Diversity Technology for Helicopter IFB and Fade Resistant
Terrestrial Reception

News helicopters need off-air pictures and IFB - both nearly impossible in the post-analog era. Brief fades below the "cliff effect" threshold result in pixelation and frozen screens. Most often, these fades are weather-related, but aircraft and even truck traffic can cause destructive reflections. These hits can pose serious problems for signals feeding cable systems or broadcast repeaters at some locations.

Cell phones could offer a solution in helicopters, but the use of cell phones in an airborne helicopter is illegal. Most cell systems can detect operation in the air and will turn off communications within a few seconds. Although your area may afford you the opportunity to use cell phones now, you run the risk of having your cell signal dropped, leaving you without any means of IFB while airborne.

While solutions for IFB for 'live' trucks using the ATSC broadcast stream are not new, porting such solutions to helicopters has been reported to be a real challenge. ATSC signal reception has been inconsistent in helicopters.

The search for a solution led Modulation Sciences, Inc. (MSI) to an old, but powerful, technology - diversity. Diversity has been used extensively since the 1950s, with voluminous technical literature to support it.

To solve both the helicopter and rebroadcast challenges, MSI adopted an approach using four LG sixth generation tuners. The diversity selection takes place on the transport stream.

The heart of the receiver is the DSP Diversity Processor, which selects portions of the transport stream from each of the four tuners to assemble into valid ATSC packets. It does this by analyzing the Transport Error Indicator flag in the TS packets in the transport stream, while the data is held in a short buffer. The buffer must be short to minimize latency - a critical parameter for effective IFB.

The Diversity Processor can output a reconstructed Transport Stream in a variety of formats: DVB ASI, SMPTE 310M, DVB SPI and USB 2.0. The USB 2.0 format is especially important for helicopter use. A USB Transport Stream makes it easy to connect the DSPversity™ to any PC, allowing decoding of the Transport Stream to video in software. MSI reported that tests showed software decoding of ATSC signals to be far more robust than doing so in available hardware subsystems. In helicopter operations, the breaking up of pictures from non-robust decoding to video can render all the advantages of diversity nil.

Reliable IFB is a critical requirement for helicopter ENG. By employing MPEG-1, layer 2 audio for IFB, a virtually unlimited number of high-quality IFB audio channels can be implemented, using encoding facilities easily available to the station (this implementation uses two channels per "ghost" PID).

The antenna system is critical for both helicopter and ground applications of diversity reception. For helicopters there are no commercially-available TV antennas. At best, an avionics antenna designed for another frequency and application may work on your channel. If it doesn't, the process of trial and error can get very expensive, and, more often than not, especially for UHF, nothing will be found that works. FAA requirements and the cost of airframe work on a helicopter make experimenting with different antennas a costly option. MSI has developed a proprietary diversity antenna system that requires no penetration of the fuselage.

The antenna array was flight tested at two locations, each over several days, with similar results. The placement of three antennas, one looking forward and one on each side of the helicopter, yielded 360º coverage on channel 9 - good coverage based on wavelength of the signal compared to the width of the helicopter. This same configuration for a channel 39 station using the same model helicopter produced about 300º of coverage with a 60º null off the tail. This null was only noticeable when flying toward a story (tail toward the tower). Once the aircraft was orbiting, the null was not apparent in either picture or IFB audio. In high speed runs (up to 150 knots) toward the station, there was no loss of lock unless the pilot jogged the aircraft. In either instance, VHF or UHF, there was no interference from rotor blade modulation of the signal.

Ground applications of diversity not only offer vastly increased reliability of reception within the radio horizon but also increased coverage beyond the radio horizon. Over-the-horizon, or troposcatter, reception was perfected by the military before the era of communications satellites. For extended-coverage ground applications of diversity for feeding cable and repeaters, the ideal antenna system has four antennas, each with as much gain as possible. High gain is imperative in over-the-horizon applications. MSI suggests that the four antennas form a box 50 to 100 wavelengths on a side. For mid-UHF channels, this is only 75 to 150 feet. Many variations on this arrangement are possible. If a tall tower is available, one can try stacking the four antennas at different heights and have no horizontal spacing. Generally a box arrangement is the best but polarization diversity is a viable alternative in some instances. While requiring a good antenna installation, a Net Present Value calculation for the system (that is factoring in the annual cost of either a satellite or fiber link for comparison) can show which to be more cost effective, with long hauls favoring use of antennas.

The DSPversity ATSC four-channel diversity receiver appears to offer an innovative solution to both program video and IFB in the air. This diversity receiver subsystem also may offer another alternative for reliable terrestrial reception to feed multiple service operators' redistribution systems.

2010 NAB Show Call for Speakers

Call for Technical Papers – NAB Broadcast Engineering Conference

The 2010 NAB Show will host the 64th Broadcast Engineering Conference. This world-class conference addresses the most recent developments in broadcast technology and focuses on the opportunities and challenges that face broadcast engineering professionals. Each year hundreds of broadcast professionals from around the world attend the conference. They include practicing broadcast engineers and technicians, engineering consultants, contract engineers, broadcast equipment manufacturers, distributors, R&D engineers plus anyone specifically interested in the latest broadcast technologies.

Do you have something to share?
If you feel qualified to speak at the NAB Broadcast Engineering Conference, we invite you to submit a technical paper proposal. Not all acceptable submissions can be included in the conference, due to the large number of submissions that are received and the limited number of available time slots.

PLAN TO ATTEND!
The IEEE Broadcast Technology Society
59th ANNUAL BROADCAST SYMPOSIUM
14 -16 October 2009
The Westin Alexandria
Alexandria, VA, USA
www.ieee.org/bts/symposium

Sign Up for NAB Satellite Uplink Operators Training Seminar
October 5 - 8, 2009
Washington, DC

Satellite interference costs your station time and money. If you or your operations staff has not received formal training for operating your uplink facilities, sign up for NAB's Satellite Uplink Operators Training Seminar. The course will be offered Oct. 5 to 8, 2009, at NAB's headquarters in Washington D.C.

This four-day course is designed to instruct about uplink operational practices, which minimize the risk of satellite transmission interference. This is an important course since the FCC rules require that a trained operator be present at all times during transmissions, either an earth station site or designated remote control point. Go to http://www.nab.org/satelliteSeminar/ or contact NAB Science & Technology Department at (202) 429-5346 or ccolerid@nab.org for information about the NAB Satellite Uplink Operators Training Seminar. If you are interested in sponsorship opportunities for this event contact NAB Advertising at (800) 521-8624 or advertising@nab.org.

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Now Through September 15

For a limited time, you can purchase The NAB Engineering Handbook from the NAB Store for only $159 (Book & CD) with the promo code: TechCheck (applied at checkout). The offer expires September 15, 2009.

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