Digital 'clean' switches can mean life or death on air

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 (, Evertz (, Harris (, Miranda Technologies ( and T-VIPS ( 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.

ASI stream clean switching
"This [smart switching] can be tricky in an ASI bit stream because it has to look at the next packet or group of pictures to align itself to and then make the switch seamless so no one outside the station notices," Zuelsdorf said, adding that engineers can set the box to switch based on different parameters, such as a missing audio Packet Identifier (PID), reduced bit rate and loss of video content (the picture can't be viewed or checked while it's part of the ASI stream, unless it's decoded, but data presence can be monitored).

The 4445 acts as a time base corrector for ASI streams by removing all jitter and putting a new clock onto the output stream. A more accurate clock is also beneficial to downstream devices such as MPEG encoders and transmitters. Other applications include STLs, satellite feeds, transoceanic cable links, terrestrial fiber and single-frequency networks (SFNs).

The switch has to have some intelligence included because it might have to switch from a baseband-to-ASI stream to a secondary fiber or IP stream, which results in timing difference (because the path is different) that have to be resolved before the signal goes out to the public. The switch can momentarily delay the first feed to "catch up" to the backup, and then go out to air.

The 4445 can be set up to tightly synchronize the primary signal with the secondary signal and find a good place to make the switch, so that viewer never notices a difference. Those stations outputting an ASI stream could also use an IP feed as the backup signal. These smart switches can automatically recognize the difference between the two signals and implement the correct one.

Engineers can also be alerted via cell phone when a problem arises, and they can activate the secondary feed from any remote location. It can also be set up so that if just part of the audio fails, a lower level technician will be alerted, but when there's a catastrophic loss, the chief engineer is alerted. Engineers can also set up their systems to communicate with other sister stations and monitor them remotely as well.

Invaluable protection
Don Mackay, chief engineer at Oregon Public Broadcasting (OPB), uses the Ensemble product to alert him on his cell phone. The PBS station also uses a number of the company's Avenue "glue" (signal processing) products.

"There's no way to describe the value of a switch like this, except to say, when it's working, you never know it, and when it's not, everybody knows it," Mackay said. "We've had success with all of the Ensemble Design products we use, but this switch has really been a great addition. In fact, it sometimes provides more information than I need, and I'm having to decide when to alarm and when not to alarm. Sometimes I'll get an alarm and call into the station to find out it was a false, or noncritical, alarm."

OPB maintains five full-power transmitters and numerous translators spread out across the state, so its signal is distributed via ASI over a fiber line. The broadcaster transmits one HD and two SD channels, which all go through the Ensemble 4445 card. There was an issue where it went off the air, due to weather, about a year ago and the card switched over without anyone ever knowing it.

"I'm more interested in 5-second interruptions at the transmitter," said Phil Olvera, a maintenance engineer at OPB who set up the automatic switch three years ago and uses it manually for maintenance activities from time to time. "Sometimes we have momentary interruptions, due to patching, but the viewer never knows it. I need to be alerted when the transport stream disappears. This card can tell me that within seconds."

Olvera agrees with Mackey when he says that a failure-proof switch is one of those products you install but hope you never have to use.