fig 1 below (derived from CQ, Jan 1970, p. 51).

This audio spectrum is represented schematically by Fig.2

Assume that the scrambling is done by audio spectrum inversion. This can be done by means of a balanced modulator and low-pass filtering. A balanced modultor is in essence an analog multiplier circuit.

If two sine-waves are multiplied the following is obtained:

i.e the sum and difference frequencies.

So, if an audio spectrum is multiplied by a constant carrier frequency to side-bands appear, but the carrier is suppressed (Fig.3):

 A steep low-pass filter only transmits the signal below F3 and F3 is chosen so that he resulting spectrum falls within the original pass-band of the voice channel. Assume that F1=0.3 kHz and F2=3.3 kHz. Then F3-F2 must be equal to 0.3 kHz and F3=F2+0.3=3.6 kHz. The inverted audio spectrum transmitted by the Salyut voice transmitter and received and recorded on the ground is therefore (Fig. 4):

To unscramble this the spectrum inversion procedure is applied once more (Fig. 5) with the new carrier frequency F4:

If the resulting spectrum is low-pass filtered around F4 and F4=F3 then the original audio spectrum is recreated, i.e. unscrambled.

Practical unscrambling

In the experimental unscrambling made on July 9, 1978 the best choice of F4(=F3) was found to be 3.763 kHz, rather close to the theoretical value 3.6 kHz deduced above. The balanced modulator was an Analog Devices AD 530 IC analogue multiplier. The experimental set-up can be described by the block diagram below (Fig. 6)

The level from the tape recorder was 200 mV and the carrier oscillator level 5 V RMS at 3.763 kHz. The low-pass filter corner frequency was set at 3.1 kHz and the maximum flatness 8-pole Butterworth filer mode was selected (48 dB/octave roll-off). 20 dB additional gain was needed to drive the low impedance loudspeaker in series with a 40 ohm resistor. The setting of the carrier oscillator frequency is crucial, and the wrong setting results in a garbled audio signal much like the wrong BFO setting when receiving SSB. Otherwise the unscrambled voice sounds rather good considering the quickly assembled character of the unscrambling set-up.

Sample Recordings

Recording of scrambled voice (starts at 1604.24 UT on July 9, 1978)
(does not exactly correspond to unscrambled recording): Unscrambled signal


Let me quote from an abstract of an article in Pravda, May 6, 1979 p 3 by V Merscheisky, under the headline "Improved communications for orbiting station includes 'speech inversion' device":

".....With regards to space-to-earth communications, the author points out that flights lasting for months require increasingly frequent consultations between cosmonauts and medical specialists, and the cosmonauts often wish to keep their talks with physicians confidential. Therefore the station's communications system now includes a small instrument called 'Konvertor'. Its function is to 'invert' the frequency spectrum of ordinary speech. An analogous instrument is installed in the medical office of the Flight Control Center. By simply pushing a button, a cosmonaut can make his speech comprehensible only to the attending physician. It is noted that sometimes the cosmonauts also use this instrument to talk about 'domestic matters'...."


The crew aboard Salyut 6 at this time consisted of commander Col. Vladimir V Kovalyonok and flight engineer Alexander S Ivanchenkov, launched to Salyut 6 on June 15, 1978 aboard Soyuz 29. The recording starts at 1604.24 UT on July 9, 1978, when the cargo ship Progress 2 had been docked to Salyut 6 for about 3 hours and the crew had obviously started to unload it. The translation below has been provided by Gene Kozin:

[begins in the middle of a phrase]

"... after taking Eunoctin (1). Are you clear about that? [pause] . ..

[Now a different voice] (Kovalyonok's probably because he mentions something his crewmate Sasha {=Alexander, i.e. Ivanchenkov} had said)

"I have checked the cargo ship as well. On the cargo ship there is no... There is just the same Eunoctin. Although the medical kit on the cargo ship should have been put together considering our crew. Sasha made the same observation. I ask you to log this, for it was authorized with him, pass it up the chain of command and report it to the 19th tomorrow so that he would find who's responsible and that measures would be taken to them appropriately. Up to the penalty! And inform us about the outcome. OK?"


  1. "Eunoctin"(Nitrazepam) is a sedative medicine used to treat insomnia. See this link for details. Thanks to Vladimir Grigorjev for this information.

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