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Rhemaradio has operated with an Inovonics 222 into a Hamilton Rangemaster for several years. The functions of the processor are very simple. First, it is a single wide band mono audio processor to be used explicitly with AM broadcast transmitters.
Human voice and some musical content contains asymmetry that causes many AM transmitters to actually sound as though the peak modulation has been reduced during some asymmetrical content. To ameliorate this situation, the input circuit does a look ahead function and smoothly flips the audio 180 degrees (back and forth) to optimize the positive modulation. This function is common in most AM processors over the past 50+ years.
The 222 then runs the audio through a limiter to level out the average audio peaks (0 db to -12 db, adjustable). The audio is then pre-emphasized where the audio above 1 KHz is amplified on a special 75 micro-second R/C curve (NRSC standard). The audio pre-emphasis adds more clarity to the high frequency audio to improve intelligibility.
The entire audio package is then hard limited by a circuit that allows the user to control the negative peaks (<99%) with out controlling the positive peaks. This keeps the modulation circuit from over modulating and creating unpleasant distortion in receivers. By allowing the positive peaks to reach an average in access of 100 percent, the audio signal to noise ratio through the transmitter is improved. In the old days we used to accomplish this process by back biasing opposing diodes between negative and positive audio leads.
And finally a smoothing filter removes the square wave content from the audio then onto the 10 KHz low pass audio filter to comply with NRSC band pass specs.
This is an over simplified brief explanation of the working of the Inovonics 222 AM audio processor.
Higher priced audio processors also have multi-band compressor limiters. The 222 does not. However, with computer generated multi-band pre-processing or a stand alone multi-band compressor limiter at the input to the Inovonics unit, the 222 can be a very potent addition to any AM broadcast station.
With average positive modulation of 110+ percent, the output audio signal to noise improvement can be pretty impressive. This can make for more range and a more listenable signal.
