In the 1980's licensed AM broadcasters became concerned with the dwindling listener numbers with the overall acceptance of FM and its signal quality, low noise and superior receiver design.
The National Radio Standards Committe was formed for the purpose of recommending solutions to qualitative signal challenges of radio. AM radio was its first arena for research, analysis and standards to be implemented to improve signals.
NRSC-1, published in July 1998, embraced the age old challenge of high frequency audio quality in AM broadcasting. A modified 75 u/sec preemphasis/deemphasis curve was recommended, much like the long used system used in FM exciter and receiver designs. In addition, the standard recommended limiting audio bandwidth to 10 Khz (-30db @ 10.5 Khz., -40db @ 11.0 Khz., -50db @ 15. Khz.). This "audio masking" scheme reduced percentage of modulation at 15 Khz. down to as little as .32 percent. This produced substantially less 2nd adjacent channel sideband interference in wideband receivers. Other advantages included better signal to noise in the "listenable" pass band of most average AM receivers and a finite audio standard for AM receiver manufacturers. NRSC-1 was a voluntary standard.
NRSC-2, published in June 1998, further described the reduction of AM broadcast spectrum bandwidth as a product of limiting audio frequency bandwidth. This original voluntary standard is referred to today as the AM "audio mask".
NRSC-3, introduced in June 1990, established audio bandwidth and distortion standards for AM receivers.
All of these standards have now been accepted by the FCC as mandatory for licensed AM broadcast stations.
You will find copies of the published studies on the NRSC standards site:
So, you might be asking, what the dickens does this have to with Part 15 broadcasting? Well, if you don't want to interfere with licensed radio stations and want to be accepted as just another station on the dial, you might consider complying with the standard. Does your station use preemphasis, audio masking and reduced bandwidth? And can you transmit your signal without "splattering" outside the 10Khz bandpass? All that audio above 10Khz is probably being spalttered all over the spectrum due to the narrow bandwidth and high reactance of Part 15 antenna systems.
I will point out Part 15 stations are NOT required to comply with any of the NRSC standards. However, if a local broadcast engineer captures your signal on his air monitor and spectrum analyzer, he might be motivated to make a phone call to the FCC. I have read where several Part 15 AM broadcasters have "solved" this problem with equalization and keeping the modulation percentage to a bare minimum. All of which makes your signal muffled sounding and not very listenable in comparison to the big boys.
My solution has been the Innovonics 222 AM audio processor which takes care of everything but the bandpass of the antenna and it's associated tuning network. This processor produces nearly perfect modified 75 u/sec preemphasis, limits audio bandpass at 10 Khz and keeps the average percentage of modulation at an acceptable level.
This piece equipment of does more to extend the range of my station legally than any other improvement I have attempted.
I welcome your feedback. What is your experience?