There's really two big secrets to making VST or MIDI strings sound realistic.
First, you need libraries that are up to date and well sampled. There's a lot of them out there, but the two favorites here are by Spitfire and Project SAM. SAM has some orchestral libraries are available through Ableton for and not super expensive.
Spitfire products definitely cost a bit more, but they're worth it. It's not really possible to polish something that sounds like crap to begin with, so it makes sense to just start with the best material you can get your hands on.
Second, once you have some solid string libraries to worth with and some basic melodic or chordal content happening in the piano roll, you're going to need to automate a lot of parameters to mimic what happens in a real ensemble.
Think about it. A violin player moves the bow back and fourth, which will never sound the same at the tip or at the frog of the bow.
(Yes, the 'handle' of the bow where it is held by the player is called the frog)
Except you have dozens of people all playing different instruments all on slightly different parts of the bow at all times. And each of those individual players is moving around as they play.
It's a very complex system and even if you have the entire orchestra just play a middle C together, there's a lot of movement baked into the cake by default.
So we need to replicate that as much as possible in the computer. Some of that human messiness is what makes things sound good and ultimately, realistic.
Except computers need to be told *exactly* what to do and as producers we need to get creative with how we add motion and movement. For example, on synthesizers, we often use LFOs to modulate static waveforms or filters to get interesting motion to happen.
Here are the parameters you should start with:
One trick to darken and thicken strings is to duplicate your lower strings (like bass and cello, possibly viola) and make one track a normal patch and another on con sordino (muted). Or alternatively if you want to brighten up a section, make your duplicated track play ponticello (by the bridge).
In real life you have a fixed number of players and all you can do is divide them up in different ways. In the computer, if you want to copy and paste another 10 cello players into the 'group' you do that with a couple of clicks.
There are other ways to go further beyond this, but usually only apply in specific situations, but these are the bread and butter techniques you can use all over the place. Even beyond orchestral VSTs. Looking for subtle ways to add motion to a synth patch or drum kit will add a sense of depth and 'organic-ness' to your sounds.
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