Like in October 2007, the OBORO workshop was aimed at giving participants a historical and theoretical overview of Ambisonic surround sound and a demonstration of the hardware and software production tools that can be used for Ambisonics. From its inception, Ambisonics was always an inclusive technology and although the SoundField microphone is often viewed as "the only Ambisonic microphone", any audio signal can be encoded to Ambisonic B-Format. Back in the late seventies, the Neumann QM69 quad microphone was planned as a source of B-Format, once its quadraphonic signal was converted. The QM69 hasn't been in production for a long time, but nowadays Swedish microphone manufacturers Pearl and Line Audio Design are both offering a quad mic model. For the October 12 recording, we had the opportunity to try out the Line Audio QM12i along with a SoundField ST350.
The Line Audio QM12i microphone is around $1000 and the SoundField ST350 is around $8000.
The QM12i has four cardioid elements arranged at 90° from each other to uniformly cover a 360° horizontal field. It's interesting to note that each cardioid element in the QM12i is in fact made up of three smaller capsules. The SoundField microphone has four cardioid capsules mounted as a tetrahedron producing an audio stream called A-Format: the SoundField control unit must be used to convert the stream to B-Format.
As it can be seen from the following pictures, we also installed a Zoom H2 surround microphone/recorder, but last minute formatting problems meant that it recorded the recital at different settings than the main recording device. We will not use the result for this review then: read the 2007 report for the Zoom H2 performance in similar conditions.
The workshop at OBORO's surround sound control room. Dynaudio Air 20 monitors are positioned in typical 5.1 fashion with left/right at 30° and Ls/Rs at 110°. On screen is a picture of the 1976 prototype SoundField microphone revealed by Michael Gerzon himself. The picture is from the Michael Gerzon Photos website.
In OBORO's Studio 1, checking the recording the gear. The Line Audio QM12i is installed on top the mic stand and about 10 cm in front of it. The four mic-level cables from the QM12i are connected to two Metric Halo ULN-2 audio interfaces. The ST350 is under the QM12i and about 15 cm in front of the mic stand. The four line-level cables out of the ST350 control unit are connected to a RME Fireface 400 audio interface. The Metric Halo and RME audio interfaces are connected to an Apple MacBook Pro 2.4 GHz running Mac OS X 10.5.5 and Steinberg Nuendo 4.2.2 DAW. The audio interfaces share the same wordclock and are grouped with the Mac OS X Aggregate Audio Device feature to present only one audio interface to the recording software. The recording is done at 24 bit and 48 kHz.
