Routing switchers - TvTechnology

Routing switchers

Video routing is still the hub of all broadcast and production centers. An examination of multi-format, HD and SDI routing techniques and technology
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Grass Valley Trinix digital video router

Signal types

Routing switchers are a key element of modern video facilities of all types and sizes. From single buss switchers to behemoths as large as 2048×1024 capable of switching one to many levels, these routers represent critical elements without which few facilities could operate. They provide many approaches to workflow and technical design, simplifying what would otherwise be far more complicated, and in some cases impossible. Thus, it is appropriate that we review the state of the art periodically.

Analog routing switcher technology dates back at least four decades, and serial digital routing has been available since shortly after SMPTE and EBU published the specifications for the SDI interface known generically as SMPTE 259M. SMPTE 259M defines a copper interface on coax and includes specifications for levels, jitter, connector type (75Ω BNC) and other important data for equipment manufacturers. HDTV signals with a data rate of 1.485Gb/s are similarly defined in SMPTE 292M.

Another important signal to note are MPEG compressed signals, which are often carried over a DVB-ASI infrastructure at 270Mb/s. With a shorthand nomenclature of ASI, it has the same bit rate as SMPTE 259M, but is coded as NRZ, as opposed to SMPTE 259M, which is NRZ-inverted (NRZI). What is important is that NRZ signals are polarity sensitive. Most common digital hardware can pass ASI today, but caution is always wise when making that assumption.

The audio bugaboo

SMPTE 310M defines a lower bit rate (19.3Mb/s or 38.6Mb/s) explicitly for carrying ATSC bit streams, which also can fit in many routing switchers. Together these standards, along with composite analog 525 and 625 NTSC and PAL, represent the majority of digital signals that end up being created or passed in a modern plant.

There are many other types of signals, most of which can be properly routed with today's multirate switchers. Even so, it is important to know exactly what types of signals your router will encounter so bandwidth requirements, connectors, format and signal conversion, and analog and digital factors can all be carefully considered before making a selection.

With today's programming video, audio is no longer a second-rate signal, or as video engineers used to say, “The noise that accompanies the picture.” Rather, audio is often equally as important as the image. Audio routing can be handled by embedding it in the SDI signal (up to 16 tracks in an HDTV signal) or by carrying it as a discrete signal. Both analog 600Ω audio and AES-EBU digital audio are common, with AES over coax as an unbalanced 1V signal or on 110Ω twisted pair interface when desired.

The ability to route signals like these obviously requires sophistication and care to insure the integrity of the signal. All signals should be switched synchronously if possible. So long as all signals are locked together, that is imminently doable. Video signals are switched on line 10, but AES signals must be sample aligned to make noiseless switching possible. Techniques, sometimes called soft switch for AES, are often employed, which perform a brief cross fade to avoid instantaneous impulse distortion in the reconstructed analog audio.


Routers, like this one from Pro-Bel, offer the ability to handle analog and digital audio as one interconnected standard.

However, with today's increasingly complex audio mixes, simple XY switches may not be sufficient. This has lead some audio routing equipment manufacturers to develop time domain multiplexing technology. These routers offer interesting possibilities, including the ability to pull an AES pair apart and treat them as discrete mono signals, which can later be recombined or even mixed within the router. This is a powerful feature for several reasons, but when combined with an input circuit that allows for analog inputs, the implication is huge.

New video solutions

Such routers, available from Grass Valley, Leitch, NVISION, Pro-Bel and others, effectively allow a facility with both analog and digital audio interfaces to act as if it were all operating on one interconnection standard. Legacy hardware needn't use converters, multiple levels of routing, tie lines or other approaches. This can be quite effective as facilities transition from legacy analog to digital systems.

Embedded audio offers another way to simplify a routing structure. Large facilities, where audio and video are treated as a single program element, can simplify from multiple levels. There is, of course, a down side: The cost of embedding and de-embedding hardware necessary for some sources and destinations can offset part of the gain in cost efficiency a single level digital video router might offer. In addition, it is important to look at the total cross point count needed for each signal type. If only a handful of audio sources are needed with analog I/O, it might be best to embed. However, if a large amount of conversion hardware will be needed, it may well be cheaper to just buy a second level for the router. It's important that a thoughtful analysis be undertaken before making any decisions here.

Optical and compressed signals

Similar to the above audio router designs, several manufacturers offer internal conversion of analog and digital signals in their video routers. In general, these SD video conversions are acceptable for monitoring needs. However, it is best to evaluate performance of these circuits to be sure that the quality of the internal conversion meets your monitoring needs. Some products may use less sophisticated conversion for both space and cost reasons. If high-quality conversion is needed, it may still be best to use external high-quality converters. Even so, this approach can have significant impact on monitoring, permitting the use of lower-cost monitors with analog inputs to view digital signals in a mixed format router.


The LDS Church Conference Center in Salt Lake City uses two interconnect-ed NVISION 8256-Plus 256x256 expandable video routers.

Large systems made up of multiple frames usually require input distribution amplifiers. Though not unique, Grass Valley offers passive splitters and combiners with internal circuits that can recover the gain lost in the splitters. This is cost-effective and can be done as part of any installation by purchasing wide bandwidth RF components readily available from more than one manufacturer. Consider that this approach may be more reliable because fewer active components are needed.

In a similar vein, some manufacturers have added optical I/O to their products. Optical interfaces allow for the use of much longer cables, especially for HD signals. For example, you could feed a remote secondary router in a system of managed tie lines without the need for external electrical-optical conversion hardware. In the future, we will likely see photonic routing, i.e., optical only, as a serious option for this industry. However, at this time, the cost of photonic routing may be too high for wide scale deployment.

Reliability

At least two manufacturers are exploring adding compressed outputs to routing switchers, with IT Ethernet connections for trunking lightly compressed signals between islands. It is not clear how ubiquitous such a strategy might become, but it is worth keeping an eye on.

Today's routers must be sophisticated and highly reliable, resulting in strategies for improving reliability that take into account real world needs and MTBF for individual components.

Today's large routers are often built on crosspoint architectures that use chip sets as large as 256×256 in a single module. The impact of a complete failure of such a module could be devastating. With so many circuits out of service at once, an entire facility could be rendered inoperative. Fortunately, the more likely failure modes would result in a much smaller impact.

However, when evaluating routers, consider the impact of a single board failure. For instance, the impact would affect only eight sources if inputs are grouped eight to a card. If the input card supports 64 signals, a single board failure could affect 64 sources, representing a much larger impact. Careful assignments of inputs to distribute the risk across multiple boards and systems can help prevent a catastrophic impact.

Router control

Main and backup feeds should not go through the same electronics. NVISION, Utah Scientific and others offer hot spare modules, which can replace cross points and in some cases inputs. With both the main and backup feeds continuously monitored, such systems can detect failure and automatically switch between inputs. Fortunately, an argument can be made that even though the total number of circuits within a router may increase, by virtue of redundant electronics and monitoring circuits, the multiplicative effects of MTBF results in a system no more likely to experience signal failures than traditional designs.

Despite all the fancy monitoring and signal processing that routers may offer, operators see only a control panel. A decade ago most control systems connected using proprietary, or at least broadcast-specific, communications over coax or multipin cable using low bit-rate asynchronous communications. Today, many routers rely on TCP/IP communications, facilitating interconnections using ubiquitous IT-based network structures.

Whether on a common switch fabric with other services, a segregated VLAN or a separate IP network reserved for routing control and status, this approach leverages inexpensive and robust bandwidth that can be extended easily across wide area networks as needed. This can add power and flexibility to a facility's workflow. In fact, new and enriching complexity becomes available because of the expanded bandwidth that is available to control new router functions.

Using IT technology facilitates graphical user interfaces and software panels that can reside on general-purpose computers. Although some manufacturers still use proprietary control busses for reasons that should not be ignored, be sure to explore the full range of options TCP/IP technology can provide. Compare the strengths and weaknesses carefully based on your particular application.

Finally, today's routers often use SNMP and proprietary schemes to become self-monitoring. A router can provide remote monitoring of its internal health and status, including temperature, power supply voltages, fan speeds and module failures, reporting all this and more back to a centralized control point. With system management software packages, engineers can be provided with a full complement of control, monitoring and logging features to keep tabs on what is really the backbone of most broadcast and recording facilities.

John Luff is senior vice president of business development at AZCAR.

ROUTING 4:2:2 DIGITAL SIGNALS

ITU-R BT601 defines the picture coding standard, using the shorthand notation of 4:2:2. This represents the method used to sample the picture before being coded for transmission over SMPTE 259M. The intent of the shorthand is to describe the relative sampling rates for the Y, CR and CB samples, with Y being sampled at roughly four times the color subcarrier (actually 13.5MHz), and the color difference channels sampled at half that of the luminance (6.75MHz).

Readers are cautioned that 4:2:2 is also a nomenclature used in MPEG standards to denote a completely different concept related to the relative coding of Y, C in the horizontal direction, and C in the vertical direction. The net result is a data rate of 270Mb/s for either 525 or 625 pictures, an important fact for much of our technology today. SMPTE 292 defines a similar NRZI bit stream with similar 4:2:2 coding based on a sample rate of 74.25MHz. SMPTE 292 also defines an optical interface. The picture coding standards for HDTV are 296M for 720 line systems and SMPTE 274M for 1920×1080 systems.