Earthing and Screening - White Paper

EARTHING AND SCREENING: DESIGN CONSIDERATIONS

Much has been written about the options that exist when designing a technical installation for a studio. Choice of cable, number of connectors in any circuit path, distribution and cabling of microphone connections in the live room, distribution and flexibility of clock and other synchronisation signals are but a few of the contentious areas that provoke debate. White Mark Limited has defined the way it approaches all of these areas and also what is perhaps the most contentious, but fundamentally one of the most important, areas of discussion: earthing and screening. 

A comprehensive approach to earthing is taken throughout the design of our studio system installations. This is reflected in the Electrical Specification documents issued at the inception of all projects and is reflected in all subsequent documentation. It is centred on an essentially simple set of rules on the basis that it can be easily understood and realised and that allows any earthing anomalies that arise during the connection of user equipment to be easily resolved. Normal use of the installation should never result in earth problems and screening effectiveness is maximised. More importantly for both the studio owners and ourselves it minimises the uncertainties when connecting large quantities of hired in or client equipment to the studio systems and therefore cuts out the mid-night calls for help from distraught technical staff! 

A single earth point in each studio should be defined by the bonding to an earth distribution bar in the machine room of a direct (home run) low loss earth cable. This should run to the incoming earth point of the building which, in turn, should be connected to a significant metal object buried deep in the earth such as an incoming water pipe, steel piling or preferably, a purpose laid earth spike. All technical mains supply earth pins should be run to this point as should the earth star point of the console. It is a matter for the mixing desk supplier to decide if the earth point is connected by separate earth wire or via the earth leads of the power supplies. 

Screening is generally projected with the output of a signal and tied back at the input. Thus machine sends from the console are tied down at the DL/EDAC/ELCO (or other) interface and tied back at the tape machine panels whereas machine returns are tied down at the machine panel and back at the console DL interface. This is generally true of all sends and returns. Thus machine to machine dubs patched through the machine panels will be over cables screened, by default, in an exactly consistent way. Experience has however taught us to allow one exception to this scheme and this is in the case of the distribution of timecode cables from the machine room patchbay. 

All timecode connections are earthed from the patchbay end due to the dubious earthing schemes and unbalanced nature of some equipment used in this area. Thus all screens are bussed at the jackfield and tied back at the remote connectors, wherever their destination. Similarly data and control ties are screened from their central patch ends, being tied back at the remote connectors. Bussed earths are run back via earth cables to the central earth bar. Inter area ties are screened as in the above input/output method. Inputs are tied back and outputs are taken down. DL ties are carried through as the source/destination relationship will be fixed at their respective destinations and sources. 

Extensive and unambiguous documentation of the general policies and specific techniques used in an installation are incredibly important; connectors are listed individually in the wiring schedule with screening and other pertinent information. Adherence to this simple but thoughtful screening plan results in a loop free installation and superior low noise performance of all equipment connected to it. 

A significant exception to this scheme is the Sony OXF R3 console. Full flexibility of this console’s systems dictates that the internal phantom power be available to all inputs. This in turn dictates that the screens are carried out on the input side and so we have successfully reversed the standard scheme described above with results in both New York and at Real World that measured exceptionally well. Anomalies arise in the paralleling of feeds to power amplifiers and care was taken that these were earthed at only one point. The input earth was disconnected at the amplifier connectors and taken down to a local ground at one point. 

Whereas experience has shown us that this scheme is the best one we have come across - it has been used in over 30 major rooms with consoles as diverse as SSL, Neve, SONY (digital and antique), Yamaha and Euphonix - we also draw on the large body of learning that exists in this area for its justification. The June 1995 Issue of the AES Journal [volume 43 number 6] draws together much of the finest writing on the subject. This includes what in our opinion is the definitive system diagram of a studio wide installation 1 & 2. Other collected works in this volume include illuminating tests on cable types for external electromagnetic interference rejection 3 and a number of investigations as to the correct implementation of balanced and unbalanced equipment connection 4 and equipment design 5. 

White Mark is always keen to assimilate experience on these issues and welcomes extensive input from technical managers at the inception of projects to clearly define house and interfacing standards where new studios are to be integrated into existing complexes or where particular operating practices already exist. 
 

1. "Fundamentals of Grounding, Shielding and Interconnection" by K.R.Fause. 
2. "Grounding Systems and Their Implementation" by Atkinson and Giddings. 
3. "Noise Susceptibility in Signal Processing Systems" by N.A.Muncy 
4. "Considerations in Grounding and Shielding Audio Devices" by S.R.Macatee 
5. "Balanced Lines in Audio Systems; Fact, Fiction and Transformers" by Bill Whitlock