[Lowfer] Audio recording of a PLC

[JD]

Which pitch of whistle would you prefer to hear? :)

That's all most PLCs sound like, most of the time.  The vast majority of the 
continuous background carriers you hear at 1750 meters, and nearly any such 
at 2200 meters, are PLCs.  They tend to be clustered around steps of 100 Hz, 
and at night you can often find multiple carriers in a cluster separated by 
a hertz or so on Argo at QRSS3 or 10 speed.  Most of the signals simply mean 
"I'm still here, everything is OK," with a fault being indicated if the 
signal drops out.  Every so often, some of them do on/off keying to indicate 
status in more detail or to issue a command...sometimes sounds a little like 
a letter or two in slow Morse, except for the dit-dah timing not being in 
the right ratios.  Some of the more advanced systems utilize brief 
intermittent bursts of FSK, reportedly, but I've never encountered one of 
those myself.

John