Using VST Plugins
Virtual Studio Technology (VST) is a standard for software synthesizer and effect plugins invented by Steinberg (maker of the Cubase DAW).
To get started, here is a remarkable and very popular free synth from German vendor U-He: https://u-he.com/products/zebralette/
Download the Linux version and unzip it to your ~/.vst directory. If the ~/.vst directory doesn't exist, you can create it.
One of the programs that can be used to run and manage VSTs is Carla. Here's how to get it:
Install and Run Carla
Carla is available in Ubuntu Studio from 19.10 (The Eoan Ermine). It is also available in the Ubuntu Studio backports ppa from 18.04 (The Bionic Beaver). However, the ppa is now frozen for 18.04 and will receive no further updates.
Configuring and Using Carla
Once you build up a collection of VST plugins and have a number of projects, you should take advantage of the ability to save and restore a Carla rack combination, which can load several instruments at once. Your plugin rack will be saved with a file extension of .carxp. If you have a favorite rack configuration, or used certain plugins for a project, storing a rack is a great time saver. JACK configurations can be stored, too.
On the far right of Carla's top menu, click "Configure Carla" to see the Settings pop-up window. Go to "Paths" and make sure "VST" is selected in the dropdown at the top center of the Settings pop-up. Add the full path to the directory you created earlier, unless it's there by default. It might look something like this: ~/.vst
Also check the project path under Carla's "Main" tab. Click "OK" to save your changes and dismiss this window. Finally, click the "+ Add plugin" button, and click "Refresh" on the subsequent pop-up window. Carla will search for Zebralette and any other VST plugins you've installed under the VST path. These may take a while to load, during which time it will look like Carla isn't doing anything. If you get an error message, loading may have simply timed out - try it again a few times. When the plugin becomes available, it will also show up in JACK, where you can connect it to System and your MIDI controller. Check that everything works. Back in Carla, you can see the GUI for this and other VSTs by clicking the gear icon (white gear at top left the following screenshot).
Zebralette and other free VSTs may have been released a few years ago, but that doesn't mean you can't make fresh-sounding music with them. Always check out commercial offerings and/or consider a donation to the developer if you end up making heavy use of a particular plugin. Most have a PayPal tip jar. Carla's wrench icon is important because it allows you to bind synth parameters like filters or the VCA rate to knobs on your controller, such as the modulation wheel. Just find the Continuous Controller number for a given knob, and set that as the CC# for a given parameter in the wrench's pop-up window. Your controller's manual will show the CC assignments. In the following example, the modulation wheel is CC# 1, and is being used to control the Hi-cut Frequency (circled in red).
Zebralette's rich and varied sounds come from the ability to stack and draw custom waveforms. The Presets tab on this "morphing" synth is the third tab from the left on the bottom tier.
Any parameter can be bound to a given CC number. Multiple parameters cannot be assigned to the same CC number. Settings can be stored and recalled.
It's not difficult to find free VSTs, but here are two sites that have hundreds, if not thousands of free plugins: http://vst4free.com which sorts by name, rating, and last added (in addition to giving a random list when you first roll up on the site) and http://vstplanet.com. Note that not all freeware uses legally-obtained samples. Additional reviews of some VST plugins appear in the appendix.
Do know that many VSTs are built for Windows, which requires the WINE Windows emulator and a plugin bridge. A part for that process may be added to this section at a later date. Be sure to use the latest version of WINE.