253 lines
9.3 KiB
HTML
253 lines
9.3 KiB
HTML
---
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layout: default
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title: Stereo Panner
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---
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<p>
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The default stereo panner distributes two inputs to two outputs. Its
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behaviour is controlled by two parameters, <strong>width</strong> and
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<strong>position</strong>. The
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default settings for the stereo panner are <em>width=100%</em> and
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<em>position=center</em>.
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This stereo panner assumes that the signals
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you wish to distribute are either uncorrelated (i.e. totally
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independent), or that they contain a stereo image which is
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<em>mono-compatible</em>, such as a co-incident microphone recording, or a
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sound stage that has been created with pan pots.<sup><a href="#caveat">*</a></sup>
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</p>
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<div class="well">
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With the default values it is not possible to alter the position,
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since the width is already spread entirely across both outputs. To
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alter the position, you must first reduce the width.
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</div>
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<h2>Stereo Panner User Interface</h2>
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<img src="/images/stereo-panner-annotated.png" alt=""/>
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<p>
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The panner user interface consists of three elements, divided between
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the top and bottom half. Click and/or drag in the top half to
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control position; click and/or drag in the bottom half to control
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width (see below for details).
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</p>
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<p>
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In the top half is the position indicator, which shows where the
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center of the stereo image is relative to the left and right
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edges. When this is the middle of the panner, the stereo image is
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centered between the left and right outputs. When it all the way to
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the left, the stereo image collapses to just the left speaker.
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</p>
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<p>
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In the bottom half are two signal indicators, one marked "L" and the
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other "R". The distance between these two shows the width of the
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stereo image. If the width is reduced to zero, there will only be a
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single signal indicator marked "M" (for mono), whose color will
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change to indicate the special state.
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</p>
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<p>
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It is possible to invert the outputs (see below) so that whatever
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would have gone to the right channel goes to the left and vice
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versa. When this happens, the entire movable part of the panner
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changes color to indicate clearly that this is the case.
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</p>
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<h3>Position vs. L/R</h3>
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<p>
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Although the implementation of the panner uses the "position"
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parameter, when the user interface displays it numerically, it shows a pair of numbers that
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will be familiar to most audio engineers.
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</p>
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<table>
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<tr><th>Position</th><th>L/R</th><th>English</th></tr>
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<tr><td>0</td><td>L=50% R=50%</td><td>signal image is midway between
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left and right speakers</td></tr>
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<tr><td>-1</td><td>L=100% R=0%</td><td>signal image is entirely
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at the left speaker</td></tr>
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<tr><td>1</td><td>L=0% R=100%</td><td>signal image is entirely
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at the right speaker</td></tr>
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</table>
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<p>
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One way to remember this sort of convention is that the middle of the
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USA is not Kansas, but "Los Angeles: 50% New York: 50%".
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</p>
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<h3>Examples In Use</h3>
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<table>
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<tr><th>Appearance</th><th>Settings</th></tr>
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<tr><td><img src="/images/stereo-panner.png"></td><td>Width=100%,
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L=50 R=50</td></tr>
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<tr><td><img src="/images/stereo-panner-zero.png"></td><td>Width=0%,
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L=50 R=50</td></tr>
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<tr><td><img src="/images/stereo-panner-inverted.png"></td><td>Width=-100%, Position = 0 (center)</td></tr>
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<tr><td><img src="/images/stereo-panner-right.png"></td><td>Width=36%,
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L=44 R=56</td></tr>
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<tr><td><img src="/images/stereo-panner-hard-right.png"></td><td>Width=0%,
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L=0 R=100</td></tr>
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</table>
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<h4>Using the mouse</h4>
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<p>
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Mouse operations in the upper half of the panner adjust the position
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parameter, constrained by the current width setting.
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</p>
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<p>
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Mouse operations in the lower half of the panner adjust the width
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parameter, constrained by the current position setting.
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</p>
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<p>
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To change the position smoothly, press the right button and drag
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within the top half of the panner, then release. The position will
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be limited by the current width setting. <em>Note: you do not need
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to grab the position indicator in order to drag</em>
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</p>
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<p>
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To change the width smoothly, press the right button and drag
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within the lower half of the panner, then release. The width will be
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limited by the current position setting.<em>Note: you do not need to
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grab the L/R indicators in order to drag</em>
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</p>
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<dl class="faq">
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<dt>Reset to defaults</dt>
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<dd>Click <kbd class="mod3">right button</kbd></dd>
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<dt>Change to a "hard left"</dt>
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<dd>Double click <kbd class="mod2">right button</kbd> in the upper left half
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of the panner</dd>
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<dt>Change to a "hard right"</dt>
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<dd>Double click <kbd class="mod2">right button</kbd> in the upper right half
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of the panner</dd>
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<dt>Move position as far left as possible, given width</dt>
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<dd>Double click <kbd>right button</kbd> in the upper left half of the
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panner</dd>
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<dt>Move position as far right as possible, given width</dt>
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<dd>Double click <kbd>right button</kbd> in the upper right half of the
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panner</dd>
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<dt>Set the position to center</dt>
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<dd>Click <kbd>right button</kbd> in the upper middle of the panner</dd>
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<dt>Reset to maximum possible width</dt>
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<dd>Double click <kbd>right button</kbd> on the lower left side</dd>
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<dt>Invert (flip channel assignments)</dt>
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<dd>Double click <kbd>right button</kbd> on the lower right side</dd>
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<dt>Set width to 0°</dt>
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<dd>Double click <kbd>right button</kbd> in the lower middle</dd>
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</dl>
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<h4>Keyboard bindings</h4>
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<p>
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When the pointer is within a stereo panner user interface, the following
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keybindings are available to operate on that panner:
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</p>
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<dl>
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<dt><kbd>↑</kbd> / <kbd class="mod1">↑</kbd></dt>
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<dd>increase width by 1° / 5°</dd>
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<dt><kbd>↓</kbd> / <kbd class="mod1">↓</kbd></dt>
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<dd>decrease width by 1° / 5°</dd>
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<dt><kbd>←</kbd> / <kbd class="mod1">←</kbd></dt>
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<dd>move position 1° / 5° to the left</dd>
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<dt><kbd>→</kbd> / <kbd class="mod1">→</kbd></dt>
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<dd>move position 1° / 5° to the right</dd>
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<dt><kbd>0</kbd></dt>
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<dd>reset position to center</dd>
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<dt><kbd class="mod2">↑</kbd></dt>
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<dd>reset width to full (100%)</dd>
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</dl>
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<h4>Using the scroll wheel/touch scroll</h4>
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<p>
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When the pointer is within a stereo panner user interface, the scroll
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wheel may be used as follows:
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</p>
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<dl>
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<dt>left / <kbd class="mod1">left</kbd></dt>
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<dd>increase width by 1° / 5°</dd>
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<dt>right / <kbd class="mod1">right</kbd></dt>
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<dd>decrease width by 1° / 5°</dd>
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<dt>up / <kbd class="mod1">up</kbd></dt>
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<dd>move position 1° / 5° to the left</dd>
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<dt>down / <kbd class="mod1">down</kbd></dt>
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<dd>move position 1° / 5°to the right</dd>
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</dl>
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<h2><a name="caveat"></a>Stereo panning caveats</h2>
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<div class="well">
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The stereo panner will introduce unwanted side effects on
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material that includes a time difference between the channels, such
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as A/B, ORTF or NOS microphone recordings, or delay-panned mixes.<br />
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When you reduce the with, you are effectively summing two highly
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correlated signals with a delay, which will cause comb filtering.
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</div>
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<p>
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Let's take a closer look at what happens when you record a source at 45° to the
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right side with an ORTF stereo microphone array and then manipulate the width.
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</p>
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<p>
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For testing, we apply a pink noise signal to both inputs of an Ardour stereo
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bus with the stereo panner, and feed the bus output to a two-channel analyser.
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Since pink noise contains equal energy per octave, the expected readout is a
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straight line, which would indicate that our signal chain does not color the
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sound:
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</p>
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<img src="/images/stereo-panner-with-ORTF-fullwidth.png" />
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<p>
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To simulate an ORTF, we use Robin Gareus' stereo balance
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control LV2 to set the level difference and time delay. Ignore the Trim/Gain
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— its purpose is just to align the test signal with the 0dB line of the
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analyser.
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</p>
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<p>
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Recall that an ORTF microphone pair consists of two cardioids spaced 17 cm
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apart, with an opening angle of 110°.
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For a far source at 45° to the right, the time difference between the capsules
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is 350 μs or approximately 15 samples at 44.1 kHz. The level difference
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due to the directivity of the microphones is about 7.5 dB (indicated by the
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distance between the blue and red lines in the analyser).
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</p>
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<p>
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Now for the interesting part: if we reduce the width of the signal to 50%,
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the time-delayed signals will be combined in the panner. Observe what
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happens to the frequency response of the left and right outputs:
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</p>
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<img src="/images/stereo-panner-with-ORTF-halfwidth.png" />
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<p>
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You may argue that all spaced microphone recordings will undergo comb
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filtering later, when the two channels recombine in the air between the speakers.
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Perceptually however, there is a huge of difference: our hearing system is
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very good at eliminating comb filters in the real world, where their component
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signals are spatially separated. But once you combine them
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inside your signal chain, this spatial separation is lost and the brain will
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no longer be able to sort out the timbral mess. As usual, you
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get to keep the pieces.
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</p>
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<div class="well">
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Depending on your material and on how much you need to manipulate the width,
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some degree of comb filtering may be acceptable. Then again, it may not. Listen
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carefully for artefacts if you manipulate unknown stereo signals — many
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orchestra sample libraries for example do contain time-delay components.
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</div>
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