Key and velocity mapping

Press play to hear a slow climb up the keyboard, one semitone at a time, on a small sampled instrument. Every note is perfectly in tune. Listen to how long each one lasts.

Up to B, each note gets a little shorter and quicker, fading before the next one arrives. Then at C the sound suddenly blooms: longer, rounder, slower. Nothing went out of tune. Before reading on, try the transpose buttons: move the run up a step, or down a fifth. Does the jump move with the notes, or stay on the same key?

It stays on the same key, because it belongs to the keyboard, not the melody. That's a zone seam: the point where the sampler stops stretching one recording and starts using the next. Being in tune doesn't make neighbouring notes sound like the same instrument.

How a sampler fills the keyboard

Recording every key at every strength would mean thousands of files. So a sampler records a handful of notes and stretches each one to cover the keys around it. It does that the way a tape machine does: play the recording faster and the pitch goes up, slower and it goes down.

That's why the stretch changes more than pitch. Speed a recording up by an octave and it plays at double speed: it lasts half as long, the attack is twice as quick, any vibrato is twice as fast. Slow it down and everything drags.

This little instrument is a deliberate extreme: just two recordings stretched across four octaves. Real libraries use many more, so the seams are smaller, but they're always there.

Before samplers, the Mellotron solved this the expensive way: a separate strip of tape for every key, so nothing was stretched. The price was that every note ran out after about eight seconds, which is why Mellotron players learned to re-press long chords.

Same idea, different place: stretch a voice up too far and you get the "chipmunk" sound, because its natural resonances move up with the pitch. Formant-preserving pitch shifters exist to avoid exactly that. See Pitch shifting.

The root is a fact

A recording's root note is the pitch it was made at. If the source is E4, its root is E4, and the sampler works out every other key from that.

It's tempting to nudge the root to make a seam less obvious. Don't: that changes what pitch the sampler thinks it's playing, so every key in that zone goes out of tune. If a recording is a little sharp or flat, use fine tuning. The root says what the recording is; tuning and gain handle the rest.

Drum samples are different. A drum kit usually has keyboard tracking turned off, so each key plays its hit at the original speed and the root doesn't change pitch at all.

A zone says where a recording plays

A zone is one recording plus the range of keys (and strengths) where it plays. A first map with three recordings might look like this:

Recording	Lowest key	Highest key	First seam to check
C4	C4	D4	between D4 and D♯4
E4	D♯4	F4	between F4 and F♯4
G4	F♯4	B4	the stretched top notes

Every key needs exactly one zone at each strength. A gap means a key that plays nothing; an accidental overlap plays two recordings at once.

Find the seams by ear

Play a slow chromatic run at one steady strength, both up and down, and listen at every boundary. Name what changes: the note is shorter, the attack sharper, the tone brighter, the room different, the level jumps. "These keys sound wrong" doesn't tell you what to fix; "F♯ is suddenly brighter" does.

Then fix one thing at a time:

• A level jump? Adjust that zone's gain.
• One side stretches better than the other? Move the boundary toward it, so the better recording covers more keys.
• Neither side survives the stretch? You need another recording in between.
• One note starts late? That's a preparation problem; trim its start.

Strength picks a recording too

Zones also have a velocity range. Play softly and you get the soft recording; play hard and you get the hard one. That's how a sampled piano gets darker and rounder when you play gently.

Where you put that boundary is a musical decision. Test it by playing right around it, 61, 63, 64, 66. Does a tiny change in touch cause a big jump in tone? If the hard layer is very bright, move the split higher so it only appears when a player really means it. If you put it too low, every phrase sounds aggressive.

Many samplers also scale level with velocity, separately from choosing the layer. That's two dynamic behaviours at once: level within a layer, and character between layers. Test both.

Crossfades smooth one seam and can open another

A velocity crossfade blends the soft and hard recordings across a range instead of switching at one value. It can make the change feel gradual.

But for a moment you're playing both recordings at once. If their attacks are a few milliseconds apart you can hear a little flam; if they're similar in tone, they can partly cancel and sound hollow. Try the boundary with and without a crossfade, and listen to both the attack and the sustained part. If the two recordings disagree strongly, a wide fade won't make them agree; re-record or move the boundary.

Not every key has every layer

Libraries aren't always even. A bank might have two layers on some notes and one on others. If one key jumps out when you play hard and its neighbours don't, it may be missing a layer. You can live with it, limit the range you use, or record the missing layer.

Make it yours

Take three notes you've recorded and prepared, say C, E and G, and map them across one octave in Maru's Sampler. Set each root to the pitch you actually recorded.

Play a slow chromatic run up and down at three strengths: quiet, medium and strong. Write down every seam you hear and the one thing you changed to fix it. Save the instrument and your seam notes together. The notes are the instrument's manual: when you come back to it, you'll know which boundaries you already argued with.

Quick check

1. A one-second recording is played an octave higher by a sampler. How long does it last? Half a second. Pitch, attack and decay all speed up together, which is why a far-stretched note sounds thin and quick.
2. Your mapped instrument is perfectly in tune but D♯4 sounds like a different instrument from D4. Change D♯4's root? No: the root describes the recording's real pitch, and changing it would detune the zone. Move the zone boundary, adjust gain, or add a recording between the two.
3. You add a velocity crossfade between soft and hard layers, and medium-strength notes now sound slightly doubled. Why? In the crossfade range both recordings play at once, and their attacks don't line up exactly. Narrow the crossfade or trim the attacks to match.

Next steps

• Go deeper: Loops and releases. Your mapped notes stop when the recording runs out. How do you make a note last as long as the key is held, and end naturally?
• Use it: Composing with a sampled library. Write parts that sit happily inside the seams.