AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by Max Powe » Mon, 25 Apr 2005 02:22:58


AM radio multiplexing question: transmitting SSB in sync with AM (numbers
stations, clandestine broadcasting)

I understand that it is possible to multiplex SSB transmissions on top of AM
transmissions, especially on SW.
The AM transmission is decoded by the envelope or PLL coherer, the SSB
transmission is ignored.

On a properly tuned SSB receiver, the AM transmission only partially
interferes with the SSB transmission.

Some numbers stations use this transmission technique, but it just as valid
if say Radio Free Asia were to use this technique by transmitting over
Chinese domestic radio.

1. What is the math behind this transmission working (SSB is a form of Angle
Modulation, not Amplitude Modulation)?

2. Are some receivers more affected by (interference where) SSB
transmissions superimposed over AM than others? (PLL versus SYNC vs Envelope
Detectors)

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by Frank Dresse » Mon, 25 Apr 2005 13:18:01



Quote:> AM radio multiplexing question: transmitting SSB in sync with AM (numbers
> stations, clandestine broadcasting)

> I understand that it is possible to multiplex SSB transmissions on top of
AM
> transmissions, especially on SW.
> The AM transmission is decoded by the envelope or PLL coherer, the SSB
> transmission is ignored.

SSB is a form of amplitude modulation.  SSB is just one sideband of a
standard AM signal, without the carrier.

Quote:

> On a properly tuned SSB receiver, the AM transmission only partially
> interferes with the SSB transmission.

If a SSB and standard AM signal are sharing the the same frequencies, they
will interfere with each other.

Quote:

> Some numbers stations use this transmission technique, but it just as
valid
> if say Radio Free Asia were to use this technique by transmitting over
> Chinese domestic radio.

Huh?  Is this supposed to be some sort of scheme for getting propaganda
broadcasts into China?  How many Chinese own these imagined fancy radios?

Which numbers stations use this technique?

Quote:

> 1. What is the math behind this transmission working (SSB is a form of
Angle
> Modulation, not Amplitude Modulation)?

Single Sideband IS amplitude modulation.  One of the AM stereo methods used
independent sidebands for each channel.  This was purely amplitude
modulation.  Other methods used phase modulation.

The Radio Amateur's Handbook has a good description of the various forms of
amplitude modulation and frequency/phase modulation.  There may be a copy at
your local public library.

There's a few websites about AM stereo modulation.

Quote:

> 2. Are some receivers more affected by (interference where) SSB
> transmissions superimposed over AM than others? (PLL versus SYNC vs
Envelope
> Detectors)

AM interferes with SSB and vice-versa.  If there were ways to seperate them,
then radio amateurs would use those ways.  As it is, some SSBers are annoyed
by AMers because AM uses twice the bandwidth as SSB, as well as throwing a
big heterodyning carrier into the mix.

Frank Dresser

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by Doug Smith W9W » Mon, 25 Apr 2005 13:18:01


> AM radio multiplexing question: transmitting SSB in sync with AM (numbers
> stations, clandestine broadcasting)

> I understand that it is possible to multiplex SSB transmissions on top of AM
> transmissions, especially on SW.
> The AM transmission is decoded by the envelope or PLL coherer, the SSB
> transmission is ignored.

> On a properly tuned SSB receiver, the AM transmission only partially
> interferes with the SSB transmission.

Doesn't make sense to me.  The SSB signal *will* be decoded by the AM
detector, even if it isn't intelligible.  There will be mutual interference.

A common multiplexing technique is to use an AM signal where the upper
and lower sidebands carry different modulation.  On a regular AM
receiver you'll hear a mix of both programs, but if you use an
appropriate filter (commonly found in SSB receivers) you can select
either program without interference.
--
Doug Smith W9WI
Pleasant View (Nashville), TN  EM66
http://www.w9wi.com

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by Davi » Mon, 25 Apr 2005 13:18:04



Quote:>AM radio multiplexing question: transmitting SSB in sync with AM (numbers
>stations, clandestine broadcasting)

>I understand that it is possible to multiplex SSB transmissions on top of AM
>transmissions, especially on SW.
>The AM transmission is decoded by the envelope or PLL coherer, the SSB
>transmission is ignored.

>On a properly tuned SSB receiver, the AM transmission only partially
>interferes with the SSB transmission.

>Some numbers stations use this transmission technique, but it just as valid
>if say Radio Free Asia were to use this technique by transmitting over
>Chinese domestic radio.

>1. What is the math behind this transmission working (SSB is a form of Angle
>Modulation, not Amplitude Modulation)?

>2. Are some receivers more affected by (interference where) SSB
>transmissions superimposed over AM than others? (PLL versus SYNC vs Envelope
>Detectors)

If there's a carrier, the SSB signal would be demodulated on a regular
AM radio, wouldn't it?  Are you thinking of ISB?
 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by m.. » Mon, 25 Apr 2005 13:18:03

Numbers stations are meant to be cheap and dirty, at least as far as
the folks in the field are concerned. So what numbers station is going
through such hassles to do some multiplexed transmission when there
isn't exactly a high premium on their broadcast time. That is, if they
want to broadcast more signals, there is plenty of time to do so.

I'm guessing by angle modulation you mean phase modulation, and SSB is
not phase modulation.

If you understand mixers, you generate AM by feeding an audio signal
that is riding on a bit of DC  into a mixer. The DC into the mixer
produces the carrier, and the AM into the mixer produces the sidebands.
Without the presence of DC,  you would get DSB (double side band),
which is like AM with the carrier suppressed. To generate SSB, you
could either filter off one of the sidebands of DSB, or uses two mixers
in quadrature and feed the audio through a Hilbert transformer to get
it into quadrature, then sum the right signals to get either LSB or
USB. The diagram should be in any basic communications book and
probably on the net if you do some searching. The Hilbert transformer
is a mathematical notion, so the book might just show a box that takes
0 degrees and outputs 90 degrees independent of frequency. In real
life, how this is done depends on the relative bandwidth over which you
want to achieve the 90 degree phase shift. Often the audio goes into
two filters designed such that the difference between these filters is
90 degrees.  Modern writings on Hilbert transformers are likely to be
DSP, so you would need to find a book (i.e. pre-Geek) to see this done
with analog filters.

I'm going to pass on the demod question as you phrased it, but I think
if I had to recover some stealthy USB riding on top of AM, I would
recover the LSB of the AM signal as one signal. Then recover both USB
signals (which are really sharing the same spectrum) as another signal,
then subtract the demod LSB from the demod USB combo to yield the
stealthy USB audio.

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by matt webe » Mon, 25 Apr 2005 13:18:03



Quote:>AM radio multiplexing question: transmitting SSB in sync with AM (numbers
>stations, clandestine broadcasting)

>I understand that it is possible to multiplex SSB transmissions on top of AM
>transmissions, especially on SW.
>The AM transmission is decoded by the envelope or PLL coherer, the SSB
>transmission is ignored.

No way. The SSB signal will in fact interact constructively and
destructively with the AM carrier, just as the AM sidebands do.
Quote:

>On a properly tuned SSB receiver, the AM transmission only partially
>interferes with the SSB transmission.

Only as long as none of the original AM sidebands occupy the same
spectrum as the AM signal. If it doesn't occupy the same spectrum,
then it isn't being muxed.
Quote:>Some numbers stations use this transmission technique, but it just as valid
>if say Radio Free Asia were to use this technique by transmitting over
>Chinese domestic radio.

>1. What is the math behind this transmission working (SSB is a form of Angle
>Modulation, not Amplitude Modulation)?

SSB is NOT angle modulation. It is in fan an amplitude modulated
signal (watch the S-meter whlie receiving an SSB signal). It is AM
less one set of sidebands and a carrier.
Quote:

>2. Are some receivers more affected by (interference where) SSB
>transmissions superimposed over AM than others? (PLL versus SYNC vs Envelope
>Detectors)

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by Ron Hardi » Tue, 26 Apr 2005 06:17:30

If you wanted to transmit stealhily, you'd put modulation power in the difference between
the sidebands, ie. the AM stereo difference channel.  So you'd modulate one up and one
down, so a receiver would pick off the same audio signal with or without your modulation.

You decode it by listening asymetrically to the sidebands, eg. SSB or even synchronous
detection on a single sideband, or AM detuned nearly to distortion.

In fact WFAN 660 NYC had a 120 Hz hum on its signal for a couple of months that was
only in this difference channel, that apparently went undetected by even the engineers;
and WOWO 1190 Ft Wayne I think had some strange low frequency modulation on theirs the
same way a couple of days ago (can't check them now because there's a thousand stations
on the frequency, at 4am) audible only if you receive a single one of the sidebands.
--
Ron Hardin

On the internet, nobody knows you're a jerk.

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by m.. » Wed, 27 Apr 2005 14:14:24

Check out "The Phasing Method of SSB Generation" on
http://members.tripod.com/michaelgellis/mixerscom.html
It has a diagram of how I was explaining to generate SSB. The diagram
has the math for the signals, so you can see how the SSB is eventually
generated by generating two DSB signals, then cancelling one of the
sideband by summing or subtracting the signals.

I'm going to let it slide, but I really don't like those comments about
SSB being AM if you blah blah blah. They are true, but I don't believe
they are useful in comprehending the modulation scheme. It is better to
think of SSB, well at least USB, as just the voice signal (baseband
audio) shifted up to a radio frequency. [If the voice signal was a 1Khz
tone and the carrier was 10Mhz, then there would be a signal at
10Mhz+1Khz. It's that simple.] When you demodulate USB, you basically
shift it back down to audio with a mixer. If you don't shift it
correctly, you get the Donald Duck sound since the frequencies are
offset a bit. LSB is slightly different in that the audio spectrum is
reversed, much like that cheap frequency inversion used in "secure"
communications.

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by m.. » Wed, 27 Apr 2005 14:14:24

I understand your scheme, though it seems to me for demod you would
subtract the sidebands assuming you had two demodulators.Or just
generate say L-R if you had a stereo AM radio.  In any event, your
method would be the best way send a stealth signal on AM.

WWV plays games with the sidebands, i.e. they are not identical.

 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by Scott Dors » Thu, 28 Apr 2005 04:23:26


>WWV plays games with the sidebands, i.e. they are not identical.

That's the first time I have heard that!  What are they doing with them?
--scott
--
"C'est un Nagra.  C'est suisse, et tres, tres precis."
 
 
 

AM radio mutliplexing question: transmitting SSB in sync with AM (numbers stations, clandestine broadcasting)

Post by Scott Dors » Thu, 28 Apr 2005 04:23:35


>AM radio multiplexing question: transmitting SSB in sync with AM (numbers
>stations, clandestine broadcasting)

>I understand that it is possible to multiplex SSB transmissions on top of AM
>transmissions, especially on SW.
>The AM transmission is decoded by the envelope or PLL coherer, the SSB
>transmission is ignored.

It's not multiplexing, really, it's sort of a phantom signal.  The two
sidebands both broadcast the AM signal.  The difference between the
two sidebands gives you the phantom.  You can probably hack up any of
the modern dual-sideband-with-reimpressed-carrier "fake AM" transmitters
to do this.  The bandwidth of the phantom channel is poor, but fine enough
for CW or RTTY.

Quote:>On a properly tuned SSB receiver, the AM transmission only partially
>interferes with the SSB transmission.

Depends on the sort of detector the receiver has.

Quote:>Some numbers stations use this transmission technique, but it just as valid
>if say Radio Free Asia were to use this technique by transmitting over
>Chinese domestic radio.

I am not sure why numbers stations would do this.  Numbers stations _want_
folks to be listening in.  One of the big things about numbers stations
is they can be used to make it look like your country has a huge espionage
network, when they actually just have a guy in a closet with a KWM-2.

Quote:>1. What is the math behind this transmission working (SSB is a form of Angle
>Modulation, not Amplitude Modulation)?

Check out the Kahn AM stereo system references.  The system is basically
the same, with the L-R channel being carried as the phantom signal on the
Kahn system.

Quote:>2. Are some receivers more affected by (interference where) SSB
>transmissions superimposed over AM than others? (PLL versus SYNC vs Envelope
>Detectors)

This is very well addressed in the Kahn AM stereo docs.  The answer is yes.
--scott

--
"C'est un Nagra.  C'est suisse, et tres, tres precis."