LV2

Any plugin that analyses input to output some useful information.

Ports of this type will be connected to an array of length sample_count with elements of C type float.

Ports of this type have the same buffer format as an AudioPort, except the buffer represents audio-rate control data rather than audio. Like an ControlPort, a CVPort SHOULD have properties describing its value (e.g. minimum, maximum, and default), and may be presented to the user as a control.

It is generally safe to connect an audio output to a CV input, but not vice versa. Hosts SHOULD take care to prevent data from a CVPort port from being used as audio.

A specific channel, e.g. left or right. A channel may be audio, or another type such as a MIDI control stream.

Ports of this type will be connected to a pointer to a single value of C type float.

Any plugin that converts some form of input into a different form of output.

Plugins that intentionally delay their input signal as an effect.

The designation (or assignment) of an input or output. A designation is metadata that describes the meaning or role of data. By assigning a designation to a port using designation, the port's content becomes meaningful and can be used more intelligently by the host.

Plugins that alter the envelope or dynamic range of audio.

Additional data and/or functions a plugin may return from LV2_Descriptor::extension_data() which can be used to add additional API beyond that defined by LV2_Descriptor.

An additional feature which a plugin or other resource may use or require.

Any plugin that generates sound internally, rather than processing its input.

Ports of this type will be connected to a pointer to some value, which will be read by the plugin during their run method.

Any plugin that is intended to be played as a musical instrument.

A string in Markdown syntax.

As an extension, LV2 documentation tools may interpret references to known vocabulary or identifiers, like lv2:Plugin or LV2_Descriptor, as links to the corresponding documentation.

A plugin which mixes some number of inputs into some number of outputs.

Ports of this type will be connected to a pointer to some value, which will be written to by the plugin during their run method.

A parameter, i.e. a recognized property. A parameter is a designation for a control.

A parameter defines the meaning of a control (not the method of conveying its value). The standard way of exposing a plugin parameter is via an ControlPort, which can be given a parameter designation with designation. Other methods, such as setting dynamic parameters via messages, are possible but not defined here.

The class which represents an LV2 plugin.

To be discovered by hosts, plugins MUST explicitly have rdf:type Plugin listed in their bundle's manifest, e.g.:

<http://example.org/my-plugin> a lv2:Plugin .

Plugins SHOULD have a doap:license property whenever possible. The doap:name property should be at most a few words in length using title capitalization, e.g. Tape Delay Unit. Use doap:shortdesc or doap:description for more detailed descriptions.

An abstract plugin-like resource that MAY not actually be an LV2 plugin (e.g. may not actually have a plugin binary).

PluginBase SHOULD be used as a base type for any resource that may have ports or otherwise mimic the structure of a Plugin (e.g. a preset), since hosts and other tools already understand this structure.

A Point describes an interesting value in a Port's range (much like a labeled notch on a physical knob).

• A Point MUST have at least one rdfs:label which is a string.
• A Point MUST have exactly one rdf:value with a type that is compatible with the type of the corresponding Port.

The class which represents an LV2 port.

All LV2 port descriptions MUST have a rdf:type that is one of Port InputPort or OutputPort. Additionally there MUST be at least one other rdf:type which more precisely describes type of the port (e.g. AudioPort).

Hosts that do not support a specific port class MUST NOT instantiate the plugin, unless that port has the connectionOptional property set (in which case the host can simply connect that port to NULL). If a host is interested in plugins to insert in a certain signal path (e.g. stereo audio), it SHOULD consider all the classes of a port to determine which ports are most suitable for connection (e.g. by ignoring ports with additional classes the host does not recognize).

A port has two identifiers: a (numeric) index, and a (textual) symbol. The index can be used as an identifier at run-time, but persistent references to ports (e.g. in a saved preset) MUST use the symbol. A symbol is guaranteed to refer to the same port on all plugins with a given URI. An index does NOT necessarily refer to the same port on all plugins with a given URI (i.e. the index for a port may differ between plugin binaries).

Similar to PluginBase, an abstract port-like resource that MAY not actually be a fully specified LV2 port. For example, this is used for preset "ports" which do not specify an index.

A property of this port that allows a host to make more sensible decisions (e.g. to provide a better interface).

A single float Point (for control inputs).

Plugins that aim to duplicate the effect of some environmental effect or musical equipment.

Plugins that manipulate the position of audio in space (e.g. panning, stereo width, surround encoding, etc.).

An LV2 specification (i.e. this specification, or an LV2 extension).

Specification data, like plugin data, is distributed in bundles so hosts may discover all present LV2 data.

Plugins that alter the spectral properties (e.g. frequency) of audio.

A short restricted name used as a machine and human readable identifier. The first character must be one of _, a-z or A-Z and subsequent characters can be from _, a-z, A-Z and 0-9. This is a valid C identifier, and compatible in most other contexts with restricted string identifiers (e.g. file paths).

Includes things like mathematical functions and non-musical delays.

Specifies that a resource is related to a plugin. This is primarily intended for discovery purposes: bundles that describe resources that work with particular plugins (e.g. presets or user interfaces) SHOULD use this predicate in manifest.ttl to relate the resource to the applicable plugin(s), e.g.:

<thing>
    a             ext:Thing ;
    lv2:appliesTo <plugin> ;
    rdfs:seeAlso  <thing.ttl> .

Particularly for large amounts of data, this is preferable to extending the plugin description with rdfs:seeAlso since the host may choose if/when to load the data, knowing that it describes an additional resource and not the plugin itself.

The binary of an LV2 resource. The value of this property must be a URI that resolves to a shared library object (the actual type of this library is system specific).

This is a required property of a Plugin which MUST be included in the bundle's manifest.ttl file. The binary of an Plugin is the shared object containing the lv2_descriptor() or lv2_lib_descriptor() function which can be used to access the descriptor for that plugin. This property may be used similarly by extensions to relate other resources to their implementations.

The default value that the host SHOULD set this port to when there is no other information available.

Indicates a channel or parameter designation.

This property is used to give the port's contents a well-defined meaning. For example, if a port has designation eg:gain, then the value of that port represents the eg:gain of the plugin instance.

Ports should be given designations whenever a well-defined designation exists. This allows the host to act more intelligently and/or provide a more effective user interface. For example, if the plugin has a BPM parameter, the host may automatically set that parameter to the current tempo.

Relates a Resource to documentation markup. The value of this property MUST be a string literal which is a valid XHTML Basic 1.1 fragment suitable for use as the content of the <body> element. This can be used by hosts to provide rich online documentation or by tools to generate external documentation pages. The standard language tagging facility of RDF can be used to provide multi-lingual documentation.

XHTML Basic is a W3C Recommendation which defines a basic subset of XHTML intended to be reasonable to implement with limited resources (e.g. on embedded devices). See XHTML Basic, Section 3 for a list of legal tags.

Whether or not processing is currently enabled, that is, not bypassed.

If this value is greater than zero, the plugin processes normally. If this value is zero, the plugin is expected to bypass all signals unmodified. The plugin must provide a click-free transition between the enabled and disabled (bypassed) states.

Values less than zero are reserved for future use (such as click-free insertion/removal of latent plugins), and should be treated like zero (bypassed) by current implementations.

Signifies that a plugin provides additional data or functions (as defined by some extension) via LV2_Descriptor::instantiate().

Whether or not processing is currently free-wheeling. If true, this means that all processing is happening as quickly as possible, not in real-time. When free-wheeling there is no relationship between the passage of real wall-clock time and the passage of time in the data being processed (e.g. audio frames).

A non-negative zero-based 32-bit index.

The latency introduced, in frames.

A hint to the host for the maximum useful value that the port will use. This is a soft limit; the plugin is required to gracefully accept all values in the range of a port's data type.

The micro component of a Resource's version.

Releases of plugins and extensions MUST be explicitly versioned. Correct version numbers MUST always be maintained for any versioned resource that is published. For example, after a release, if a change is made in the development version in source control, the micro version MUST be incremented (to an odd number) to distinguish this modified version from the previous release.

This property describes half of a resource version. For detailed documentation on LV2 resource versioning, see minorVersion.

A hint to the host for the minimum useful value that the port will use. This is a soft limit; the plugin is required to gracefully accept all values in the range of a port's data type.

The minor version of an LV2 Resource. This, along with microVersion, is used to distinguish between different versions of the same resource, e.g. to load only the bundle with the most recent version of a plugin. An LV2 version has a minor and micro number with the usual semantics:

• The minor version MUST be incremented when backwards (but not forwards) compatible additions are made, e.g. the addition of a port to a plugin.
• The micro version is incremented for changes which do not affect compatibility at all, e.g. bug fixes or documentation updates.

Note there is deliberately no major version; all versions with the same URI are compatible by definition. Replacing a resource with a newer version of that resource MUST NOT break anything. If a change violates this rule, then the URI of the resource (which serves as the major version) MUST be changed.

Plugins and extensions MUST adhere to at least the following rules:

• All versions of a plugin with a given URI MUST have the same set of mandatory (i.e. not connectionOptional) ports with respect to symbol and rdf:type. In other words, every port on a particular version is guaranteed to exist on a future version with same symbol and at least those rdf:types.
• New ports MAY be added without changing the plugin URI if and only if they are connectionOptional and the minor version is incremented.
• The minor version MUST be incremented if the index of any port (identified by its symbol) is changed.
• All versions of a specification MUST be compatible in the sense that an implementation of the new version can interoperate with an implementation of any previous version.

Anything that depends on a specific version of a plugin (e.g. a serialisation that references ports by index) MUST refer to the plugin by both URI and version. However, implementations should be tolerant and extensions should be designed such that there is no need to do this (e.g. indices should only be meaningful for a particular plugin instance at run-time).

When hosts discover several installed versions of a resource, they SHOULD warn the user and load only the most recent version.

An odd minor or micro version, or minor version zero, indicates that the resource is a development version. Hosts and tools SHOULD clearly indicate this wherever appropriate. Minor version zero is a special case for pre-release development of plugins, or experimental plugins that are not intended for stable use at all. Hosts SHOULD NOT expect such a plugin to remain compatible with any future version. If possible, hosts SHOULD hide such plugins from users unless an experimental option is enabled.

A display name for labeling in a user interface. Unlike lv2:symbol this is unrestricted and may be translated. The lv2:name MUST NOT be used as an identifier. This property is required for Ports, but MUST NOT be used by the host for port identification. The plugin author may change the values of this property without changing the Plugin URI.

Signifies that a plugin or other resource supports a certain feature. If the host supports this feature, it MUST pass its URI and any additional data to the plugin in LV2_Descriptor::instantiate(). The plugin MUST NOT fail to instantiate if an optional feature is not supported by the host.

A port (input or output) on this plugin.

Relates Ports to PortProperties. The PortProperty may be ignored without catastrophic effects, though it may be useful e.g. for providing a sensible interface for the port.

The project this is a component of.

This property provides a way to group plugins and/or related resources. A project may have useful metadata common to all plugins (such as homepage, author, version history) which would be wasteful to list separately for each plugin.

Grouping via projects also allows users to find plugins in hosts by project, which is often how they are remembered. For this reason, a project that contains plugins SHOULD always have a doap:name. It is also a good idea for each plugin and the project itself to have an symbol property, which allows nice quasi-global identifiers for plugins, e.g. myproj.superamp which can be useful for display or fast user entry.

The prototype to inherit properties from.

This property can be used to include common properties in several descriptions. If a plugin has a prototype, then the host must load all the properties for the prototype as if they were properties of the plugin. That is, if :plug prototype :prot, then for each triple :prot p o, the triple :plug p o should be loaded.

This facility is useful for distributing text-only plugins that rely on a common binary, by referring to a prototype which is installed by the corresponding software, along with the plugin binary.

Signifies that a plugin or other resource requires a certain feature. If the host supports this feature, it MUST pass its URI and any additional data to the plugin in LV2_Descriptor::instantiate(). The plugin MUST fail to instantiate if a required feature is not present; hosts SHOULD always check this before attempting to instantiate a plugin (i.e. discovery by attempting to instantiate is strongly discouraged).

A scale point of a port or parameter.

A short display name for labeling in a user interface. The same rules for port names apply here, with the exception that short names should not be longer than 16 characters.

The value of this property MUST conform to the rules for Symbol, and MUST NOT have a language tag.

A symbol is a unique identifier with respect to the parent (e.g. a port's symbol is a unique identifier with respect to its plugin). The plugin author MUST change the plugin URI if a port symbol is changed or removed.

Indicates that this port does not have to be connected to valid data by the host. If it is to be disconnected then the port MUST set to NULL with a call to the connectPort method.

The primary control channel. This should be used as the designation of ports that are used to send commands and receive responses. Typically this will be an event port that supports some protocol, e.g. MIDI or LV2 Atoms.

Indicates that a port's only reasonable values are the scale points defined for that port. A host SHOULD NOT allow a user to set the value of such a port to anything other than a scale point. However, a plugin MUST operate reasonably even if such a port has an input that is not a scale point, preferably by simply choosing the largest enumeration value less than or equal to the actual input value (i.e. round the input value down).

Indicates that the plugin is capable of running not only in a conventional host but also in a hard real-time environment. To qualify for this the plugin MUST satisfy all of the following:

1. The plugin MUST NOT use malloc(), free() or other heap memory management functions within its Audio class functions.
2. The plugin MUST NOT attempt to make use of any library functions in its Audio class functions, unless those functions themselves adhere to these rules (i.e. are hard realtime safe). The plugin MAY assume the standard C and C math library functions are safe.
3. The plugin will not access files, devices, pipes, sockets, IPC or any other mechanism that might result in process or thread blocking within its Audio class functions.
4. The plugin will take an amount of time to execute a run() call approximately of form A + B * sample_count where A and B depend on the machine and host in use. This amount of time MUST NOT depend on input signals or plugin state.

Note these rules apply to the connect_port() function as well as run().

Indicates that the plugin may cease to work correctly if the host elects to use the same data location for both input and output. Plugins that will fail to work correctly if ANY input port is connected to the same location as ANY output port MUST require this Feature. Doing so should be avoided as it makes it impossible for hosts to use the plugin to process data in-place.

Indicates that a port's reasonable values are integers (eg. a user interface would likely wish to provide a stepped control allowing only integer input). A plugin MUST operate reasonably even if such a port has a non-integer input.

Indicates that the plugin has a real-time dependency (e.g. queues data from a socket) and so its output must not be cached or subject to significant latency, and calls to the run method should be done in rapid succession. This property is not related to hard real-time execution requirements (see hardRTCapable).

Indicates that a port is a "sidechain", which affects the output somehow but should not be considered a main input. Sidechain ports should be connectionOptional, and may be ignored by hosts.

Indicates that the port is used to express the processing latency incurred by the plugin, expressed in samples. The latency may be affected by the current sample rate, plugin settings, or other factors, and may be changed by the plugin at any time. Where the latency is frequency dependent the plugin may choose any appropriate value. If a plugin introduces latency it MUST provide EXACTLY ONE port with this property set which informs the host of the correct latency. In fuzzy cases the value output should be the most reasonable based on user expectation of input/output alignment (eg. musical delay/echo plugins should not report their delay as latency, as it is an intentional effect).

Indicates that any bounds specified should be interpreted as multiples of the sample rate. For instance, a frequency range from 0Hz to the Nyquist frequency (half the sample rate) could be requested by this property in conjunction with lv2:minimum 0.0 and lv2:maximum 0.5. Hosts that support bounds at all MUST support this property.

Indicates that the data item should be considered a Boolean toggle. Data less than or equal to zero should be considered off or false, and data above zero should be considered on or true.