Real-time preview of effects

Real-time preview enables you to change effect settings while playing and listening to the result in real-time, rather than having to preview, tweak settings then preview again. A real-time preview effect therefore includes Play/Stop, Skip Backward and Skip Forward buttons instead of a Preview button.

Currently, the only effects shipped with Audacity that support real-time preview are Hard Limiter and SC4, but you can download additional LADSPA, VST and Audio Unit (for Mac OS X) effects, almost all of which will support real-time preview.

Real-time preview differences

An effect that supports real-time preview is different from regular effects in the following ways.

• It can be opened when audio is playing or paused.
• It can be opened even when no audio is selected, as long as the track is selected.
• One regular effect can be opened and applied while a real-time preview effect is open but not playing.
• Real-time preview settings can be changed while audio is playing.
• When in Graphical Mode, live meters or graphs should respond when audio is playing.
• You can start and stop audio playback, change the selection, apply gain or pan to tracks or Mute or Solo them while the effect dialog is open (a selection must be present to apply the effect).
• You will hear a preview of applying the effect to the selected track(s). Unselected tracks will not have the effect applied to them but unlike in regular effects, unselected tracks will still be heard unless you use the Mute/Solo state to silence them.
• The effect may be applied without closing the effect.

Gale 03Feb15: "You will hear the affected track in the context of the mix" is not likely to be clear to novices. I think we need an actual section here - maybe even an example - about what user hears when they have two tracks and one track or the other or both tracks are selected.

• Gale 05Feb15: Downgraded to ToDo. Changed to "You will hear a preview of applying the effect to the selected track(s). Unselected tracks will not have the effect applied to them but unlike in regular effects, unselected tracks will still be heard unless you use the Mute/Solo state to silence them". I can live with that now if others can.

Real-Time preview with graphical icon-based controls

• Peter 03Jan15: I cannot update this image as I don't now seem to have any effects with graphical controls.
• Gale 03Feb15: The text needs changing too. There is no On/Off button any longer.
• Bill 07Feb2015: ToDo-2 I've faked up an image of RTP graphical controls in a windows dialog - feel free to provide me with a real screenshot and I'll update it. Changed text to correspond to current control layout.

Gale 03Feb15: ToDo-2 I see no mention here or on Keyboard Shortcut Reference of the special shortcut(s?) to toggle between RTP and main window.

Gale 03Feb15: ToDo-2 I see no mention here that the RTP playback shortcuts P,B,F work on Windows when either graphical or text RTP window has focus, without the ALT access key. I think that should be the case on all OS'es, but another story.

When you open an effect that supports real-time preview with graphical buttons it will have these icons at the bottom of the effect dialog.

•   Manage Effect menu:
  • User presets: If any user presets exist for this effect this menu will list them. Select one of the user presets to load it into the effect.
  • Factory presets: If any factory presets exist for this effect this menu will list them. Select one of the factory presets to load it into the effect.
  • Delete preset: Select this option to delete the currently-loaded user preset.
  • Save as... Select this option to save the current settings as a user preset.
  • Export... Select this option to export one or more presets (for sharing with other users or to move to another computer running the same effect).
  • Import... Select this option to import a presets file.
  • Options... Displays the options dialog for this type of effect. See the sections for each type of effect for a description of the options available.
    • LADSPA Options
    • VST Options
    • Audio Unit Options
  • About: Displays the effect type, effect name, version, vendor, and description.
•   Play/Stop button: Click this button to begin playback. It changes to the Stop button  . Click it again to stop playback.
•   Skip Back button: When playback is stopped, clicking this button will move the playback cursor to the beginning of the track. During playback, each click of this button will perform a short seek back.
•   Skip Forward button: When playback is stopped, clicking this button will move the playback cursor to the end of the track. During playback, each click of this button will perform a short seek forward.
• Enable checkbox: When this is checked the effect is enabled. Uncheck it to disable the effect (also known as "bypassing" the effect). This is a convenient was to compare the non-effected with the effected sound.
• Close button: Click this button to close the effect dialog.
• Apply button: Click this button to apply the effect to the selected audio. The effect dialog will remain open after the effect is applied.

The Import and Export options behave differently for the different classes of effect that support them. See the appropriate sections below in the descriptions of each effect class. LADSPA effects do not support Import and Export.

Real-Time preview with text-based controls

If you use the Manage button then Options... to disable graphical interface, the Manage button and the Real-Time Preview playback controls display as text-based buttons instead, as in the below image:

As with the graphical interface, a single button starts and stops playback, reading Start Playback when there is no playback and Stop Playback when there is playback.

Text-based controls and textual interface will display automatically even if graphical interface is chosen if the effect does not provide a graphical interface of its own.

Notes and Limitations

• Currently, real-time preview doesn't compensate for the latency many plugins introduce in order to provide smooth processing of audio. Real-time preview therefore may have small gaps in the audio, or timing artifacts may be audible when previewing multiple tracks. However by default Audacity compensates for latency when applying the effect (unless you disable compensation in the options for the effect). Therefore if you hear latency problems in real-time preview, apply the effect, leaving it open, then use the playback controls in the main window to listen to the result.
• So that real-time preview plays how the audio will sound immediately after applying the effect, any existing volume envelopes or Time Tracks and the positions of gain or pan sliders are applied before preview is heard.