The world of terrestrial live shots exists largely in metropolitan canyons between steel and concrete, which have long caused havoc with terrestrial analog microwave transmission in the form of multipath interference. A single carrier bouncing off buildings arrives at the receiver with mutually canceling time delays, causing fading or complete dropout. Static multipath is tricky enough, but when an attempt is made to move the transmitter, dynamic multipath problems cause such wireless links to be tenuous.
ESPN used Link Research’s LinkXP1 digital wireless camera to broadcast the Stanley Cup.
The primary reason for official adoption of the COFDM as the European DVB-T standard has been its performance in severe multipath environments. American TV news engineers also noted that its use of multiple carriers, each of which is modulated at 90 degrees to those adjacent, provided less range but better multipath reception than the Canadian/American 8-VSB, though gains in 8-VSB performance have been made. COFDM actually eats multipath signals and grows stronger. With larger transmitters, news helicopters have reported an effective throw of 50 miles or more with omni patterns, compared with 90 to 100 miles with directional analog microwave under similar conditions.
A small amount of the total information stream is put on each of nearly 2000 narrowband carriers, and enough data is included to enable reconstruction and even reinforcement of the signal among the multipath variants seen at the receiving end. The secret of its robust RF performance is in the limited bandwidth and data rate on each carrier. Another advantage of wireless ENG cameras is the selection of data rates for specific conditions. Operators can choose between QPSK, 16 QAM or even 64 QAM modulation to custom-tailor the data rate for varying degrees of signal/noise performance vs. spectral efficiency, with QPSK offering the lowest bit rate and most robust performance.
Another selectable parameter exists in the transmission pattern. Small, omni-directional antennas for COFDM transmission have even made live shots from moving vehicles a relatively simple task. In static situations, however, range can be increased by using antennas with switched 90-degree segments, enabling the concentration of power over a narrower polar pattern. This represents a significant ENG advance because antenna alignment has typically stolen valuable time in setting up live shots. Recent developments in auto-tracking antennas promise to further increase range without sacrificing RF durability. In situations where multiple mobile cameras are needed inside a large building, receiving antenna arrays can be strung though the rooms in a manner long used by TV sound people for continuous pickup of wireless microphones. In fact, one new offering for wireless ENG cameras is diversity reception with two tiny truck-mounted rooftop antennas.
Operators must become familiar with the effects of combining MPEG-2 parameters such as 4:2:2 and 4:2:0 chroma sampling with the selectable modulation schemes for COFDM to suit local conditions because results can vary substantially. Adjustable group of picture (GOP) structure, the type of forward error correction (FEC) and horizontal picture resolution have dramatic effects on picture quality, so good results are anything but automatic. Longer GOP intervals will increase latency, a vital factor in cutting these shots with those from wired cameras. Over the past year, latency rates have decreased to less than two frames. Among the current models, the DVCPRO format and wavelet compression are being used, but most manufacturers have stayed with MPEG processing for its ability to get a quality picture onto a QPSK or 16 QAM bitstream.
As the technology has evolved, its box has shrunk and moved from trucks to the cameras themselves. The latest stage has seen standard battery mounts applied to the transmitters for mounting directly on the back or side of the camera batteries. Makers of dockable cameras have enabled substitution of the recorder section by a matched-mount COFDM transmitter. Of course, this trades local recording capability for increased live mobility.
Bennett Liles is a writer and TV production engineer in the Atlanta area.
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