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WLS-TV's on-air lessons

ABC O&O WLS-TV Chicago has put the finishing touches on a $5 million build-out to support digital television operation and new master-control facilities. The multiyear project was aimed at experimenting with new technologies and learning from those who had gone on-air with DTV before.

The station is now operating two channels: its current NTSC channel (ch. 7) and its DTV channel (ch. 52). The latter, offering upconverted NTSC programs and limited weekend feature films in HD from ABC (plus weekly NYPD Blue episodes in HD), runs in 720p, the format of choice for the network.

The building that houses WLS was originally a furrier in the 1920s and is located on the corner of State and Lake streets, across the street from the historic Chicago theater. The station used to occupy the top floors of the 13-story building, the 12th housing most of the equipment for its NTSC operations. Over the years, the building had been purchased by station management and new studios and control rooms built on the first and third floors. It was on the third floor that "TV3," its all-digital studio and control room, was built last year.

"Having the equipment room on the 12th floor, far above the new studio, created many problems with the long cable runs that were necessary," says Director of Engineering Kal Hassan. "Any time you're dealing with power on different floors that far apart, you get hum in the system."

One of the first things he did was to move the equipment room from the 12th floor to the fifth floor, which was empty. This made the cable runs shorter and solved a number of problems. The fifth floor is approximately 12,000 square feet. The master control is now located there, with all the equipment (for both analog and digital) racks rewired and taking up about 5,000 square feet.

One critical lesson Hassan learned—one that anyone creating a serial digital infrastructure should be aware of—is latency: Because of the digital-to-analog conversion, the video picture often lags behind the audio signals. To prevent this, two control rooms—which have Sony 7350 production switchers, each with three channels of digital effects—were complemented with Solid State Logic Aysis Air digital audio consoles, with audio delays added to the outputs to compensate for the video latency.

To manage the multiple feeds, WLS purchased a large (160x192) Venus2001 digital router with a JupiterPlus control system from Philips Digital Networks' Media Networking and Control division. This serves as a complement to its existing Venus 128x128 analog video router and Venus 128x128 stereo audio router. Hassan notes that the station has already outgrown the digital I/O capacity and is looking to expand it. "One thing I would recommend is that stations should install the biggest router they can afford."

WLS has also added an ADC Broadcast 256x256 digital AES audio router for standard-definition distribution and a high-definition (32x32) router to accommodate an HD island, including feeds from the network. The advantage of JupiterPlus, according to Hassan, is that it can easily control routers from other manufacturers, such as the ADC system.

The second phase of the rebuild, accommodating both NTSC and DTV, was a new master-control room, based on the Philips Saturn switcher. Designed as a digital island in the building, it has serial digital video and multichannel AC-3 digital audio. The rest of the facility is not AES-stereo-compatible yet because not all the feeds are stereo audio. The NTSC channel is served by an SDV601 Saturn with four channels of digital audio (stereo, SAP and DVS). The DTV channel is served by an HD-video Saturn with six channels of digital audio (for surround sound). Both the NTSC and the DTV channels are under Florical automation control.

Up to 34 Digital Processing Systems 470 and 475 frame synchronizers are used.

The ADC HD router provides WLS with the capability to handle six channels of Dolby Digital (AC-3) audio. When the network sends a movie with multichannel audio, it can be decoded into its component six channels and passed through the Saturn switcher on six discreet channels. The audio is then sent to a Dolby AC-3 encoder, which feeds a Harris/Lucent encoder. The encoder combines the HD video, audio and data to produce the "transmission stream" of 19.39 Mb/s for distribution to the transmitter and, ultimately, to consumers' homes.