Applied Technology: Videotek's ShARC aspect ratio converters

Before Super Bowl XXXVI, Fox Sports was issued a technological challenge for the broadcast: deliver high-quality coverage in both 16:9 and 4:3 aspect ratios — in essence, one show, two formats. Fox turned to Videotek for the solution: its new ShARC aspect ratio converter.

Fox’s approach was straightforward: produce the Super Bowl in 16:9 and run it through an aspect ratio converter. Fox’s production people initially were concerned that converting it might compromise their work. So the production team rehearsed by producing games in 16:9 but broadcasting in 4:3. Fox integrated the converters into their digital trucks, wiring them directly into the device’s SDI inputs and outputs.

By shooting in 16:9 and protecting for 4:3, Fox created elements such as the graphics and the “Fox Box” (the strip that contains the score) so that all the pertinent information remained within the 4:3 screen while extending all the way to the edges of the 16:9 screen. The fixed-mode, aspect ratio converter was optimized for a single conversion. The unit converted the 16:9 video signal to 4:3 by horizontally stretching the central 75 percent of the input picture, with no vertical processing involved and no need for black bands at the top and bottom of the final picture.

Since vertical processing is absent in the converter, there is no degradation in vertical performance, and the circuit has a delay of only eight microseconds. The device has a wide and flat frequency response — 5.5 MHz (4.25 MHz after conversion) with a passband ripple of 0.1 dB. It exhibits very low levels of measurable conversion artifacts such as aliasing, banding and ripples. There are two operation modes: convert and bypass. Bypass delays unconverted video by the same amount as converted video so no timing changes occur when switching. The ShARC passes embedded audio and auxiliary data with the appropriate compensating delay to preserve lip sync or audio-video timing. EDH generation can be switched on or off. The system has one input and two outputs and can be configured to provide 12 channels in 2RU of rack space.

The video input accepts an SDI 270 Mbit signal to SMPTE 259M and EBU 3267 standards. It offers cable equalization of more than 200 meters for Belden 8281 or equivalent cable, and features an auto 525/625-line selection. The output is SDI 270 Mbit to SMPTE 259M and EBU 3267 standards with EDH. Picture processing is 10-bit with a 10-bit data path in the horizontal and vertical blanking intervals to accommodate embedded audio and data.

The GPI input level for active state requires a ground connection. For inactive, it requires a high impedance or +5 volts. GPI input current is less than 50µA. The local control is set by DIP switches. The remote control has two RS-422/485 serial ports — one connected to the front-panel frame, and one that can be linked to the rear of the frame instead of GPI signals. Frame panel control is available from an active front panel or a remote panel.

Over the next 10 years, as more content becomes available in DTV and the number of homes with 16:9 equipment continues to grow, the preparation that Fox has taken for Super Bowl XXXVI will continue to push the network further ahead of the curve. The Videotek ShARC 102 aspect ratio converter helped Fox make a visually compelling 16:9 and 4:3 production.

Mark J. Everett is vice president of advanced technology at Videotek. Additional reporting by freelance writer J. B. Duke.