NATO 39-Tone Radio Modem

NATO uses this variant of the Harris MFSK 39-tone radio modem, which does not have a pilot refrence tone. Clearly visible are the 4- and 16-tone preambles, followed by the parallel data in all 39 tones, narrowly spaced between 675 and 2812 Hz, just fitting a USB/LSB voice channel.

During the 16-tone phase, we can see the initiation of all 39 tones (count 'em), though not all keyed. Soon the application of phase shift keying turns all 39 into visual noise, corresponding to the aural buzz which is all we hear on standard radios.