An LV2 Resource (e.g. plugin, specification, or any other LV2 related thing)
An LV2 specification (i.e. this specification, or an LV2 "extension"). Specification data, like plugin data, is distributed in standardized bundles so hosts may discover all present LV2 data. See http://lv2plug.in/docs for more details.
Overview There are a large number of open source and free software synthesis packages in use or development at this time. This API ("LV2") attempts to give programmers the ability to write simple "plugin" audio processors in C/C++ and link them dynamically ("plug" them) into a range of these packages ("hosts"). It should be possible for any host and any plugin to communicate completely through this interface. This API is deliberately as short and simple as possible. The information required to use a plugin is in a companion data (RDF) file. The shared library portion of the API does not contain enough information to make use of the plugin possible; the data file is mandatory. Plugins can operate on any type of data. Plugins have "ports" that are inputs or outputs and each plugin is "run" for a "block" corresponding to a short time interval measured in samples. The plugin may assume that all its input and output ports have been connected to the relevant data location (using the connect_port() function) before it is asked to run, unless the port has been set "connection optional" in the plugin's data file. This "core" specification defines two types of port data, equivalent to those in LADSPA: control rate and audio rate. Audio rate data is communicated using arrays with one float element per sample processed, allowing a block of audio to be processed by the plugin in a single pass. Control rate data is communicated using single float values. Control rate data has a single value at the start of a call to the run() function which is considered valid for the duration of the call to run(). Thus the "control rate" is determined by the block size, controlled by the host. Plugins reside in shared object files suitable for dynamic linking (e.g. by dlopen() and family). This file provides one or many plugins via the lv2_descriptor() function. These plugins can be instantiated to create "plugin instances", which can be connected together to perform tasks. This API contains very limited error-handling. Threading Rules Certain hosts may need to call the functions provided by a plugin from multiple threads. For this to be safe, the plugin must be written so that those functions can be executed simultaneously without problems. To facilitate this, the functions provided by a plugin are divided into classes: Discovery Class lv2_descriptor(), extension_data() Instantiation Class instantiate(), cleanup(), activate(), deactivate() Audio Class run(), connect_port() Extensions to this specification which add new functions MUST declare in which of these classes the functions belong, or define new classes for them. The rules that hosts MUST follow are: When a function is running for a plugin instance, no other function in the same class may run for that instance. When a function from the Discovery class is running, no other functions in the same shared object file may run. When a function from the Instantiation class is running for a plugin instance, no other functions for that instance may run. Any simultaneous calls that are not explicitly forbidden by these rules are allowed. For example, a host may call run() for two different plugin instances simultaneously.
An abstract plugin-like resource that may not actually be an LV2 plugin (e.g. may not actually have a plugin binary). A Template is a Resource that may have ports, and otherwise mimic the structure of a plugin. This should be subclassed by extensions that define such things.
A Plugin MUST have at least one rdf:type that is lv2:Plugin.
A Plugin MUST have at least one doap:name that is a string with no language tag.
The class which represents an 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.
A Port MUST have at least one rdf:type with object either lv2:Port, lv2:InputPort, or lv2:OutputPort.
A Port MUST have at least two rdf:type properties with objects that are some subclass of lv2:Port (one for lv2:Port, lv2:InputPort, or lv2:OutputPort, and another to describe the specific data type, e.g. lv2:AudioPort).
A port MUST have a single lv2:index which is of type xsd:decimal (e.g. a literal integer in Turtle).
A port MUST have a single lv2:symbol which is of type xsd:string with no language tag.
A port MUST have at least one lv2:name which is of type xsd:string.
The class which represents an LV2 port. In order for it to be used by a host it MUST have at least the following properties: rdf:type (with object one of lv2:Port, lv2:InputPort, lv2:OutputPort) rdf:type (more specific port class, see below) lv2:index lv2:symbol lv2:name All LV2 port descriptions MUST have a property rdf:type where the object is one of lv2:Port lv2:InputPort or lv2:OutputPort. Additionally there MUST be at least one other rdf:type property which more specifically describes type of the port (e.g. lv2: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).
Ports of this type will be connected to a pointer to some value, which will be read by the plugin during their run method.
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.
Ports of this type will be connected to a pointer to a single value conforming to the 32bit IEEE-754 floating point specification.
Ports of this type will be connected to an array of length SampleCount conforming to the 32bit IEEE-754 floating point specification.
Relates a Template or Plugin to the Ports it contains
The revision of an LV2 Resource. If a plugin's port indices change, the revision of the plugin MUST be increased. Note that if port symbols change or are removed, the plugin URI MUST be changed, the revision acts as a 'minor version' to distinguish otherwise compatible revisions of a plugin. A plugin that has changed indices MUST have a lv2:revision property, if a plugin has no revision property it is assumed to be 0. Anything that refers to a specific revision of a plugin (e.g. a serialisation that depends on specific port indices) MUST refer to the plugin by URI along with the revision. This property may be used for other objects, in this case it should be used in a similar way to represent a 'minor version', and NOT as a major version to distinguish incompatible objects (use the URI for that).
A very basic subset of XHTML for use with lv2:documentation, intended to be reasonable for hosts to support for styled inline documentation. A literal with this data type is an XHTML 1.0 fragment containing only tags from the following XHTML modules: text, hypertext, list, basic tables, image, presentation. See the XHTML and XHTML Modularization specifications for details. A literal with this data type must be legal to insert as the body of a tag (free text is allowed). If only basicXHTML documentation is given but a host has no facilities for handling tags, simply stripping tags and inserting newlines after appropriate tags will yield a somewhat readable plain text version of the documentation.
Relates a Plugin to some text/audio/video documentation either online or included with the plugin package. The value of this property may be either a URL, or a literal of any type. Literal documentation SHOULD be either plain text, or lv2:basicXHTML. More advanced documentation should be linked to instead.
Specifies the index of the port, passed as an argument to the connect port function. The index uniqely identifies the port on an instance of the plugin.
A short name used as a machine and human readable identifier. The first character must be one of _, a-z or A-Z and subsequenct characters can be from _, a-z, A-Z and 0-9. A language tag MUST NOT be used on this property. The symbol uniquely identifies the port on a plugin with a given URI (i.e. the plugin author MUST change the plugin URI if a port symbol is changed or removed).
A display name for labeling the Port in a user interface. 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.
Used to describe interesting values in a Port's range. To be valid it requires two properties: rdfs:label and rdf:value. There are 3 specially defined Points in the LV2 specification (default, minimum, and maximum), though future extensions may define more.
A single lv2:float Point (for control inputs)
The default value that the host SHOULD set this port to when there is no other information available.
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 lv2:float.
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 lv2:float.
An additional feature which a plugin may use or require.
Signifies that a plugin or other resource supports a certain features. If the host supports this feature, it MUST pass its URI and any additional data to the plugin in the instantiate() function. The plugin MUST NOT fail to instantiate if an optional feature is not supported by the host. This predicate may be used by extensions for any type of subject.
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 the instantiate() function. 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). This predicate may be used by extensions for any type of subject.
A port property - a useful piece of information that allows a host to make more sensible decisions (e.g. to provide a better interface).
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.
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 lv2:hardRTCapable).
Indicates that the plugin may cease to work correctly if the host elects to use the same data location for both audio input and audio output. Plugins that will fail to work correctly if ANY input buffer for a port of the class lv2:AudioPort is set to the same location as ANY output buffer for a port of the same class (with connect_port()) MUST require this Feature. Doing so should be avoided as it makes it impossible for hosts to use the plugin to process audio "in-place".
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 within its Audio class functions. All new memory used in Audio class functions must be managed via the stack. These restrictions only apply to the Audio class functions. (2) The plugin will not attempt to make use of any library functions in its Audio class functions, with the exceptions of functions in the ANSI standard C and C maths libraries, which the host is expected to provide. (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*SampleCount) where A and B depend on the machine and host in use. This amount of time may not depend on input signals or plugin state. The host is left the responsibility to perform timings to estimate upper bounds for A and B. The plugin will also take an approximately constant amount of time to execute a connect_port() call.
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.
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 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".
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 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 a port's only reasonable values are the scale points defined for that port. Though a host SHOULD NOT allow a user to set the value of such a port to anything other than a scale point. 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).
Any plugin that generates sound internally, rather than processing its input.
Any plugin that is intended to be played as a musical instrument.
Includes things like mathematical functions and non-musical delays.
Any plugin that converts some form of input into a different form of output.
Any plugin that analyses input to output some useful information.
A plugin which mixes some number of inputs into some number of outputs.
Plugins that aim to duplicate the effect of some environmental effect or musical equipment.
Plugins that intentionally delay their input signal as an effect.
Plugins that alter the envelope or dynamic range of the processed audio.