Total posts : 45366
In the early 1950’s AM broadcast audio processors were simple compressor-limiters. Eventually engineers played around with clipping the negative-going side of the audio, understanding that phase amplitude shifts in the human voice, after a compressor-limiter, produces positive sine wave excursions less than 100% or levels equal to negative side. A couple of very smart broadcast engineers developed audio processing that sensed the most advantageous phase shift and flipped the audio polarity automatically. Eventually, the circuits shifted the audio without any audible processing artifacts.
Today, processors like the Innovonics 222 do all the sensing and shifting and clipping digitally, maintaining very natural sounding audio with 95% negative peaks and 125% positive peaks. Price is about $700.00, much less than the $5-10,000 original units in the 60’s.
The FCC does not regulate percentage of modulation any more, except to establish maximum limits. The Part-15 reg’s determine power by the voltage/amperage input to the final stage(PA) of the transmitter. All done minus any modulation. Otherwise power would be determined by peak-to-peak or peak envelopepower which is beyond the capability of most low power broadcasters.
Someday, take a look at an asymetrically processed audio sine wave on an O-scope. That is the picture that has been making very smart audio engineers scrotch there heads for decades.
Around the middle ’80’s, the FCC adopted the NRSC audio-mask standard for AM broadcast narrowing the audio bandwidth to 10Khz plus or minus the carrier frequency. This standard also prescribes audio pre-emphasis (75 u/sec) just like FM broadcast to make up for the poor receivers used to pick up broadcasts. Commercial and non-commercial licensed full-power AM radio stations are required to comply with this standard now. Part-15 stations are not required to use the NRSC standard. However, on most receivers the non-NRSC audio will sound tubby and life-less.
Theoretically, all of this introduces considerable sine-wave distortion. However, without the processing many AM signals simple are not “competitive” in the listener’s ear. So, for all intents and purposes, listeners have demonstrated over and over they would rather have less noise and acceptable levels of distortion instead of pristine audio. Even FM broadcast stations carry up to 5%-10% processing distortion on their final audio product these days.
Being a licensed broadcast engineer for the past 38+ years has allowed me to work with all kinds of audio processing and transmitters. Not everything I have had to do to please the program director, general manager and listeners had anything to do with good audio practices. Its just my opinion, but getting a tenth of a watt to the listener will require some exceptional measures in light of the poor quality of most modern day AM receivers and the susceptibility of AM to atmospheric noise.
And finally, I too have an AM-25 transmitter. I have had to modify it with bigger heat sinks and take extra care with supplying low-ripple DC power, a good ground system and adequate audio processing. Matching the antenna to this transmitter is well beyond most Part-15 users though. Without a network analyzer for tuning the antenna, providing this transmitter with a proper 50 ohm/resistive load is close to impossible. Much of the challenge is because of the shortness of the radiator itself in relationship to wavelength and the tenth of a watt power level. My AM-25 works satifactory. However, because of a recent FCC action against Ramsey, I am moving to the more expensive FCC certified AM-1000 transmitter.