Real-time preview of effects

• Peter 03Feb15: I added an H2 section for text-based RTP controls and made some changes to the existing page.
  • Gale 03Feb15. Tweaked, removed the P2.
• Peter 03Feb15: ToDo-2 We don't really tell users here (or on the Effect Menu page) what Real-time preview is, what you'd use it for and what its limitations are.
  • Gale 03Feb15: I agree. Does my added text to the Intro and at the bottom help?

LADSPA, VST and Audio Unit (Mac OS X) effects in Audacity support real-time preview. This lets you 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 VST, LADSPA or Audio Unit effects, almost all of which will support real-time preview.

Effects that support real-time preview are different from regular effects in the following ways:

• They can be opened when audio is playing or paused
• They can be opened even when no audio is selected, as long as the track is selected
• The effect 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, and solo or mute tracks while the effect dialog is open (a selection must be present to apply the effect)
• You will hear the affected track in the context of the mix
• The effect may be applied without closing the effect.

Gale 03Feb15: ToDo-2 "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.

Real-Time preview with graphical icon-based controls

• Bill 04Jan2015: ToDo-1 We need a new image of the "control panel" of a VST effect in graphical mode. We may also want an example of a LADSPA effect in text mode.
• 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.

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.
•   On/Off button: When this button is lit up (blue) the effect is on, otherwise the effect is off (inactive or bypassed). This is a convenient way to do a quick A/B comparison of how your project sounds with and without the effect.
•   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.
• 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 what the audio will sound like immediately after applying the effect, any volume envelopes, Time Tracks and the positions of gain or pan sliders are applied before preview is heard.