The "serious" Part 15 AM community broadcaster, if he doesn't already know, will soon discover that in-band interference can be caused by over modulation. Other factors might included bandwidth of transmit audio, bandwdith of antenna circuit, power levels, 2nd and 3rd adjacent channel frequency separation, signal cleanliness, ect.
In regards to the modulation issue, I recently added an Inovonics 222 audio processor to the AM audio chain. When installing the unit and going through setup, I found it frustrating not being able to tell how much signal was being pumped out on the positive peaks. I bought the box to get the last little drop of signal out of my Rangemaster transmitter. Furthermore, the unit is capable of getting consistantly right up to and not exceeding 98 percent negative peaks. But with real-world programming, how close can you get with an O-scope? Depending on the screen size and the graticle grid, probably not very close. This would be particularly true when setting the 125 percent positive maxium peaks. Any over modulation on the negative peaks (over 100%) produces all kinds of garbage that, even at micropower, can get you in trouble with the neighbors and licensed broadcasters. One caviat: wave shape (positive and negative) can determine the amount of unwanted signal transmitted. An O-scope is very helpful in looking at how clean the signal is.
With all of this in mind, I have discovered a new radio-related box being offered that will help determine the modulation package for my station. It is designed and manufactured by Radio Engineering Associates. It was developed primarily for use on AM HF amateur radio transmitters. Their web site might deserve a look-see.
http://www.radioassociates.com/
A recent answer to my email says the box could be available as early as June 2006 and available in kit form or wired and tested. And, the price seems within reason ($300-$400). Not only does this unit have separate displays for negative and positive program peaks with a peak delay, but it also has a "peak flasher" enabling the broadcaster to literally "push" the envelope and do it legally and cleanly. The flashers can be set to user specified points as well. The AMM-HF1 can monitor several different transmitters using remote amps and detectors, of which the plans are available from REA.
My plans are to acquire one these boxes in the near future. I will gladly let everyone know how it works with Part 15 transmitters.