manual/include/create-region-fades-and-crossfades.html

<p>
  Every Region has a fade-in and fade-out. By default, the region fade
  is very short, and serves to de-click the transitions at the start and
  end of the region. By adjusting the regions fade length, a more
  gradual transition can be accomplished.
</p>

<h2>Region Fades</h2>
<p>
  <dfn>Region fades</dfn> are possible at the beginning and end of
  all audio regions. In object mode, a grip appears at the top left and
  top right of an audio region when the cursor hovers over it. Placing
  the cursor over the top of the grip displays the region fade cursor
  tip. Clicking and dragging the grip left or right in the timeline adjusts the
  length of the fade.
</p>

<h2>Crossfades</h2>

<p>
  <dfn>Crossfades</dfn> refer to the behavior of two audio regions transitioning
  smoothly (mixing) from one to another on the same
  track. Historically, this was done by splicing two pieces of analog
  tape together, and this concept was carried forward into digital
  editing. Each track is a sequence of sound files (regions). If
  two regions are butted against each other, there needs to be a method
  to splice them smoothly together. The crossfade allows one region
  to fade smoothly out, while the next region fades smoothly in, like two
  pieces of tape that have been cut at an angle, and overlapped.
</p>
<p>
  But Ardour uses a more refined "layered" editing model, and
  therefore it is possible for multiple regions to be stacked on a single
  location with arbitrary overlaps between different layers. For
  this reason, crossfades must be implemented differently. It can't be
  assumed that a crossfade is an entity that exists between two regions;
  instead each region must have its own associated crossfades at each
  end, and the topmost region must always crossfade down to the
  underlying region(s), if any.
</p>
<p>
  Ardour solves this problem by putting a crossfade at the beginning
  and end of every region. The fades of the bottom-most region are
  first rendered, and then each region is rendered on top of the one
  below it, with fades at the end of each region providing a crossfade to
  the region(s) beneath it.
</p>
<p>
  It is important to understand that region fades <em>are</em> crossfades.
  When one region has another region or multiple regions beneath its fade area,
  then what will be heard is the topmost region fade-out mirrored as a fade-in
  on the underlying region(s). The grip for the topmost region will allow
  changing the length and type of the crossfade into the underlying
  region(s). In this way complicated series of crossfades can be created, and
  then another region layered atop the others, and faded into a complicated
  series.
</p>
<p>
  If a region doesn't have any region(s) under it, then the region is
  crossfaded to silence; for convenience this is called a "fade"
  rather than a crossfade.
</p>

<h2>Fade Shapes</h2>

<figure class=right>
  <img src="/images/crossfade_menu.png">
  <figcaption class=center>The fade shape context menu.</figcaption>
</figure>

<p>
  To activate/deactivate or change the shape of a region's fadein or
  fade-out, the cursor has to be hovered over the region fade grip until the
  cursor tip indicates region fade editing, then <kbd class="mouse">right</kbd>
  clicked to bring up a context menu. In the context menu is a list of options
  for the region fade. <kbd class="menu">Activate/Deactivate</kbd> enables and
  disables the region fade.
</p>
<p>
  Because each fade is also a crossfade, it has an inverse fade shape
  for the audio beneath the fade. It is important to know how the
  shapes differ, and which are most suitable for various editing tasks.
</p>
<p>
  The different types of fades are:
</p>
<table class="dl">
  <tr><th><kbd class="menu">Linear</kbd></th><td>A simple linear coefficient
    decrease, and its mathematical inverse. A Linear fade starts attenuating
    quickly, and then cuts off even more abruptly at lower levels. When used as a
    crossfade, the signals are each -6dB attenuated at the midpoint. This is the
    correct crossfade to use with highly-correlated signals for a smooth
    transition.</td></tr>
  <tr><th><kbd class="menu">Constant Power</kbd></th><td>The constant power
    curve starts fading slowly and then cuts off abruptly. When used as a
    crossfade between 2 audio regions, the signals are symmetrically attenuated,
    and they each reach -3dB at the midpoint. This is the correct crossfade to
    use when splicing audio in the general (uncorrelated) case.</td></tr>
  <tr><th><kbd class="menu">Symmetric</kbd></th><td>The Symmetric fade starts
    slowly, then attenuates significantly before transitioning to a slower
    fade-out near the end of the fade. When used as a crossfade, the Symmetric
    curve is not mathematically correct like the Constant Power or Linear curves,
    but it provides a slower fade-out at low volumes. This is sometimes useful
    when editing 2 entire music works together so that the transition is more
    gradual.</td></tr>
  <tr><th><kbd class="menu">Slow</kbd></th><td>The Slow curve is a modified
    linear decibel fade. The initial curve starts more gradually so that it has
    a less abrupt transition near unity. After that, it sounds like a perfectly
    smooth fader or knob moved to silence. This shape is excellent as a
    general-purpose fade-out. When used as a crossfade, the inverse fade curve
    maintains constant power but is therefore non-symmetric; so its use is
    limited to those cases where the user finds it appropriate.</td></tr>
  <tr><th><kbd class="menu">Fast</kbd></th><td>The Fast curve is a linear
    decibel fade; It sounds like a perfectly smooth fader or knob moved to
    silence. This shape is excellent as a general-purpose fade-in. When used as
    a crossfade, the inverse fade curve maintains constant power but is
    therefore non-symmetric; so its use is limited to those cases where the
    user finds it appropriate.</td></tr>
</table>

<p>
  Although these fade shapes serve specific purposes, any of the shapes is
  usable in any situation, so the final decision is mostly an artistic choice.
</p>

<p>
  These fade curves are developed to provide a range of common uses, and
  are developed with the least possible amount of changes in the "slope"
  of the line. This provides artefact-free crossfades. Some
  DAWs provide complicated fade editors with parametric "spline" controls
  of the fade curves. While it might be interesting to develop a
  fade curve with a faster cutoff, the mathematical difference between
  this and simply shortening the fade is vanishingly small; and the
  amount of effort to shorten the fade is much easier than messing with a
  crossfade editor dialog.
</p>