To help HD broadcasters meet the challenge of producing audio in 5.1, SoundField introduced the DSF-2 digital microphone system. It offers a means of creating surround in a variety of formats with a simultaneous, phase-coherent stereo feed for SD transmission. The DSF-2 digitally processes audio for outputting phase-coherent audio in simultaneous mono, stereo, mid-side and surround formats.
How it works
SoundField microphone systems all comprise a multicapsule microphone and associated decoder, and output audio as a proprietary four-channel signal known as SoundField B-Format. Three of these channels describe the space around the mic in three dimensions, as though recorded by three figure eight microphones placed at right angles to one another. The fourth provides an overall reference signal for the others, as though recorded by an omnidirectional capsule.
The four channels in the B-Format are phase coherent. This is achieved by mounting the four matched capsules in the microphone head as closely together as possible in a tetrahedral array. Of course, the capsules cannot be located at precisely the same point in space, which is ideally what is required to create point source phase-coherent output signals.
However, because the distance of the four capsules from the center of the tetrahedral array is small, known and constant, it is possible to post process the output of each capsule, generating four signals that correspond to those that would be created by recording at exactly the same point at the center of the array. This eliminates phase problems between the four constituent channels of the B-Format signal, and is central to the SoundField concept, as it allows them to be combined without any unpleasant-sounding phase artifacts, the usual drawback of multimicrophone or other multicapsule systems.
Simultaneous decoding into different formats is also possible. The B-Format signal can even be recorded to a suitable four-channel storage medium and decoded later. This allows broadcasters to determine the precise output format at a later date, and leaves the door open for future broadcasting standards by decoding into new surround formats such as 7.1.
By altering the balance of the four channels in the B-Format signal, it is also possible to change the mic's effective pickup pattern continuously from omnidirectional through cardioid to figure eight, and to change the virtual orientation of the mic. Thus, for example, a mic may have its pickup pattern tailored to become more or less directional, and shut out or admit more background noise, or it may be virtually moved or rotated without requiring the operator to touch the mic itself.
These options are easily accessed from controls on the microphone's associated decoder and are useful in outside broadcast situations, where the mic often cannot be accessed or adjusted once broadcasting has begun. Interestingly, this audio zooming and virtual rotation of the mic can still be carried out by post processing a stored four-channel B-Format signal with a SoundField hardware processor or software plug-in long after the recording has been made.
Another advantage of the design is that the microphone head can be placed at distances of up to 300m from its decoder without a drop in signal quality. Moreover, the decoder can be up to 1km from its associated broadcast vehicle, which is particularly useful at some sports venues, where the trucks can never get very close to the events themselves.
Recent projects
The DSF-2 first saw use at the 2006 World Cup. It was installed by Host Broadcast Services at the 12 German stadiums involved in the tournament and provided crowd and venue ambience in 5.1 and stereo, respectively, for HBS' HD and SD coverage. The stereo SD output was taken straight from the outputs on the mic's processor, while the mic's digital B-Format signal was routed to the International Broadcast Centre in Munich, passed through a SoundField SP451 decoder there to convert it into 5.1, and then distributed worldwide as part of the HD transmission.
Pieter Schillebeeckx is the head designer for SoundField.
