Just when Cuban numbers seemed to be on the decline, they came up with this completely new mode for their intelligence broadcasts. It started in late November of 2012, and at this time it is in regular daily use. ENIGMA 2000 has already designated it HM01, for Hybrid Mode.
This mode is the first of its kind. A transmission begins with the traditional "female" machine voice giving six 5-figure header groups for just under two minutes. This is followed by a the first header group, again "spoken," followed by a file transfer in a weird old ham radio mode called RDFT. When this is finished, the next header group is sent, followed by another RDFT file transfer, and so on for about a half hour. At this point, the whole thing typically repeats, though since it's Cuban numbers, nothing is for sure.
The purpose of the I/Q plot (left) is to show that the transmission mode is indeed AM. The same intelligence is in both sidebands.
The structure of RDFT is visible here. It's a strange mode to begin with, with its nine phase states (zero change being meaningful information) and 8 orthogonal carriers. It was ultimately implemented by DIGTRX, a fussy and highly unstable little ham radio program, for the purpose of what was called digital slow-scan TV. It lost out to HamDRM many years ago.
Under the hood, though, we're really looking at a straight binary file transfer, which is exactly how Cuba uses it. Each little burst transfers a different little binary file, purposely mislabeled with the .txt extension. but printing gibberish if you open it as text. (Hex dumps, however, can be rather illuminating.)
Early SK01 transmissions even had the DIGTRX waterfall picture, but that's long gone. Today, it's very likely that SK01 and HM01 are the only RDFT applications left in the world. While HM01 is actually a pretty nice piece of engineering, it's still often sent over some truly wretched transmitters.
Click here for a very informative, annotated spectrogram of a complete HM01 transmission.
Plots made with Spectrum Lab