Surround Microphone Technique for the Direct Sound Approach

For perspectives that include "inside the band" the microphone technique for the surround channels differs little from that of the microphones panned to the front loudspeakers. Mixing technique optimally may demonstrate some differences though, due to the different frequency response in the ear canal of the surround speakers compared to the fronts. Further information about this is in Chapters 4 and 6.

Special Microphones Arrays for 5.1-Channel Recordings

A few 5.1-channel specific microphone systems have appeared on the market, mostly using a combination of the principles of the various stereo microphone systems described above extended to surround. There are also a few models that have special utility in parts of a 5-channel recording, and setups using conventional microphones but arranged specifically for 5.1-channel recording.They are, in alphabetical order:

• Double M-S'. Here three microphone capsules located close to one another, along with electronic processing, can produce left/center/

right/left surround/right surround outputs from one compact array. Two cardioids or preferably super- or hyper-cardioids face back to back, with one aimed at the primary sound source and the other away from it. A bidirectional mic. is aimed perpendicular to these first two, with its null in their plane of greatest output. By sum and difference matrixing as described above, the various signals can be derived. What keeps this from being an ideal technique is the fact that the angle of acceptance of each of the capsules overlaps one another too much due to the use of first-order microphones. However as a single-point recording system with very good compatibility across mono, stereo, and surround, this technique is unsurpassed.

• Fukada array. This setup for front channel microphones developed at NHK is derived from the DeccaTree, but the omnis are replaced with cardioids with specific distances and angles. The reason to change from omni to cardioid is principally to provide better isolation of front channel direct sound from ambient reverberation. Two cardioids are placed at the ends of a 6-ft long line, facing outwards. Thus these cardioids must have good off-axis performance since their 90° axis is aimed at the typical source. This polar pattern requirement dictates that they should be small-diaphragm microphones. A third caridioid is placed on a perpendicular line bisect-ing the first 5 ft from the first line and facing the source.This setup won rather decisively an informal blind listening test comparison among various of the types.⁴ However, the authors are clear to call their paper informal in its title, and the number of sources is limited and various microphone models were employed for the various setups thus not quite making a comparison among apples but in some ways apples to oranges. Nevertheless this result is intriguing, as several authors have written against spaced microphones arrays of any sort. The Fukada array would normally be supplemented by backwards-facing cardioids spaced away from the array, an IRT cross, or a Hamasaki square, for surround. Of these, the Hamasaki square produced the best results in the informal listening test.

• Holophone Global Sound Microphone system {multiple models):

This consists of a set of pressure microphones flush mounted in a dual-radius ellipsoid, and a separate pressure microphone interior to the device for the Low Frequency Enhancement (LFE) channel. Both wired and wireless models have been demonstrated. Its name should not be confused with Holophonics, an earlier dummy head recording system.

⁴Rafael Kassier, Hyun-Kook Lee, Tim Brookes, and Francis Rumsey, "An Informal Comparison among Surround Sound MicrophoneTechniques," AES Preprint 6429.

• Ideal Cardioid Arrangement INA-3 and INA-5: Three (for front imaging) or five (for front imaging plus surround) typically cardioids are placed on arms and spaced in directions like those of the ITU Rec. 775 loudspeaker array. One example is the Gefell INA-5. Another is the Brauner ASM-5 microphone system and Atmos 5.1 Model 2600 matching console. The Brauner/Atmos system consists of an array of five microphones that additionally offers electrically adjustability from omnidirectional through Figure-8. Their mechanical configuration is also adjustable within set limits, and the manufacturer offers preferred setup information. The console provides some special features for 5.1-channel work, including microphone pattern control, LFE channel extraction, and front all-pass stereo spreading.

• OCT array. This array was developed by GuntherTheile at the IRTThis array consists of two hypercardioids mounted at the ends of a line and facing outwards, and a cardioid capsule mounted on a line centered on and perpendicular to the first. The dimensions can be varied to meet a requirement that may be set for included angle. The objective is to produce better phantom images at left-center and right-center locations than other array types, since most setups overlap the outputs of the various microphones employed in the array so much as to make the half-images "mushy." Very good hypercardioids must be used since the principal sound is at 90° off axis and this requirement dictates the use of small-diaphragm microphones. In addition a pressure mike may be added in the mix for below 40 Hz response by low-pass filtering an omni included with the array, making a total of four mikes and mike channels to record this front array. It would normally be used with spaced backwards-facing cardioids, or an IRT cross or Hamanaki square array located behind it relative to the source as the channels to employ for surround envelopment and spaciousness. Thus a total of eight recorded channels would be used. The OCT array did not do very well in the informal listening test referenced above, however, it was realized in that case with large-diaphragm microphones that are not good in this service, so can't be counted too much against this rig. I have heard this setup In one AES conference demonstration and it seemed to image sources moving across the front sound stage quite well. In fact, its reason for being is this sharp frontal imaging, of particular use in small-ensemble music recording, and in some kinds of stereo sound effects recordings that must match to picture.

• Sanken l/l//V7S-5:The 5-channel microphone, which is a variation on double M-S. A short shotgun mike is used for center front, and two cardioids and a bidirectional mike are combined with it and an M-S style matrix to produce a 5-channel output. Note that the shotgun devolves to a hypercardioid at medium and low frequencies,

• The IRT cross, an arrangement of four spaced cardioids facing outwards arranged in a square about 10 in. (25cm) on a side at 45° incidence to the direct sound has been found to be a useful arrangement for picking up reverberation in concert halls; one could see it almost as double ORTR Also, it may be used for ambi-ences of sound effects and other spatial sound where imaging is not the first consideration, but spaciousness is. The outputs of the four microphones are directed to left, right, left surround, and right surround loudspeakers. A limitation is that some direct sound will reach especially the front facing microphones and pollute its use as a pickup of principally reverberation.

• The Hamasaki square array is another setup useful in particular for hall ambience. In it, four bidirectional mikes are placed in a square of 6-10 ft on a side, with their nulls facing the main sound source so that the direct sound is minimized, and their positive polarity lobes facing outwards. The array is located far away and high up in the hall to minimize direct sound. The front two are routed to L and R and the back two are routed to LS and RS. Side-wall reflections and the side component of reverberation are picked up well, while back wall echoes are minimized. In one informal blind listening test this array proved the most useful for surround.

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