Velocity
A note's velocity is how vigorously it's played. Whether your keyboard can send such information, and whether your other equipment can make use of it, is whether or not they are velocity-sensitive.
In theory, it should measure how hard you pressed the key on your keyboard. In practice, it's cheaper to measure how quickly the key goes from slightly pressed to fully pressed, hence velocity. As the two correlate well, this works just fine as a substitute.
As with subtly changing the tempo throughout a song, varying the velocity of notes is a chance to make your music more expressive, to add a whole new dimension to it.
Velocity-sensitive synthesis
The usual example of what to do with velocity information is to make the higher-velocity notes louder and the lower-velocity ones quieter. On a modular synthesiser, this can be accomplished with a second VCA, either by changing the volume of the signal once per note, either before or after the envelope generator does so with the main VCA, or by making the second VCA in turn affect how much the envelope generator affects the main one. With a hardwired synthesiser, if it's velocity-sensitive, then this is probably taken care of for you.
I think a more satisfying use of the velocity is to send it to the lowpass filter, so that higher-velocity notes are brighter, and lower-velocity ones are duller. This is especially true for analogue synthesisers, whose quirky filters are often amongst their greatest assets.
Velocity-sensitive sampling
When it comes to samplers, an especially good one (such as the Studio 440 or S1000) should let you specify how much the velocity offsets the sample start. That way, louder notes can play the entire sample, while quieter ones can miss off the beginning, making them softer. (This may result in a click at the start, as the speaker jumps straight to a mid-cycle peak, but you can compensate for this by also slowing the VCA's attack for softer notes.)[1]
You can combine this with the sampler's lowpass filter, if it has one. Although it's likely to be digital and not as interesting as on an analogue synthesiser, it's certainly adequate to dull the sample as necessary.
A velocity-sensitive sampler won't sound quite as expressive as a velocity-sensitive analogue synthesiser, but it'll still sound interesting in its own right.
MIDI
MIDI supports two velocities: the Note On velocity is how quickly you press a key on your keyboard; and the Note Off velocity is how quickly you let go of it again. While the Note Off velocity is barely supported, the Note On velocity is often an integral part of electronic music.
If you play the keyboard, and your MIDI keyboard's velocity-sensitive, all you need to do is be mindful of how you play. If you prefer to draw or otherwise step sequence notes on a grid, you'll need to enter the velocities manually.
Accent
The simplest type of velocity is accents, such as on Roland's venerable TB-303, TR-606, TR-808, and TR-909. They're Boolean, as in either on or off, nothing in between, so you only have two intensities of note: loud or quiet. Even then, you might be surprised just how much of a difference using accents makes. It's a good habit to form.
Another way to look at it is that music is patterns of sounds, and accents let you add another layer of patterns on top of the others. (For example, one thing I do far too often is draw in arpeggios consisting of an even number of notes before repeating, then accent the first of every three notes, with a bit of extra variation on top of that to keep things interesting.)
When drawing on notes in a MIDI sequencer or DAW, the usual way to enter accent-style velocities is to highlight all the quiet notes in a phrase and set their velocities to half the regular amount. This is what I use when sending notes to my DrumStation, for instance.
If you're using a modular synthesiser for a particular part, you might find it easier to simply set the non-accented notes to the minimum velocity, all the way down to 1. This is quicker to do with the mouse, and also easier to patch up: you can set your quiet notes just as you like them, then dial in how much the velocity should affect any given parameter. With the quiet notes at 50% velocity instead of 1%, changing how much the velocity affects the sound also affects how the quiet notes sound.
In summary, velocities are an important way of making your music more expressive. It's up to you what to do with them, but you should make use of them often, because they'll really bring your music to life. Even rigid, grid-based music like house and techno benefits from accents, which are the simplest kind of velocities.
Quotes
One of the reasons I began using acoustic drums with pickups on them rather than synthesisers was because early instruments were not touch sensitive, and therefore hardly suitable for drummers. I'm still amazed that people make systems without touch sensitivity. It's a fundamental requirement for bringing electronic instruments in line with traditional instruments.
— Richard Burgess, Landscape, 1981[2]
References
- "We Can't Go On..." Matt Isaacson, Music Technology, May 1987, pp. 60—63
- "Landscape Explored" Mike Beecher, Electronics & Music Maker, Nov 1981, pp. 6—10
Music theory: Arpeggio | Block chord | Broken chord | Circle of fifths | Interval | Music | Pitched tempos | Polymetre and polyrhythm | Rest | Swing | Velocity