As a PBS member station and the single largest producer of PBS television — including “NOVA,” “Antiques Roadshow,” “Frontline” and “Masterpiece Theatre” — and Web content, WGBH in Boston produces a variety of programming for our viewership throughout New England. While we transmit a strong HD/SD signal over the air, we also rely upon cable and satellite providers, such as Thames Valley Communications in Groton, CT, to deliver our channel to regional viewers, especially to those outside of our coverage area.
The challenge
Thames Valley, a provider of digital cable, Internet and digital phone services, previously received our signal from a satellite service operated by Shaw Media, which uplinked major Boston channels for rebroadcast to its Canadian viewers.
In the spring of 2010, Shaw Media discontinued its satellite service. Thames needed to quickly find an affordable, reliable alternative means of receiving our signal.
With only a one-month lead time and little budget to work with, WGBH engineers began searching for an easily accessible, affordable, real-time, broadcast-quality transport system. We initially thought we would just secure dedicated telco circuits between our Boston plant and Thames Valley's headend, but that proved to be beyond our budgetary means.
The tests
I then remembered a demo I'd seen at NAB of the T-VIPS TVG420 ASI-to-IP video gateway, which sends a real-time ASI transport stream over IP networks. I called the manufacturer's U.S. office in Millburn, NJ, and they immediately loaned us two gateways that we could use for two months to conduct a test.
One gateway, which was installed in the equipment racks in our station's machine room, was set up to take a 6Mb/s MPEG-2 feed of our SD signal. It encapsulated that signal into IP packets and sent it over the Internet to Thames' headend. The other gateway, installed in Thames' headend, received that MPEG-2 stream and fed it to an MPEG-2 decoder that converted it back to baseband video and audio for rebroadcast.
Since WGBH needed to filter out its secondary services, we soon replaced our gateway with a different T-VIPS device, the CP510 transport stream processor. The processor automatically identifies and filters the specific PID component for WGBH-SD from our SMPTE-310 ASI transport stream and outputs an IP signal for real-time transport over IP networks including the Internet. This additional signal processing — filtering a specific PID from our ASI multiplex — spared us from having to dedicate an expensive ATSC encoder to solve this problem. The processor filters select services from the WGBH-DT transport stream for delivery to Thames Valley and enables flexible adaptation and filtering of MPEG transport streams and component filtering, as well as PSI/SI/PSIP table updates.
The results
For both transport tests — the gateway to gateway and processor to gateway — we found that Thames Valley successfully received broadcast-quality video and audio suitable for rebroadcast in real-time. Problems occurred at times between 8 p.m. and midnight when the IP-delivered WGBH signal showed signs of intermittent problems, such as black flashes and audio dropouts. During this time, Thames' data customers were heavily using their PCs to access the Internet, consuming significant bandwidth on the Thames circuit to the Internet. The problem was resolved when engineers increased the bandwidth allocated to the device.
Despite the fact that WGBH has more than 500Mb/s of bandwidth in and out of our plant, once that transport stream reaches the public Internet, it is subject to the vagaries of Internet traffic and packet routing. The processor and the gateway both solve this problem by applying advanced FEC to the signal to ensure a high quality of service. This results in a solid video picture and high-quality audio even over the public Internet in the presence of packet loss, latency and other defects caused by Internet routing.
After one full year of operation, this T-VIPS system has proven to be extremely cost-effective and reliable. WGBH purchased the processor and gateway, and we are free from the recurring monthly charges common to other broadcast systems. Thames Valley now benefits from this real-time signal transport system for a significantly lesser monthly charge than it was paying for Shaw Media's satellite service, and for far less than the cost of dedicated fiber circuits.
We considered it critical to retain WGBH programming within Thames' TV package. The CP510 and TVG420 made it possible to do that cost-effectively, within our tight timeframe and without quality compromises.
Michael Foti is director of engineering, WGBH Educational Foundation.
