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".