Thanks for clarifying that, Glen. An excellent read. So from that, I'm thinking that the efficacy range of an expander must lie somewhere in the realms of a sensibly compressed signal to begin with? I assume that, while we could restore the range of an instrument, that's been limited as far as one normally would, to make it sound better, that trying to restore full dynamics to a voice, that's been sqaushed to telephone quality, would be more than we could reasonably expect?
Don't over-think it. An expander is not a magical restoration device. All it is is a compressor in reverse.
With a standard downward compressor you set the gain ratio to something-to-1 (e.g. 3:1, 8:1 etc.). Using 3:1 as an example, that means that for every 3dB going into the compressor over the threshold setting, 1 dB will come out. Simple enough in concept. Upward expansion would simply be the opposite of that; i.e. it's simply reversing the compression ratio to 1-to-something, meaning that for every 1dB going in over the threshold setting, some number of dB greater than one comes out. The exact opposite of 3:1 compression would be 1:3 expansion.
This means that theoretically one could "restore" a signal for which only simple compression has been done by simply performing the exact opposite expansion at the same threshold. The problem is that in real life, rarely is that one simple step of compression all that has been done to a mastered signal. Once you start compounding several steps of processing, including multiple compressions or intermediate EQ steps, and so forth, exactly undoing and precisely restoring the original signal becomes virtually impossible. You may get close enough for rock and roll, but exact deconstruction is usually unreasonable at that point.
And, as we alluded to earlier, if everything has been smashed against a wall by a limiter or by clipping, then there are no dynamics left to restore. Trying to expand a brick wall results in nothing other than a louder brick wall; in that case expansion is no different than just pumping up the volume. any attempt at restoration at that point would need a lot more than just passing it through an expander.
I'd like to experiment, but I don't think I have one in my VST bank.
VST Expanders are much harder to find than compressors, but in fact most DAWS come with native expansion built in to their native tools. Most DAWs come with something called a "Dynamics Processor" or some name similar to that. These are not always the easiest tools for everybody to learn, but they are actually very powerful, and once you get used to them, they can actually become pretty natural to use. You can do all sorts of different types of both compression and expansion with them. Search your DAW help files for "Dynamics" or "Dynamics processor" and see what you can find.
G.