Mystery Signals of the Short Wave

Dedicated to the more unusual, strange, bizarre and apparently meaningless signals on the short wave bands !

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The Slot Machine   -   XSL   (formerly XMEW)

Frequency:   (Various)

Known Frequencies
4231.5 kHz 6417 kHz 8588 kHz
4291 kHz 6445.1 kHz 8704 kHz

A relative newcomer to the shortwave bands, this signal was first reported by monitors around October/November 2000.

The signal sounds like a series of musical tones consisting of one low tone followed by two high tones, repeated but made up of varying lengths and spacing, against a constant pulse tone. This would seem to be the signal in idling mode. At times the pattern changes into a burst of rapid pulses and tones.

  "Slot Machine"  (49 kb)

This signal appears to be sending intelligent data, and looks to be a new type of communication mode. The clock pulse constantly present indicates a form of fax or teletype system, but could also be a multi-tone system.

The signal transmits continuously on pairs of frequencies in the 4MHz, 6MHz and 8MHz bands, which places it firmly in the ITU bands allocated for Maritime use.

Reports show the signal to be stongest in the Far East, indicating an origin in that part of the World. Although the signals are weak in Europe, they can be monitored in the evenings on both the 6MHz and 8MHz frequncies. The poor reception makes analysis of the signal difficult.

An article in Monitoring Times, December 2002 was the first to identify these signals as Japanese Navy. Writing it the "Utility World" column, Hugh Stegman outlines his reasons for this claim. Firstly, direction-finding fixes indicated Japan at the source of the signals, although China and Russia were not ruled out.

Secondly, the frequencies correlate with those previously used by the Japanese Navy for eight-tone radio modems, some of which disappeared at the right time.

Finally, monitors travelling to Japan identified additional strong local frequencies, some of which were only operating on a part daily basis.

The "Slot Machine" tones are believed to be synchronising pulses, or idling states, with the bursts of data heard being high-speed MPSK. (Multitone Phase Shift Keying) - a technically advanced form of radio teletype system.

Hugh gives the full list of frequencies as;
3058 kHz 3075 kHz 4152.5 kHz 4231.5 kHz 4291 kHz
5643 kHz 6249.5 kHz 6417 kHz 6445.1 kHz 6500 kHz
6693 kHz 6768 kHz 8312.5 kHz 8588 kHz 8704 kHz

Following Hugh's article, more monitors became involved in the study of these strange signals. It is now generally accepted that Hugh was correct in his identification, and that these signals are indeed a complex digital communication system used by the Japanese Navy.

It still remains a challenging and difficult signal to monitor at any reasonable strength in Europe.

Update July 2004