It would be easy to assume that the least important technology in a broadcast facility is the piece that only serves to keep equipment from falling to the floor — the ubiquitous equipment rack. It is metal, does not (normally) move and must match a well-defined standard. Henry Ford would have painted them any color as long as it was black, and indeed most racks are black or grey.
Chief engineers often have a strong preference for one brand or another, and additions to an older facility must match. For that reason, over the years, manufacturers have created extremely stable product lines with the same appearance and function. But in this age, as technology transitions significantly, there are consequential changes in the way we mount, ventilate, wire and power equipment.
Standard rack dimensions
The 19in rack is standardized (EIA-310-D, IEC 60297 and DIN 41494 SC48D) across many uses. From home theater to power plants to broadcast, everyone worldwide has adopted a common standard. The Electronic Industries Association has specified the dimensions for the front-mounting surface of racks. The standard is available for purchase on the Web, but here are the most critical dimensions, some of which are quite familiar:
- The panel mounting flange is 19in exactly.
- The holes repeat in a regular pattern (0.625in, 0.625in, 0.500in) every 1.75in, commonly called one rack unit (abbreviated RU).
- The left-right spacing between holes is specified as 18.312in, or 465mm.
- The clear space between the mounting rails is specified as 17.72in, or 450mm.
For the most part, these dimensions are all one needs to know to build a rack. How cable access is provided, rack depth, construction, structural integrity and other matters are left to the manufacturer. Usually there is space outside the mounting area for power and signal cabling, as well as air movement. Wire access is often planned both top and bottom. The screw holes are not specified, but 10/24, 10/32, 12/24 and clip-style holes are common, and their spacing is always the same. Things we often take for granted, like rear or intermediate rails, power strips, cable lacing bars, wireways and structural rack attachments are left entirely to the creativity of the manufacturer — at least almost.
Other specs that affect the design and installation of racks include earthquake protection, grounding, structural weight loading, etc. Some of these are specified in local ordinances. For example, in California and other earthquake-prone areas, rack bases must be attached to the structure to prevent entire rows of racks from tipping over when the building structure moves beneath them. The details of this can be quite complicated, and often a structural engineer with appropriate expertise is needed to review plans before an installation can proceed.
Modern technology has made miniaturized equipment attractive and practical. There is, however, a pervasive downside of this trend. Equipment mounting — particularly deep computing servers, power density and wire count — has changed the function of equipment racks. It is not unusual to have as much as 12kW of power draw in a single equipment rack full of servers. This presents engineering challenges for both power distribution and cooling.
Effective strategies include building a plenum for removing heat, but even then the density of the equipment and the high temperatures that can result lead to the conclusion that high-density racks, which worked for video facilities with loads of cable but manageable heat loads, may no longer be acceptable design practice. IT server racks sometimes include intermediate narrow racks between populated racks, which contain fans to help evacuate the heat. By building a hot side, cold side approach (enclosing the rear of the racks and forcing cool air in from the front and out the back into a low pressure return duct area) and keeping the front of the racks at cooler than normal temperatures, it is possible to considerably drop the temperature inside the racks.
This can help to solve other issues as well. With systems routinely requiring as much IT gear as baseband video equipment, it is common to deal with large bundles of fiber and Cat 5/6 cable. Handling these cable types is quite different than bulk quantities of video and audio cable. Technicians must be trained in proper installation techniques, with controlled bend radius and less use of tight cable ties required. Often it is best to separate the fiber and data cable in distinct cable paths to make handling effective. With intermediate racks designed to handle cooling, it is sometimes possible to create more useful wireways for all cable types.
Of course, with high-bandwidth video systems, like the newer systems capable of 3GHz (SMPTE 424M-2006), it is critical to manage cable with similar effective control over bend radius and anything that might deform the cable and create return loss issues. Some routing switchers today are capable of handling more than 1700 cables in a single 32RU frame, and almost 2400 in two adjacent racks in a 1152 × 1152 matrix, so cable management is a huge and growing concern. Deep racks can help make space for cable, and intermediate racks mitigate cable density and heat loading problems.
Beyond the engineering of effective equipment room rack layouts, the rack itself has become a source of innovation in the last decade. Racks that hang on walls and pivot for service are handy for studio systems, or perhaps for wireless microphones. In tight spaces, there are racks with bases that can be pulled away from walls and even rotated. These specialized racks come with limitations, like restricted use in earthquake zones and lower weight loading capacity, but offer utility not available in the past.
Finally, there was a time when broadcasters had a choice of consoles made from slope-fronted racks or custom-designed furniture, which usually cost an order of magnitude more. In a move that comes full circle, some custom control room furniture suppliers now offer standard designs, which provide great utility at a more modest cost by leveraging standardized partial assemblies. The availability of custom features at a more modest cost is certainly attractive, and in a complementary move, some rack manufacturers now offer more appealing and functional metal products to compete with fully custom furniture. The biggest advantage of both of these approaches is that they offer attractive and operationally functional products that are designed to handle the mounting of computers and peripherals, as well as cable management and heat load issues, in an effective manner.
John Luff is a broadcast technology consultant.
Send questions and comments to:firstname.lastname@example.org