The AM Forum

I don't know how well this would work or if there may have been someone who has tried it before, but would it be possible to send an AM signal and an FM signal on a single carrier, but have two different conversations transmitted? So one conversation would be on FM and the other would be on AM. I thought this over for a while and the way I think it could be done is to modulate an exciter with FM, then use that to drive a plate modulated amplifier (it wouldn't have to be plate modulated, another method of modulation could work). On receive, the FM should come through clearly, since the receiver's limiter would take care of the amplitude variations, but my question is, what would happen to the AM on receive? If the receiver is tuned right on the carrier, the FM audio shouldn't be detected, unless the receiver was tuned to the side of the carrier, then the receiver would pick up the FM audio through slope detection.
Shelby KB3OUK

Build an "SBE".   :P
http://amfone.net/index.php?ind=media&op=file_view&iden=226

The AM receiver would slope detect the FM.  And the FM receiver, due to imperfect limiting would probably also detect some of the AM.

If you want to send 2 streams of intelligence, ISB with carrier might be one way.

Actually, some forms of AM  stereo work that way.  L + R was used to amplitude  modulate the carrier normal fashion.  L-R was used to phase modulate the carrier.  Phase modulation is very closely related to FM.

SSB can be thought of as a combination of AM and phase modulation.  The AM component is seen in on an oscilloscope in the envelope pattern of the SSB signal.  The PM component can de derived by hard limiting the signal to remove all the AM component.  There were some experimental systems back in the early 50's that tried to generate SSB just that way.  The PM component was used to drive a class-C final and the AM component was fed into the modulator.  The idea was to generate SSB using a minimally modified AM transmitter.

It reportedly met with some success.  I'm not sure how they maintained carrier suppression during long pauses in the audio.  Perhaps they didn't worry about suppressing the carrier.

Then there is the so-called compatible SSB that was tried with AM broadcasting.  It supposedly was copyable with perfect fidelity using a normal envelope detector.  I believe WOR, KDKA and WBZ tried some experiments with it.  It was mainly tried for reducing interference when two major stations would mutually interfere with each other, by shifting all of the signal to one side of the carrier.  It was a complex combination of phase modulating the carrier and simultaneously pre-distorting the AM modulation.

I was wondering about that today. I figured that transmitting the signal would be easy, but receiving it would be where the problems are. Running the same signal through it would be easy, but trying to feed different signals through would be difficult. It might be possible if the level of modulation on both the AM and FM were very low, then the frequency and amplitude variations would be as large. That might reduce the chance of one picking up the other.

Here is an article (patent information) on the subject:

http://www.freepatentsonline.com/4593410.html