bradhowes / sf2libau

AUv3 MIDI instrument with sound font (SF2) rendering

SF2LibAU - AUv3 MIDI instrument with sound font (SF2) rendering [BETA]

The code defines an AUAudioUnit MIDI component in Swift that uses the SF2Lib engine for rendering audio samples.

Currently, the AUv3 component:

• responds to MIDI messages and can render audio
• supports loading of SF2 files via custom sysex MIDI command
• supports selecting of a preset by bank/program as well as index position

The unit tests contain examples showing all of the above.

NOTE: requires Swift 5.9 for it's Swift/C++ interoperability facility.

Parameter Control

An SF2 file contains a collection of instruments and presets each of which contains a set of values for the SF2 synthesizer parameters (generators) and modulator mappings; those for instruments are absolute values, while the preset values are always relative, adding to an instrument's value. Many of these parameters can be modified in real-time by two means:

• AUv3 parameter changes communicated via the AUParameterTree instance provided by the AUv3 instrument.

• MIDI control messages using "NRPN" MIDI control. From the SF2 spec:

NRPN stands for Non Registered Parameter Number. The MIDI specification has defined this series of continuous controllers to permit General MIDI compatible synthesizers to take advantage of their proprietary hardware by using these messages to control the non-General MIDI compatible aspects of their hardware. The SoundFont 2.01 specification uses these messages to allow arbitrary real-time control over all SoundFont synthesis parameters.

Note that some MIDI control messages will by default also effect a parameter change. For instance, MIDI pitch bend messages affect the pitch of the note being played, and MIDI controllers 64 (sustain pedal), 66 (soft pedal), and 67 (sostenuto pedal) affect the envelope behavior of a playing note.

AUParameterTree

The AUParameterTree for the SF2 engine contains entries for all of the implemented generators. See the SF spec for descriptions of these generators and their valid value ranges.

Address	Name
0	startAddrsOffset
1	endAddrsOffset
2	startloopAddrsOffset
3	endloopAddrsOffset
4	startAddrsCoarseOffset
5	modLfoToPitch
6	vibLfoToPitch
7	modEnvToPitch
8	initialFilterFc
9	initialFilterQ
10	modLfoToFilterFc
11	modEnvToFilterFc
12	endAddrsCoarseOffset
13	modLfoToVolume
15	chorusEffectsSend
16	reverbEffectsSend
17	pan
21	delayModLFO
22	freqModLFO
23	delayVibLFO
24	freqVibLFO
25	delayModEnv
26	attackModEnv
27	holdModEnv
28	decayModEnv
29	sustainModEnv
30	releaseModEnv
31	keynumToModEnvHold
32	keynumToModEnvDecay
33	delayVolEnv
34	attackVolEnv
35	holdVolEnv
36	decayVolEnv
37	sustainVolEnv
38	releaseVolEnv
39	keynumToVolEnvHold
40	keynumToVolEnvDecay
43	keyRange
44	velRange
45	startloopAddrsCoarseOffset
46	keynum
47	velocity
48	initialAttenuation
50	endloopAddrsCoarseOffset
51	coarseTune
52	fineTune
54	sampleModes
56	scaleTuning
57	exclusiveClass
58	overridingRootKey

NOTE: any address not listed above will not be found in the AUParameterTree due to gaps in the SF spec.

All values for the elements in the AUParameterTree are floating-point values which will be converted into integer values that conform to the spec. For boolean (true/false )settings, values < 0.5 will be treated as false and values >= 0.5 will be true.

There are additional parameters definitions for MIDI control state:

Address	Name	Description
1000	portamentoModeEnabled	Portamento mode (aka glide)
1001	portamentoRate	How must time to transition for each step
1002	oneVoicePerKeyModeEnabled	When enabled, playing same key will cancel previous voice
1003	polyphonicModeEnabled	When enabled, supports playing multiple notes at same time
1004	activeVoiceCount	Reports the number of active voices (read-only)
1005	retriggerModeEnabled	When enabled, playing same voice restarts the envelope of the voice