AES British Section lectures - April 2008

Stereo and Surround Microphone Techniques

Helmut Wittek, Schoeps

The British Section hosted Helmut Wittek on April 15, who lectured on "Stereo and Surround Microphone Techniques". Wittek is head of engineering and technical support at Schoeps GmbH in Karlsruhe, Germany.

At this well attended lecture, Wittek gave an overview of psychoacoustical principles pertinent to spatial sound perception. He pointed out that current theories still do not explain fully the perception of phantom sound sources reproduced by a pair of loudspeakers. In the second part of his lecture Wittek explained the principles underlining the design of both two-channel stereo and surround microphone systems. He gave an interesting demonstration showing how the optimization of a microphone technique can be guided by analysis of the inter-channel diffuse field correlation.

Wittek also explained the benefits of using surround sound over two-channel stereo in terms of preservation of spatial cues included in early reflections and the reverberation tail. He emphasized that in a well designed surround microphone array there should be no more than two microphones capturing correlated signals.

One of the highlights of Wittek's lecture was the introduction of a microphone technique called Double MS. This utilizes a triplet of coincident microphones consisting of a figure-of-eight microphone and two back-to-back cardioid microphones. The original microphone signals are converted either to a two-channel stereo or a standard five-channel loudspeaker setup. There is a high degree of flexibility in this conversion process allowing the recording engineer to simulate different virtual microphone techniques.

During his lecture, Wittek gave several interesting demonstrations. He presented a software utility allowing optimization of a two-channel microphone system to enhance the decorrelation of diffuse field signals between the channels. In a second demonstration Wittek showed the Image Assistant Java applet (available at www.hauptmikrofon.de) which can be used to optimize two- and three-channel stereo microphone arrays in terms of their coverage angle and the homogeneity of a captured sound image. Finally, he demonstrated a VST plug-in that can be used for conversion and rendering of a sound captured by the Double MS system. His lecture was followed by a lively question-and-answer session.

Those interested may obtain a recording of the lecture from the British Section web site at www.aes.org/sections/uk/. Further information on the mentioned microphone configurations can be found on www.schoeps.de.

Report by Slawek Zielinski