Oli Larkin's Formant Filter is a free VST plug-in for Windows operating systems. It's not available in any other formats, unfortunately. You can download the plug straight from their website, and it's easy enough to install it as well, as it should only take a couple of minutes from start to finish. This isn't your average filter, as it's to be used with virtual software synthesizers. It's not the easiest plug-in to use in the world, but once you get the hang of it it's not too bad. It's been a couple of years since I've even used the plug-in however, since I don't have a compatible system for it any longer. The interface is made up of main sections for the five band pass filters, an LFO, and a main section. It has preset vowel sounds for getting your sound shapes, but you can also put your own in there as well, although I've only used the built in settings. A manual probably wasn't made for this software.
All systems should be able to handle the Oli Larkin Formant Filter plug-in just fine. I was running it on a Hewlett Packard Pavilion dv8000 Notebook with a 3.0 Ghz processor and 2 GB of RAM and in Cubase SX software. I used it with some free synths, although I don't remember exactly which as it's been a good while now. I wish I could run this on my current system but it's not compatible. I'd be surprised if anyone was having trouble running it if you're already able to run a virtual synth smoothly.
The Oli Larkin Formant Filter is definitely a nice freebee for those who have access to it. If you have a virtual synth that doesn't have a formant filter built into it, this is going to be quite helpful. It sounds good and there's a lot that you can do with it between the five filters. I like that it has the vowel sounds built right into it as it makes it easy to get those kind of tones right out of your synthesizer. The only bad thing about this is that you can't get it unless you're running a PC and a VST compatible platform. If you meet these requirements, go ahead and get this plug-in now!