---
<p>
-The default stereo panner distributes 2 inputs to 2 outputs. Its
-behaviour is controlled by two parameters, width and position. The
-default settings for the stereo panner
-are width=100%>, position=center (L=50%,
-R=50%). This panner assumes that you have 2 uncorrelated,
-independent signals that you wish to distribute to two outputs.
+ The default stereo panner distributes two inputs to two outputs. Its
+ behaviour is controlled by two parameters, <em>width</em> and
+ <em>position</em>. The
+ default settings for the stereo panner are <em>width=100%</em> and
+ <em>position=center</em>.
+ This stereo panner assumes that the signals
+ you wish to distribute are either uncorrelated (i.e. totally
+ independent), or that they contain a stereo image which is
+ <em>mono-compatible</em>, such as a co-incident microphone recording, or a
+ sound stage that has been created with pan pots.<sup><a href="#caveat">*</a></sup>
</p>
<div class="well">
-<p>
With the default values it is not possible to alter the position,
since the width is already spread entirely across both outputs. To
alter the position, you must first reduce the width.
-</p>
</div>
<h2>Stereo Panner User Interface</h2>
<img src="/images/stereo-panner-annotated.png" alt=""/>
<p>
- The panner user interface consists of 3 elements, divided between
+ The panner user interface consists of three elements, divided between
the top and bottom half. Click and/or drag in the top half to
control position; click and/or drag in the bottom half to control
width (see below for details).
-<p>
+</p>
<p>
In the top half is the position indicator, which shows where the
center of the stereo image is relative to the left and right
In the bottom half are two signal indicators, one marked "L" and the
other "R". The distance between these two shows the width of the
stereo image. If the width is reduced to zero, there will only be a
- single signal indicator marked "M" (for mono), and whose color will
+ single signal indicator marked "M" (for mono), whose color will
change to indicate the special state.
</p>
-
<p>
It is possible to invert the outputs (see below) so that whatever
would have gone to the right channel goes to the left and vice
left and right speakers</td></tr>
<tr><td>-1</td><td>L=100% R=0%</td><td>signal image is entirely
- at the left speaker </td></tr>
+ at the left speaker</td></tr>
<tr><td>1</td><td>L=0% R=100%</td><td>signal image is entirely
at the right speaker</td></tr>
</table>
<p>
-One way to remember this sort of convention is that the middle of the
-USA is not Kansas, but "Los Angeles: 50% New York: 50%".
+ One way to remember this sort of convention is that the middle of the
+ USA is not Kansas, but "Los Angeles: 50% New York: 50%".
</p>
-<h2>Examples In Use</h2>
+<h3>Examples In Use</h3>
<table>
<tr><th>Appearance</th><th>Settings</th></tr>
L=0 R=100</td></tr>
</table>
-<h2>Using the mouse</h2>
+<h4>Using the mouse</h4>
-<p>Mouse operations in the upper half of the panner adjust the position
+<p>
+ Mouse operations in the upper half of the panner adjust the position
parameter, constrained by the current width setting.
</p>
-
-<p>Mouse operations in the lower half of the panner adjust the width
+<p>
+ Mouse operations in the lower half of the panner adjust the width
parameter, constrained by the current position setting.
</p>
-
-<p>To change the position smoothly, press the right button and drag
+<p>
+ To change the position smoothly, press the right button and drag
within the top half of the panner, then release. The position will
be limited by the current width setting. <em>Note: you do not need
to grab the position indicator in order to drag</em>
</p>
-
-<p>To change the width smoothly, press the right button and drag
+<p>
+ To change the width smoothly, press the right button and drag
within the lower half of the panner, then release. The width will be
limited by the current position setting.<em>Note: you do not need to
grab the L/R indicators in order to drag</em>
<h4>Keyboard bindings</h4>
-When the pointer is within a stereo panner user interface, the following
-keybindings are available to operate on that panner:
+<p>
+ When the pointer is within a stereo panner user interface, the following
+ keybindings are available to operate on that panner:
+</p>
<dl>
<dt><key>↑</key> / <key class="mod1">↑</key></dt>
<dt><key>0</key></dt>
<dd>reset position to center</dd>
<dt><key class="mod2">↑</key></dt>
- <dd>reset width to 180° (100%)</dd>
+ <dd>reset width to full (100%)</dd>
</dl>
<h4>Using the scroll wheel/touch scroll</h4>
-When the pointer is within a stereo panner user interface, the scroll
-wheel may be used as follows:
+<p>
+ When the pointer is within a stereo panner user interface, the scroll
+ wheel may be used as follows:
+</p>
<dl>
<dt>left / <key class="mod1">left</key></dt>
<dt>down / <key class="mod1">down</key></dt>
<dd>move position 1° / 5°to the right</dd>
</dl>
+
+<h2><a name="caveat"></a>Stereo panning caveats</h2>
+
+<div class="well">
+The stereo panner will introduce unwanted side effects on
+material that includes a time difference between the channels, such
+as A/B, ORTF or NOS microphone recordings, or delay-panned mixes.<br />
+When you reduce the with, you are effectively summing two highly
+correlated signals with a delay, which will cause comb filtering.
+</div>
+<p>
+Let's take a closer look at what happens when you record a source at 45° to the
+right side with an ORTF stereo microphone array and then manipulate the width.
+</p>
+<p>
+For testing, we apply a pink noise signal to both inputs of an Ardour stereo
+bus with the stereo panner, and feed the bus output to a two-channel analyser.
+Since pink noise contains equal energy per octave, the expected readout is a
+straight line, which would indicate that our signal chain does not color the
+sound:
+</p>
+<img src="/images/stereo-panner-with-ORTF-fullwidth.png" />
+<p>
+To simulate an ORTF, we use Robin Gareus' stereo balance
+control LV2 to set the level difference and time delay. Ignore the Trim/Gain
+— its purpose is just to align the test signal with the 0dB line of the
+analyser.
+</p>
+<p>
+Recall that an ORTF microphone pair consists of two cardioids spaced 17 cm
+apart, with an opening angle of 110°.
+For a far source at 45° to the right, the time difference between the capsules
+is 350 μs or approximately 15 samples at 44.1 kHz. The level difference
+due to the directivity of the microphones is about 7.5 dB (indicated by the
+distance between the blue and red lines in the analyser).
+</p>
+<p>
+Now for the interesting part: if we reduce the width of the signal to 50%,
+the time-delayed signals will be combined in the panner. Observe what
+happens to the frequency response of the left and right outputs:
+</p>
+<img src="/images/stereo-panner-with-ORTF-halfwidth.png" />
+<p>
+You may argue that all spaced microphone recordings will undergo comb
+filtering later, when the two channels recombine in the air between the speakers.
+Perceptually however, there is a huge of difference: our hearing system is
+very good at eliminating comb filters in the real world, where their component
+signals are spatially separated. But once you combine them
+inside your signal chain, this spatial separation is lost and the brain will
+no longer be able to sort out the timbral mess. As usual, you
+get to keep the pieces.
+</p>
+<div class="well">
+Depending on your material and on how much you need to manipulate the width,
+some degree of comb filtering may be acceptable. Then again, it may not. Listen
+carefully for artefacts if you manipulate unknown stereo signals — many
+orchestra sample libraries for example do contain time-delay components.
+</div>
+
projects / ardour-manual / blobdiff