Atmos 5.1
Model 9843
Microphone Array | Surround Console | We Have You Surrounded |
Line Up - June 1999
Surrounded By Microphones -a novel approach to recording surround sound
HUGH ROBJOHNS MIBS reports on the results of an interesting collaboration between German companies SPL and Brauner.
The impending domination of surround sound formats - particularly the 5.1 arrangement - in virtually every aspect of commercial sound production is something of which I'm sure every LINE UP reader is very much aware. After all, discrete channel surround sound has been a standard requirement of feature film soundtracks for several years now, is already making regular appearances in some big budget television productions (particularly in America), and all kinds of music recordings are now being produced with surround sound capability in anticipation of the European launch of DVD-A and SACD. However, effective, pragmatic techniques for recording and post-production in the various surround sound formats are still envolving and a concensus on suitable methods and practices has yet to be reached. Nevertheless, a significant new proposal for a surround sound recording and production technique has recently emerged.
Two German pro-audio companies, Sound Performance Lab (SPL) and Brauner Röhrengerätemanufaktur have been collaborating on an innovative solution consisting of a purpose designed array of matched microphone capsules and a dedicated recording, mixing and pre-mastering surround sound signal processor.
The bespoke microphone array is dubbed the ASM 5 and employs five of Dirk Brauner's hand-built capacitor mic capsules - the same as those used in his very impressive VM 1 valve microphone. For this application, the carefully matched dual-diaphragm capsules are mounted in an adjustable 'star' formation which has apparently been arrived at following extensive academic research by Volker Henkels and Ulf Herrmann. It is claimed that their INA 5 arrangement (Ideale Nierenanordnung 5 - 'ideal 5-cardioid setup') has been shown through blindfold listening tests, to be superior to various other published surround and ambisonic recording techniques, such as the IRT-Surround and ABC systems. However, the Brauner ASM 5 implementation of the original INA-5 concept incorporates adjustability of the capsule polar patterns and their positioning, making it a considerably more flexible system for recording musical events in the real world!
The ASM 5 array consists of five radial spokes supporting the VM 1 capsules (from above), joined at a central hub which provides the stand mounting point and the 12-pair cable connection. For the standardised INA-5 configuration the front three microphones are arranged much like a 'Decca Tree' on 175 mm arms and facing outwards at 0,90 and 270 degrees (relative to the frontal axis). The two rear microphone capsules are on longer 596 mm arms facing 150 and 210 degrees. However, the added flexibility of the ASM 5 allows the front three microphones to be orientated in a straight line (perpendicular to the front axis) if preferred, and the length of the rear arms is adjustable over quite a wide range too. Furthermore, the capsules can all be rotated +/- 45 degrees relative to the axis of their support arms, and their polar patterns altered remotely and independently between omni and figure-of-eight.
The SPL Atmos 5.1 console is an equally interesting device, although it is more of an elaborate signal processor than a 'console' in the accepted sense. It has been specifically designed to complement and contol the ASM 5 mic array, as well as providing suitable facilities for the post-production of all surround sound material, irrespective of the intended surround encoding format (ie. DSD, AC3, ISO/MPEG2, DTS or SDDS).
The Atmos 5.1 unit occupies 5U of rack space and provides five microphone preamplifiers, image width processing, bar graph metering monitoring facilities, and polar pattem controls for all of the ASM 5's capsules. The microphone preamplifiers feature SPL's high-precision, low noise 'Triple Gain Stage', and are equipped with motorised gain controls (+ 20 to + 70 dB) and VU meters. There are also the usual switches for phantom power, high-pass filtering (100 Hz, 50 Hz and off), a 25 dB attenuator, and polarity reverse.
A fader knob and full surround sound panning facilities (left/centre/right, front/back and divergence) complete the channel controls. The motorised gain knobs can be staved to a master control in Channel 5 which ensures accurate level matching across the five signals should it become necessary to adjust the overall gain after an initial line-up. A direct output (tape send) and tape return is also provided on each mic preamp, as is an insert point.
The 5.1 surround format includes a 'low frequency effects' channel (the '.1') and the Atmos 5.1 unit accommodates this with a matrix section to collect LF information from either the left and right, centre, surround left and right, or a combination of these main channels. There is also a switchable low pass filter restricting the LFE information to below 130Hz. A pair of balanced line level inputs provide a means of combining an external stereo signal with the surround programme, and switching is available to route this external signal in to the left/right, surround left/right, or centre channel outputs.
To provide total control over the surround sound stage the Atmos 5.1 even includes a pair of stereo spreaders, one across the front left/right signals and the other for the rear surrounds. They both offer continuously variable width (from narrow to extra wide), and have bypass and mono switches, plus built-in LED phase correlation bar graph meters. As mentioned earlier, there are also independent polar controls for the ASM 5 microphones, with continuous pattern adjustment from omni, through cardioid, and on to figure-of-eight.
Completing the exhaustive specification of the Atmos 5.1, a monitoring section provides outputs for a full surround monitoring loudspeaker system with the volume of all six channels controlled from a single knob. There is also a dedicated output for a Sennheiser 'Surrounder' headset, as well as switchable auditioning of the left/right pair, rear surround pair, centre, and sub channels for conventional stereo headphones. Six LED bar graphs provide levd metering of the LCR, SL/SR, and Sub channels.
The Atmos 5.1 and ASMS combination is, as far as I am aware, the first dedicated 5.1 surround sound recording/processing system and offers a novel, innovative and comprehensive approach to the problem of recording genuine surround sound imaging of live acoustic events. The nearest comparable system is probably the new processor from Soundfield Technology which adapts the B-Format output of a Soundfield microphone to create the equivalent 5.1 formatted signal. It would be most interesting to compare the results from these two systems - something I would hope to achieve and report on in the future. In the meantime, why not have some fun experimenting with a Heath Robinson replica of the INA-5 arrangement and try live surround sound recording yourself?
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