MARQUETTE AUDIO LABS

* Since 1994 *

Soundfield ST250 Microphone System


What is it?
The SoundField ST250 is a very unique microphone that applies the SoundField concept to "on location" applications. In its most basic form, it is a mono and variable angle stereo Microphone with remotely variable pick-up patterns plus the option to deliver Mid/Side and B-Format output formats. The ST250 System consists of two parts that are used together, a SoundField microphone and a miniaturized DC powerable SoundField Processor. Sharing design principles and sound character with the entire SoundField family, it is the model most chosen by engineers and hobbyists who record live concerts, shows and documentaries.

What makes it different?
The SoundField microphone itself contains four capsules, mounted in a tetrahedral array (tetrahedral describes an equilateral triangle). This tetrahedral capsule array, combined with the SoundField Processor/Preamp, allows the ST250 to become different kinds of microphones by varying individual capsule combinations. It can be a conventional mono omni, cardioid, hyper-cardioid, figure eight or anything in between. It can be a stereo variable angle omni, stereo cardioid, or stereo figure eight. It can also be a complete Mid/Side system for those with sum/difference matrix decoders. And it is the one and only portable B-Format microphone for use as a multi-channel source with the SP451 Surround Processor. Or, when ST250 B-Format recordings are played back through a MKV Processor in Post Production, you can alter microphone position within an existing B-Format recording to yield a final stereo or mono output.

Perhaps the key advantage of the SoundField system is that unlike separate or even stereo microphones, the error between capsules is resolved within the SoundField Processor. This capsule error is similar to the time arrival error experienced by a listener offset between 2 or more spaced loudspeakers.

The benefit in resolving this error is significant in that you can simply "point and shoot" a SoundField microphone regardless of mode. SoundField avoids lengthy set-up time typical of separate microphones and there is no phase cancellation when shifting between mono and stereo modes. Stereo imaging is rock stable and absolutely breathtaking. With multi-channel looming on the horizon, the SoundField ST250's B-Format is already delivering an output that can be decoded into 5.1 and beyond. In fact, the forthcoming SoundField SP451 Surround Processor which is a dedicated B-Format to 5.1 surround converter will make the ST250 the only 5.1 location microphone on the market.

How does it work?
The SoundField System relies upon B-Format and its processing at its core. The advantage of B-Format is that everything begins as a phase accurate 3 dimensional representation of an acoustical event. Phase accurate means that all sound is recorded from the same central "single" point. This is the core difference between SoundField and traditional stereo or multi-mike set-ups, in that all other methods (other than a single mono mike) capture audio from different points in space. From SoundField's single "point source", all three acoustical dimensions are defined, not just two or one as in other microphone set-ups. The 3 dimensions are: front to back (X), left to right (Y) and up and down (Z). B-Format offers each of these 3 dimensions as an individual output plus the point source reference called W, across 4 channels. A complete three dimensional graphical representation of B-Format is shown in the drawing to the right. The beauty of B-Format is that it through processing, any type of output (omni, cardioid, hyper-cardioid or figure eight) in any form (mono, stereo, or extra wide stereo) can be derived. In addition, special output forms such as mid/side and B-Format are just a push button away. This makes the SoundField the most flexible microphone on the planet, and one that offers a long future in a multi-channel world.

How do I use it?
SoundField is used as a normal mono omni microphone in that you just set it on a stand and go. Moving the microphone around can vary the relationship between ambient and direct sound, so some experimentation with placement is likely. It can be pointed directly at the source like a shotgun microphone (end fire mode), hung upside down from a fishpole or boom (invert mode), used in a "blimp" Rycote Kit (for location or outdoor work) or set on a stand like an ordinary mike. The Processor stays with the user, connected via a supplied 5 metre SoundField cable (longer lengths available), so pattern and stereo angle (width) can be changed and the result monitored with headphones.

The ST250 can be powered in three ways: from 2 "C" Batteries (installed inside the Processor), from externally supplied phantom power (to the Processor), or from AC mains power. All three modes yield the same performance. The ST250 is designed to operate at lower voltages for DC capability, so you cannot plug an ST250 Mike into any other SoundField Processor (SPS422B or MKV), nor can you use any other SoundField Microphone (SPS422B, MKV) on an ST250 Processor. B-Format recordings made with a ST250 are compatible with the MKV Processor for remixing.

About the future of multi-channel and microphones
The future of multi-channel presents some serious problems in microphone technique as documenting a real event in 5 channels is not so easy. 5 microphones at 5 different locations will present significant capsule error, and will make such a recording not compatible with mono or even stereo playback or broadcast systems without significant post work. The potential of SoundField microphones to function equally well in multi-channel, stereo and mono environments is a key advantage. Backward compatibility is likely to be an important practical issue as recordings may be distributed to the International and rapidly evolving Domestic marketplace.

Because SoundField starts with B-Format as a phase accurate 3 dimensional representation, deriving multi-channel is a matter of decoding only. Right now, most SoundField multi-channel users apply analog matrix devices for this purpose. Some use a console, with advanced matrix options built-in. Some use custom built analog matrix devices. Shortly you will be able to purchase a dedicated SoundField B-Format to 5.1 Multi-channel Decoder from our dealers to go with the ST250 and MKV SoundField Systems.

SoundField advantages
1) Only a SoundField via B-Format acts as a "single point source", all but eliminating phase error between capsules, making it backward compatible down to stereo or mono without cancellations in response.
2) As a small "handheld size" integrated multi-capsule microphone, it offers ease of handling and mounting compared to traditional stereo mikes, making recording a "point and shoot" process.
3) Variable Pattern control and stereo X/Y Angle (width) controls are remotely located with the user on the shoulder carried ST250 Processor.
4) The B-Format potential of the ST250 reaches beyond the capabilities of all other stereo mikes to recover and document 3 dimensional information for use in future multi-channel playback environment.

Upgrades and multi-channel
A ST250 System can be expanded in the post environment in two ways. First, a 4 track B-Format recording from the ST250 can be processed by the studio MKV Processor for alteration of B-Format signals to alter mic position within the recording after it was recorded and generate a final stereo output. See the MKV pages for details on the processor [link to MKV]. Second, if you look toward 5.1 field recording, the SP451 Surround Processor is compatible with the B-Format recordings originating from the ST250. This means that 5.1 playback is possible from a "single" ST250 microphone.

Where is the SoundField made?
The SoundField ST250 system is fully manufactured in Wakefield (England) UK. All SoundField capsules are hand made, the microphone hand assembled and the processor hand wired. The quality of construction and sonic performance are world class. Failure rates are extremely low, and it is a rarity to see a SoundField come back for service. With normal care, the SoundField should provide many years of problem free service.

How does it sound?
While SoundField specializes in imaging, it is no less effective in the studio or the field for faithfully recording the sound exactly as it is without additional bass or high frequency. The ST250 is a true "flat" performance microphone system free of sharp peaks or dips, bumps in response that change the tone of the real event. It is designed to be flat in distant miking situations so the ST250 is ideal for documentaries, wildlife recordings, concerts in halls or open air, ambient recovery such as ENG when it must sound like "you are there".

System Components


1) The ST250 Microphone itself looks and sounds identical to other SoundField microphones. However, it is a different microphone than the SPS422B or MKV, as it is designed to operate at a lower supply voltage. The SoundField mike "head" is larger than normal (to hold the 4 capsule array), with a narrow handle and special 12 pin output connector (to use with the supplied 5 Meter SoundField 6 pair cable). All 4 capsules are hand made and fitted in the UK at SoundField factory. The microphone body has no switches or controls on it, as all operational controls are located on the SoundField ST250 Processor. Note that the ST250 mike and ST250 Processor are not interchangeable with other SoundField microphones and Processors.

2) A Mic Clip, a standard plastic stand mounted mic clip to hold the ST250 Microphone body to a conventional mic stand, fish pole or boom. Many people opt to purchase the optional Rycote Accessories, including a blimp and woolly windscreen.

3) A 5 Metre SoundField Cable is included (about 55 feet long) to connect the microphone to its 1U processor. It is a flexible 6 pair cable, which is a similar diameter to standard mic cable and manufactured for SoundField by the Belden Cable Co.

4) A ST250 Portable Processor/Preamp accomplishes all the math and matrix operations to make it extremely easy to operate with virtually no training or study. You do not need to have in-depth understanding of B-Format to successfully use the ST250 as XLR's on the back supply a normal stereo (or mono) output. The ST250 Processor is essentially microphone preamplifiers (delivering a very hot mic level, almost line level output), capsule compensation, capsule [array] pattern control, stereo width control (stereo X/Y angle) and a built-in headphone monitor.

System Controls


A) "Bass" switch, a 120Hz roll-off filter for wind noise or low frequency control

B) "Atten" (attenuation) switch, a 20 dB input pad, for increasing maximum input from 115dB SPL to 135dB SPL.

C) "End" switch: Sets the processor to operate the microphone when it is horizontally "pointed" at a source, allowing the pattern and stereo width controls to operate normally from this mic position. This is important for blimp mounted uses or handheld applications. Correct microphone position in end fire mode is logo facing down.

D) "Invert" switch: Sets the processor to operate the microphone when it is vertically hung (head end pointed down, connector up). This is important for fishpole or boom operation. Correct invert position is logo facing out.

E) "M/S" mode switch: This sets the ST250 to deliver 2 channel Mid/Side mode recordings. Mid Side output requires a user provided "sum and difference" matrix for decoding.

F) "B" (Format) switch: This sets the ST250 to deliver unbalanced B-Format when used in conjunction with the optional ST250 B-Format cable. Basically, B-Format offers 4 outputs that are X, Y, Z, and W (an absolute pressure reference). The purpose of B-Format is the ability to create a 3 dimensional sphere of audio originating from a single point in space. The advantage of B-Format is that because all information is referenced to a single point, all output modes (mono, stereo, Mid /Side or B-Format ) are phase consistent. B-Format also opens up multi-channel realms, where custom matrixes can be built to pull off X/Y/Z point specific information used in directionally accurate multi-channel playback.

G) "Battery" switch: switches power from external AC or Phantom to Internal Battery power using 2 ordinary C Cells user installed within the ST250 Processor. Battery life is impossible to predict as it varies with batteries, use and signal level, but it is estimated at between 3 hours and 24 hours. Typical is 5 hours.

H) Pattern Control: This control varies the polar pattern of the entire array as a single point stereo device, and is continuously variable from Omni to Figure Eight. It is extremely useful in tuning the "acceptance angle" of the array. In figure eight mode, the microphone "sees" more information from the front(direct information) and the rear (ambient information), rejecting information from the sides (direct and/or ambient). In Cardioid mode, it is rejecting more information from the rear. In Omni, it picks information equally in all directions. This is helpful in deciding what balance of direct vs. ambient information is desired, and from what direction that ambient information is gathered. It is also helpful in operating the microphone (in mono) as a conventional variable pattern condenser when close miking.

I) Width Control: This control varies the stereo image from zero (mono) to extra wide. It is perhaps easier to envision the SPS422B as a stereo pair that can be gradually decreased in angle to deliver mono (0 degrees) or increased in angle to stereo (90 degrees) or "wrap around stereo" (180 degree). You'll love this control for the creative options it opens up to you in close and distant miking situations.

J) Headphone Amplifier: A 1/4 inch front panel headphone connector and variable output level control are provided for monitoring of the ST250 output. The amplifier provided is for monitoring of the stereo L/R (X/Y) output only, regardless of front panel settings. It is not an audiophile grade output and is best when used with medium impedance headphones (above 75 ohms), as the output is 75 ohms. Keep in mind that in internal battery DC power mode, how hard you drive the headphones and their "draw" will impact battery life. For this reason it is important to use the right impedance phones.




Please call Dave at (510) 581-3817 for Soundfield product information and your price.