+<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