On-Demand Loading

On-Demand Loading is an optional faster method of importing audio. Instead of a progress bar appearing while Audacity waits to import the entire file, the normal Audacity waveform starts to display almost immediately. You can play or edit wherever the normal waveform is displayed while the remaining waveform is still being drawn as a process running in the background. On-Demand Loading is available as an option when importing most uncompressed files like WAV and AIFF (and on Linux, FLAC).

Uncompressed files
On-Demand Loading is used for most uncompressed files when choosing to read these files "directly from the original (faster)" instead of copying them in. The "read directly" option can be chosen when the warning appears for importing uncompressed audio files:


 * [[Image:PreferencesWarningsUncompressed_set_directly_Win7Basic.png‎|Warning for importing uncompressed audio files set to "read directly"]]

and also in the Import / Export Preferences:
 * [[Image:EnableOD.png|Import / Export Preferences set to "read directly"]]

The warning can be turned off in the Warnings Preferences, in which case the current import setting in Preferences will be used automatically.

On-Demand Loading of uncompressed files is much faster than copying in the audio, even if you wait for the waveform drawing to complete before playing or editing.

Uncompressed files shorter than 30 seconds long will always import using the normal progress bar even if you choose to "read directly". However on most machines the import will still seem to be immediate unless it is a slower machine or the processor is very busy with other tasks.

Using On-Demand Loading
When you import an On-Demand file by any means (, or dragging in), a sequence of patterned blocks appears almost immediately, representing the uncomputed waveform. The blocks are then progressively replaced from left to right by the normal Audacity waveform as soon as it has been computed. You can play or edit the track at once wherever that normal waveform is displaying, without waiting for the remainder of the waveform to be drawn.

You can also change the focal point from which the waveform is computed. To do this, either click in the track at the leftmost point from which you want the waveform drawn, or drag a selection in either direction. In the image below, we clicked to place the cursor in the waveform at a time of 23 minutes. The waveform then started being drawn rightwards from the cursor point:
 * [[Image:ODinprogress.png|On-Demand loading focal point moved to just after center of track (Mac OS X image)]]

Using read-directly On-Demand, you can now edit or play from the new focal point. You can also start playback from a new position without changing the focal point of the waveform by clicking or dragging a region in the Timeline.

Once the computation point has reached the end of the track, any uncomputed waveform will be drawn from right to left, unless interrupted by clicking elsewhere at an uncomputed point.

The percentage of the waveform that has been computed is displayed on the Status Bar, as shown at bottom left of the image above. When multiple files are being imported, the Status Bar will display the overall percentage of the multiple tracks that has been computed. To make the Status Bar display the percentage completion of a particular track, move the mouse over that track.

Slow seeking when moving the focal position
If the drive the file is being imported from is not able to seek rapidly through the file, the On-Demand imported track may be slow to respond when moving the focal position. This will be especially noticeable before the waveform has been fully computed. Slow seeking could occur on Mac computers when importing CD tracks from the Finder, or when using older external USB drives.