The AM Forum

Of course, they have a fancier name for it, Modulation-Dependent Carrier Level.  One system reduces the carrier at low percentages of modulation while another reduces the carrier up to 3 dB at high percentages. Harris Corp. and WOR have recently claimed success with experimental broadcasts.  Reportedly, it has been in use by short wave broadcasters in Europe for several years.

The purported advantage to broadcasters is a substantial reduction in the electric power bill.

http://www.broadcast.harris.com/productsandsolutions/RadioTransmission/AMTransmitters/AMTransmitterPowerReductionAlgorithms.asp

they might as well just run double sideband suppressed or reduced carrier.

i think part 15 operators have been allowed to do carrier current for years (with severe power limitations of course).

i was under the impression that carrier current is only useful in small enclosed areas like office buildings or university dorms. is that not the case?

Who is talking about carrier current? The last time that was popular for hams was during WW2.
Currently the power utilities are trying to get Internet working on the wires

Aw, Sol is just cornfused. :-[

Controlled carrier ain't the same as carrier current radio. ???

Controlled has the carrier power linked and following the syllabic rate of the modulation.  An old trick to save watts on AM.  Sounded like crap back then.

He knows what carrier current is already.

73DG

ahh whoops i misread that pretty badly. can i blame that on a sinus headache?

Given the programming and listners on AM these days, it won't matter much what method they use so long as it is understandable.

Now, now! We're not allow to claim AM broadcast is dying here.   ;)

Nah, it's not dying, but it IS getting absolutely unintelligible... :P

Even with perfect fidelity, if one can get it anywhere...

73DG

I hate it when some damn bible thumper from Albany overides my favorite oldies station on 1540 :o

WOR?  That is surprising.... I wonder how controlled carrier modulation works
with the HD sidebands????  WOR was one of the first NY stations to adopt
the IBOC (I'm Big On Crap) technology. These are dark days for AM.

Pedro

People in Alaska are using it for some time I got an Email about from ARRL be Like screen modulation but using transisters not Tubes..

Has anyone actually monitored a station using this newer method of carrier control??
Or is it just conjecture from the old days that it's gonna suck??

IBOC is dead for AM broadcast. There wasn't clear thinking on that one. With skip and mixing / interfering with other markets.
I will say that IBOC on AM hangs in there better than FM. I can drive 25 miles listening to the one local AMer using IBOC and no drop-outs. FM is constantly flipping back and forth from the multipath.....IF you're in a big city with many high power FMs then IBOC shines.........nice to hear other programming choices on the HD-2....I swear it's CD quality audio.......not squashed processed FM audio.
Fred

If this is the case, why doesn't it go away? Minor hi-jack going on here ;D ;D ;D

CBS stations are major investors in the technology.

Hard to admit defeat?  It's like falling in love with a stock that you're
losing your shirt on. You know you sell and recoup some losses,
but you can't letgo :)  AM Stereo made more sense.

this what you Talking about!


http://www.youtube.com/watch?v=7KtJxHN2cFE

Dennis,
 So with "AMC",  During no modulation, the carrier is a full 10KW.  But as modulation increases the power Drops down to 5KW.  His wording is confusing me. 

A 10K Transmitter would be 10KW carrier with just under 30KW peak power (according to BIRD).   

Does the carrier drop down to 5KW but the peak output power modulated stay the same at just under 30KW?  In other words, ONLY the carrier is the only thing that is changing or is the peak power Droping down with the carrier?  The idea being that the BIG carrier would clear the frequency at no to low modulation but with the high level modulation, you do not need that much over all power?

I would really enjoy having a KW of carrier during pauses and no mod and then when I was speaking still have the correct carrier to 1500 watt ratio.  The only way I see to do this is to vary my plate volts. HMMM.

C

Back when the Christian Science Monitor was running a SWBC station, they used controlled carrier to save on the power bill. I believe that Brown Boveri made such transmitters at the time, though I don't recall if that was the kind that CSM was using.

It did compromise the reception quality to the point where it annoyed me, but the vast unwashed probably couldn't tell the difference.

Controlled carrier, even when it isn't inherently distorted as it often is, adds hard-to-predict AGC system artifacts at the receive end. It takes away one advantage of AM: a steady reference for receiver gain setting so that the received audio levels track the transmitted audio levels, all without a rise in received noise between syllables.

If the broadcasters want to save power, they'd probably be better off just reducing their power by half, though a 3 dB worse signal to noise ratio isn't exactly desirable these days, with rising digital hash everywhere (which could have been prevented by the FCC; another topic though). But then Harris wouldn't be able to sell any shiny gizmos.

The fact that the power bill is a much larger consideration than it used to be is quite an indicator of the immense decline of the Standard Broadcast Band.

Had independent broadcasters and the FCC insisted on migration to a dedicated digital band twenty or more years ago instead of following the wishes of the big chains and hanging their fortunes on the almost unbelievably retarded IBOC turkey, then the stations would probably be prospering, the remaining Standard Broadcast Band users would have far less crowding to deal with -- and there would be no buzzsaws driving even more people into the arms of mp3 players.


Happy 2012,


Kevin, WB4AIO.

That's been my experience too:


http://www.youtube.com/watch?v=91BPPnozeIs


Well, maybe not quite the same come to think of it ... KLIF has since discontinued IBOC on AM and is/was riding an FM last I knew and the FM was marginal from the get-go where there was at least a 'clean' AM IBOC when there was no lightning ...

ok, first, dropping the carrier level during modulation then running it back up full during no modulation sounds a little dumb to me but i could see the point behind doing it that way. running full carrier during pauses in modulation would keep the background static noise down between syllables, but would seem to me like the amount of noise during modulation would increase. by reducing carrier between periods of no modulation and bringing it back up for modulation, you would have background static bleeding through when there was no modulation, which doesn't matter since there is obviously nothing to listen to when the transmitter isn't being modulated, but then there wouldn't be as much noise during modulation because the carrier was back up to full power. second, yes, the carrier is the only thing that is being cut, the peak power is the same. a good example would be how some of the transmitters at WBCQ work. 7490 is the only full carrier AM transmitter they have. 5110, 9330, and 15420 are running compatible sideband. FCC HF rules say a station running SSB for HF BC work has to have 50 kw PEP or more. those transmitters are putting out 50 kw peaks on one sideband, but are running a carrier that is down something like 6 dB from the peaks. it's not using as much power as full AM, but it is still receivable on an AM receiver as well. and as for having a KW carrier but dropping it back to 375 watts for 1500 watt peaks on modulation, it would be neat to try rigging up a VOX circuit that would drop the B+ on the tube down to what is needed to make 375 watts as soon as you strt talking, or what may be a little more realistic would be simply controlling the amount of drive going to the tube. pick a tube that can do a KW out then rig up a VOX circuit or something that way when you start to talk, part of the drive from the exciter is dumped through a load, and only enough to make 375 watts is passed into the final amp.
shelby

You guys have it backwards.

Carrier ramps up to match the modulation.

Goofy thing it is.

73DG

Dennis. Watch the Video.. on the scheme called AMC, the carrier is full power with no mod but Drops during modulation. The man explains it. I wanted to know if the peak power is staying the same with this scheme or if they are JUST lowering the carrier?

C

Oh that will work well on a weak station - what with the AGC 'opening up' IF gain amplifying adjacent channel (usually mild) splatter or IBOC sidebands (like when listening to KLBJ on 590 from north of Dallas and IBOC hash is ever-present from 620 KMKI) on 'quiet' program segments (due to pauses in speech, etc).

There goes the "AM Quieting" effect.

Is this the AM band station owners making their last stand, the paring back of the last 'direct expenses' necessary (for electricity) to run the broadcast transmitter of the station, as they are now into directly affecting 'transmit signal characteristics' (without outright filing for a transmit power reduction) ... until the last listener migrates to FM, MP3's, XM/Sirius (?) or 4G/LTE program content streamed to their cell phone?

(That was a question above BTW.)

Jim de WB5WPA

i would assume the peaks are staying the same, and the carrier is just being reduced. it would be dumb to reduce the carrier AND the peaks at the same time, really in that case it would be the same as if i were talking to someone on my transmitter and turned the carrier knob back when i started to talk then turned it back up when i was done.

control carrier transmitt the carrier is reduced but sidebands are not the carrier get greater when the sandband power amplitude like screen modulation  ???

OK, the technology is a bit diffused here.  I was referring to the older system.

The photos show the vintage "controlled carrier" modulation results of yore.  The carrier RISES under modulation, drops in quiescent periods.

The new idea for AM BC operation may be similar, I have no interest in it presently.  In other words, neither do any of my client BC stations.

The first idea didn't pan out.  The latter may, time will tell.

AM IBOC is reportedly incompatible with it as well per conversations with my peers. 

73DG

As insane as it sounds, that is exactly what they are doing.

Death throes and self-delusion go together.


73,


Kevin, WB4AIO.

Tain't no free lunches.... ;)

73DG

On Solid state this would be much easier.  But the idea of a BIG Carrier of say 1000watts that would fall back to 400 when I was talking and 1500 PEP is really kind of neat. It would sure clear the frequency and keep the pauses between words clear.

C

It seems like it's working for the Alaska Public Broadcasting and the engineer says they are saving about 30% on their electric bills for the transmitter.

I'd like to watch an 'S' meter with this at work.

I'd also like to audition it 'before & after' on a good vintage receiver.

Come to think of it, I guess good vintage receivers all had 'S' meters.

73DG

ps...Clark TRP can't possibly be worried 'bout his electric bill..... ;)

They could save 50 per cent. if they just ran half power, and with no audio/AGC artifacts, and with a lot less complexity.

What do they gain for all this complexity? -- a barely discernible 3 dB of quieting between words and between songs? -- seems pretty silly to me.

My interpretation: They're really running their 10 kW transmitters at 5 kW but refusing to admit it -- and paying for the privilege.


With all good wishes,

Kevin, WB4AIO.

http://www.nab.org/xert/scitech/pdfs/rd092611.pdf

http://transition.fcc.gov/Daily_Releases/Daily_Business/2011/db0913/DA-11-1535A1.pdf

Carrier appears to rise with modulation similar to controlled
carrier from what I can see??

Pete

One explanation I read was that the amplitude of the carrier is digitally controlled to more accurately follow the syllabic rate of the audio than what is obtainable with conventional controlled carrier, and algorithm is designed so that with low percentages or no modulation the carrier rises back to or near full amplitude to keep the channel quiet, but in the region of average modulation density (somewhere around 20-30% percent), the carrier is at a minimum so that the audio is actually modulating the reduced carrier near 100%, and as the percentage of modulation of the equivalent steady carrier goes up, the actual carrier is increased to accommodate the  audio on up to what would be 100% modulation on a normal full carrier transmitter.

PERFECT video and explanation!!!

And they use the old fashioned 'O scopes to monitor the new variable power transmitters.

Don’t know about all this, thing around here that’s killing AM is that the land required for the tower site and transmitter is worth more then the revenue that the station can produce. Have a couple Harris DAX-5 solid state monsters at my two AM stations that are fairly efficient and way cheaper to run then the old MW-5 tube transmitters that they replaced because they would want new modulator tubes every year so now the only expense is the electric. Talk or sports radio all runs from a small server in a rack back at the studio so there are no labor cost or other operational cost but would assume if your running a 50 kW station in a real market and knowing how cheap all the AM people are every cent they can save is important but most stations bought new solid state transmitters in the last ten or so years and cant see them running out and buying a new transmitter, maybe this can be adapted to the carrier control in modern PDM or PCM transmitters? Back in the day of analog television someone came out with this stupid box that you had to install on your klystron that did something during the sync pulse to give you increased efficiency and from what I recall it involved lots of HV wiring and danger! Fortunately my one big television site used IOX tubes and did not have that but tell the GM they can save $50 a year for the owners and being that they don’t have to do the work in the middle of the night yea you’ll be installing it.

Thank you, Dennis! ! ! ! ! ! ! !  The carrier starts out low then ramps up as the modulating audio level increases! (Just think DX-60) It can be a real pain in the arse to get tweaked out right at times, but can be made to sound good if one is patient and dilligent in it's set up.


controlled carrier and screen modulation are often used together, but not necessarily one and the same. They are two seperate animals. the DX-60 (and a few others) have made them synonymous. But that is not always the case!

A good example is my big HB transmitter. It runs along at 375w of carrier  ;)  all day
and easily makes 125% positive peaks. And it is screen modulated!! but not controlled carrier. (Actually it is screen and control grid biass modulated) And no one ever complains about how it sounds. (Which also breaks the old myth that you cant make 100% mod with screen modulation)

Broadcashters going to controlled carrier systems only seems natural in these days of everything "going green". As far as I am concerned, the only thing green about it is the feeling it all gives you in the pit of your stomach as you are about to hurl!

really? can't hardly hear them here in the Albany area... ha!

                 _-_-bear

Some of us have carrier controlled txers like the DX-60 well it be like hearing that or far worst?

Maybe not relevant, but I remember the DX-40, the bassy audio and the S-meter bouncing around with modulation, a lot worse than the DX-60.

It would not be as bad as the older Ham transmitters. With the time constant of a BC AM receiver it is un-noticeable. It's only changing the TX power by 3dB...The video link to Youtube is lengthy but very informative from engineers who are Amateurs and aware of the 'old style' carrier controlled transmitters.

The first controlled carrier rigs when the concept was introduced in the 1930s were plate modulated.  One scheme was to let the class B modulator plate current control the carrier level. The plate current passed through one winding of a saturable reactor, while the other winding was in series with the primary of the plate transformer for the rf final power supply.  The increase in modulator plate current caused the core to saturate and thus decrease the inductance of the winding in series with the final amplifier plate transformer, which in turn decreased the inductive reactance and voltage drop to give the rf plate voltage a boost.  Somehow the winding with the DC passing through it was balanced for a.c. to keep 60~ from being introduced back into the class B DC plate supply.

A cheap and dirty method was to use one big power supply for modulator and final, with approximately double the DC output voltage of the normal power modulator or rf final power supply, and simply wire the DC plate circuits of the final and modulator in series.  The class B modulator DC plate circuit acted like a variable resistor in series with the rf final plate supply.  With increased modulator plate current, the effective series resistance dropped, supplying more voltage to the rf final. Or another way of looking at it, since the modulator and final were in series across the same power supply, any increase in modulator plate current had to result in an identical increase in rf final plate current. Basically, series modulation at the syllabic rate. It probably had a lot of distortion, since the class B modulator was operating with extremely poor voltage regulation.  It probably worked better using zero-bias modulator tubes.

The first controlled carrier rigs when the concept was introduced in the 1930s were plate modulated.  One scheme was to let the class B modulator plate current control the carrier level. The plate current passed through one winding of a saturable reactor, while the other winding was in series with the primary of the plate transformer for the rf final power supply.  The increase in modulator plate current caused the core to saturate and thus decrease the inductance of the winding in series with the final amplifier plate transformer, which in turn decreased the inductive reactance and voltage drop to give the rf plate voltage a boost.  Somehow the winding with the DC passing through it was balanced for a.c. to keep 60~ from being introduced back into the class B DC plate supply.

A cheap and dirty method was to use one big power supply for modulator and final, with approximately double the DC output voltage of the normal power modulator or rf final power supply, and simply wire the DC plate circuits of the final and modulator in series.  The class B modulator DC plate circuit acted like a variable resistor in series with the rf final plate supply.  With increased modulator plate current, the effective series resistance dropped, supplying more voltage to the rf final. Or another way of looking at it, since the modulator and final were in series across the same power supply, any increase in modulator plate current had to result in an identical increase in rf final plate current. Basically, series modulation at the syllabic rate. It probably had a lot of distortion, since the class B modulator was operating with extremely poor voltage regulation, and  likely worked better using zero-bias modulator tubes.

The quoted figure for a remote generator 10KW transmitter, his diesel cost is $6.50/gal, at 23 gallons a day works out to a cost of about $822/day and a total fuel cost for a year at $300,000.

The engineer interviewed did bring up some good points about the dynamic loads presented to the AC line depending on the type of 'power control' mode selected (as there are two types), and in the case he cited with the generator, this can drive the generator's voltage regulation control bananas as it 'hunts' for the proper drive to the generator exciter winding ... also the various substations use solid-state 'tap selection' (on transformers) and can be driven nuts with highly dynamic loads (like voice or speech).

Interesting vid.

de Jim WB5WPA