Surround Front System OCT Set
- microphone set for the front surround channels
OCT stands for "Optimized Cardioid Triangle."
It is a method for picking up the three front channels of a surround recording. It can be combined with any of several possible methods for obtaining the rear channels (and thus "OCT surround"), as will be shown on the following pages.
The preferred setup for OCT uses a forward-facing cardioid for the center channel. For the front L and R channels, two supercardioid microphones are placed at opposite ends of an imaginary line running about 8 cm behind the center microphone. These two microphones should be 40 – 90 cm apart, depending on the required recording angle*, and must face squarely outward, away from center (see diagrams below and on the following pages).
Good separation between the center-to-left sector and the center-to-right sector is obtained with this method. For example, sound originating from half right is picked up only very weakly by the left microphone. Sound from the extreme right will be picked up directly on-axis by the right-facing supercardioid (0 dB) and by the forward-facing cardioid (attenuated by 6 dB due to its directional pattern). Finally it will be picked up, with a delay caused by the increased distance, on the rear lobe of the left-facing supercardioid. The polar pattern attenuation for this will be 10 dB and the polarity will be inverted. These factors prevent the formation of annoying “phantom images“ of sound sources in the wrong sector during playback.
A clean center channel is conveyed by this system since front-incident sound is picked up mainly by the cardioid in the center. The left and right microphones, with their high directivity, pick up front-incident sound only at a much lower level.
If the cardioid is placed 8 cm forward, the following recording angles* will result, depending on the distance between the supercardioids:
40 cm: 160°
50 cm: 140°
60 cm: 120°
70 cm: 110°
80 cm: 100°
90 cm: 90°
It is better to err in the direction of greater spacing so that one can be sure to avoid center-heavy images.
The fact that the supercardioids receive so much of their sound from off axis necessitates the use of small-diaphragm condenser microphones, as only this type of microphone has the requisite independence of frequency response from the angle of sound incidence. The CCM 41V and the MK 41V are particularly well suited for this application, but the CCM 41 or MK 41 can also be used.
This Set comprises:
2 × CCM 41 Lg compact microphone (supercardioid),
1 × CCM 4 Lg compact microphone (cardioid),
1 × MAB 1000 mounting bar,
For more information on OCT and other surround recording arrangements please see our prospectus Surround Recording Techniques.