Audio processing is a topic that can go many directions. I would like to briefly touch on the basic principles.
Once you have set up a transmitter to be able to pass the audio bandwidth, and make the positive peaks that you desire to achieve, you must tailor the audio to fit within the boundaries of the transmitter, good engineering practices, and the receiver(s) you are trying to cater to.
In Amplitude Modulation, there is a tremendous noise floor, and limited dynamic range. Beyond 150% positive peaks, most receivers respond to bigger peaks as noise. This detracts from Intelligibility, and Loudness due to amplitude alone is limited.
Loudness is perceived to be greater when the density or average percentage is increased. Generally, the higher the average, the less bass you can run. Thus the audio will start to sound thin, or more strident. It might cut through noise better, but at the expense of quality.
Audio processing is critical in making sure that the negative peaks do not exceed 100% modulation. Exceeding 100% might make you sound a bit louder, but it cuts the carrier off at an audio rate, causing splatter. Also, it creates more area under the curve causing RMS Watt meters to read higher than they should. Looking at an oscilloscope will show this as the Peak to Peak does not change, but the RMS meter keeps climbing (More on the Scope in another section). Also, the flat topping of positive peaks will increase area under the curve.
The idea of audio processing is to control the audio in a manner that is acoustically pleasing, while confining it into the parameters that are defined.

Audio Processing

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