Allowed positive modulation percentage in commercial AM stations and other questions

[Moleculo]

The FCC permits commercial AM broadcast stations' to transmit a signal that has up to 125% positive modulation but the negative modulation cannot exceed -100%. The best description I have seen for why exceeding -100% at the modulation troughs is not allowed is this quote:

"[The] effect, commonly described as "pinch off", produces at the "corners" at the start and finish points in time of the straight zero line, a spectrum of spurious emissions that typically `spatter` beyond the band limits and potentially interfere with adjacent channels in violation of broadcast regulations."

Follow that statement up with this quote from the same material:

"In recognition that the interference problem originates at "pinch-off" at the envelope troughs and not at the envelope peaks, regulations have been relaxed to allow some amount of "upward" modulation, e.g. to a level corresponding to 125% modulation as long as "pinch-off" and resultant out-of-band "splatter" interference are avoided."

So my first question is, if the problem with splatter is not caused by the positive modulation, why is it regulated to be cut off at 125%? Are there some ill effects by exceeding that percentage that the FCC (and other agencies) are wanting to avoid?


On a related note, I'm reading that because the human voice (and other things) that are broadcast are assymetrical, the phase of the source (normal or inverted) becomes a factor in maintaining at least 100% modulation. Because of this, stations use sophisticated equipment to monitor and detect whether or not the input signal has inverted or normal phase and then automatically switch the output phase to maintain modulation percentage. A write-up on the issue and (of course) a device to handle this issue is described here: Posi-Phase. My question is, does this issue apply to the typical AM ham station? I mean, since we're typically using our same mic, transmitter, amp, etc., do we need to concern ourselves with phase shift?

I think I have more questions...

 

[W5LZ]

I think I would revise that question by deleting "AM ham" and substituting "any". I know that there are different 'standards'/rules for each service. Very seldom do those 'standards' overlap services and they don't with ham/CB services in regard to AM mode.
All of this is because AM mode is just not all that fantastic or efficient when compared to other voice modes. And it really doesn't matter what you are talking about, there is only so much 'improvement' before that 'improvement' turns to no improvement/trash.
Something else to think about is that ham/CB voice modes do not require the necessary audio bandwidth that commercial broadcast stations require. Sort of a music versus voice thingy.
It only gets 'so' good, then you gotta regroup and try something else or you end up shooting yourself in the foot.
- 'Doc

 

[Captain Kilowatt]

The 125% limit was imposed,for one reason, to prevent a "loudness" war on the AM band.Without any limits all hell would break out. I once tested a Nautel 1Kw TX that had a modulator problem. Once it was fixed I was able to test it out at 100% neg mod and 225% pos mod. Ever hear ,or see on a scope, what 225% pos mod with a 1000 hertz tone looks/sounds like. It's .The other thing is power limits.Exceeding the 100% pos limit also exceeds the normal pep rating of the transmitter. If we consider that 4Kw pep is the same as 1Kw of carrier at 100% mod then anything over that is exceeding the actual power limits of a station's license. The extra 25% mod was determined to be an acceptable trade off of license limits versus solid audio. Not all TX's are capable of achieving audio in excess of 100% without flat topping BTW.

As for the ham bands, unless you are a purist you need not bother with phase. The small differance in percentage is negligible.

 

[freecell]

"Not all TX's are capable of achieving audio in excess of 100% without flat topping BTW."

or carrier "foldover".

AM_ Mod_Overmodulation

 

[HiDef]

Good stuff. The broadcast regulation for +125% was not always there. Before it existed some stations opted for the solid wall of sound with increased distortion (which was due to receivers) in order to keep up with other stations who were allocated more power. I know one person who worked for a 5KW outfit in the #1 market early 70s using 200% positive. Station kept up with 50KW outfits in advertising sales for a while. He later defected to very well known AM outlet where he is still staph announcer. The idea is to have the signal jump out of the speaker while tuning around the dial. FM processing wars in the 70s were similar. WPLJ in NYC always had a sig that you could pick out without even looking at the dial. My opinion: Kids brains can fix distortion and even off key vocals.

Harmonic distortion sounds good to lots of people. You like 1966-on hard rock?

Natural asymmetry occurs when the transmitted bandwidth allows fundamental and voice harmonics through AND the subject has a measure of testosterone.
Pinch the audio to narrow bandwidth and asymmetry disappears. Certain heavy smoker chicks have this quality.

 

[Captain Kilowatt]

"Not all TX's are capable of achieving audio in excess of 100% without flat topping BTW."

or carrier "foldover".

AM_ Mod_Overmodulation

THANK YOU! I used to have a copy of that very piece of info when I was still at the radio station and wished I had taken it when I left. I have saved a copy to my HD.


I only mentioned flat topping because the tube transmitters would not produce enough power under high level modulation conditions to achieve 125% pos mod in a lot of cases and would flat top except when new tubes were first installed.

THANK YOU!