Create sympathetic_resonance.md

Very initial version, just writing stuff down while it's fresh in my mind.

docs/tutorials/sympathetic_resonance.md

@@ -0,0 +1,187 @@
+---
+title: Sympathetic Resonance
+---
+
+## Basics
+
+Many instruments have elements which will resonate after a note is stopped. A piano
+with the sustain pedal down is an obvious example. In sampled instruments, this is
+usually dealt with by recording one set of pedal-up samples and one set of
+pedal-down samples. Some sampled pianos will also support half-pedaling, though this
+will have to be documented later.
+
+The current version of this tutorial is based on an implementation of sympathetic
+resonance for a nyckelharpa, which is, more or less, a violin with a key mechanism
+and a set of 12 sympathetic strings. This was also reused for another instrument
+with a smaller number of diatonic sympathetic strings.
+
+The basic concept is simple: when a sustained note ends, a sympathetic resonance
+sample is played as a release sample. In the case of the nyckelharpa, the resonance
+samples were created by using only the tails of short notes. There is a set of
+round robin samples for each pitch, and they are mapped to the keys as normal.
+
+```sfz
+<master>
+#include "modules/maps/strings/nh/nh_sus_f_map.sfz"
+
+<master>
+trigger=release
+ampeg_attack=0.1
+#include "modules/maps/strings/nh/nh_release_map.sfz"
+```
+
+Instead of creating a separate set of samples it's even possible to simply use the
+shorts samples with an offset, if the shorts are performed consistently enough.
+
+```sfz
+<master>
+#include "modules/maps/strings/nh/nh_sus_f_map.sfz"
+
+<master>
+trigger=release
+ampeg_attack=0.1
+offset=6600
+#include "modules/maps/strings/nh/nh_short_map.sfz"
+```
+
+## Controls
+
+We can add a simple volume control.
+
+```sfz
+<master>
+#include "modules/maps/strings/nh/nh_sus_f_map.sfz"
+
+<master>
+trigger=release
+ampeg_attack=0.1
+amplitude_oncc52=100
+locc52=1
+#include "modules/maps/strings/nh/nh_release_map.sfz"
+```
+
+To prevent the buildup of voices with fast playing, a polyphony group can be
+used.
+
+```sfz
+<master>
+#include "modules/maps/strings/nh/nh_sus_f_map.sfz"
+
+<master>
+trigger=release
+ampeg_attack=0.1
+amplitude_oncc52=100
+locc52=1
+off_time=0.4
+group=14
+note_polyphony=1 
+#include "modules/maps/strings/nh/nh_release_map.sfz"
+```
+
+## Multiple articulations
+
+There could be a separate set of resonance samples captured for each technique, but
+in practice one set of samples cut from shorts works reasonably well for most.
+However, tremolo seemed to agitate all the strings more. In other words, playing an
+A didn't just make the A and a little E resonate, even the A# and G# strings
+resonated a bit, perhaps as a result of the whole instrument shaking a bit. So,
+tremolo got its own set of samples.
+
+Shorts and pizzicato didn't use separate resonance samples, they were simply set
+to one-shot mode, so the entire sample played, resonance and all. This has its
+downsides, of course, but again it seems to work well enough in reality.
+
+However, with the jete technique, the same set of samples was used with the
+addition of rt_decay. Keeping it simple, here is the polyphonic implementation:
+
+```sfz
+<master>
+#include "modules/maps/strings/nh/nh_jete_map.sfz"
+
+<master>
+trigger=release
+off_time=0.4
+rt_decay=12
+ampeg_attack=0.1
+#include "modules/maps/strings/nh/nh_release_map.sfz"
+```
+
+## Legato
+
+The above works fine for polyphonic instruments, but with a mono instrument it's
+not enough to have a sample play on release; we also need to play a sample when
+a note is muted and a new note starts playing. At that point, we need to trigger
+the resonance based not on the incoming MIDI note but based on the previous pitch
+that was playing.
+
+```sfz
+//Mono mode
+
+<master>
+group=1
+off_by=1
+#include "modules/maps/strings/nh/nh_sus_f_map.sfz"
+
+//Mono mode, keypress restrictions so only triggered on last note of phrase
+
+<master>
+trigger=release
+amplitude_oncc52=200
+locc52=1
+off_time=0.4
+ampeg_attack=0.1
+group=14
+note_polyphony=1 
+//Trigger only when one key was down
+lohdcc153=0
+hihdcc153=1.1
+//To keep from over-triggering when unison is on
+group=199
+note_polyphony=1
+#include "modules/maps/strings/nh/nh_release_map.sfz"
+
+//Mono mode, legato triggers
+//Loop mode is one shot to keep these from dying when the next key is released
+
+<master>
+trigger=legato
+amplitude_oncc52=200
+locc52=1
+locc105=64
+off_time=0.4
+ampeg_attack=0.1
+loop_mode=one_shot
+pitch_keytrack=0
+#include "modules/maps/strings/nh/nh_release_swdown_map.sfz"
+```
+
+## Multiple voices
+
+When one key triggers multiple samples, whether they are crossfaded dynamic layers,
+mic posotions or unison voices, each sample playing is a voice which can trigger
+a release. In other words, with four mic positions having four sets of releases,
+sixteen release voices would be triggered for each note. This not only uses up
+unison voices but also is too loud and doesn't sound good. Fortunately this is
+easily fixed by creating a separate polyphony group for each voice's releases,
+and limiting its note polyphony. If groups are already created as discussed above,
+just make sure each set of samples has its own group number.
+
+This is not a consideration limited to sympathetic resonance, of course, but a
+general principle of using release triggers with multiple voices.
+
+```sfz
+<master>
+trigger=release
+ampeg_attack=0.1
+//To keep from over-triggering when unison is on
+group=199
+note_polyphony=1
+#include "modules/maps/strings/nh/nh_release_map.sfz"
+```
+
+## Other methods
+
+At least in theory, a better way to do this would be to capture an impulse response
+of the resonance, perhaps by giving the instrument a whack and letting the
+sympathetic strings ring. However, this would require an sfz player with
+convolution reverb implemented.