ANDVT

This is the Advanced Narrowband Digital Voice Terminal used by the US military for secure voice communication on HF. It's heard a lot, usually associated with KY-99 or other voice encryption modules, during exercises and drug operations. It's controlling more or less the same 39-tone modem seen elsewhere in our NATO spectrogram, but in a different manner, using a pilot tone and more complex preambles.

This particular transmission is using ANDVT's "short interleave" mode (MIL-STD 188-110A). The various synching tones last around six tenths of a second, after which we see all tones initiate and begin to transfer data using tribit (8-phase) TDPSK keying. This creates the hissy, buzzy, noise associated with 39-tone modems. The red line near the bottom is the pilot tone, which makes this ANDVT mode sound a little more beepy than the NATO burst plotted. At the end there's another short beep as the interleaver is flushed, but we don't see this here.