Time Tracks

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Pistes de temps Zeitspuren

Gale: 19Dec12: Needs complete rewrite with new images following r12077.
  • Gale: I added images. I thought the track background looked nicer in the audio with time track image as we usually don't show background, but the background can easily be removed.
  • Bill 06Jan13: What, exactly, did r12077 do that changed functionality? AFAICT it works as described on this page.
  • Gale 07Jan13: ToDo-2 Set Range no longer changes the pitch/speed. It changes only the range. I have made changes and demoted the main content to P2 for review. A P1 query remains about the converters used has been resolved (see the bottom).
  • Gale 10Jan13: Fleshed out the "Tutorial" and added advice div about closing the Time Track after render (this removes the other P2). Please check.

A Time Track is used in conjunction with one or more audio tracks to progressively increase or decrease playback speed (and pitch) over the length of the audio. Speed changes are controlled by manipulating the blue "time warp" line with the Envelope Tool used for making gradual volume changes. If the blue line is dragged above the horizontal, the audio plays faster; if dragged below, it plays slower.

Time Track showing the vertical scale, warp line and control points

General method of use:

  • Using Envelope Tool, drag the time warp line up or down, or click above or below the line at the point to which you want to move the line.
  • By default you can warp playback speed by a maximum increase of 10% (110 on the vertical scale to left of the warp line) or a maximum decrease of 10% (90 on the vertical scale).
  • To warp the speed by greater amounts, click in the name of the Time Track (in the panel that contains the downward pointing arrow) and choose "Set Range". Here you can specify the lower and upper speed limit of the vertical scale:
Lower speed dialog, showing default value of 90 Upper speed dialog, showing default value of 110

Changing the range merely rescales the Time Track to the new limits, preserving the warp value of any envelope points that were there before. Having changed the range, you can now move the warp line up or down anywhere within the new upper or lower limit of the range.

  • Only one Time Track can be used in a project, and it affects all audio tracks on screen.
  • Time Track has no effect when audio is speed-adjusted using the Play-at-Speed slider in Transcription Toolbar.

Menu

Time Track Drop-Down menu

Clicking in the panel containing the downward-pointing triangle (to the left of the track) opens a menu containing display choices for the track.

  • Name...: Displays the "Track Name" dialog where you can give the track a new name.
  • Move Track Up: Moves the track up.
  • Move Track Down: Moves the track down.
  • Linear: Sets the vertical scale of Time Track to a linear display; this is the default.
  • Logarithmic: Sets the vertical ruler to a logarithmic display.
  • Set Range...: Opens dialogs where you can first set the minimum and then the maximum speed change value to be displayed on the vertical scale. Values are expressed in percentages relative to the "no change" value of 100%. Only integer values (whole numbers) can be set. The lowest speed change value that can be set is 10% and the highest speed change value that can be set is 1000% .
  • Logarithmic Interpolation: The curve between control points is by default calculated using logarithmic interpolation. Uncheck this option to use linear interpolation between control points, so drawing an exactly straight line between points (assuming the vertical scale is set to "Linear").
Bill 04Jan 13:
  • The Linear / Logarithmic scale selections in this menu could be confusing. The dimmed item is the current selection. Should we say this? An improvement would be to add a checkmark to the current selection.
    • Peter 6Jan13: I have added glossary links to logarithmic and linear to provide a bit of clarification.I don't think we need to hand-hold them by telling them that the dimming indicates "selection". There are plenty of other places in the manual and the GUI where that occurs and we don't tell them there. Grayed-out means you cant select it - and you can't select it because it's already selected.
  • Changed bullet list of menu items to standard "bold plus colon" format.
  • Do we need to describe the first three menu items?
    • Peter 6Jan13: Yes, I think so (for completeness) - so I went ahead and did it
    • Gale 07Jan13: IMO the dimmed menu is not a P anything unless we want a Px to explain dimmed menus everywhere they occur.

Bill 05Jan13:

  • Above it says that the user sets the min and max pitch change. Is this correct? Or are they setting the min and max speed change?
  • 06Jan13: I tested this today and it is definitely changing the speed (it plays faster and the pitch goes up, and vice versa), so I changed the above text. Also the into says it changes the "playback speed". Otherwise this would be a P1 "function incorrectly documented"

Gale 07Jan13:

  • Moving the curve up or down changes pitch and speed. Set Range does not. The text used to say "Set Range..." lets you set first the minimum and then the maximum pitch change". This is the fundamental thing that changed in the code and was blocking release of this page. I think my text has removed that P1 and so this P2.

A Tutorial on Time Warping

Gale 07Jan 13: I've somewhat duplicated this with my "General Usage" (which I think is essential given Set Range" no longer changes the speed/pitch). Is some of the below now repetition?

Does it need some examples like unwarping an LP that is warped for the first few revolutions, or making a glissando?

Time Track with Audio Track below

Key points about the image above:

  • The Time Track curves are drawn with default display types (linear vertical scaling with logarithmic interpolation).
  • The range of the vertical scale has been increased to a lower limit of 50 and an upper limit of 200.
  • The horizontal ruler underneath the Timeline warps as you change the warp line. It shows when the speed-changed playback will reach each time point in the audio. Thus the green playback cursor moves faster or slower than normal as it moves along Time Track's horizontal ruler.
  • In our example:
    • we started playing twice as fast as normal
    • we slowed down fairly suddenly to a speed twice as slow as normal
    • then increased very gradually to a steady speed a little faster than the normal speed of 100%.

    After all the changes, the resultant audio ends up with only one second shorter playing time than it had before (about 1 minute 29 seconds, where the end of the audio lies against the Time Track horizontal ruler). Had we decided to delete the audio to right of 1 minute 15 seconds on the Timeline, the audio remaining would be 1 minute 15 seconds long whether we decided to speed change it or not.

Important Steps:

  1. You must first have at least one audio track to manipulate.
  2. Choose Tracks > New Time Track.
  3. If you want to warp the speed by more than plus or minus 10%, open the Time Track drop-down menu and choose Set Range.... The dialog will appear twice, the first time to choose the lower limit of the range and the second time to choose the upper limit of the range.
    Gale: Are the comparisons with Change Speed more confusing than they're worth?
    • For example if you set a Lower Speed Limit of 50%, you can make the audio play up to twice as slowly (an envelope point at the bottom of the Time Track would make the playback twice as slow, just like a Change Speed effect of -50%).
    • If you set an Upper Speed Limit of 200%, you can make the audio play up to twice as fast (an envelope point at the top of the Time Track would double the playback speed, like a Change Speed effect of 100%).
  4. Before you can change the speed you must select Envelope Tool The envelope tool from Tools Toolbar which lets you shape the time warp line. Creating one envelope point only changes the speed of the whole track by the same amount, just like the Change Speed effect. To make a speed change that varies over time you must create at least one more point. Thereafter when you drag a point the line on each side of that point moves to reflect your change. Drag points off the track to delete them.
  5. You can press SPACE at any time to listen to how the warping sounds and change the points on the warp line further.
  6. If you wish to continue editing after completing your warp points, it is often recommendable to click in the Track Control Panel of the audio track to select it (hold SHIFT and click to select multiple tracks) then Tracks > Mix and Render. This writes the resampled audio data directly to the project rather than resampling the speed-changed audio back to the current sample rate in real-time. This may therefore make project playback more responsive.
    • The quality setting of the resampling library used for playing or rendering Time Tracks is chosen automatically by Audacity. Therefore changing the converter quality settings in Quality Preferences will not affect Time Tracks.
  7. To use the speed-changed audio in other programs, choose File > Export..., which opens the File Export Dialog.
Warning icon Time Track is always active (and therefore modifies audio speed when playing or exporting) whenever it is visible in the project. This applies even after rendering the audio. Therefore after render (or as soon as you want to work with audio that you do not want to speed-change), use the [X] "Close" button to left of the Time Track to remove it.
Warning icon Older computers may be incapable of playing audio in real-time that makes use of a Time Track. However you can always export the file as a WAV and then play the WAV file later.
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