Mexican Military Scrambling

This hooty signal popped up all over HF on practically the same day the Mexican Army's loud "radiogrammas" vanished from the air. Since nothing much else changed about this traffic, which is heard constantly in the southwestern US, we decided it was almost certainly the same people.

This stuff sounds a bit like analog inversion, and a bit like really old Telco "vocoder." This is because the encoder first slices the speech into several narrow, very ringy bands, as we see clearly. We also see how quite a bit of inflection makes it through the process, allowing the brain to pull out an occasional Spanish word. Never underestimate "wetware" decoding!

The amplitude graph at the top of the first picture is interesting in that the received waveform is completely symmetrical. This is not a characteristic of natural human speech, and it further explains the mechanical sound of this mode.

The highly averaged frequency plot below shows the precise spacing of the inverted bands, not to mention their extreme peakiness. This dramatically shows the reason for the hooty sound.

All this is so the bad guys can't hear some VERY mundane traffic about recruiting "guarnacions" and toilet paper requisitions (as I heard once in a radiogramma). An army travels on something else besides its stomach.