Space vs Time in a Decca Tree

[boojum]

I have followed most of the Decca Tree threads on the board. The discussions seem to focus on mic types and array dimensions. And there is the additional problem that the original engineers had no hard and fast rule for dimensions; they sort of winged it.

Fine. But if I were to arrange three omni's in a line say six feet/two meters across with the center mic in the exact center but in the line, not forward of it, and adjusted the theoretical distance in post by delaying the L and R mics appropriately, would it be Decca? In short, must the C mic be the physical appropriate distance ahead of L and R?

I think on the one hand this is an easy way to fiddle a bit with the array and the other hand think this is so simple that it must have been tested and quite possibly discarded. Has anyone experience with this approach or knowledge of it??

Thanks.

[mpdonahue]

In theory this should work for sound sources directly in front of the center microphone in an anechoic chamber. However, in the real world you will find that the sources coming from off center will be realized differently when compared to a microphone physically spaced from the back of the tree.
This is much like trying to time align your spot microphones to your main array. It would work if there was no interaction between the individual spot microphones being delayed, but back here in the real world it causes many more problems than it fixes, IMNSHO.
As always YMMV,
All the best,
-m

[boojum]

Mark, thanks for YNSHO. If I knew this stuff I would not have to ask. It seemed so easy but I had never read of ot being used, so I figured there must be a reason. Now I know it.

Now, three in a line a la Mercury Living Presence works if the distances between mics are great enough? I am trying to solve a venue problem which allows me few mic'ing options and less setup time.


TIA!

[Larry Elliott]

Boojum,
The Mercury 3 mic system is a great one to adopt if you only have a limited setup time. I have used it regularly at the concert hall in Auckland. It does help that the local radio company has cables in the ceiling that I am allowed to use. The results are excellent.
Larry

[Petrus]

If you look at the Decca tree as a three to two vector summing system it is easy to see that by delaying the signals of a three-in-a-line the end result will not be the same as the real Decca tree.

By vector summing I mean a situation where L and M are summed, say, 1:1, then a virtual L is created at half way between L and M. Certainly it is not the same as a real mic in the same spot, but basically something like it. By adjusting timing only, not location, the same effect can not be created, only an approximation. If it sounds better, though, why not use it?

I have made Decca tree contraptions from carbon fiber tubes (about 16 and 14mm, fitting nicely inside each other). T-midsection from thicker tube with stand attachment spigot and then 2 sets of thinner tubes with epoxied screws at the ends, lengths 120 and 90 cm and the front pointing one with one screw 60 and 45 cm (working distance, actual length is a bit less, as the spigot is glued about 5 cm in front of the junction for balance reasons). These are taped in place with black gorilla tape. I can use the side tubes also alone for longer AB miking. These are light, easy to take apart, quite cheap and only one stand is needed.

[muziekschuur]

Use a phasemeter on the spot to check the incoming signals.

[Gaston69]

This is much like trying to time align your spot microphones to your main array. It would work if there was no interaction between the individual spot microphones being delayed, but back here in the real world it causes many more problems than it fixes.

Above mentioned statement is exactly what Onno told me when I visited him 2 months ago, by the way I was sitting on a Soundmirror chair and he told me he had worked with you (Mark Donahue) ?

Gaston