There are plenty of stereo expansion plug-ins out there and a lot of them are free. They work via a few different means, some play with the phase between the L and R channels, some add a delay between the L and R channels, some convert from L+R to Middle+Side and then play with the Side volume, some do things with the EQ (different in each channel). In the end, what they do is make you feel that the left and right channels are farther apart or sending your ears different information. They expand the stereo field. They make the sound feel wider.
They can be particularly useful in taking a mono source (where you have no access to the stereo source) and creating a stereo illusion. They can also widen the feel of a stereo mix. This is useful when you only have the final mix available and not the basic tracks that it is composed from. However, when applying them to a mixdown that is already stereo, where you have the basic tracks available to you, then it might be a lot better just to go back to your basic tracks and pan them differently and play with the relative volumes differently. As a rule of thumb, if you can fix something at a more basic stage of the production, then fix it and don't wait to apply a bandage at a later stage. If you can fix something during recording (with mic placement, etc.) or midi programming, then fix it there first. Don't wait to "fix it in the mix". And similarly, if you can fix something by balancing your mix differently, then fix it there, don't "fix it in the mastering".
That being said, the final decision is "if it sounds better, if it gives you what you are searching for, if it makes you smile, then go ahead". Guidelines, rules of thumb, etc are there to get you started because they usually give something pleasing. On the other hand, you as an artist are free to explore and if something does "it" for you, then why not!
Here are some interesting free VST/PC plug-ins that you might find helpful
Big Tick Hexaline
Beta Bugs Wide Bug
Joeren Breebaart Spatial Image Processor
Braindoc Stereo Enhancer
If you google for them, I'm pretty sure you'll find them, otherwise PM me and I'll send you more dertails. The Hexaline is one of my personal favorites.
Another plug-in that is essentially a mastering compressor, but because it allows you to play with mid and side signals can be used to play with the stereo field is Endorphin by Digital Fishphones.
You can also get similar effects with more simple tools. For instance, if you put a very slight delay (1-9 ms) on just one channel of a stereo mixdown, that will give some sort of spacing illusion. One that I particularly like is to use slightly different EQ on the L and R channels of the stereo buss. The OvertoneEQ from Voxengo, that you mentioned in another post, is a useful tool for this (set up your routing so that you have one side taking care of the L channel and the other the R channel). This EQ trick can also sometimes help on the overall clarity of mixdown.
Potential Pitfall: Listen in mono. All these tricks do things to the phase of the L and R signals and when summing these back into mono you might get surprises. What sounded good in stereo might sound very different in mono due to phase cancellation. If you absolutely don't care about mono, then don't worry about it, but you never know when your tune might get played on mono-only equipment.
But once again, I'd like to reinforce that a stereo expander is not at all obligatory and that in fact, if you can achieve a sense of space by changing the panning, volume or reverb in your mix, then that is probably the best way to work. (emphasis on "probably", your mileage may vary)
Thank you for this answer. Now I also know why Voxengo Overtone uses more channels and how to deal with them. I will experiment with it a bit.
About Stereo Expansion, I can use the one within Adobe Audition, namely the Pan/Expand where it uses a stereo signal as having four channels, L, R, Center and Surround. It goes too far to explain it here, but I meant this one, not only that dry stereo filed expander, which gives me a very strange stereo effect.
and I think that I understand a little bit about what is going on.
When Adobe Audition speaks of "center" and "surround" they mean what a lot of other people mean when they say "middle" and "side". If you take your L and R channel and sum it (L+R), then you have what is known as the middle channel (or center channel). This is essentially the mono version of your stereo track. If instead of summing them, you take their difference (L-R), this is the side signal (or surround signal). It is the sound that makes the L and R channels sound different. This M/S language is related to a mic technique as well. This center/surround or mid/side method of changing the stereo field is usually a fairly mono-safe way of treating the sound. It usually will not introduce phase issues so when taking the stereo and playing it mono, there should be no surprises.
How realistic is it? Like all of these things ... it depends. It depends on where the listener in real-life is sitting, how loud each instrument is playing, the orchestration ... it will all effect the listener's perceived sound stage.
So with the Pan/Expand plug, you can place the center and surround signals where you want in the L-to-R panorama and thereby affect the perceived separation. At the end of the day, the realism depends on what you are going after. Very exagerated settings will most likely not be realistic, but if you keep things subtle or just a little bit pushed, then you are probably simulating something that could be realistic. Finally, if it sounds good to you and to a reasonable sample of perspecitve listeners, then go for it. If it sounds bizarre, unrealistic, unmusical or unpleasant, then leave it out. It is not something that "must" be used and you are not "lowering the quality" of your mix if you don't use it. Trust your ears and maybe the ears of a few careful listeners and go with that.
Or another method is to carefully listen to other recordings that you want to imitate. Listen to how others have treated their stereo field and when you find the one that you think will sound best for your piece, then try to reproduce that with your tools. A good way to do this is to rip the recording into a wav and load it into your editor along with the piece that you are mixing/mastering. Every now and then give your chosen reference a listen and then think about how that sound compares with what you are trying to do. Tweak your tools and then see how it compares and cycle through this until you're happy with the results. Then go away for a few hours or a few days, then come back to it and see if you still like it. Sooner (hopefully ) or later you'll converge (or go crazy ) and then you're done . Anyway, that's how I usually do it.
That Adobe doc is exactly what I meant. You are right about "doing it very subtle". It tends to a huge space, like those Hollywood sounds. All other recommendations were very welcome. Rendering takes time and pace.
Raymond - and I saw that I can have an update of Audition 2.0