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Having been a broadcast engineer for the past 28 years, knowing the technical capabilities of AM and FM broadcast audio may be instructive.
Years ago, AM broadcasters did not have audio spectrum limitations put on the transmitted signal. Many times audio frequencies would be transmitted in excess of 12 KHz; that is if the transmitter audio section was capable of working at that level without going into meltdown. Modulation percentage was limited to 100 percent, both positive and negative. Today with NRSC audio standards, both for transmitters and receivers, 10 KHz audio is the highest audio frequency to be transmitted. This is done to limit the sideband interference so that more stations can operate peaceably side by side on the band. Modulation limits have changed however from 100 percent negative peaks and positive peaks to the contemporary positive peak standard not to exceed 125 percent “occasionally”. The new standards were also implemented to improve signal to noise ratio of the received audio and to more closely duplicate real-world transmitter and receiver audio. Most cheap AM receivers have never performed beyond 7 to 8 KHz in audio frequency with nearly no low end audio at all. AM with the proper pre-emphasis of audio and wave amplitude control processing can be made to sound nearly FM in mono. And finally, AM stereo transmitters do exist and work well. However, the diminishing supply of AM stereo receivers makes broadcasting in the mode purely audiophile experimentation.
The FCC considers the AM band utilitarian in nature and uses its “custodial” powers to keep the operators on the band legal and in check. Part 15 certified AM transmitters are considered intentional radiators and noise makers. As a micro-broadcaster, the more professional the operation, the less trouble the operator will have with inspectors and regulators.
FM broadcasting started as strictly monaural. The antenna systems were very narrow in bandwidth and caused the FM signal to not be of constant amplitude. The signal was prone to “AM-ing” causing received signal problems. This problem persisted until about 10 years ago when broadcasters began developing test equipment to measure the AM component of the FM carrier. FM stereo brought its own set of issues. The maximum modulation for program audio was about 85% of the mono signal (not as loud) and was limited in maximum frequency to less than 15 KHz so that the program audio wouldn’t interfere with the stereo pilot signal (19 KHz) and to limit the total channel bandwidth. So audio response suffered due to the stereo technology. The original stereo signal process exists today in a single chip. Plus, multi-band audio processors are used today, even on classical music radio stations, to permit the received audio to be heard over the ambient room or vehicular noise. Not exactly the “purist” approach.
The FM band is the “money” band to the FCC. And as such, they guard it like the crown jewels. Hence, the more narrow field strength rules for Part 15. Mess with the signal of an FM broadcast station and the full weight of the FCC will be on its way. To prove a point, it took an act of Congress, literally, to cause the FCC to create the Low Power FM (LPFM) service. It took years for the feds to finally put the service together. It took congressional arm twisting to finally get the service implemented just a few years ago. Many of the modern day Part 15 rules have been driven by agency compromises with the National Association of Broadcasters and broadcasting lobbyists in Washington, D.C.
As a final note, the better quality the audio supplied to the transmitting system and the better the receiver design, the better the over all quality of the listened to radio product. So, when you use a blanket criticism of FM over AM based on audio quality, consider there are caveats. Neither are capable of broadcasting “studio quality” audio. I have designed and built recording studios with equipment much more expensive than you mention here. Remember, even a Stradavarius in the hands of anyone but a master still produces, at best, so-so music.
Rev. Marshall Johnson, Sr.
President & General Manger
Rhema Radio – The Word In Worship
The Dalles, Oregon, USA