Digital 'clean' switches can mean life or death on air
Dec 1, 2011 9:28 AM, By Michael Grotticelli
Ensemble Designs' 4445 "clean switch" ensures a station stays on the air in the event of a system failure.
In the world of broadcasting, there's nothing worse than loosing the main program feed and going off air. In fact, to the viewer, there's nothing, period. For the station, it means lost revenue, lost viewers and maybe a lost job.
To protect against this in the "old days," a station's chief engineer would get a call and they would then frantically go to where the main patchbay was located and manually switch it over to a secondary feed. This could sometimes take a minute or two before the station was back on the air. Meanwhile viewers saw color bars, a "stay tuned" message or, worse still, a black screen.
In today's highly competitive world, seconds are like hours. That's why over the last few years broadcasters have begun to install various types of digital "clean" or "quiet" switching systems that automatically move over from the main or primary signal to a preselected backup signal in the event of equipment failure. A switch that isn't clean could produce a hiccup in the signal, which could or could not be noticeable to viewers on their living room screens.
The device is usually located on the output of the terrestrial station, after the encoder (if there is one) and just before it hits the transmitter. Or, it could be use coming out of master control, so the main input could be the master control signal into the clean switch and then the backup might be a bypass signal. In the most common cable, satellite or telco plants, the switch might be located right before it is sent out to a program distributor, over a fiber link. In both scenarios, the idea is that when one signal goes down, a second one instantly replaces it.
Lots of options
Several companies offer products in this category, including Ensemble Designs (ensembledesigns.com), Evertz (www.evertz.com), Harris (www.broadcast.harris.com), Miranda Technologies (www.miranda.com) and T-VIPS (www.t-vips.com). All offer a full range of fail-safe bypass protection switches — because stations have a variety of ways in which they want to use them — and all of the switching products are typically part of an infrastructure line of cards that fit into a standard frame.
As part of its Avenue line of signal processing modules (cards), Ensemble Designs (Grass Valley, CA) offers switches for both baseband and ASI signals. The switches typically have two inputs and a single output. In situations where a station might have its switch installed before the final output, it might also use a HD-SDI switch before the encoder.
Ensemble's newest "smart switch" (Avenue 4445) is designed for ASI transmission streams. It includes new MPEG analysis and switch technology that combines detailed analysis of the transport stream and removes all jitter in DVB-ASI streams. The company said the 4445 is both a clean switch and a fail-safe bypass protection switch that uses internal buffers to align the two streams to matching points to allow the clean switching. It switches at the beginning of an I-frame so that downstream equipment is never disrupted.
Cindy Zuelsdorf, marketing czar, at Ensemble Designs, said clean switching is critical to enable downstream equipment such as MPEG encoders and home set-top decoders to recognize a new signal in the event of a system failure at the station.
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