I've started listening to some of the HF feeds (especially the new caribbean one) and just have one question.
What is the two tone "pings" I hear every so often?
Those are SELCAL tones and allow aircraft to be "called" back on frequency when the HF controller needs to speak to that aircraft. SELCAL allows pilots to turn down the HF frequency volume (because it would be pretty annoying to listen to static the whole time!) but can still be reached.
SELCAL consists of 2, 1 second tones that corrispond with 2 sets of 2 digits (letters) of the alphabet. Aircraft have SELCAL codes (eg: DJ-RS) which the controller uses to send the SELCAL to that aircraft. If you run a quick google search on "SELCAL" you will be amazed at what you will find (and learn!). A SELCAL also activates a visual light/button for the pilot(s) to see.
I have all 121 of the tones in digital format (.wav to be specific) and I can send them to you if you'd like. E-mail/PM if you're interested.
Hope this helps,
Jason
P.S. Also note that SELCALon VHF is available, but rarely used (...at least here in the US).