From HF Underground
The Backwards Music Station, also known as Whales, is nothing more than an audio feedback loop. It is caused by a keyed up transmitter and co-located receiving equipment. Although it is classified under ENIGMA designation XM, it is not a spy numbers station. The transmission does not contain any sort of a message. It is not that interesting.
The Croughton-Sigonella circuit
An audio sample illustrating the way many of these circuits work and why they create audio feedback loops is the following:
The recording was made on an ISB circuit from Croughton (AJE) to Sigonella (NSY).
The circuit sent continuous KG-84 encrypted MIL-STD 188-110B serial tone data traffic on the upper side band (USB), while the lower side band (LSB) was used for voice coordination between Croughton and Sigonella. If you listen closely, you can hear the voice of the operator at Sigonella coming in via the Croughton audio circuit, despite the fact that Sigonella actually transmits on a totally different frequency. So what you are hearing is simply audio from the return circuit being retransmitted by AJE's transmitter that in this case is constantly keyed up. The same setup is used at Sigonella, hence the audio feedback whenever NSY starts to transmit).
To illustrate this further, the setup at Croughton was:
Transmitter on frequency A Receiver on frequency B
The Setup at Sigonella was of course the other way round:
Transmitter on frequency B Receiver on frequency A
There are similar setups on several HF circuits, hence the large occurrence of these audio feedback loops.
- One good example of such a constantly keyed up circuit is the Russian Naval aviation circuit on 11354 USB. You'll hear it keyed up for hours without any voice traffic being sent - only retransmitting atmospheric noise. Then suddenly, it will spring to life with analogue voice, mostly accompanied by fierce audio feedback loops. http://signals.taunus.de/PUB/11354_PRIBOJ_FEEDBACK.WAV
- Another good example of XM transmissions are the French Air Force circuits on 6700, 6712, 8992 and 18012 kHz USB. They leave their transmitters keyed up for hours even if there is no traffic on hand (easily recognized by their constant hum/buzz).
- Recently (September 2010) similar phenomena appeared on Turkish Navy HF radio transmitters on 4382, 5453 and 6485 kHz. Turkish Navy circuit sending feedback, then switching to data and then back to feedback, the data mode is 600 Bd / 400 Hz KG-84 encrypted RATT
- On a Polish air defense circuit, 5066.5 khz USB, you can clearly hear voices in the background coming in via the open mike, then followed by intentional traffic from the radio operator: http://signals.taunus.de/PUB/POL-MIL_FEEDBACK.WAV