Stereo and Surround Microphones

Most audio from interviews is captured and recorded in mono.

Music, atmos and other effects tracks will often be in stereo or in a surround format. The mono recorded dialogue can be manipulated by panning and phase splitting for use in stereo or surround field productions. This technique enables greater creativity in sound design. The music, atmos and effects tracks can be mixed with the dialogue etc in an almost infinitely adjustable way within the post environment to suit the final edit. 

Location recordings in stereo using a boom remain one of the most popular sound acquisition techniques. There are two microphone configurations that are most popular for this operation - XY stereophony and MS microphone rigs. Both systems offer a high performance acquisition setup. For high-quality finishing, the MS arrangement is very popular. MS microphones record from two different channels, using at least two separate mics – an M signal comes from a Mid microphone that is pointed at the sound source and uses a cardioid microphone, the S signal is a differential Side recording that uses a bi-directional microphone to capture the other sound. Using a decoder matrix allows the post operator to combine the two signals into a stereo signal with variable base width. While the orientation of the microphone capsules must be optimally adjusted before it is included in an XY configuration, the mid-side stereophony also allows subsequent changes of the stereo image. Portable mixers and recording devices usually have a built-in MS decoder circuit to monitor the MS microphone stereophony while recording.

Double-MS arrangement for surround recordings

In the concert hall and in the studio, surround recordings often use complex microphone arrays and many additional microphones. For location sound, and TV live broadcasts from sports and other event sites, such arrangements are often far too complex. Instead, easy-to-deploy, portable yet high-end solutions are needed. Compact surround microphones are especially popular. These single-piece units can be mounted directly on the camera, or, for music and sound recordings, on a boom pole or suspended from the ceiling. For these types of applications, compact systems such as the INA-5 array, the Holophone system or the IRT Cross have become successful in recent times.

surround microphone SoundField DSF-2 in the windscreen

Because of its special properties, many film and television sound engineers will work with versions of the SoundField microphone. The SoundField works with four capsules in the assembly, aligned in the shape of a tetrahedron. The four microphone signals are arranged as "B-format" (W, X, Y, Z). The microphone signals are fed to a controller which allows configuration of the microphone system including gain, axis of working etc. The signals can either be directly recorded raw on four tracks, configured for a twin track mode, or passed to a processor for further encoding. The spatial information provided by these four microphones makes it possible to convert the signal immediately or during post production into any desired channel configuration, such as mono, stereo, MS-Stereo, 5.1 Surround, and more. Even a virtual tilting or rotating of the microphone (pan, tilt) in the desired direction and a subsequent change of the focus (zoom) is possible by changing the phase and level relationship of the capsules. 

Surround sound productions use a variety of microphones and different placements techniques to create the soundscape. These include spot microphones for individual sound sources and room microphones. At football games, for example, microphones are often placed on the side lines for crowd and player sound.

Author: Karl M. Slavik, Arte Cast Vienna