Surround Sound for Post Production - White Paper

SURROUND SOUND FOR POST PRODUCTION: A REAL WORLD DESIGN APPROACH

Since the formation of White Mark Limited , the company has been involved with many rooms designed for use in a surround mixing capacity. We have been almost exclusively occupied with the reconciliation of operational opinions on how these rooms should be configured, constrained as always by the limitations of real buildings. The imposition of additional criteria by the various format licensing bodies also adds to the equation, with the various different stipulations often being at odds both with each other and with many of the modern thoughts on studio design. 

Whilst there has been much discussion in the music recording community about the possible methods of taking advantage of the opportunities afforded by the emergence the new technologies, it is in the area of post production where most of the real surround work is actually being done. It is therefore post production studios, working on both features and commercials and originating product for release into cinemas and for television broadcast, who have been confronting these issues directly for some time. 

These studios have clients who require that the surround mixes of their work excel both in the surround equipped cinemas, even when placed amongst the finest feature film material, and on the largely stereo television set ups in the domestic market. Whilst there is a current battle to get commercials shown in the cinemas in the correct sound format, there is none the less a strong and immediate need for the studios that originate this work to achieve the highest standards in limited time and with great creativity. It is not unusual for a commercial reel to arrive totally silent, the music, effects and dialogue needing to be sourced and mixed from scratch in a matter of hours. This real commercial pressure has led to the design of rooms that have to accommodate all of the various standards of surround sound and also work well in stereo. This means not only achieving a high level of performance for engineers and producers, but also includes client areas; all of this whilst satisfying a number of separate and fundamentally different surround standard licensing companies. This licensing requirement is necessary in order that clients can be happy that their commercials are acceptable for issue into the cinema distribution chain and so that the mixes will be dealt with correctly by the end user cinemas. 

White Mark Limited has recently been involved, in conjunction with monitor designer Matt Dobson, in the engineering of a control room and monitor system to meet these exacting requirements for a top flight commercial post production house in London’s Soho. The requirement was that a comprehensive surround monitoring system be accommodated, whilst allowing uninterrupted viewing for engineer/producer/artist voice over 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 as shown in the two drawings and the detailed design of the acoustics and monitoring system commenced. 

The monitors designed by Matt Dobson, for Grand Central Studios, consist of five full range units configured as four way cabinets. These are based on the 15-inch ATC SB75-375 SC, the 9 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 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 within which and forming the acoustic boundary of the room are mounted two diffuser arrays. These diffuser arrays serve to widen the acoustic dimensions of the space and, very effectively, cause these surround monitors to become diffuse sources. Alternative methods of achieving this can involve the use of dipole devices but these solutions have yet to prove popular with engineers and clients (to say nothing of the licensing bodies). All amplification is by Chevin Research. Please see the drawings for clarification of these locations. 

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 programmed logic controller (PLC) system configured by specialist designers, Industrial Control Solutions. The crossover units are BSS FDS 355 models and have their control inputs cascaded to allow addressing of the whole set-up serially from the PLC. Each main loudspeaker driver and passive unit has its own dedicated BSS FDS 355 , capable of independent control over muting, delay or equalisation. 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 voice over 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 120Hz upwards. Two closely positioned cabinets mounted at 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. Sub-bass is handled by all low frequency units to maximise headroom and minimise any effects of localisation. The rear left and right loudspeakers have low bass boxes that are configured to fit a slightly differently shaped space but which achieve an identical audio performance to the front wall units. 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 closely aligned. 

The smaller full frequency passively crossed over units are mounted as shown. The rear two units are radiating directly forward into the room from high on the back wall. The side units are positioned approximately three feet from the edges of the room and radiating towards the side wall mounted diffuser arrays. This produces a very diffuse radiation pattern originating from an acoustic point beyond the walls of the room. Care has been taken that no direct radiation from the boxes should be monitored from any seating position within the room. 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 near field 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 distributed to all of the low frequency boxes. This allows for both good headroom capability and minimises any of the unwanted sub bass localisation often encountered in small rooms.

The room geometry has been set to minimise early reflections and produce an even decay time with frequency. The diffusor elements are RPG Skyline units manufactured under license from RPG Europe. Bilateral symmetry has been maintained with the room by matching the hard surfaces caused by windows and doors on opposite walls. The geometry and extent of these surfaces has been controlled to minimise both early and lateral reflections which would be damaging to imaging and response of the room.

As can be seen from the elevation drawings, there are two client-seating areas, one to the front of the room and one to the rear. The AMS Neve Logic 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.

The room has been tested by Dolby and full 5.1 and Dolby Stereo accreditation has been gained. For this there 1/3 octave equalisers were added to the main monitors but these, although still in circuit, were circumvented by the in house equalisation capability in the BSS units. Reverberation time is well controlled and even across the frequency spectrum, whilst imaging is both solid and maintained over an extended area. 

As surround monitoring is very delay critical to the listening point, the system has a further advantage. Typically, room set ups are optimised solely 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 invariably and, as with all control room design practices, has slight but definite exclusionary effects on other locations within the room. During the project, this issue has been addressed by providing extra set ups. These are identical to those calibrated for the engineer but are optimised for the two other significant listening 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 his/her location. This is instantaneous, silent and as accurately calibrated as the other room settings. Thus when asking for an opinion from the clients or other contributors, the engineer can select the set up that gives that person the ability to comment based on a correctly aligned sound-field. 

For further information see:

• White Mark Clients: Grand Central Studios (Studio 1)
• White Mark Clients: Grand Central Studios (Studio 3)
• White Mark News: The Last Round Up