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Old AM Modulation Rules

S

scottwmro

Guest
On the commerical AM Band, did the FCC limit broadcast stations to 100% positive modulation, and 100% negative modulation, before the rules were changed and 125% was allow on the positive end? Seems like I recall hearing this when the old tube rigs were running.
 
The initial requirement was to not exceed 100% positive or negative. Exceeding 100% negative causes Bad Things. Increasing positive only addsa some distortion to the recovered audio. Increasing positive does to a degree improve coverage. So the limit of 125% positive was set when assymetric audio became achievable and transmitter power supplies and drive systems became robust enough to produce it.
It can also be argued that this loosening was the beginning of the end of concern for quality audio on AM, a concept which has been deader than yesterday's news for some years now.
 
littlejohn said:
The initial requirement was to not exceed 100% positive or negative. Exceeding 100% negative causes Bad Things. Increasing positive only addsa some distortion to the recovered audio. Increasing positive does to a degree improve coverage. So the limit of 125% positive was set when assymetric audio became achievable and transmitter power supplies and drive systems became robust enough to produce it.
It can also be argued that this loosening was the beginning of the end of concern for quality audio on AM, a concept which has been deader than yesterday's news for some years now.
Click to expand...

When me & my dad bought out little AM daytime about 17 years ago, we had a CCA 1 KW rig for daytime, and a Gates BC-250GY for Pre-Sunrise & Post Sunset. I was never able to get the Gates to do over 100% modulation on the positive end, even with all new tubes. I figured there waw once a rule that did not allow an operator to go over 100% modulation either way.
Thanks for the info! I use a BE AM1A not, and the old tube transmitters are gone.

Scott
 
There used to be NO limits on positive modulation. The positive modulation was REDUCED to 125% after Gates and RCA both came out with 5 kW transmitters that had 10 kW modulators. With these rigs it was not uncommon for stations to modulate to 200% or more positive. That and the combination of excessive pre-emphasis made many stations (particularly at the top half of the band) very 'wide'.

NRSC has made the 125% limit redundant, but since the FCC is full of blowhards, don't expect the limit to be removed soon....
 
I don't know what the time frame of the earlier replies is, but during the period from the mid sixties to about 1973, I believe there was no limit on positive peaks. It was a wonderful time, especially for those with some transmitter headroom.

We did about 180 percent at night on the positives at the station I worked at. Phase the mics correctly, make the gain reduction detector in the Volumax half wave, and the resultant mic sound was huge, actually scary. Really amazing to see (on the mod monitor) studio voice negatives up at 70 to 90 percent, then switch to the positives and the meter slams like it is going to break. Back the RF input down to 50 percent and notice you are over 200 percent on the positives on raw voice.

Normally the voice negatives would be about 30 to 50 percent, the game was phasing the gain reduction before the clipper to respond only to negatives, allowing the positives (on asymmetrical source material) to fly, and the negatives would come up to where the music normally was.

Perhaps some might wonder why engineers at a mono AM station would be paying attention to absolute phase everywhere. That was to make the mics come out right on the air.
 
As has been posted, there were no limits on positive modulation. Most transmitters had trouble modulating positive anyway.

And there is no technical reason to limit positives. As was also mentioned, voice is asymmetrical to some degree, so phasing allowed higher average modulation on voices.

Kahn made a phase scrambler so that the negative and positive ended up pretty equal. Others took advantage of the rule if the transmitter could handle it.

In the late 60s into the early 70s, Wenatchee Consulting engineer George Frese produced about 3 dozen of his "Frese Audio Pilot". Although solid-state gear was beginning to come out, George built it with tubes. The AGC sections produced a compression ratio of something like 100:1. He used a "float clipper" for negative peaks, relay switching to change the polarity so the positives were always higher., and feedback from an RF sample so the negative peaks were always just under 100%. George also had a presence boost, as well as high-pass and low-pass filters to limit the frequencies below 100 Hz and over 8 kHz. Later models added a control to limit positives, but the earlier models let 'er rip.

At KOL, we modified the modulation transformer circuit to make it easier to modulate the Collins transmitters, and the modulator tubes didn't last as long!

At one point our competitor blew the whistle and the FCC office tracked us doing 160% plus on a regular basis and "peaks not infrequently exceeding 200%".

I hope the Statute of Limitations has run out...
 
I believe positive modulation above 100 percent puts more ERP out there. This seems logical considering the increased strain on many transmitters... overheated or blown transformers, popping circuit breakers, tube plates glowing more and thermocouple antenna current meters moving up to levels never seen before.

That is the reason to do it (in the context of that era), and probably one of the reasons the FCC pulled the plug.

These days, besides not being legal, extreme positive modulation makes little sense. Most AM stations do not run local mics all day long. Since phase scrambling is built into most processors now, positive modulation is a result of asymmetrical clipping only, and does not include the kick of one- sided source material.

Despite the crude, unsophisticated behavior of some of today's music content producers and CD mastering decision makers, the technology of clipping in the broadcast industry has vastly improved from the days of the '60s.

So this is a discussion about the old days, not a suggestion for now.
 
Modern transmitters also don't have all the iron in the circuit, and thus usually do a much lower amount of IM distortion than the older stuff.That being said, a BC-50F running flat out was a thing of beauty, and with good source material sounded really nice on the radios of the day.
 
The RCA 50F was one of the best rigs I ever heard back in the day. CKLW and the old WGAR 1220 had very good audio and punch playing top 40 and oldies. KRMG in Tulsa also was very good with their 50F, which I understand still is in place and working..No need to have positive peaks in excess of 100%, but the 50F could modulate positive in excess of 125% if desired. Not only that, but a very nice looking rig.
 
Yeh. Jim Henry was at KRMG when the 50F was the backup to a 50J. He used to put it on now and again because it sounded sweet. And the power meter made warp speed.
 
Discovered the secret by accident to modulate the GE Green Monster (4BT50A1) 125% when the C-QUAM was installed. Seemed kind of backwards but load for least IPM and off-tune it for licensed power output (when the RF tubes were good). Then feed it 304-TL tubes every 6 months.
 
I was at CKLW when Ed Buterbaugh was CE and the main was an RCA (Victor! It was built in Montreal...) BTA-50F. The rig, dating from the late 1940s, looked like it had been uncrated two weeks ago - and this was 1973. The floors at the Tx site gleamed like a hospital's.

The 50F routinely ran 150% + positive being driven by Ed's own homebrew discriminate multiband processor, which IIRC was 3-band and used some kind of soft clipping. The bass register, picked off-air via RF amp and a General Radio mod monitor guarding the Harrow, Ontario Tx site some 30 miles distant, would rattle the fillings out of your teeth.

I still have reel-type airchecks of the station from my tenure there. The quality is astonishing, 35 years later. Beats most of what you hear on FM these days.
 
Today, as the radio manufactures careless about building quality AM radios, it's not important to us not to be in a loudness wars. Those days are over. The best thing to do is to set the audio where it sounds best and if the transmitter is of a older model, don't try to push 125% positives peaks down it when it won't do it. People care more about how good it sounds than how far it's getting out with a bunch of distortion.

Some of us in our area believe that we can sell advertising outside out 5 mv/m on AM. With that being said, especially the smaller, local stations, strive for better audio quality than loudness, and don't try to sell ads where your signal is not strong.

I have a preacher on Sunday Mornings, and as he prays, he says: "Lord boost up the power on this station and have it get out futher than it normally does". We'll we all know that's not going to happen. Preachers think that the more power an AM station has the more listeners there will be. That's not true, as in most markets, there are too many on at one time and the listener is going to listen to the one that is coming in the strongest, which is the closest station to them.
 
Savage said:
I was at CKLW when Ed Buterbaugh was CE and the main was an RCA (Victor! It was built in Montreal...) BTA-50F. The rig, dating from the late 1940s, looked like it had been uncrated two weeks ago - and this was 1973. The floors at the Tx site gleamed like a hospital's.

The 50F routinely ran 150% + positive being driven by Ed's own homebrew discriminate multiband processor, which IIRC was 3-band and used some kind of soft clipping. The bass register, picked off-air via RF amp and a General Radio mod monitor guarding the Harrow, Ontario Tx site some 30 miles distant, would rattle the fillings out of your teeth.

I still have reel-type airchecks of the station from my tenure there. The quality is astonishing, 35 years later. Beats most of what you hear on FM these days.
Click to expand...

Reelradio has a 1971 CKLW aircheck featuring their superb audio. It was recorded in Cleveland using a receiver with a modified detector yielding full fidelity.
 
Positive modulation over 100%, if done right, makes the audio stage huge. Unfortunately, it's a toss-up whether the listener will get to hear the
advantage or may even suffer distortion from this practice.

If the listener has a radio with a passive diode detector following sine wave IF detection, the audio may be improved if the receiver has an
AGC in which the timing is not so fast as to "follow" the audio.

If the receiver has square wave detection ( IF mixing) as many, many, do, there will be actual distortion as "more than 100" cannot be
expressed once we "refine" the IF signal into something akin to what always happens in FM (with a limiter stage).

For reasons that would require mathematics to explain, you can't get an extended linear response out of a system response that's not linear to begin with.
I'm no mathemetician, but I do know that once you define a limit for one variable in the equation, that limit will define a range in the solution.
In sine wave detection there is NO given max on either the incoming signal OR the local oscillator. This allows the "room" for the audio voltage
to extend beyond 100%.

My part 15 AM does about 150%, which sounds great on the true hifi AM radios I use. On anything "digitally tuned" with a PLL, etc, it sounds pretty
darn bad. The punch becomes crunch and the "open air" sound becomes flat and dead. Even the ones which will tune by 1 khz steps, while they can at least be tuned to pick up the higher frequency audio, still sound flat compred to any of my radios with passive detectors.
 
What a great thread!! I did not know about the PLL or square wave factor, although it does make sense. I'll have to listen carefully to AM's I know are asymmetrical.
Back in the day, I had a BC-1T that, at 250 watts, would honestly modulate 175% positive, At a kilowatt, it would do 125-130%, but all that made it hungrier for new tubes...

I would love to hear those CKLW tapes... Will have to listen to the one on Reel Radio.
 
Someone already said this was a great thread -- and having been on the air back in the late 60's and early 70's, I remember the difference good modulation made on a 1kw AM where I worked. We had the then new gates solid state limiter that had phase detection built in and it would flip the polarity to favor any asymetrical material. I don't remember the exact settings, but it was built right around the time the rules changed, and had a switch that would set positive peaks at 100%, 125%, and a couple higher settings. It was installed a few months before the new rules went into effect, and we were able to use the higher settings for awhile. I have a very asymetrical voice, and I loved that unit. However, it took some getting used to, since it took a noticable fraction of a second to flip phase, and when I would open the mic, and the phase was wrong for my voice, I would sound "flat" for the first word or so, then I could hear it flip, and my audio would pop right up. Our TX, when tubes were properly balanced, would easily do 140 - 150 % positive modulation. Ah, the great days when AM was King !!!
 
When the rules changed to 125 percent positive, most of the fun was gone. If the wiring polarity of the mics was correct, polarity would be correct for nearly all male voices. An engineer could watch the modulation monitor or polarity light (on processors with one) and flip the local mics that were consistently "wrong" phase over to the correct phase.

Just want to note again that this is from another era, when some stations were doing huge positive modulation.
If I was adjusting processing at an AM station today I would likely go symmetrical and work on the RF system instead.
 
I remember more than one jock who's voice would go the oposite polarity of everyone else. Mostly the way they "worked" the mic. First processor I remember that rolled the phase was an Inovonics (don't remember the model). The shifting phase when they started talking drove the jocks nuts.
 
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