I agree that the default configuration of a typical Omnia or Optimod product will try to compress out all the dynamics of the original content, but many of these at least have some presets for things like "Classical Music" or perhaps even "Talk Radio", that could serve as a starting point to get where you need to go. Then one would likely have to learn how to adjust these boxes further to really get what you desire (I expect a default "Talk Radio" preset would still be too processed for your purpose).
I can't find the comment that resulted in the "A Compeller does that?" comment, but it is a dual channel (stereo) single-band compressor that can be adjusted to be "fast" to provide somewhat heavy processing or "slow" to just provide consistent leveling (AGC). I think it works really well. It also has all the gating functions I talked about to prevent some side effects compressor/AGC units that don't have this ability. A LPFM transmitter probably at least has a "final limiting" stage to keep the modulation legal, but you should check for sure. That said you may not like the sound of it if you "hit" this limiter too much. You should also make sure the LPFM box also has the pre-emphasis curve appropriate for your country's standards. If any of this is missing, the LPFM manufacturer then expects you to use an outboard processor like the Optimod or Omnia (there are many others too) box for this. If you run the station in true "mono" mode the signal will go a *LOT* further, but some people might wonder why the stereo light doesn't light on their receivers. So "side-chain" may be a lesser known term for the control signal (a DC level in the old analog days) that commands the audio stages as to what amount of audio gain it should provide at any moment. Sophisticated compressors often add a lot of "smarts" to this signal by varying the speed of how it reacts to certain signals, whether or not it should be "linked" to some degree to another related signal (such as the other channel in a stereo pair), or finally when it should simply "hold" at a current level during a brief period of silence (as in my "side-chain gate" function mentioned earlier). The 1st stage of those expensive Omnia and Optimod processors generally do all this too, but perhaps some LPFM transmitters with simple/little/no processing will not so a Compeller might be a simpler/cheaper choice for you as long as the LPFM transmitter will take care of the pre-emphasis, stereo generation (if desired) and final limiting for you. Finally as it sounds like you are willing to go through each audio file separately, perhaps you can find and use the processing effects found in whatever editing software you are using to deal with your problem that way? Finding a compressor/AGC effect that can be adjusted to do what you want would be the key. If you like the Orban 424 processor recommendation mentioned earlier, perhaps you can find a used one? My worry is that since this is quite an old product, that you should get one that someone has fully "refurbished". I worry that they might use hard-to-find "LDR" (a calibrated incandescent light & photocell combination) modules that burn out at times. I've also had problems with noisy/intermittent controls that are on the front of these units.
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