Sound Performance Lab . Atmos 5.1 . Symphony in Surround

Atmos 5.1
Model 9843

Symphony in Surround

Studio Sound June 1999
Author: Dave Foister

The full scale of what 5.1 surround can do is only just becoming apparent, along with the new methods we need to develop in order to exploit it. Faced at last with the reality of surround systems in the homes of the record-buying public, the race is on to make music work on the five speakers of a home cinema set-up.

This has given Wolfgang Neumann of SPL the chance to realise an idea going back 20 years: a dedicated surround microphone array optimised for natural pickup of the acoustic event it is placed in. The idea became hardware with the involvement of Dirk Brauner, the microphone specialist whose top-flight models have caused such a stir, and the result is a striking piece of kit that looks like a space station or a chandelier depending on your predisposition. Brauner provides the microphone array, SPL the control electronics and the whole system is known as Atmos 5.1. Early showings aroused a high degree of interest, one of the keenest would-be users being Thore Brinkmann of Swiss recording outfit Sound Arts. Thus it is that I came to be sitting in one of two temporary control rooms at the Kultur- und Kongresszentrum in Luzern/Lucerne, as Sound Arts conducted their boldest experiment yet with surround recording of an orchestral concert.

But first, back to the system. The Brauner ASM 5 array will be familiar from pictures here and elsewhere; less familiar perhaps is the SPL control console that interfaces it to the outside world. This is a 5U box, well laid out with sensible controls whose functions are readily apparent once you know what the system is doing. Although SPL is known for extremes of technology from valve to all-digital, the Atmos electronics are solid-state analogue. The microphone array is connected to its controller via a heavy multicore that carries signals, power and control voltages.

The microphones are side-firing capsule assemblies mounted on the ends of five arms that place them in positions corresponding to the conventional 5-speaker playback layout. Thus there are three fairly closely spaced at the front for left, centre and right, and two further away at the back for left and right surround. But this is not a take-it-or-leave-it system that makes you do things in one particular way; a surprising amount of fine tuning and adjustment is available to adapt the behaviour to a specific job. It might be thought that given a reasonably standard playback speaker layout, a fixed microphone rig specifically set up for that layout would do the job best, but the Atmos system aims to allow for a wide range of acoustic conditions, microphone placements dictated by circumstance rather than choice, and different arrangements of the source material - in this case the orchestra.

Each capsule can be physically swivelled from side to side (though not up and down) on its mount so that it doesn't have to just point out in line with its arm, and a great deal of further flexibility is built in to the control unit. For a start, each capsule can have its polar pattern adjusted independently. This is an infinitely-variable adjustment, not switched, all the way from omni to figure-of-eight via cardioid in the middle. Thus the centre capsule could be cardioid, the left and right wide cardioid, and the surrounds omni, if that is what suited the circumstances. From here the signals pass through five preamp modules, each fitted with the full complement of necessary functions - input gain, pad, phase reverse, high pass filters and output fader (actually a rotary pot). An illuminated vu meter at the top of each channel shows its output level. There's a neat trick fitted to allow for the problem of adjusting all the gains to achieve correct modulation without disturbing the balance of the channel outputs: the input gain pots are motorised and channel 5's can be set to be the master, all the other four following its changes.

Under many circumstances (and the Swiss concert was one of them) the preamplified microphone signals go directly to the five corresponding surround output busses for recording on a suitable multitrack. Further flexibility is added, however, by a full-blown surround panning module on each channel. Any microphone can be routed to any bus, and panned LCR with divergence control, and between front and surround. I was puzzled as to why this would ever be useful, until it was suggested that an over-reverberant room could be compensated for by panning the left and right signals a little in from the centre, and conversely exaggerated effects could be created if desired. These facilities are augmented by controls for the stereo spread of the front left-right and surround left-right pairs, each having an associated correlation meter.

Metering is also provided for the bus outputs, and the sharp-eyed will notice that there are in fact six. These allow for a sixth channel derived from a selection of the existing five, the obvious use for this being a low frequency .1 channel. With this in mind the relevant output is fitted with a low pass filter set at 130Hz, although this is switchable to allow other filter characteristics to be added externally.

Below this on the panel is a small set of controls for a two-channel external input to be added to the surround mix. It can be sent to the front or back pairs or summed to the centre bus, or any combination, and has a single gain control.

But it doesn't stop there. Each channel has its own separate microphone input to allow use with arrays of other microphones, and two insert points to allow the use of EQ and dynamic processing. As Wolfgang Neumann noted, there is a growing need for ganged 5-channel signal processing since it would be a nightmare to EQ this lot unless it were simultaneously adjustable.

Sound Arts is a three-man team of Detmold Tonmeisters, Clement Spiess (unavailable for this job), Koichiro Hattori and Thore Brinkmann. Their presence near Lucerne led to an early involvement in the new concert hall and the recording of a series of concerts given by the Lucerne Symphony Orchestra under Jonathan Nott. Nott's enthusiasm for surround coupled with the excellent variable acoustics of the hall prompted the idea of a series of recordings to establish a preferred way of capturing the performances in 5.1 surround, and after some trials with various ad hoc arrays this was to be the first with a dedicated surround system. Extra spice was added by the need to provide a conventional stereo recording for Swiss radio, and this necessitated two control rooms. Hattori was to handle the stereo mix while Brinkmann dealt with the Atmos system, with Wolfgang Neumann and Andre Indefurth of SPL on hand to assist.

The Atmos controller sat in one room, its outputs fed to an O2R in the stereo control room. Hattori created his own stereo mix from around 20 overhead and spot microphones, and the spots were mixed with the Atmos feeds on to a Genex 8000 while the stereo mix ran separately to another Genex and DAT. The O2R allowed all the spots to be independently panned to a stereo bus and to a 5-channel surround group, although the balance between them had to remain the same on both. The surround mix was returned to the surround room for monitoring, and meanwhile the raw Atmos outputs were recorded on a DA-88 for comparison with the full mix.

The surround monitoring was done on the new KS Digital ADM-2 active monitors, with features that made them in many ways ideal for the job. These speakers have both analogue and digital inputs, automatically dividing themselves into stereo pairs when fed with a looped AES/EBU input, and have global remote control of volume and input selection. For this session the digital signals from the full mix were fed to one input and the analogue output of the Atmos controller to the other, allowing simple switching between them with accurate overall volume control. The Atmos control unit was routed to the DA-88 and back again via its insert points, allowing tapes of rehearsals to be played back for checking and comparison. This also meant that the Atmos's monitor level control could be used to match its volume with that from the complete mix when the monitors were switched over.

Lucerne's new concert hall goes to elaborate lengths to provide adjustable acoustics. Its interior feels intimate for a 1900-seater, but there is a further 8000 cubic metres of space behind the walls to act as reverberation chambers. Hydraulic doors can be opened and closed to allow these chambers to become part of the acoustic space or not, and the chambers also have heavy curtains that can be drawn across their walls to control their effect. The setting for the concert being recorded gave a delightful bloom to the orchestra with a consistently pleasant sound throughout the various seating areas.

The concert programme could certainly be described as adventurous, ranging from a Beethoven symphony to Adams' A Short Ride in a Fast Machine, and beginning with a bizarre piece by Ligeti that could have been designed as a surround demo. The piece requires 100 metronomes to be set up all round the hall, set to different speeds, and started while the audience is coming in. They are then left to run down, the concert starting properly when the last one (the slowest) stops. This meant that the audience sat for around a quarter of an hour surrounded by ticking metronomes with the orchestra on stage waiting to play.

The results in the control room were remarkable. The sensation of being immersed in metronomes was completely convincing, and as the faster ones began to drop out it became possible to pinpoint individual metronomes all around the listening position. But perhaps more important is the way the system presented the orchestral works.

Many will agree that a good surround recording of an event like this will not attempt to place the listener in the middle of the orchestra, but in the best seat in the house, recreating the ambience of the hall in the listening room. It was immediately obvious in rehearsals that the Atmos system was doing just that. Newcomers to surround often expect dramatic effects from the rear speakers, but this kind of recording will rarely deliver them; indeed, it is often hard to tell just what the surround channels are contributing until they are turned off. The immediate collapse of the image into a flat line across the front is almost as dramatic as any deliberate effect would be. More than that, the presence of the rear channels seemed to enhance the depth beyond the front speakers, adding to the illusion in an unexpected way.

The big issue with any surround system is the size of the sweet spot. Some are happy to be strapped into a chair in the centre of the speakers, while others prefer a bit more latitude as to where reasonable results can be heard, and this is obviously hard to deliver. It is not, however, impossible, and the Atmos system shows this well; there may be an optimum position to hear the recording from, as there might be a best seat in the hall itself, but the reproduced soundstage works well from anywhere within the loudspeakers and even outside them. Another test of a surround system is whether the individual loudspeakers draw the ear, and the combination in use here passed that one too. With everything running properly, the loudspeakers disappeared, leaving a seamless sound stage enveloping the listener.

The ability to add panned spot mics into the Atmos signal without special equipment, and without compromising the surround image too much, is a significant benefit of the system. The mixes with and without the spots both had merits; the spots inevitably added touches of detail, but the Atmos alone won for me in terms of the realism of the surround and the sense of space and depth. The Brauner microphones are superb, helping to deliver a memorably convincing impression of the actual acoustic experience.

The conductor, Jonathan Nott, was similarly impressed, sharing the view that the Atmos system as it stands was all that was required for a faithful rendition of what he was hearing as he stood in front of the orchestra. Sound Arts plan to try further ideas in the magnificent Lucerne hall, but the Atmos package certainly proved itself on this occasion. As a dedicated surround microphone system it is not alone, but has perhaps only one serious competitor, and its simplicity and flexibility make it very attractive to those who want to exploit the growing surround market for serious musical recording.