Greetings All,
I have a friend who's an advocate of the Decca Tree mic arrangement. Many of
his recordings (a lot of choir and guitar) sound quite nice, so I looked into
aspects of the Decca Tree technique. For those who may not be familiar, the
*traditional* Decca Tree arrangement is comprised of three spaced
omnidirectional mics. A center microphone is spaced slightly forward. From what
I've read thus far (Spatial Audio by Francis Rumsey, Focal
Press; and selected articles in the AES Stereophonic Techniques Anthology), the
slightly advanced time-of-arrival for the center mic stabilizes the central
image due the precedence effect. However, the existence of the third (center)
mic can result in exacerbated comb-filtering effects that can arise with spaced
pairs. So, to avoid these filtering effects, bring on a Soundfield / Ambisonic
mic...??
As I understand, Ambisonics already takes into consideration known
psychoacoustical principles, and is why shelving is used to *optimize* ILDs and
ITDs above and below 700 Hz, respectively. But as many readers may know, there
are some nearly unpredictable ILD/ITD effects at approx. 1.7 kHz (for example,
see Mills, 1972, Foundations of Modern Auditory Theory). Creating a virtual
Decca Tree seems straightforward. To move the center channel, or a virtual mic
*forward* would require little more than offline processing. I wonder whether
anybody has tried the following: Slightly delay all channels except the signal
(or feeds) that make up the forward-most (central) channel. Using an Ambisonic
mic would eliminate combing effects. I realize a number of Ambisonic plug-ins
have built-in crossed-cardiod, Blumlein, and spaced omni functions, but not
sure I've seen any of them give *precedence* to the precedence effect or Decca
Tree arrangement.
Two-channel playback (both convention and binaural) is here to stay for a
while, so optimizing Ambisonics for stereo is desirable to me. In fact, one of
my favorite recordings from the late 80s was made with the band (The Cowboy
Junkies) circled around a Calrec Soundfield mic. I've never heard whether the
Trinity Session recording was released in a surround format, or if the mic's
hardware decoder converted straight to stereo from the get go. That particular
recording made me aware of the Soundfield mic, though surround sound wasn't an
interest for me at that time.
If anybody I had attempted the Decca Tree using an Ambisonic mic (even with
addition of a separate and forward omni mic), I'd be interested in knowing what
your experiences were.
Many thanks for your time.
Best,
Eric C. (the C continues to remind readers that this post submitted by the
*off-the-cuff* Eric)
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