The Last Round Up

Reproduced with kind permission from Audio Media, Feb 1999.

DESIGN CONSIDERATIONS FOR MIXING SURROUND FOR ADVERTISING

Most design work, and most of the theory for surround sound, has previously been associated with pure music or sound for film. If you take mixing audio for commercials in surround to its logical conclusion you might end up with a studio like Soho's Grand Central Studio One with its 20 monitors. JULIAN MITCHELL reports, and designers DAVID BELL and MATT DOBSON explain how and why the studio turned out the way it did.

We may all be convinced, to some degree or another, that mixing sound in surround is a major step forward, but the problem lies in convincing the clients. Music studios have been known to offer free surround re-mixing when a record company books for a stereo mix - one in the bank for the inevitable re-release on DVD. The advertising world has its own set of criteria when dealing with the client, and the idea of selling a room to mix your commercial in surround exhausts all of them and probably introduces some new ones. Take it for granted that the advertising types know little or nothing of the excitement that is surround mixing; the selling of a good soundtrack itself is usually difficult enough without introducing a new concept to them.

It is with this knowledge that Grand Central Studios in London's Soho started to sketch out how a surround room could work physically and, perhaps more importantly, business-wise. A problem for the agencies was to sell the concept on to their clients who may well be operating under the same degree of surround sound ignorance. Grand Central's hope is that the experience of a film seen in surround would be enough to persuade advertisers to want their product looking and sounding as good as the main picture. Ironically, one of the main problems the agencies face is the cinema's engineers actually checking that the material is being played across all the speaker channels; this is apparently a huge problem and may be a by-product of how cinema commercials have been sold up to now.

Studio One
As with most of the studios at Grand Central, the allowance for the client is prized above all else, and this philosophy is the basis for Studio One. As a studio that is intended for voice recording as well as mixing in surround, the line-of-sight to the VO was not to be compromised; this becomes difficult when, later on, you will need a centre channel for mixing. The solution: a monitor that lowers and raises!

On some mixes the clients can out-number the engineer ten to one, and nobody is ever told that they can't attend, so sofas have been placed in front of the desk as well as behind. With the selling of the surround concept in mind, however, each area has its own set of monitors or the ability to have the main system reconfigured to be optimal for that area or both. The engineer also has three sets of nearfield monitors 20 monitors in all!

The Design
The design requirement for Studio One was that all surround monitoring systems needed to be accommodated whilst allowing uninterrupted viewing for engineer/producer/artist voiceover work in stereo. There were to be two large client areas; one in front of the mixing desk and one to the rear of the room. A range of outboard equipment and a large keyboard were also required in the room close to the console.

In association with interior designers AKA Design and the clients, the room was laid out and the detailed design of the acoustics and monitoring system commenced.

The monitors, designed by Matt Dobson, consist of five full-range units configured as four-way boxes. These are based on the 15-inch ATC 5B75-375 SC, the nine-inch ATC SB75-234 LC, the ATC SM75-150S soft dome mid-range and the ATC HD13D34 soft dome tweeter. Additionally, four further full-frequency, passively crossed over two-way ATC A7 units are mounted towards the rear and sides of the room. One pair of the smaller units is situated in the rear wall, radiating directly forward, and the other pair are mounted in the ceiling facing the side walls upon which are two diffuser arrays. All amplification is by Chevin Research Q6 35OW into 8* models that are configured as four-channel units mounted in the machine room.

Moving Monitors
The monitor array is controlled from a centrally located touch screen. This communicates with the serial control inputs of a group of digital crossovers via a PLC system configured by specialist designers Industrial Control Solutions. The crossover units are BSS FDS 355 digital models and have their control inputs cascaded to allow the whole set-up to be addressed serially from the PLC. Each main loudspeaker driver has its own dedicated BSS channel that is capable of muting, delaying or equalising it. Thus any set-up can instantly be recalled from the control room and applied via the crossovers to the monitor array as a whole.

The placement of monitors in the room has been carefully considered. The central loudspeaker unit lowers from the ceiling, behind the motorised projection screen, in order to allow uninterrupted vision to the voiceover booth when working in stereo. This three-way cabinet is matched to those for the left and right front and left and right rear units and carries frequencies from 1OOHz upwards. Two closely positioned cabinets mounted at a low level handle the low bass for each of the left and right front units. Two matching cabinets handle the bass for the centre channel. The rear left and right loudspeakers have similar low bass boxes that are engineered to a slightly different shape but exactly the same performance criteria. These are mounted to either side of the rear higher frequency cabinets. The horizontal plane of the front loudspeaker array is such that the high-frequency drivers are all aligned to the optimal 5/8s of screen height. The sub-bass information is distributed across all of the bass cabinets to prevent any chance of localisation.

Rear
The rear two A7 full range units radiate directly forward into the room from high on the back wall above the main rear diffuser array. The side two A7 units are positioned approximately three feet from the edges of the room and radiate towards additional side wall-mounted diffuser arrays. This allows them to produce a very diffuse radiation pattern originating from an acoustic point beyond the walls of the room. Care has been taken that minimum direct radiation from the boxes should be monitored from any seating position within the room, and the balance between the rear direct radiating units and the side diffuse ones was carefully considered. Thus it can be seen that the room can operate in many modes including:

• Stereo - only the front main monitors left and right operational. There are two further stereo nearfield choices accommodated with monitors mounted at the desk.
• Four-way - three front full-frequency monitors driven with a mono rear channel. This can be configured to be from any combination of the two small units in the rear, the two main units in the rear and the two diffuse side units. These can have equalisation and delay applied to them in any combination required.
• Five-way - as above with the rear channels split left and right for stereo surround. This, again, can be configured with any of the rear drivers in use and accommodates all of the standards that require stereo surround, be it diffuse or direct and matching the front units or band limited.
• Sub bass (.1) - any of the above standards can be supplemented by the addition of the sub bass channel, which is fed to all low-frequency cabinets to avoid any auditory localisation.

The room geometry has been set to minimise early reflections and produce an even decay time with frequency. The diffuser elements are RPG Skyline units manufactured under license from RPG Europe. Bilateral symmetry has been maintained as far as possible within the room.

There are two client seating areas; one to the front of the room and one to the rear. The AMS Neve Logic 2 console is mounted centrally in the room with its level being raised one step with respect to the front seating area. The rear seating area is raised a further two steps in level.

Local Delays
The room has been tested by Dolby and full 5.1 and Dolby Stereo accreditation has been awarded. This allows the Dolby acceptable curves to be applied when that set-up is requested, not when other standards are worked to.

As surround monitoring is very delay-critical to the listening point, the system has one further advantage. Whilst all set-ups are optimised for the engineering position (for the obvious reason that this is the position of the most important listener with respect to getting the final sound balance at the conclusion of the session), this has exclusionary effects on all other locations within the room. This has been addressed by providing extra set-ups to the main choices calibrated for the engineer but optimised for the two other significant locations in the room. Thus the client, who makes judgements on the final mix from his/her sub-optimal position on the couches to front or rear, can be given a measure of greater accuracy in the audio that is presented by selecting a setting based on her/his location. This is instantaneous, silent and just as accurately calibrated as all other settings in the room.

Great care has been taken to allow the room to be as flexible as possible and to allow multiple, repeatable and carefully calibrated choices as to the set-up chosen for each session or, indeed, for different standards satisfaction within one session.

Monitor System Development
The Studio One main monitor system was the result of having to meet the client's very specific requirements while providing the flexibility to meet multiple, and sometimes seemingly conflicting, format specifications, together with the recording engineers' preferences. The approach taken involved combining different surround sound philosophies into a reconfigurable system, controlled by selecting one of several pre-programmed set-ups. Once it was accepted that, to achieve this, all sound sources had to be programmable in terms of level, muting, time delay and equalisation, several new possibilities became apparent.

Components And Specifications
Table 1 shows the components used in the system. This uses a total of 24 channels of BSS FDS 355 digital crossover and 24 channels of Chevin Research Q6 amplification with a total R.M.S. output power of 8.4kW.

All drivers are direct radiators, the LF and sub cabinets comprise 15-inch reflex loaded drivers, the MF/HF cabinets comprise a nine-inch LM cone driver in a sealed enclosure, HM is a three-inch soft dome and HF is a 34mm soft dome. The full-range surround cabinets comprise passive five-inch LF in a sealed enclosure and one-inch soft dome HF. Whilst the LF cabinets are all identical in volume and tuning, the two rear cabinets have a different shape to the front four. The front L & R and rear L & R LM/HM/HF cabinets are identical, the front centre cabinet is of a different shape (but identical volume) as it is raised into the ceiling when the room is being used for over dubbing. It therefore required a shallow but wide physical shape. When in use, the centre cabinet is lowered into a position where it is in contact (via an isolating foam slab) with the glass of the booth window to maintain the effective front wall baffle.

Operation Details
The BSS FDS 355 digital crossovers are linked to a Studio Systems computer that has a touch-sensitive selection screen. Programs are available that optimise the system for different seating positions in the room, select the correct time delays for Dolby Stereo or Dolby Digital operation and allow the engineer to use point source or diffuse source rear channels. There is capacity for several additional set-ups when required.

The additional surround channels comprise one pair of L&R full-range cabinets mounted in the rear wall and one pair mounted at ceiling height firing onto a 3-D diffuser array on the side. The concept of pointing a full-range loudspeaker away from the listener and towards a diffuser array which is, in turn, directed towards the listener, derives from work done by Peter D'Antonio of RPG Systems and has been executed several times previously, with varying degrees of success. The diffused sound source does provide an excellent spread of sound and greatly improved perception of panned sources behind the listener but suffers from frequency response limitations caused by the bandwidth of the diffuser and differing degrees of reflected versus direct response over the frequency range.

In Studio One, the concept was to mix the diffuse image with the direct image from the speakers on the back wall, in degrees determined by experiment, while the total sound level remained constant. This was made possible by the programmable nature of the system control and an excellent result was reached when some 70 percent was from the diffuse source and 30 percent from the direct source. Further improvement was attained by complementing the LF of both with a specifically equalised feed to the LF section of the main rear speakers. All sources are time delayed to be coherent at each selected listening position. The decision to mount the diffusers on the side walls as well as the back wall provides the additional benefit of a perceived increase in room width. In point source mode, these sound sources are muted.

In addition to the above main monitor system, there is a further surround sound system at the front of the room. This is used with the wide screen TV mounted below the booth window and consists of five Genelec 1029 self-powered monitors, three in the front wall and two surrounds mounted behind the front sofa, just in front of the console. This system allows the mix to be checked on a system more typical of a home surround system and is a great success with the clients.

Lastly, for traditional stereo work, the engineer has the use of a three-way active nearfield system, based on the unique Bronze speakers developed for Grand Central. This is complemented with a small Fostex two-way active nearfield and, last but not least, a portable TV with the usual poor quality mono speaker for final checking of the mix in a worst case playback situation.

In designing the system, the arguments between separate feeds to each of the five full-range channels and summing these feeds below a certain frequency to allow the room-dependent LF mutual coupling to be equalised fiat, were carefully considered. It was decided that the feeds would not be summed but that provision for this would be made. Crossover frequencies for the four-way cabinets are 100Hz, 500Hz and 3.3kHz. The sub bass is set up to the usual Dolby room curves. Dolby Digital and Dolby Stereo licences have both been issued for the room.

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