This weird-sounding mode, which is now thought to be from the Japanese government or Self-Defense Force (Navy), is still heard clearly on six frequencies in the western US, and thirteen in Asia. Its idler makes a continuous chopping and beeping noise, which manifests itself as a little tune something like "Nosiree Bob, nosiree Bob, nosiree Bob Bob Bob." The resulting audio frequencies sound like a broken old-school video game. The "Slot Machine" name, however, was given by yours truly because at time of discovery, it sounded creepily like the little tune played by one of the older pre-video gambling machines at Lake Tahoe, as it spun its wheels preparatory to taking your money.
All frequencies audible in the US are a perfect simulcast, too perfect to be fed by satellites or landlines, perhaps using GPS time for sync. Frequencies we hear come and go with the skip, but listeners in Japan have reported part-time operation on lower-coverage channels. This doesn't sound like anything else on short wave.
The underlying mode being used here is quadrature phase-shift keying (QPSK). The top picture shows this station's weirdest feature, a full on-off or phase-shift keying at a relentless 11 Hz. The "tune" itself derives from the QPSK idler's unique characteristic (seen at right) in which some phases disappear completely. In my (slightly mistuned) spectrum examples, we see the 800-Hz tone by itself, followed by the two tones 160 Hz out (and plenty of IM distortion), then the 3 outer tones, then a chopped-up steady chorus.
Steven A. Harlow has done some good research on this signal, which he summarizes in the manual to his Sigmira program, which includes a decoder which can identify unique frames and sequences being transmitted by the signal. He understands the underlying theory a lot better than I do. He concurs with the other experts with whom I'd previously discussed this, coming to the actually rather common-sensical conclusion that the payload is actually in the several types of hiss that interrupt the idler periodically. It's almost certain that this is encrypted using a system which requires close sync, hence all the other sound and fury. The actual data sent in the payloads, of course, can be anything you can change into binary bits.
The amplitude distribution at bottom shows the waveform's 6 tones more clearly, minus all the distracting noises. All this phase distortion and intermod make these signals about 3 kHz wide - in a narrowband maritime allocation! And why so badly distorted, regardless of propagation, when this is otherwise such a state-of-the-art operation? Strange.
Updated 2012 -
Spectra made with GRAM.EXE; Phase plane from Sigmira.