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Stereo, Surround & More: A Look At The Core Sound TetraMic Microphone

TetraMic is the first portable, coincident, stereo and surround-sound Ambisonic soundfield microphone under $1,000; here's a thorough look at how it works and applications

By Bruce Bartlett August 8, 2009

The Core Sound TetraMic

The Core Sound TetraMic is the first portable, coincident, stereo and surround-sound Ambisonic soundfield microphone under $1,000, and it compares very favorably with similar soundfield mics selling for $3,000.

This pencil-sized microphone offers amazing sound and flexibility at an affordable price. Some applications include stereo or surround recordings and broadcasts of musical ensembles, sound effects, news events, sporting events, environmental sounds, and general ambience.

How It Works
The TetraMic is a soundfield system that utilizes a patented four-capsule microphone and decoding software.

On the top of the tiny mic are four sub-cardioid electret-condenser capsules of 12 mm diameter mounted 90 degrees apart.

The capsule grilles define the faces of a tetrahedron. Below the capsules is a slim brass handle with a built-in 6-pin tiny XLR connector.

Other cables and connectors (described later) let you connect the mic to a recorder of your choice.

The signals coming directly from the four capsules are called A-format. While these signals are not usable as they are, they feed into Core Sound’s processing software which creates a four-channel signal called the soundfield B-Format.

Those four channels are known as:
X (front-back)
Y (left-right)
Z (up-down)
W (omnidirectional, a reference for the other three channels).

The X, Y and Z channels are effectively three figure-eight patterns at right angles to each other.

The Core Sound TetraMic

By summing and differencing those B-format channels in varying amounts, the processing software can create a wide variety of mono, stereo, mid-side, or surround polar patterns—even in post-producton after the 4-channel recording has been made.

Surround formats include 5.1, 6.1 and 7.1; loudspeakers in a square, and loudspeakers in an octagon. Almost any loudspeaker arrangement can be user-defined.

Although the capsule diaphragms are separated about 1.3 inches, the processing results in phase-coherent signals, as if the capsules occupied the same point in space.

The four B-format channels capture the 3-D sound field all around the microphone, as picked up at a single point. These signals can sum to mono with no phase cancellations.

The processing also matches the capsule sensitivities, and equalizes the capsule signals to give them a wide, flat frequency response.

So you start with a four-channel recording of the mic-capsule signals. Then using the processing software in post, you can effectively point or steer the “effective” or “virtual” microphone(s) as desired.

The software allows modeling of essentially any number of coincident first-order microphones, each pointing at any arbitrary angle and each having an independent pickup pattern. 

For example, suppose you set the software to create a virtual figure-eight mic. As you move a slider in the software to turn the virtual mic off-axis, you can hear the recorded source become more distant as the null of the figure-eight pattern sweeps toward the source.

Or suppose you create a virtual Blumlein array of two figure-eights angled 90 degrees apart. As the sound source moves from center to 45 degrees to the right, you hear the sound image move the same way to the right monitor speaker.

When the source moves beyond 45 degrees, you hear the sound imaging becoming out-of-phase and diffuse, just as you would with a real Blumlein pair.

It’s as if you had a mono mic, stereo mic, or surround mic that could be rotated, tilted, or zoomed at will—after the recording is made.

The Physical Mic & Cables
A TetraMic system of mic, cables and adapters can be connected in various ways. Figure 1 shows a typical system.

Figure 1. The parts of one TetraMic system (click to enlarge)

The TetraMic is connected to an extension cable, which goes to a breakout cable, which is connected to four PPA phantom-power adapters with XLR plugs.

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The signal at those XLR connectors is low-Z balanced.

The PPA adapters plug into a mixer, audio interface, or mic preamps that supply 48-volt phantom power, or they can plug into an Audio Technica AT8531 power module, and many others.

The front of the ruggedly constructed mic is indicated by the “Core” logo on the machined brass handle.

A Switchcraft TB6M connector in the mic handle mates with a TA6F in the extension cable, and this extension cable plugs into a “four 3-pin mini-XLR-F breakout cable”, which divides into four 3-pin mini-XLR plugs.

The signal chain of this connection option is:
TetraMic > extension cable > breakout cable > PPA phantom power adapters >  mic preamp with phantom power.

Because the mic-capsule signals are medium impedance unbalanced, it’s best to limit cable runs to 20 feet before converting to low-Z balanced with the PPA adapters.

Core Sound offers another connection option:
TetraMic > extension cable > dual 5-pin adapter cable > Core Sound 4Mic power supply/mic pre/A-D converter > 2-track digital recorder with S/PDIF input.

The Core Sound 4Mic ($899) is a handheld, battery operated preamp and A/D converter that provides four discrete outputs or a matrixed 2-channel signal that can be decoded later.


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About Bruce

Bruce Bartlett
Bruce Bartlett

Recording Engineer
   
AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are “Practical Recording Techniques 7th Edition” and “Recording Music On Location 2nd Edition.”
http://www.bartlettaudio.com

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Erik Sikkema says

About the Tetra mic, why subcardioids? As far as I know such microphone types use cardioids instead, subcardioids, (semi-omni) would have too less separation.

marklee says

Thanks for the mention, This shoes that you put a great deal of care to your work.  insurance Thanks for the links and all informative stuff you provided.

jody peterson says

The late Brad Miller (the true father of surround) (Mobile Fidelity Sound Labs, Mystic Moods Orchestra, HDS-High Definitation Sound) had developed a spectacular mic known an the Miller Surround 4 or MS4 that was a virtual 3D “bubble”. Some of the recordings done with this mic are the various aircraft surround efx on Alan Parson’s excellent DTS release “On Air”. I have the mics and have used them over and over with spectacular results both for both recording and live surround performances.  I have often thought they should be manufactured affordably for the project studio user. These mics also yield vertical information. 
Nice to see someone addressing the price range that makes VIRTUAL surround affordable.  4 channels of full bandwidth is all you need (“QUAD ON THE POD” anyone??)...then you don’t have to worry about making everything below 80Hz,or even worse, below 120Hz(!) MONO.  I thought we left that behind with the cutting lathes.
Best wishes with your new mic!

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