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.<br></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.<br>
</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.<br>
</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.<br>
</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. <br>
</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 _that_ complicated series. <em>An image here would
probably help.</em><br>
</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.<br>
</p>
<h2>Fade Shapes<br>
</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.<br>
</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.<br>
</p>
<p>The different types of fades are:<br>
</p>
<ul>
  <li><kbd class="menu">Linear</kbd>: 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.<br>
  </li>
  <li><kbd class="menu">Constant Power</kbd>: 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.<br>
  </li>
  <li><kbd class="menu">Symmetric</kbd>: 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.<br>
  </li>
  <li><kbd class="menu">Fast</kbd>: 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.</li>
  <li><kbd class="menu">Slow</kbd>: 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.<br>
  </li>
</ul>
<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 an artistic choice rather than a <br></p>
<br>
<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.<br></p>