Anybody know just what mode this is?

[Ben Hutchinson]

Sounds like some kind of MFSK mode but with timing/sync pulses used as well (not typical of normal MFSK modes). I haven't found much info on it, but it has been refered to on mystery signal websites as "Slot Machine" or XSL. My spectrogram shows it as having a much wider band than normal MFSK modes used for ham communications (uses about 3khz bandwidth). It's also in a non-ham band, ruling out experimental ham mode as a possibility. From the one or two sites that say anything on it at all, it is said to be suspected to be from Japanese military. I suppose if that's true, then that would mean that there's no way to decode it, and even if someone wrote digital mode software that could decode it, it would probably be illegal (though hopefully if it indeed did carry secret information it would have been encrypted with something like AES or Triple-DES before transmission).

I heard it a few times on my ICOM-PCR1000 receiver, in the 6000khz-7000khz band and the 8000khz-9000khz band. However I unfortunately never made a recording of it, and now those bands are getting a bit noisy, making it nearly impossible to find it now. But I did find someone else's MP3 audio recording of it here:
http://www.brogers.dsl.pipex.com/slot.mp3

If anyone here has any more info on it, please post it here.

[John_D]

Check it out here:-
http://www.nonstopsystems.com/radio/radio-sounds.html

[Ben Hutchinson]

Thanks but that isn't any of the modes on that web page. Although I did bookmark that page for future reference.