BDK wrote:

In article , _
says...

Greetings:

Seeking opinions on an old, commonly heard signal (AM) on HF, of some
considerable power, with a very memorable sound.

snip, see thread

It's called several terms FDM "Frequency division multiplexed" is the
most common term. Also called VFT. There are several formats.

It's basically a bunch of RTTY channels all blended together into one
signal.

Here is a purported sample of a VFT modulated signal (from a site with
digital mode sound samples):

http://www.kb9ukd.com/digital/vft.wav

One can hear at least one rtty channel and the background drone (of
FDM rtty 'spacing' on the other channels?).

This is pretty much what I thought VFT would sound like with low baud
rate channels; I can only extrapolate how faster rate channels would
sound and I suspect that it would be very multispectral. The signal
from the 1960s had only a few tones, closely spaced and harmonically
unrelated, was very much harsher sounding and these tones had a
phase-modulated characteristic sound with a period of about one second.
(and it was non-random in character, not like qpsk, modem noise, etc.)
The wide bandwidth it occupied didn't seem to be FDM related but
appeared to be a consequence of its high percentage of modulation
(and didn't appear to be restricted to one set of sidebands). It
contained no audible frequency shifting as one would expect from FSK
rtty. It may have been a self-clocked (manchester style) signal or
set of muxed signals.

One respondent suggested LORAN; even with the recent enhancements
I don't think the description of LORAN modes is much like my old signal
and the carrier frequencies are wrong.

Geolocating it would help to narrow down the potential
users of this signal; please, if you remember this signal, post a
message about signal strength and where you were located.

I will appreciate more recollections and comments

Regards,

Michael