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

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<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
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gradual transition can be accomplished.
</p>
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<h2>Region Fades</h2>
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<p>
<dfn>Region fades</dfn> are possible at the beginning and end of
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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.
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</p>
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<h2>Crossfades</h2>
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<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
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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>
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<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;
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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>
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<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>
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<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
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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>
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<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"
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rather than a crossfade.
</p>
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<h2>Fade Shapes</h2>
<figure class=right>
<img src="/images/crossfade_menu.png">
<figcaption class=center>The fade shape context menu.</figcaption>
</figure>
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<p>
To activate/deactivate or change the shape of a region's fadein or
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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
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for the region fade. <kbd class="menu">Activate/Deactivate</kbd> enables and
disables the region fade.
</p>
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<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>
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<p>
The different types of fades are:
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</p>
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<table class="dl">
<tr><th><kbd class="menu">Linear</kbd></th><td>A simple linear coefficient
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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>
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<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
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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>
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</table>
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<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.
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</p>
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<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"
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of the line. This provides artefact-free crossfades. Some
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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>