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. Click and drag the grip left or right in the timeline to
  adjust the length of the fade.
</p>

<h2>Crossfades</h2>

<p>
  <dfn>Crossfades</dfn> refer to the behavior when you want to make
  a smooth transition (mix) from one audio region to another on the same
  track. Historically, this was done by splicing 2 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 2
  pieces of tape that have been cut at and 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. We can't
  assume that a crossfade is an entitry that exists between 2 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 you will
  hear the topmost region fade-out be 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 you can create a complicated series of
  crossfades, and then layer another region atop the others, and fade
  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 we call this a "fade"
  rather than a crossfade.
</p>

<h2>Fade Shapes</h2>

<p>
  To activate/deactivate or change the shape of a region's fadein or
  fade-out, hover the cursor over the regionfade grip till the cursor tip
  indicates region fade editing and context-click to bring up a context
  menu. In the context menu there is a list of options for the
  regionfade. <kbd class="menu">Activate/Deactivate</kbd> enables and
  disables the regionfade.
</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 attentuating 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 symetrically attenuated, and
they each reach -3dB at the midpoint. This is the correct crossfade to
use when you want to splice 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 Equal 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">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>
  <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>
</table>

<p>
  Although these fade shapes serve specific purposes, you might find that
  any of the shapes is usable in your situation. 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 artifact-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>