On-Demand Loading

Ready for Proofreading

-- Ed 16 November 2009 updated the graphic images

• Peter 17Nov09: Checked for spelling/typos/consistency
• Peter 24Nov09: Checked Links

With normal file import, you must wait for a progress dialog to complete while Audacity computes the waveform. This can take several minutes with very long uncompressed files like WAV. When "On-Demand Loading" is enabled in Preferences, you can play, edit and view the audio almost immediately from any chosen point, while the waveform image is still being computed in the background.

Usage

To use On-Demand Loading, go to the Import / Export tab of Preferences, and in the "When importing audio files" section, check "Read uncompressed audio files directly from the original":

Then import an uncompressed file, for example WAV or AIFF, via any means (drag-and-drop, File > Open... or File > Import > Audio...). To activate On Demand loading, a file most be at least 30 seconds long. A sequence of patterned blocks representing the uncomputed waveform appears almost immediately. The blocks are then progressively replaced from left to right by the computed waveform. To change the point from which the waveform is computed, click at the leftmost point from which you want the waveform drawn. In the image below, we clicked to place the cursor in the waveform at time 2 minutes, 40 seconds. The waveform then started being drawn rightwards from the cursor point:

ToDo Gale: Image needs replacing, calculation is more than 0% complete

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.

Multiple files may be imported simultaneously, and the waveform computation point moved from one to the other by clicking in the required track.

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.

In the future we hope to extend On-Demand processing to compressed formats such as FLAC and MP3, to formats imported via the optional FFmpeg library, and to effects.