HD routing

When considering an HD routing infrastructure build-out, you'll need to be able to answer these four questions: What specifications do I need to consider?
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When considering an HD routing infrastructure build-out, you'll need to be able to answer these four questions:

  1. What specifications do I need to consider?
  2. What are my options and considerations to weigh against my financial budget?
  3. Does the actual picture format (1080i or 720p) matter?
  4. Are there any special timing considerations for HD?

Before we find the answers to these questions, let's review the background concepts of SDI.

SDI background

There is a serial signal transport standard for routing SD digital video signals (SMPTE 259M) and a different, but similar, transport standard for HD signals (SMPTE 292M). These standards' similarity allows HD infrastructure to transport SD signals. The similarities include:

  • Electrical interface — 75Ω BNC, unbalanced signal over coaxial cable.
  • Signal level — 800mV PP signal (+/-10 percent) and 0.0VDC offset (+/- 0.5V).
  • Channel coding — scrambled NRZI.

Because HD and SD share this same basic electrical interface, circuit chips and finished products can be designed to handle both SD and HD. Any remaining differences in the specifications can be handled by equipment designers.

The key difference between the SD and the HD serial transport standards is the data rate of the serial bit stream. The SD rates are from 143Mb/s to 360Mb/s, and the SMPTE 292M HD rate is 1.485Gb/s. Both of these standards are designed as funnels in which there are multiple source formatting standards that can be turned into a compliant SDI signal. Examples of these standards are the NTSC 4fsc composite sampling for SD (SMPTE 244M) or 1080 line video (SMPTE 274M) for HD rates.

Specifications

Because the basic characteristics of SD and HD transport are similar, what aspects of the specifications are noteworthy for consideration in planning for HD? The first issue is cable length.

Today, 100m of coax cable is typically specified by HD transmission gear as the equalization length for HD. Equalization is the process of inserting specific high frequencies back into the signal to recreate a good signal.

Equalization is possible because the transport situation is well-defined and consistent. The signal transport medium is known (a coaxial cable with specified characteristics), and the signal being transported is a fixed style.Therefore, the filtering action on the signal can be reversed or equalized.

If you have an infrastructure that requires cable runs for HD equipment longer than 100m, what options do you have? By choosing the correct cable types, you can increase the cable runs about 20 percent to 30 percent.

The 100m equalization specification typically calls for Belden 1694A or equivalent cable. Belden 7731A or equivalent can increase your reach, but if significantly longer cable runs are needed, the best option is to turn to fiber.

Fiber signal transport options for HD signals are readily available. With fiber, the distance of the signal runs can be anywhere from 1km to 60km, depending on the device used. Fiber transport capabilities center on two criteria: signaling type and carrier frequency of the laser. There are two signaling types: single- and multi-mode fiber.

Multi-mode fiber has a larger pipe diameter (50 micrometers) and thus allows for multiple paths for the light to travel (hence the term multi). Single-mode fiber has a much narrower pipe diameter (9 micrometers) and essentially has only one path for the light to travel. Because of the many paths and resulting modal dispersion, multi-mode fiber has more signal attenuation and thus travels shorter distances than single-mode fiber.

The second aspect that has a major impact on the distance traveled is the carrier frequency. Fiber transmission on glass fibers has three main frequencies that are used. These frequencies are used because they fit into the locations where the least amount of light is absorbed. The three windows are 830nm, 1310nm and 1550nm.

The 830nm window is old technology and not found in new equipment. The 1310 window is used with multi-mode transmission to create cost-effective gear, sufficient for use in distances up to 10km. The single-mode fibers and the 1550nm wavelength are used in combination to push the distances up to the 30km range. There is gear on the market that can reach 60km distances while transporting HD-SDI signals.

Costs

It is also important to evaluate the total cost of ownership of an HD-SDI routing infrastructure. The total cost of ownership includes the original price of the equipment, the day-to-day costs, and the maintenance and repair costs.

Because of the similarity in the SD and HD transport standards, there is plenty of transport gear that ranges from SD-only to SD/HD-capable. The original purchase cost of HD today is typically about 25 percent to 35 percent more than SD-only gear.

This cost can be mitigated for those who need SD-only today but want to be HD-ready. The minimum required plan to prepare for HD is to have your cabling and your frames HD-ready.

The next step is to evaluate how much of your circuitry should be HD-capable today. For example, most of today's routers use a three-board architecture. This includes an input board, a matrix or crosspoint board and an output board. To save money, you could pay for the crosspoints to be HD/SD capable and select SD-only I/O boards.

One key consideration in choosing SD/HD gear in routing equipment is that the equalization distances are 300m for SD and 100m for HD. In some HD/SD gear, the equalization specification drops to 100m for SD and HD. Evaluate your signal transport layout to determine if you require the full 300m specification for your SD runs. If that is the case, seek out gear that allows for the dual-rate equalization and meets both the SD and HD specifications.

The operating costs for HD signal transport equipment is essentially the power consumption costs. Today, in many cases, you can expect not to pay a power premium for HD gear. However, a quick analysis can show that it is worth looking at the power consumption costs.

For example, if you are evaluating products where the total power delta is 1000W, you can expect to save about €1200 to €1500 per year in power and cooling costs. For 10 years worth of service, you can save as much as €15,000.

Picture format

The SMPTE 292M serial digital interface standard accepts all the common picture format scanning and sampling standards. There are 13 formats outlined in the standard.

The transport was designed to handle 1.485Gb/s, and all picture formats were designed to create 1.485Gb/s or 1.483Gb/s. The second rate is a result of accommodating the frame rates of both 30Hz and 30/1.001Hz, or 29.97Hz. To have all these picture formats achieve an equal number of bits, the horizontal ancillary data spaces are adjusted.

Thus, for signal transport reasons, products that conform to SMPTE 292M will handle any of the SMPTE standard picture formats. The real consideration in this case is to evaluate the gear for its synchronization and timing capabilities.

Synchronizing and timing

The frame rates available in HD picture formats are 24fps, 25fps, 29.97fps, 30fps, 50fps, 59.94fps and 60fps. The signal routing infrastructure needs to be able to create switch points at all these frame-timing rates. This is a vendor-to-vendor implementation decision — not something mandated by a standard. For this reason, you will need to be sure the routing gear you purchase has sufficient support to switch on a frame-rate basis.

If your situation will have multiple picture formats, you must also evaluate whether the routing gear will be able to support multiple switch points. The two most common situations are when 29.97fps and 25fps content are both included in the signal infrastructure and when SD and HD are in place.

Bringing the system together

A little bit of planning and early system design work will go a long way toward successful and cost-effective implementation of a reliable, multiformat HD routing infrastructure. Knowing your business and how your company wants to implement new digital signal distribution paths is critical to avoid wasted time and money on equipment you may not need.

Mitch Hayden is the director of product management for routers at Grass Valley.