Headers Reference

Plugin Headers are specially formatted code comments placed at the top of a Nyquist plugin script.

Any line beginning with a semi-colon (";") is entirely ignored by Nyquist, but properly formatted "header" comments provide instructions that tell Audacity how to create the plugin.

Plugin header format

Plugin headers are written as:

;keyword args

where keyword is the header command, and args is a list of values. For most header commands there must be the correct number of arguments.

Important:

• Nyquist plugin headers normally begin with one single semicolon at the beginning of each line.

• Headers must be all lower-case except for quoted text (for example: "name") which may include upper-case characters.

• Malformed plugin headers are ignored (treated as normal code comments).

• Audacity 2.3.0 adds a new style of header for plugins that are shipped with Audacity which provides multi-language translation support.

Locale Support

Translated headers begin with a dollar character ("$") rather than a semi-colon. The string to be translated is double quoted, and appears between (_ and ).

Multi-language support for a plugin's GUI is available only to plugins that are shipped with Audacity, as the translations must be compiled into Audacity's language files.

See also: Plugin Translations.

Example:

$name (_ "Name of Effect")

Header Syntax

All plugin headers have the syntax: ;keyword args, where args are the parameters ("arguments") for the header command.

Required plugin headers:	These headers are required in all Nyquist plugins.
****
;nyquist plug-in	Defines the text file as a Nyquist plugin.
;name "name"	Sets the name of the plugin.
;type "type"	Specifies the type of plugin.
;version version	Specifies the Nyquist plugin version.
Display Headers:	Display additional information about the plugin. These headers are optional.
;author "text"	Name of plugin developer. Shown in the "About" section of Manage. Also used when effects are sorted by "Publisher".
;copyright "text"	Copyright / license notice. Shown in the "About" section of Manage.
;release version	Version / release number of the plugin. Shown in the "About" section of Manage.
Functional Headers:	These headers may be used to provide additional functionality.
;codetype type	Specifies if the Nyquist code uses SAL or LISP syntax.
;debugbutton option	Specifies if the Debug button is visible.
;debugflags flags	Sets the debug message behavior.
;helpfile "path to file"	Displays a Help button that links to a plugin Help file.
;manpage "URL"	Displays a Help button that links to the plugin Help page in the Audacity manual.
;maxlen integer	Sets the maximum number of samples to be processed.
;mergeclips integer	Specifies the clip merge behavior.
;preview option	Specifies the plugin Preview options.
;restoresplits integer	Specifies the clip split behavior.
Obsolete Headers:	These headers are no longer used by current versions of Audacity and are ignored.
;action "text"	Ignored.
;categories "text"	Ignored.
;info "text"	Ignored.
Control Headers:	These headers define the plugin GUI.
;control <args>	The parameters (<args>) specify the type of widget and the widget's required arguments. See also: Nyquist Plugins Widgets

Required plugin headers

nyquist plug-in

Tells Audacity "this is a Nyquist plugin". This should normally be the first line as it defines the contents of the file.

 ;nyquist plug-in

name

Name of the plugin as it will appear in the Audacity menu:

 ;name "name"

Note that for plugins to be used in Chains, the colon character ":" cannot be used (as it is a special character in the Chain text file).

If the plugin has an interface, the name should end with three dots so as to indicate that additional user action is required before the plugin is applied. Plugins that act immediately without additional user action should not have dots at the end of the name.

type

Only one ";type" line should be used.

A plugin cannot appear in several Audacity menus at the same time, but it is possible to write several plugins with the same name and different ";type" lines. Each plugin will then appear in the appropriate menu. Using the same name for more than one plugin is not recommended and should generally be avoided.

Type header	Features	Typical Role
;type analyze	Plugin appears in the Audacity Analyze menu.	Analyzing selected track audio. Durations are relative to the selection length. Code iterates through each selected track.
;type generate	Plugin appears in the Audacity Generate menu.	Generating audio. Durations are absolute (1 "unit" of time = 1 second). Code runs once only regardless of number of tracks.
;type process	Plugin appears in the Audacity Effect menu.	Processing selected track audio. Durations are relative to the selection length. Code iterates through each selected track.
;type tool	Plugin appears in the Audacity Tools menu.	Nyquist Macros / Other. Nyquist cannot directly modify the project.

Tool type plugins are typically Nyquist Macros or plugins that don't fit well in any of the first three roles. They may also be combined with one of the other types:

Type header	Features
;type tool analyze	Appears in the Audacity Tools menu and behaves like an Analyze type.
;type tool generate	Appears in the Audacity Tools menu and behaves like a Generate type.
;type tool process	Appears in the Audacity Tools menu and behaves like a Process type.

version

Use only one ";version" line.

All new plugins should use the most recent version number so that all current features are available. The version line is required to allow Audacity to run the plugin correctly and prevents plugins with new features from being loaded in an old Audacity program that is missing required features.

Version header	Features
;version 1	Slider Widget
;version 2	Text input widget added
;version 3	multiple-Choice widget added
;version 4	Additional global variables to pass additional information from Audacity to Nyquist. New optional plugin headers: ;author ;copyright ;maxlen ;mergeclips ;preview ;release ;restoresplits

Display headers

author

Name of the plugin Author. If this line is added, its text will appear in the Audacity Effect Menu when sorted or grouped by "Publisher". The author name string must be quoted.

 ;author "Name of Plugin Author(s)"

copyright

A short statement of the copyright/license terms. For plugins shipped with Audacity, this must be compatible with Audacity's GPL v2 license. The copyright string must be quoted.

Recommended text for GPL v2 license:

;copyright "Released under terms of the GNU General Public License version 2 or later"

Additional copyright details may be included in the plugin code comments, but must not conflict with the terms declared in the copyright header.

 ;copyright "copyright text"

release

Displays a release version number for the plugin in the Manage > About menu.

Plugins that are shipped with Audacity have a release version number equivalent to the Audacity version at the time that the plugin was last updated.

Plugin authors may choose whichever versioning scheme they prefer, but should ensure that later versions of the plugin always have a later version number. If the version number has spaces, it must be enclosed in double quotes.

Any one of the following are valid (though there should be only ONE release header in a plugin):

release 1 release 0.0.1 release "1.0" release "3.5 beta"

Functional headers

codetype

Declaration of code syntax. May be either lisp or sal (lower case).

• For LISP syntax plugins this is usually omitted, but it should always be included for SAL syntax plugins.

• If the code type is not declared in the Nyquist Prompt effect, Audacity attempts to deduce the correct syntax from the code.
• The code type can only be Lisp syntax or SAL. It cannot be a mix of both.

 ;codetype type

debugbutton

Show or hide the Debug button. The default is to show the button, but for plugins that are shipped with Audacity, or other plugins that are believed to be bug free, the Debug button may be hidden by setting this to "false" or "disabled"

 ;debugbutton false

or

 ;debugbutton disabled

debugflags

debugflags header	Description
;debugflags trace	Sets tracenable (LISP) or sal-traceback (SAL) and displays debug window if there is anything to show. This may be useful when showing debug info is integral to what the plugin does, or when debugging a script.
;debugflags notrace	Disables tracenable (LISP) / sal-traceback (SAL). This prevents the debug window from opening on error, unless the Debug button has been pressed. If the Debug button has not been pressed and there is an error, the error message will be sent to Audacity's log. Note that disabling tracenable limits the debug output to only the error message with little or no additional debugging information.
;debugflags compiler	Set sal-compiler-debug to 'true'. Output from the SAL compiler is printed to the debug output. This may be viewed in the Debug window if enabled (for example, by clicking the Debug button), otherwise in the Audacity log.
;debugflags nocompiler	Set sal-compiler-debug to 'false'. Disables compiler messages from SAL. Only debug and error messages are printed to the debug output.

These flags may be used in conjunction with the debugbutton header to provide a variety of behaviors.

• debugbutton true, debugflags trace: Debug window opens if debug button pressed or if Nyquist printed to the debug window.

• debugbutton false, debugflags trace: Debug button disabled, but debug window opens if Nyquist printed to the debug window.

• debugbutton true, debugflags notrace: Debug window opens only when Debug button is pressed. If Debug button has NOT been pressed, any error messages that arise are routed to the Audacity log.
• debugflags compiler, debugflags trace: (When using SAL syntax) The Debug window always opens to display the output from the SAL compiler.

• debugflags compiler, debugflags notrace: (When using SAL syntax) The Debug window only opens to display the output from the SAL compiler if Debug button is clicked, otherwise it is printed to the Audacity log.

• debugflags nocompiler, debugflags trace: (When using SAL syntax) The Debug window always opens to display debug and/or error messages. Output from the SAL compiler is suppressed.

helpfile

A Help button may be added to the plugin GUI by giving the relative path and name of a help file. As the button can only be created if the plugin has a GUI, this header should not be used in plugins that do not have a GUI.

 ;helpfile path to file

The file path is relative to the plugin search path. Normally the help file would be placed in the same location as the plugin when it is installed. To support HTML files with images and/or media, the help file and its resources may be in a folder, and the folder included in the file path. For example, if the help file is called "my_effect.html" and it includes images, then the html file and images may be placed in a folder called "my_effect_help", and the helpfile header would be:

 ;helpfile "my_effect_help/my_effect.html"

If the help file is not found, the ? button will not appear.

manpage

This is primarily intended for use by plugins that are shipped with Audacity. It is similar to the helpfile header, except that it looks for the help file in the search path for Audacity's manual. As with ";helpfile" it should only be used in plugins that have a GUI. For more information, see Location of Manual

 ;manpage URL

maxlen

Specifies the maximum number of samples to be processed in "process" or "analyze" type plugins. This can help the progress bar if length that will be processed is determined by the plugin rather than by the length of selection. It could also be used as a fail-safe for plugins that are specifically designed for short selections. This example limits the number of samples to 1 million:

 ;maxlen 1000000

When the MAXLEN header is used, the Nyquist global variable LEN is the maximum of "length of selection in samples" and "value set by MAXLEN".

In "process" type effects, this may cause the selected audio to be truncated to the specified number of samples. In such cases it may be best to throw an error if "maxlen" is exceeded, for example:

;maxlen 1000000

(defun isok ()
  ;; Return true if selection is less than "maxlen".
  (let ((start (get '*selection* 'start))
        (end (get '*selection* 'end)))
    (<= (truncate (* *sound-srate* (- end start))) len)))

mergeclips

Allows Nyquist plugins to override Audacity's default "clip merge" behaviour. By default, when effects (including generator effects) are applied across one or more clip boundaries and the returned audio is a different length from the original selection, Audacity will add "split lines" at the ends of the returned audio. In all other cases, the returned audio is "merged" into the current audio.

This option only applies when the plugin is applied across clip boundaries (including across "split lines").

Clip Merge Options

• -1 Automatic clip merge behaviour (default)

 ;mergeclips -1

• 0 Don't merge clips. Effects that are applied across clip boundaries will not be merged into the existing audio (there will be split lines at the ends of the returned audio) whether the returned audio is the same length as the original selection or not.

 ;mergeclips 0

• 1 Always merge clips. The returned audio will always be merged into the existing audio (no split lines added).

 ;mergeclips 1

See also "restoresplits".

preview

Provides options for previewing the effect. Multiple preview options may be defined to achieve the desired behaviour.

Preview Options

• enabled (default). Preview is enabled.

 ;preview enabled

• true Same as "enabled".

 ;preview true

• disabled Preview is disabled. If Audacity is unable to provide a meaningful preview, then preview should be disabled. This may be required for effects that affect specific time regions within the selection.

 ;preview disabled

• false Same as "disabled".

 ;preview false

• linear Provides an optimisation for previewing multiple tracks by mixing the selected tracks before applying the Nyquist code. This optimisation is disabled by default.

 ;preview linear

• selection When previewing, the Nyquist code is applied to the entire selection (not just the length that will be previewed). Audacity's "Preview" then plays the first few seconds of the processed audio. This may be required for effects that vary over the duration of the selection.

 ;preview selection

restoresplits

Allows Nyquist plugins to override Audacity's default "split restore" behaviour. By default, when effects (including generator effects) are applied across one or more clip boundaries, Audacity will restore "split lines" at the position of the original clip boundaries.

This option only applies when the plugin is applied across clip boundaries (including across "split lines").

Restore Splits Options

• 1 Splits at clip boundaries are restored (default)

 ;restoresplits 1

• 0 Splits at clip boundaries are not restored (clips are joined).

 ;restoresplits 0

Note: Nyquist plugins are not currently able to distinguish between silence and "empty space" within the selection, so gaps between audio clips will be treated as if the empty space is an additional "silent" audio clip.

See also "mergeclips".

Obsolete plugin headers

These headers are obsolete and no longer used by Audacity. They should not be used in new plugins.

They may still be found in some old plugins, but are now treated by Audacity as ordinary "comments" and simply disregarded.

action

Description shown in progress window.

 ;action "text"

categories

Specifies an LV2 category for the plugin.

 ;categories "text"

info

A single line of text to be displayed at the top border of the plugin window. For multiple lines of text, a two-character sequence "\n" may be used within "text" to create a line break.

 ;info "text"

Control headers

If the plugin includes at least one properly formed "Control" header, Audacity will launch the plugin with a GUI.

Each valid "Control" header adds a widget to the GUI. If there are no "Control" headers, Audacity will attempt to run the effect without a GUI.

There can be several "control" lines in the plugin header. Add one for each widget to appear in the dialog box. The ;control headers should normally be the final headers as they define variables used by the Nyquist code.

 ;control parameters