Michael Grotticelli /
09.03.2009
Originally featured on BroadcastEngineering.com
Squeezing 20 channels through a 19Mb/s pipe

KAXT-CA, a low-power (Class A) community TV station based in San Jose, CA, is using statistical multiplexing, compression and IP distribution technology to multicast up to 20 digital subchannels through its existing bandwidth, which previously provided a single channel of analog TV (on UHF Channel 22).

More channels of digital audio and video means increased programming for viewers and a potential opportunity to generate new revenue. According to those involved, KAXT is one of the first stations in the United States to offer such a large number of channels within a 19.39Mb/s spectrum. The station began digital broadcasts July 30.

Calling it a “a technological feat”, Warren Trumbly, president of KAXT, said he was contemplating shutting down his LPTV station last year, after two years of losing money, when he discovered a way to get more value out of the station. Working with various vendors, he figured out that by deploying video processing and statistical multiplexing solutions from Harmonic, terrestrial RF transmission equipment from Linear Industries and PSIP generation from Triveni Digital, he could broadcast 12 digital video channels and four audio channels within his allotted 19.39Mb/s ATSC spectrum. And there’s room for five more audio services, which will be online in October.

Leveraging statistical multiplexing and noise-filtering algorithms developed by Harmonic, each channel gets from .3Mb/s to 5.5Mb/s of data, depending upon the content being broadcast (static scenes get less bandwidth than fast-moving content, like a music video). The total data pool includes about 17Mb/s for content and about 2.38Mb/s for audio and overhead (PSIP) data. Audio signals for all of the channels are delivered in mono.

The tightly integrated technology has facilitated a new multiplexed DTV service capable of broadcasting up to 20 MPEG-2 video and audio services within the station’s allotted ATSC spectrum. The service will include Spanish, Indian and Vietnamese-language content for the diverse population of the Bay Area that it serves.

Trumbly’s model is to enable the local communities to develop their own ethnic content and market it themselves. The way he sees it, he’s providing the 24/7 pipeline to the community.

“The quality is standard definition but quite good,” Trumbly said. “The real magic is in the statistical multiplexing. The video resolution [640] is mediocre, but the encoders don't require a lot of bandwidth. Some of our channels get a lot of bandwidth, while others get limited, like the audio channels. But all of our channels are local and serving a niche that was not filled in this area before.”

The station also distributes two shopping channels and an Internet-based channel coming in from a DSL connection, which does not require that much data throughput.

Six Harmonic DiviCom Electra 5000 Series multichannel encoders, as well as Harmonic’s ProStream 1000 with DiviTrackIP statistical multiplexing and NMX Digital Service Manager software, are used to compress and manage the bit rate of video channels. There’s also a Cisco 3500 Series switch.

Triveni Digital’s GuideBuilder PSIP generator manages the large number of virtual channels and EPG metadata files and feeds the carousel of data to the Harmonic ProStream 1000 remultiplexer over the IP infrastructure. A Linear Industries 1kW digital UHF transmitter and ATSC exciter (optimized to synchronize each of the 12 video signals) round out the system — using linear and nonlinear precorrection — to deliver what those involved (and some viewers) say is a “good-quality” signal across all of the channels.

There’s also an Alive circularly polarized TV antenna to propagate the signal, “which I love,” Trumbly said.

KAXT’s multiple channels are now available on UHF Channel 1, due to a PSIP conflict with another station (KRCB on Channel 22) that occurred when Trumbly decided to raise his antenna height from 1000ft to 3000ft, on Mt. Allison, to reach a wider audience. Now, his is the only Channel 1 (and Channel 1.1, 1.2 and 1.3) TV station in the nation.

Joel Wilhite, broadcast solutions manager at Harmonic, helped put the system together and said that KAXT now has a higher signal-to-noise ratio (MER of 34.2dBu) and one of the cleanest signals being broadcast within the San Jose DMA — that includes all full-power stations. That normally means signals devoid of noise and artifacts, so quality is high.

Wilhite said everything in the outgoing transport stream is treated like a constant bit rate (CBR), so the pool carves out 17Mb/s for the video PIDs, which vary based on the content, while the audio signals and PSIP tables run at a CBR, which all fits inside 19.39Mb/s.

“No one channel is ever starved for data,” Wilhite said. “Warren [Trumbly] has deployed ASI throughout his plant, whereas a lot of other broadcasters use SMPTE 310 interfaces up to the transmitters, so with very little investment he has done a great job.

“The video data rates are set to run at .3Mb/s to 6Mb/s. I never saw any of the encoders take more than 5Mb, and that was in an instant, not the average. The encoders average about 1.75Mb/s, while one of KAXT’s music video channels takes about 3Mb/s, due to extra motion in the picture.”

With all of his new channels, Trumbly’s new problem is to find enough content to fill those channels. Among his new bouquet, he’ll have the first 24/7 South Asian Indian channel, a Spanish Christian channel and a 24/7 Vietnamese channel that serves the largest Vietnamese population in the country.

“We see our success in providing as much niche programming and localism as we can,” he said. “Our station is now charging as much for our 24-hour channel that other stations are charging for a single hour of ad time, so we’re offering a good deal and helping these smaller communities build their identities among their respective populations. If they succeed, we succeed.”



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