MPEG-2 Channel Sharing Just Got Easier

A test last week conducted by the PBS Member Station Joint Master Control in Syracuse, N.Y., and broadcast equipment vendor Evertz demonstrated that it is possible to put two 1080i HD and four 480i SD streams on air in a single 6 MHz DTV channel.

While not a revolutionary accomplishment, the demonstration underscored that incremental advancements in MPEG-2 encoding efficiency and statistical multiplexing (stat mux) continue to be made and that those improvements will make it easier for two stations to share a single channel.

“What we were trying to find out is how much we can squeeze into a standard ATSC channel without impairing picture quality,” says Terry Harvey, director of engineering and operations of the PBS facility in Syracuse.

The Syracuse test has implications for stations looking to share channels in the wake of the FCC’s incentive auction and for the possible transition to the next-gen ATSC 3.0 broadcasting standard.

The FCC is allowing stations that sell their spectrum in the auction to continue broadcasting by doubling up on another station. Broadcasters working on the ATSC 3.0 transition are considering creating legacy stations that would continue broadcasting ATSC 1.0 signals so that consumers do not have to run out and buy new ATSC 3.0 sets.

The joint master control operation services public broadcasters in New York, New Jersey, Hawaii, Pennsylvania and North Carolina.

Participating stations include WNED Buffalo, N.Y.; WXXI Rochester, N.Y.; WCNY Syracuse, N.Y.; WPBS Watertown, N.Y; WSKG Binghamton, N.Y.; WMHT Schenectady, N.Y.; WCFE Plattsburgh, N.Y.; WLIW New York City; WNET Newark, N.J.; WHYY Philadelphia; PBS Hawaii in Honolulu; and WTVI Charlotte, N.C.

For the Aug. 11 test of the Evertz 3480TXE encoder, the Joint Master Control doubled the channel load of a single typical PBS station: one 1080i and two SD program streams, including three AC-3 audio streams — main audio, descriptive audio and Spanish language — as well as 5.1 Surround Sound for the HD stream, Harvey says.

According to Harvey, the Evertz unit appears to one-up the Harmonic Electra 9200, which Univision now uses to air two 1080i HD and two 480i SD channels. “I have seen the Harmonic [Electra 9200] demonstration and was impressed by it,” he says. “But this [the Evertz encoder] pushed the envelope a little bit further.”

For the test, the Joint Master Control created a dummy terrestrial VHF ch. 5, Harvey says. The test simulated an over-the-air transmission scenario in a closed circuit.

The signals were received on three devices: a one-year old Samsung HDTV, a 2009 National Telecommunications and Information Administration-coupon DTV-to-analog converter box and a Samsung set-top box circa 2003.

“That was an important part of our test — to prove that even legacy receivers will still function with this setup,” Harvey says. “Bear in mind this has to play to MPEG-2 decoding devices that effectively are locked in design in the mid-’90s.”

Programming fed to the Evertz encoder and stat mux varied from typical talking-head shows to more demanding nature, news documentaries and dramatic programs, says Harvey.

One aspect in particular that impressed Harvey about the performance of the 3480TXE was the way it handle on-screen artifacts, he says.

“We commented that the minor artifacts we were seeing on the video were very similar to what one would see with H.264 or the MPEG-4 compression — that is, the picture would become a bit softer rather than showing harsh macroblocking,” Harvey says.

Abdullah Merei, Evertz product manager, compression systems, says the unit takes advantage of a “superior video encoding psycho-visual algorithm” to handle scenes that are difficult to compress.

“The preprocessing stage looks at the difficult, high-motion and activity areas and works with the encoder in an adaptive way,” he explains. “The 3480TXE reduces the high blockiness effect and makes the final video pleasant to view, which creates a softer effect but not blurriness.”

Treating artifacts this way makes channel sharing a bit easier to swallow, Harvey says.

“Right now we are in a situation of choosing between being willing to live with maybe minor artifacts from time to time, or losing an entire service. It’s down to that choice because we are really up against the wall here.

“We are using an older, perhaps outmoded technology [MPEG-2], to try to sustain a service with limited resources available — namely, bandwidth — to deliver it,” he says. “Like anything in engineering, it is a compromise. What are you willing to live with?”

Neil Brydon, director of product management for the Harmonic Electra encoders, says that interest in the Electra X2, the company’s next-generation MPEG-2 encoder, is growing as the incentive auction approaches. Introduced in fall 2014 primarily for the satellite and IPTV markets, he says, the encoder achieves “25% better bandwidth” efficiency than its predecessor.

“Some folks are even considering going from 1080i to 720p, which requires about three-quarters of the bit rate,” he says. “Broadcasters are asking, ‘What constitutes good enough?’”

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