52 lines
3.4 KiB
Markdown
52 lines
3.4 KiB
Markdown
# 0009 — Parameter processors and bounded LADSPA hosting
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Status: accepted, Phases 14–15 (2026-10-27).
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Native ABI 1 remains compatible. ABI 2 adds explicitly parameter-only configuration,
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metadata, validation and sample-offset parameter/MIDI ordering. It is a public C
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facility; opaque state and automatic native descriptor registration in project
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files remain unsupported. Native instrument and gain examples exercise the
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interface before adding a dynamically loaded module lifetime.
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Select LADSPA 1.1 for the first external host. Reconsidering the Phase 9 candidates,
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LV2 needs RDF/features/URIDs/workers, CLAP needs host scheduling/extension
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negotiation, and VST3 needs a separate C++ SDK adapter and release-specific license
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review. LADSPA adds a mature Linux C ABI, installed real plugins, straightforward
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port negotiation and no RDF/SDK runtime library. Its specification is old and
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stable, not an actively expanding instrument/event ecosystem. This trades MIDI,
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editors, opaque state and modern automation for a small testable first host.
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No general multi-format abstraction is introduced.
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Use the installed LGPL-2.1-or-later header, system dynamic loader (`-ldl`), and
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user-selected, trusted module files. Neither the header nor plugin binaries are
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vendored. Header absence or `XTC_DISABLE_LADSPA` yields unsupported hosting stubs;
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metadata can still be loaded/saved. Plugin licenses are independent of the host
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and require review before distribution. See LICENSES.md.
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Support zero/one mono audio input and exactly one mono output, at most 64 ports
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and 16 input controls. Graph stereo consists of independent instances, not a
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stereo LADSPA layout. Reject incompatible layouts and identities explicitly.
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Control outputs are connected to private storage, not exposed as graph events.
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LADSPA has no MIDI or opaque state API: save control values only. Parameter
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changes rebuild plans on the control thread; sample automation is deferred to
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Phase 16. Rate-scaled bounds are recalculated at compile. Instance history/tails
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reset on recompile, not every transport seek (LADSPA has no realtime reset rule).
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Latency compensation is unsupported: the supported set is zero-latency amp,
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inverter and sine source, not arbitrary lookahead/delay/routing plugins.
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Discovery, dlopen, descriptor calls, instantiate, connect, activate, deactivate,
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cleanup and dlclose are control-thread operations. Only preallocated copies and
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run execute in audio; the usual pending/retired plan handoff owns module handles.
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Partial initialization unwinds on the control thread. The host cannot enforce
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third-party callback behavior, bound CPU time, catch native crashes/hangs, or
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undo memory corruption. There is no sandbox or crash recovery inside the audio
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process. A fault may kill xtc; previously saved documents/recording recovery files
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remain subject to their ordinary durability guarantees. Test only trusted modules.
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Project v5 stores plain identity/layout/control metadata. Parsing never opens
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modules. Compile failure preserves the source document and active plan. GUI Open
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rejects missing/incompatible modules, retains its current document, and reports
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saved module identity/path. Restore the compatible module at that path and retry;
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a metadata-only C client can preserve/save the document without the module.
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No silent replacement or deletion occurs. Same-ID/label/ports do not authenticate
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binaries or guarantee unchanged sound across plugin upgrades.
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