The AM Forum

Modulation-controlled carrier has been discussed here and there, but it really deserves its own topic because of the many methods and articles about it. Hams and commercial entities both have an interest in it.

There are analog methods including very old brute force and saturable reactor stuff and more modern methods like in combination with screen modulation, and there are flashy configurable digital methods as shown in the attached paper on Modulation Dependent Carrier Level.

In the paper there is what they call ACC, which lowers the carrier power with a DECREASE in audio input and AMC, which lowers the carrier power with an INCREASE in audio input. The paper highlights the digital methods, but the two ways of increasing efficiency could be applied in an analog manner. They also discuss what they call ACC+ and AMC+. No idea whether these are GatesAir's terms or are open terms.

Controlled Carrier is also a little controversial because some people like the idea and some don't. Personally I don't use it but I would like to try it. It must have some benefits. Maybe it's like cats. One either likes them or dislikes them, with few in-between.

I don't care so much about efficiency on the old tube stuff I run but there are times during reception when I would like silences to be accompanied by the other person's full carrier so as to reduce noise and static that seems to interfere on AM at times. Would it help to blot out the occasional SSB noise in the local receiver? Is there a fundamental difference between AMC and the unspoken schemes that were probably behind the "modulation essentially negative" ratings for some vacuum tubes?

Anyway the article is some very modern stuff, to me, I prefer simpler ways on the older ham radio gear, plastic radios and boatanchors alike.

This is (or some equivalent-effect plate mod) more my speed when trying to learn about the topic
http://www.noding.com/la8ak/f97.htm

some interesting points of view in these ... the controlled carrier (abreviated cc) Heath circuit has the ability with an added pot in the cathode ( pin 8 ) to adjust the amount of cc generated.  The amount of cc generated in the stock circuit seems to be around 12 or so db and this amount with typical rx agc action gives a 'chuffing' sound to the audio.... not very pleasant.  A lesser amount even as little as 3 db would allow significant waste heat production reductions.

I am starting to begin testing of a screen modulator for my S line stations incorporating a cc possibility, switch selected or not, depending on conditions....we shall see

The la8ak schematic is a copy of some earlier mods from contributors here and if one goes down to the 'Transmitters" forum you will see later and much improved screen modulation methods.

I don't see any real linkage between say an HT-40, Knight T-150, or a DX-60 and the modern AM broadcast methods.

Phil - AC0OB

Here's a pretty good discussion from someone whose actually implemented DCC in a shortwave system.

http://fweb.wallawalla.edu/~frohro/Dynamic%20Carrier%20Control/DCC1.html

And a schematic with his notes in pdf format is attached.

--Shane
KD6VXI

Here is another Slide Presentation on DCC by Hatfield and Dawson


Phil - AC0OB

Here is a blast from the past: The Rothman System.

http://www.electronicsandbooks.com/eab3/manual/Magazine/C/CQ%20Amateur%20Radio%20US/1952/04%20April%201952.pdf

Marmax Electronics marketed the KW52 modulator using the Rothman Modulation System. Their advertisement, basically a copy of the CQ article, can be found here:
http://bama.edebris.com/manuals/marmax/kw52

In their write-up they state:

"The system is not advantageous when used with tubes which do not have suitably related screen power to output power ratings.This limitation however, is not serious since an adequate number of suitable tubes for all power levels are available."

I wonder what 'screen power to output power ratings' would be suitable, and which tubes would be the best to use?

It could be fun to build a low power version with which to drive an amp.


Don

The Rothman system is about as unusual as this diode modulated AM Broadcast transmitter
system:

Back in the late 90's (I think) while working at shortwave station, KTWR, Guam, I designed and deployed a DCC system with the contour graphed in the article referenced by Shane.  At the time we had four Harris SW100A transmitters (100kw, PDM modulator).  Tubes in PA, PDM modulator, and their driver tubes.  (We also had a much newer HCJB HC100 but it already had DCC.)  We did it strictly for power savings.  At the time we had an electric bill of roughly $30,000 per month.  After installing DCC in the Harris rigs, our bill dropped by $5,000 per month.  Instant ROI!  When playing music, with its near-continuous sound, there was little or no savings.  But if a slow-talking person was speaking and had lots of pauses, we made money.  :-)   We had listening tests in several target areas like Indonesia and China.  By ear, no one could tell we did it. 

A ham would have to be on the air a LOT to see any real savings.  These were, of course, broadcast transmitters.

We didn't play with other contours such as the BBC-style full carrier during silences. 

Those four Harris transmitters have left Guam.  One or two may still be in service somewhere.  They've since been replaced by a couple of 250kw DRM-capable transmitters but that's another story.  There's a life lesson in there: Sooner or later, everything ends up in a landfill.  Or W3GMS's museum.  ;-)

73,
Chuck
NA3CW

To summarize, screen grid modulation (SGM) in Amateur radio transmitters using Controlled Carrier TC circuits has no relation to AM broadcast companding systems.

Since perceived loudness can be a make or break a station in being received and heard, it appears the AMC method makes the most sense.

From the NRSC report: "A key performance aspect of AMC systems is that there is no change in the perceived loudness of the signal. This is a consequence of the carrier and sideband levels tracking, unlike the systems which fall into the second category (DCS) where only the carrier level is being modified."


Phil

The Rob Frohne system linked above seems the most intelligently-designed of such systems I've seen.

But I think they're all undesirable. Simple systems (like those in the DX-60 and T4X) add lots of audio distortion, and the more complex systems don't save as much money as just dropping to half or quarter power, which sounds better and works pretty much as well. Dropping to half power only degrades S/N by 3 dB -- barely noticeable -- and distortion actually goes down, and no expensive transmitter mods are required!

73,

Kevin.