Greetings:

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

This will be a bit difficult to describe and I wonder if some folks
would have audio recordings of spectrum surveillance from the 1960s
for an actual sample, but in lieu of that I will try to convey a
sense of what it was. I had always assumed that it was photo
transmissions by the wire services, but listening to modern equivalents
I have doubts; it actually sounds more like the early telephone
voice scrambling systems that were acoustically coupled to the handset,
and which produced a continuous noise irrespective of voice level.
The sound on the air was much like hearing several P51 fighters
approaching at high airspeed and props not in phase; the spectral
power is mostly in the range of about 200 Hz to 500 Hz with a
varying heterodyne of several Hz. It had a bandwidth of at least
25 kHz (can't say more precisely due to the cheap receivers I had at
the time). I don't remember the frequencies, but at the time I would
have been concentrating on monitoring 3 Mhz to about 12 Mhz, and these
signals were strong at all hours on perhaps a dozen different frequencies.
I always regarded them as annoying QRM.

I did little SWLing from the mid '70s until somewhat recently, and of
course there is nothing like this heard nowadays.

At the time, my QTH was about 25 miles from the largest National Guard
training camp in the midwest and there were four USAF bases within 200 miles
as well, so it could well have been some modulation mode used by the
military.

I would really enjoy knowing if others remember a signal like this
and knew its origins.

Regards,

Michael

In article , _
says...
Greetings:

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

This will be a bit difficult to describe and I wonder if some folks
would have audio recordings of spectrum surveillance from the 1960s
for an actual sample, but in lieu of that I will try to convey a
sense of what it was. I had always assumed that it was photo
transmissions by the wire services, but listening to modern equivalents
I have doubts; it actually sounds more like the early telephone
voice scrambling systems that were acoustically coupled to the handset,
and which produced a continuous noise irrespective of voice level.
The sound on the air was much like hearing several P51 fighters
approaching at high airspeed and props not in phase; the spectral
power is mostly in the range of about 200 Hz to 500 Hz with a
varying heterodyne of several Hz. It had a bandwidth of at least
25 kHz (can't say more precisely due to the cheap receivers I had at
the time). I don't remember the frequencies, but at the time I would
have been concentrating on monitoring 3 Mhz to about 12 Mhz, and these
signals were strong at all hours on perhaps a dozen different frequencies.
I always regarded them as annoying QRM.

I did little SWLing from the mid '70s until somewhat recently, and of
course there is nothing like this heard nowadays.

At the time, my QTH was about 25 miles from the largest National Guard
training camp in the midwest and there were four USAF bases within 200 miles
as well, so it could well have been some modulation mode used by the
military.

I would really enjoy knowing if others remember a signal like this
and knew its origins.

Regards,

Michael


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. Universal Radio in Ohio sells decoders that will decode some of
it, but most of it is/was encrypted, and it's a real hassle to tune.
Even 10hz steps are too coarse. You need a really good receiver to do it
with, and a narrow filter is a big help too. I actually got pretty good
at tuning it.

http://www.universal-radio.com/catal...ers/m7000.html


I've heard some of it fairly recently. I don't have the decoder anymore,
so I don't know if it's readable or not. Fax signals are still common on
HF, they sound like a squeaky wheel turning when you are in USB or if
you have a "fax" mode on your receiver (Only JRC had it, as far as I
know). Squeak..squeak..squeak.

It's an excellent time waster..if you have the patience. Sadly, starting
in the late 80's, a lot of the good stuff, like press photos and ship
traffic began to disappear.

BDK

On Jul 19, 11:50 am, BDK wrote:
(snip)
It's an excellent time waster..if you have the patience. Sadly, starting
in the late 80's, a lot of the good stuff, like press photos and ship
traffic began to disappear.

That's because it all went to satellites. Satellites are much more
efficient at digital transmission. Satellite reception for home use
still has a way to go, but satellite usage for data transmission has
just about taken over except when the military wants to be
unpredictable and keep potential eavesdroppers (such as Iran or North
Korea) off guard. If you hear any sort of data today, it's almost
always military. Voice HF utes are mainly used in international
airplane travel and sometimes for older ship to shore rescue dispatch.
(The Canadians still seem to rely on HF a lot for ship rescues.) The
planes and Canadian rescue dispatch from the maritimes is ok, but can
get boring especially if you don't know the jargon.

I never really got into listening to the cops and the firefighters,
most of what they do is pretty dull. Spectacular fires and frantic
"officer down" calls are pretty rare. It got to be that the scanner
craze of the 70s petered out and the only ones listening were the
crooks, in order to avoid the cops. When Johnny Law realized that,
they replaced their radios, kicking them up to the Ghz range and
making the equipment to recieve the "trunks" costly and hard to find.
Nowadays only the old scanner diehards listen to the public servants,
and the crooks aren't organized enough to have somebody listening with
a Trunktracker at a base QTH and then radio info out to the field. I
suppose that Mara Salvatrucha and some of the more organized drug
gangs could pull it off, but they usually don't get beyond cell phone
level of electronic difficulty.

In article . com,
says...
On Jul 19, 11:50 am, BDK wrote:
(snip)
It's an excellent time waster..if you have the patience. Sadly, starting
in the late 80's, a lot of the good stuff, like press photos and ship
traffic began to disappear.

That's because it all went to satellites. Satellites are much more
efficient at digital transmission. Satellite reception for home use
still has a way to go, but satellite usage for data transmission has
just about taken over except when the military wants to be
unpredictable and keep potential eavesdroppers (such as Iran or North
Korea) off guard. If you hear any sort of data today, it's almost
always military. Voice HF utes are mainly used in international
airplane travel and sometimes for older ship to shore rescue dispatch.
(The Canadians still seem to rely on HF a lot for ship rescues.) The
planes and Canadian rescue dispatch from the maritimes is ok, but can
get boring especially if you don't know the jargon.

I never really got into listening to the cops and the firefighters,
most of what they do is pretty dull. Spectacular fires and frantic
"officer down" calls are pretty rare. It got to be that the scanner
craze of the 70s petered out and the only ones listening were the
crooks, in order to avoid the cops. When Johnny Law realized that,
they replaced their radios, kicking them up to the Ghz range and
making the equipment to recieve the "trunks" costly and hard to find.
Nowadays only the old scanner diehards listen to the public servants,
and the crooks aren't organized enough to have somebody listening with
a Trunktracker at a base QTH and then radio info out to the field. I
suppose that Mara Salvatrucha and some of the more organized drug
gangs could pull it off, but they usually don't get beyond cell phone
level of electronic difficulty.




I disagree about the cops and firefighters being dull. I recently
upgraded to my first digital handheld scanner, and will be buying a
digital base unit as soon as I can get the money together. The amount of
minor repairs I do on scanners shows that a lot of people are listening.
If they would drop the price of digital scanners to about a hundred less
than they are now, I know a lot of people that would finally make the
jump.

A friend's wife has been a recent convert. She ragged him endlessly when
he spent over $400 for the digital handheld he recently bought to
replace the conventional unit that puked out. She is now hooked big
time, and calls me and emails me a lot to explain some of the odd jargon
used by the local police and fire depts.

BDK

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

msg wrote:

Greetings:

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

snip

Thanks for the replies. I was surprised to discover these posts from
an archived mailing list that described my signal in similar terms:

From Sat Sep 9 13:18:00 1995
From:
Subject: Coded Signal Identification ?
Message-ID: Chameleon.4.01.2.950908231744.jproc@jproc

Dear BA's,

Back in the 1960's, I remember hearing very loud signals in the HF bands
which sounded exactly like piston aircraft engines. Also evident, was a bit
of alternating Doppler shift in the sound. Someone once told me this was some
sort of high speed, coded message circuit. By the 70's or 80's, it looked
like all of these signals vanished from the bands.

Can anyone identify what I was receiving?

Regards,
-------------------------------------
Jerry Proc VE3FAB
E-mail:
Radio Restoration Volunteer
HMCS Haida, Toronto Ontario

From Sat Sep 9 13:18:00 1995
From:
Subject: Re. Coded signal ident.
Message-ID:

to:

Hello Jerry.

The "aircraft engine" signals you used to hear weren't necessarily coded.
These beasts were VFT, or Voice Frequency Telegraphy.

Essentially, they were stacks of 8, 16, or 24 narrow shift AFSK signals
that were used to modulate an SSB transmitter; i.e., they were multiplexed
RTTY.

The reason most of 'em disappeared (yes, there are still a few around) is
that these point to point circuits have mostly moved to satellite. BTW,
thereis a 50 channel version of the same scheme on the birds.

73's,
Tom, K9TA


From Sat Sep 9 13:18:00 1995
From: Steve Ellington
Subject: Re. Coded signal ident.
Message-ID: Pine.SOL.3.91.950909103627.2439A-100000@iglou


The "aircraft engine" signals you used to hear weren't necessarily coded.
These beasts were VFT, or Voice Frequency Telegraphy.

Aw nuts! When I was a kid I had it all figured out that those were
communist jamming stations. I would carefully tune around them trying to
hear the Jammed broadcast to no avail. Later on I just decided they were
really airplanes. Once, me and a buddy used that noise to record a skit
for a school play involving some aircraft.

I prefer to go on believing they are airplanes. :*o

Steve




At the time, I didn't associate the sound with aircraft engines since it was
much more discordant and had a very aggressive quality when heard on AM through
a wide filter. I used to hear the sound in nightmares...

If anyone has this sound preserved in recordings from that era, I would
appreciate the chance to hear it again and analyze it.

Regards,

Michael
msg _at_ cybertheque _dot_ org

"msg" wrote in message
...
Greetings:

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

This will be a bit difficult to describe and I wonder if some folks
would have audio recordings of spectrum surveillance from the 1960s
for an actual sample, but in lieu of that I will try to convey a
sense of what it was. I had always assumed that it was photo
transmissions by the wire services, but listening to modern equivalents
I have doubts; it actually sounds more like the early telephone
voice scrambling systems that were acoustically coupled to the handset,
and which produced a continuous noise irrespective of voice level.
The sound on the air was much like hearing several P51 fighters
approaching at high airspeed and props not in phase; the spectral
power is mostly in the range of about 200 Hz to 500 Hz with a
varying heterodyne of several Hz. It had a bandwidth of at least
25 kHz (can't say more precisely due to the cheap receivers I had at
the time). I don't remember the frequencies, but at the time I would
have been concentrating on monitoring 3 Mhz to about 12 Mhz, and these
signals were strong at all hours on perhaps a dozen different frequencies.
I always regarded them as annoying QRM.

I did little SWLing from the mid '70s until somewhat recently, and of
course there is nothing like this heard nowadays.

At the time, my QTH was about 25 miles from the largest National Guard
training camp in the midwest and there were four USAF bases within 200
miles
as well, so it could well have been some modulation mode used by the
military.

I would really enjoy knowing if others remember a signal like this
and knew its origins.

Regards,

Michael

I remember what you are talking about. When I was
a preteen in the late 60's using a cheap shortwave
receiver we heard things such that we naturally said,
"That sounds like an airplane." Sounded like someone
left the mike open in the cockpit of an airplane. We
suspected that they probably weren't airplanes because
there never was a voice and who would just transmit
airplane noise.

Most likely they were, as others have said, VFT or
something like it. They were almost certainly a
Frequency Division Multiplexing or Frequency Diversity
system, i.e. multiple carriers stacked in frequency,
each modulated (with either PSK or FSK) and
shifted in time relative to each other. The diversity
is usefull against the frequency selective multipath
fading of HF propagation. If multipath causes
a dropout of one carrier the info is still available
in the other carriers.
I'm a little surprised to think they had that technology
back then but maybe I shouldn't be.

Ionospheric multipath fading can also give it
a doppler-like sound, as if it were airplanes diving
and banking.

You can sometimes hear signals like you describe
today. There is something like it now at
11455 kHz, s5, 04:55 utc, in socal.

I hear VFT at 11010.55 and 11499 kHz
occasionally.
I have demodulated them down to bits and then
run autocorrelations. The autocorrelations were
flat indicating that they are encrypted.

There are many utes using OFDM these days.
They sound similar but sound more flat,
more whitenoise-like. I've heard them at:
4.28480
4.5905
4.81020
5.0175
6.39250
6.39850
6.42320
6.4345
6.76525
6.84272
8.4884
8.541
8.553
8.6255
8.646
12.7243
12.805
13.41050
16.9435
17.07750
17.08250
17.098

all in MHz, all USB.

If you find any old recordings I'd be interested in
hearing them too.

--
rb

Ron Baker, Pluralitas! wrote:

"msg" wrote in message
...

Greetings:

Seeking opinions on an old, commonly heard signal (AM) on HF, of some

snip

Thanks Ron for your detailed reply. Some folks have reported those
signals to be loud and harsh (high power) and not associated them
with airplane engine sounds, as was also my impression of them -- the
transmitters must have been nearby or very high power but who were
the users (in a non-suburban town of 30,000 people why a dozen or
more such signals)? As I had mentioned, I lived near a large Army
training camp and thus suspected a military origin but also wonder
if they had commercial users such as news wires to the media
(newspaper, radio), telephone company, etc.

There are many utes using OFDM these days.
They sound similar but sound more flat,
more whitenoise-like. I've heard them at:
4.28480
snip freq. list

Please listen to two examples I just recorded, the filenames encode
the freq. and date/time:

http://www.cybertheque.org/homebrew/rcvr/audio

I have heard this mode even on WEFAX frequencies during idle times.
It is certainly pervasive on HF and not at all like the old signals.
I had assumed by the spectral quality and frequency hopping
character of some examples that this was a sort of spread spectrum
signal.

Anyone hearing an example of older VFT signals currently on the air, please
post a freq. and/or record some audio.

Regards,

Michael

"Ron Baker, Pluralitas!" wrote in message
...

"msg" wrote in message
...
Greetings:

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

This will be a bit difficult to describe and I wonder if some folks

snip

The sound on the air was much like hearing several P51 fighters
approaching at high airspeed and props not in phase; the spectral

snip


Regards,

Michael

I remember what you are talking about.
Back in the late 60's using a cheap shortwave
receiver we heard things such that we naturally said,
"That sounds like an airplane." Sounded like someone

snip

Most likely they were, as others have said, VFT or
something like it. They were almost certainly a

Propagation doesn't seem to have been that good
recently. (Height of summer I guess.)
But I managed to catch a VFT signal tonight at
11103 kHz. Switched to AM mode. It didn't sound
like what I remember from way back when.

Also found a signal at 8488 kHz that I think is
a mil-std-188-110 signal. Switch to AM mode. That
sounded more like what I remember from the
60's.

But mil-std-188-110 didn't exist in the 60's. Maybe there
were similar signals at that time.

Ron Baker, Pluralitas! wrote:

"Ron Baker, Pluralitas!" wrote in message
...

"msg" wrote in message
...

snip

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

This will be a bit difficult to describe and I wonder if some folks


snip

snip

Also found a signal at 8488 kHz that I think is
a mil-std-188-110 signal. Switch to AM mode. That
sounded more like what I remember from the
60's.

But mil-std-188-110 didn't exist in the 60's. Maybe there
were similar signals at that time.


I have recently located a friend from those days and the same town
who may also shed some light on this question. Also I do remember
lots of interference from medical equipment at the local hospital,
especially diathermy machines, which may possibly be the source
since the signals were _very_ strong, broadband, and at many
frequencies.

Regards,

Michael