Criteria for stereophonic arrays
Stereophonic arrays are thus the approach of choice for all 3D formats. The requirements for 3D are the same as in two- and five-channel stereophony:
- Signal separation among all channels in order to avoid comb filtering: No one signal should be present at significant levels in more than two channels.
- Level and/or arrival time differences between adjacent channels to achieve the desired imaging characteristics
- Decorrelation of diffuse-field sound for optimal envelopment and sound quality
These demands are still easy to fulfill in two-channel stereophony; a suitable arrangement of two microphones and two independent channels can provide the desired imaging curve. Tools such as the Imaging Assistant application (available as an iOS app or on the Web at www.ima.schoeps.de) have been developed for this purpose. They take into account not only the creation of phantom image sources, but also the ever-important channel decorrelation.
Fig. 2: Two-channel ORTF system in a suspension designed for use within a windscreen; two cardioids, 17 cm, 110º.
A classic, positive example is the ORTF technique, which has a 100º recording angle and delivers a stereo signal with good channel decorrelation.
The above requirements are distinctly more difficult to meet with five channels, and there are numerous geometries that fail to meet them, e.g. a microphone that looks like an egg the size of a rugby ball, with five omni capsules that can deliver only a mono signal at low frequencies.
Five independent channels simply cannot be obtained with any coincident arrangement of first-order microphones. A coincident arrangement such as first-order Ambisonics is thus a compromise for 5.1, though highly workable because of its advantages in compactness and post-production flexibility.
One optimal solution for ambient recordings in multichannel stereophony is the "ORTF surround" system, in which four supercardioids are arranged in a rectangle with 10 x 20 cm side lengths. Here the distances between microphones help with decorrela-tion, and thereby lend the sonic impression its spatial openness. The microphone signals are routed dis-cretely to the L, R, LS and RS channels. The signal separation in terms of level is ca. 10 dB; thus the sonic image during playback is stable even in off-axis listening positions.
Fig. 3: Four-channel "ORTF Surround" system; four supercardioids, 10 / 20 cm spacing, 80º / 100º angles
8 or more channels
With eight or nine channels, the arrangement of the microphones becomes very difficult if the abovementioned requirements are to be met. The simplest method for maintaining signal separation is to set up eight or nine microphones far apart from one another. Thus a large nine-channel "Decca Tree" arrangement is very well suited for certain applications, although it has severe disadvantages that limit its prac-tical usability. For one, the sheer size of the arrange-ment is greater than 2 meters in width and height. And the signal separation in terms of level difference is nearly zero; every signal is more or less available in all loudspeakers. Thus this array can represent a beautiful, diffuse spaciousness, but stable directional reproduction isn't achieved beyond the "sweet spot." This can be helped by adding spot micro-phones.