"Backwards Music" station
bbruhns at erols.com
bbruhns at erols.com
Mon Feb 1 13:38:51 CST 2010
I don't know, but I'll venture a guess.
It really sounds like an analog SSB repeater with choppy agc, and / or noise gating, hearing its own input through selective fading. Maybe deliberately, maybe by accident, there is a frequency error in the repeat chain, such that a signal is repeated just a Hertz or less lower in frequency than it was received, the net result being sporadic feedback, dominant at any time at various frequencies in the speech range, falling in pitch after their original appearance.
About fifteen or twenty years ago, I used to hear an active voice repeat channel with near-feedback that sounded like this, but much more mild, around 11.25 Mhz or so, I forget the exact frequency now. The voice traffic seemed military, and as I recall it was in US English. The feedback did not reach oscillation like this 6.7 MHz signal seems to, but you could hear it resonating the background noise. As I recall, the resonations seemed to change in pitch over time at about this rate. They sounded like a very rough peak of noise, rather than a whistle or howl. I don't remember well enough to say for sure that they were moving downward in on-air frequency as this one seems to be, but I think they were.
The initial appearance of new feedback modes seems pretty fast and sudden. The 6.7 MHz system may have noise gating, so it does not gradually build a resonance up into an oscillation, but it snaps into each new oscillation and then drifts down in pitch as heard on an upper sideband receiver ( I don't want to say USB, that has another meaning today ). If so, this system may be doing noise gating with an FFT, and working on individual bin squelch levels and maybe linking bin unsquelch together over a limited frequency distance. Maybe it is meant to repeat speech, maybe it is meant to repeat piccolo, etc.
A very slight frequency shift is sometimes deliberately used in public address systems to fight acoustic feedback. Maybe it is deliberately inserted in these systems for the same purpose.
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