Japanese "Slot Machine" Oddity (XSL)

This weird-sounding mode, which is now thought to be from the Japanese Navy, is now heard clearly on six frequencies in the western US, and thirteen in Asia. It makes a continuous chopping and beeping noise, which sounds like a broken video game or a Vega$ slot machine about to take your money. It's been heard worldwide on 8 MHz for about 3 years. 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.

Modulation looks like multitone PSK. The top picture shows this station's weirdest feature, a full on-off or phase-shift keying at a relentless 11 Hz. It also keys the 6 unevenly spaced tones, which repeat a 7-second, musical-sounding pattern forever. This "tune" is 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. It is almost certainly an idler. It stops every few minutes for a louder hiss, minus tones but with the 11-Hz chopping, that may be high-speed data.

The amplitude distribution at bottom shows the 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.